A ESICM Multidisciplinary Distance Learning Programme for Intensive Care Training

A ESICM MULTIDISCIPLINARY DISTANCE LEARNING PROGRAMME FOR INTENSIVE CARE TRAINING

Mechanical ventilation Instructions for Simulators

Module and Simulator Author GIORGIO ANTONIO IOTTI Head of Anaesthesia and Intensive Care Department Ospedale Civile “Santi Antonio e Biagio” Azienda Ospedaliera “Santi Antonio e Biagio e Cesare Arrigo” Alessandria, Italy

A. Introduction

The following instructions should be read before using any of the four simulators:

 B-Collapse v.1.0 with Expiratory Bronchial Collapse  CurviLin v.1.0 with Static Pressure-Volume Curve  EasyTrigger v.1.0 focusing on the Inspiratory Trigger  Virtual MV v.4.0

B. General information

Important

Other Excel files opened before a simulator and remaining open may affect the calculation setting of Excel and the proper working of the simulator. However, the simulators are compatible, and can be used together. We therefore recommend you close other active Excel files before opening a simulator.

Documents and application

The simulators have been developed with Microsoft Excel 98 for Macintosh. It is compatible with Excel 97/98 versions or higher for Macintosh and Windows.

Depending on the graphic settings and display of your computer, you may need to adjust some settings of Excel to optimise the full view of the Control Panel and Graphic Monitor of the simulator worksheets. If some parts are outside your screen, you should:  Turn off unnecessary Excel bars (for instance, you could just keep the Standard Instruments and Status bars, and turn off the other Instruments bars and the Formula bar) - 2 -

 Adjust the zoom setting of each worksheet, if necessary.

If you want, you can save the results of given settings by saving the active document using the normal controls of Excel. It is suggested to use the Save as ... option, and to keep a backup copy of the original Excel file just in case there is an irreversible crash.

Each worksheet is protected against accidental corruption. Removing the protection is not advisable, but possible, with no password.

The calculation of steady state data is complex, especially when patient-ventilator interaction generates dynamic pulmonary hyperinflation. In all the simulators, except EasyTrigger, the results are obtained by iterative calculation. In the simulators the default setting of calculation (see Tools/Options/Calculation) corresponds to:  Automatic calculation: checked  Iterations: checked  Iteration maximum number: 25  Iteration allowed deviation: 0.01. If your computer is very fast and you want to improve the precision of the results, you can increase the maximum number of iterations and decrease the allowed deviation.

The calculation is continuous and automatic. Normally you will not need to be concerned about calculation: just operate the controls as you want, and wait a short while for updated results. If your computer is not very fast and a simulator is running slowly, you may be interested in checking the calculation status, by turning on the Status bar of Excel.

Should you experience slow operation of a simulator or other problems, you should try the following:  Close any other Excel file,  Check the calculation setting of Excel (iteration maximum number and allowed deviation),  Close all unnecessary applications,  Increase the memory assigned to Excel (Mac OS),  Switch to a faster computer...

Remember: Other Excel documents started before a simulator and still active may affect the calculation setting of Excel: automatic calculation or iterations may be turned off, or the iterations settings may deviate from ideal values. Hence the recommendation that you close other Excel documents before opening a simulator.

If you find that a given control setup provides strange results, carefully consider your control settings. In most of cases, strange outputs will be as a result of strange inputs, exactly like in clinics with a real ventilator.

In CurviLin and Virtual-MV in order to check whether the results of a given control setup are valid, you should:  Check that each graph starts and ends at the same value level (especially, check the spirogram)  Manually launch the iterative calculation by pressing the key F9: results are valid when stable, and acceptable when reasonably stable.

If the displays show #######, it means that you have set out-of-range or conflicting inputs. Don't worry. Just return to more normal settings and wait a while: The simulator normally recovers! If not, close the active document without saving, and restart with a backup copy of the simulator.

In EasyTrigger if the message Apply sufficient Pmus,max to activate Trigger ! appears, it means that the trigger threshold is not reached within the explored time range. Before increasing Pmus,max, you can try to reduce the Zoom in order to explore a wider time range.

C. Specific instructions for the Simulators - 3 -

B-Collapse v1.0

Purpose

B-Collapse is an Excel-based simulator of mechanical ventilation designed for calculation and graphic representation of the mechanics of a single respiratory cycle in passive Volume- Control Ventilation (VCV) in a steady state.

B-Collapse simulates a COPD patient with intrinsic PEEP caused by:  High compliance due to emphysema  Increased airway resistance  Major increase in expiratory resistance due to expiratory bronchial collapse.

B-Collapse simulates a favourable interaction between external PEEP and intrinsic PEEP. Low levels of external PEEP result in reduction of bronchial collapse and of intrinsic PEEP, without increasing total intrapulmonary PEEP and pulmonary hyperinflation. Higher levels of external PEEP, on the contrary, maintain the advantage of major reduction of intrinsic PEEP, but increase total PEEP and pulmonary hyperinflation.

B-Collapse shows how the tidal breath moves along the Static Pressure-Volume Curve of the respiratory system depending on the variable effect of external PEEP.

Models

The Ventilator of B-Collapse behaves as an ideal ventilator. The effects of circuit compliance (due to elasticity and gas compression) and circuit resistance are not taken into account, and thus the ventilator moves the airway pressure and flow more rapidly when cycling from inspiration to exhalation and vice versa. Hence the ventilator of B-Collapse behaves in a similar way to those ventilators that are provided with an efficient system for circuit resistance compensation. The data shown in the graphs are pertinent to the airway opening, and hence similar to the data shown by those ventilators that are provided with proximal sensing for airway pressure and flow at the airway opening.

With the exception of the PEEP setting, all the ventilator controls are fixed in B-Collapse. The ventilator is set with a Tidal Volume of 800 ml, low Frequency of 10 b/min, and low I:E ratio of 1:3.

In B-Collapse the Respiratory System corresponds to a single-compartment model, with constant compliance, constant inspiratory resistance and variable expiratory resistance.

The Static Pressure-Volume (P-V) Curve of the respiratory system has no lower inflection, and is linear up to an upper inflection point (UIP - expressed as static pressure) of 35 cmH2O. In the entire range of the possible settings of B-Collapse, ventilation develops within the linear part of the static P-V relationship, that corresponds to a respiratory system compliance (Crs) of 80 ml/cmH2O.

The inspiratory airway resistance corresponds to 23 cmH2O/l/s. During exhalation, an additional resistance of 5 cmH2O/l/s is considered (Rexp,add), for simulation of the expiratory resistance of the apparatus, thus resulting in a basal total expiratory resistance of 28 cmH2O/l/s. After the start of exhalation, the expiratory resistance progressively increases from its basal level, for simulation of expiratory bronchial collapse, resulting in an intrinsic PEEP of 10 cmH2O when the external PEEP is set at zero. This increase in expiratory resistance is progressively attenuated by increasing external PEEP levels up to 8 cmH2O (80% of the basal intrinsic PEEP). External PEEP levels higher than 8 cmH2O are associated with no further attenuation of bronchial collapse simulation.

General description - 4 -

The File B-Collapse.xls is made up of three main Worksheets, each one designed with the same controls and a different graphic display. Each worksheet has a control panel and a graphic monitor.

The Control Panel is divided into two sections, for:  Patient's Passive Mechanics (light blue)  Ventilator (light green).

The control panel includes:  Controls  Displays, typed in blue, and showing "monitored" data that result from control settings and patient-ventilator interaction. In B-Collapse, only the control for PEEP (shown in red) can be altered the user.

Note: You do not need, and should not, directly enter numeric data in the worksheet cells. You should just operate the arrows of the control for PEEP.

The Graphic Monitor includes: - Real-time graphs for - Paw airway opening pressure - Flow airway opening flow - Volume volume change (spirogram) - Palv alveolar pressure. - or X-Y graphs for - Flow-Volume loop - Paw-Volume loop - Palv-Volume loop - Dynamic Paw-Volume loop on Complete Static P-V curve - Tidal P-V curve on Complete Static P-V curve.

All data on displays and graphs refer to a single breath in a steady state.

Each worksheet is linked to the other ones. Any change in the PEEP setting of a given worksheet is automatically mirrored on the other worksheets.

Controls

Patient's Passive Mechanics The controls for passive mechanics are: - 5 -

 Crs: respiratory system compliance (best compliance on the static P-V curve)  Rrs,insp: respiratory system inspiratory resistance  Rexp,add: additional expiratory resistance.  LIP: lower inflection point of the static P-V curve, expressed as static pressure  UIP: upper inflection point of the static P-V curve, expressed as static pressure.

Ventilator The ventilation mode is VCV, with the following controls:  Freq.: frequency  Vt: tidal volume  Ti%: inspiratory time expressed as inspiratory duty cycle in %  PEEP: positive end-expiratory pressure applied by the ventilator.

Note: Only the PEEP control, typed in red, is available for the user.

Displays

The displays, typed in blue in the control panels, show "monitored" data that result from control settings and patient-ventilator interaction.  MinVol: minute ventilation  Ti: inspiratory time, expressed in seconds  Te: expiratory time, expressed in seconds  PEEP: external PEEP  PEEPi: intrinsic PEEP  PEEPtot: total intrapulmonary PEEP  Vol,PEEP: FRC increase due to PEEP  Vol,PEEPi: dynamically trapped volume associated with PEEPi and added on top of Vol,PEEP  Vol,ee: total end-expiratory volume above atmospheric FRC, i.e. Vol,PEEP plus Vol,PEEPi.

The displays include parameters that ventilator monitors cannot provide, or usually do not provide, like:  PEEPi, expressing dynamic pulmonary hyperinflation in terms of pressure  PEEPtot, expressing total end-expiratory inflation in terms of static end-expiratory pressure  Vol,PEEP, expressing the increase in FRC due to PEEP  Vol,PEEPi, expressing dynamic pulmonary hyperinflation in terms of volume  Vol,ee, expressing total end-expiratory inflation in terms of volume above the atmospheric FRC.

Graphic monitor

Basic signals Volume. The zero of the Volume signal always corresponds to the Functional Residual Capacity (FRC) at atmospheric pressure. The Volume baseline is affected by both PEEP and intrinsic PEEP. The instantaneous Volume rises during inspiration, and decreases during exhalation.

Flow. The inspiratory flow is represented as positive, and the expiratory flow as negative.

Paw. The Paw zero corresponds to the atmospheric pressure. The Paw baseline corresponds to PEEP.

Palv. The Palv zero corresponds to the atmospheric pressure. The Palv baseline is the sum of PEEP and intrinsic PEEP.

Real-time graphs - 6 -

The upper panel combines Volume (in litres) and Flow (in l/min) on the same value scale. The lower panel combines Paw and Palv on the same scale (in cmH2O).

Dynamic loops The upper panel represents the Flow-Volume loop. The lower panel combines the Paw-Volume and Palv-Volume loops.

P-V curves The upper panel represents the Dynamic Paw-Volume loop, plotted on the Complete Static P- V curve from pressure zero to 60 cmH2O. The lower panel represents the Tidal Static P-V curve, plotted on the Complete Static P-V curve from pressure zero to 60 cmH2O.

Additional notes on graphic monitor

The graphic monitor is similar to the one of modern ventilators, but its performance has been made much higher, in order to provide data for an easier and more complete understanding of all the phenomena.

In ventilator monitors, the Volume signal is reset at zero at the start of every breath. In other words the Volume signal baseline corresponds to the end-expiratory lung volume of the previous breath, taken as zero in the volume scale. On the contrary, in B-Collapse the zero of the Volume signal always corresponds to the Functional Residual Capacity (FRC) at atmospheric pressure. Therefore, unlike the monitor of any real ventilator, B-Collapse shows a rise above zero in the spirogram and in the X-Volume loops, showing the cumulative effect of intrinsic PEEP and external PEEP. This rise corresponds to the value of the parameter displayed as Vol,ee , that describes the total increase of the end-expiratory volume above the atmospheric FRC.

Note. The Volume plot of B-Collapse is different to the one in Virtual-MV. In B-Collapse the Volume zero corresponds to the atmospheric FRC and hence the Volume baseline depends on the cumulative effect of intrinsic PEEP and external PEEP. On the contrary, in Virtual-MV the Volume zero corresponds to the FRC as modified by PEEP, and hence the Volume baseline only reflects the effect of intrinsic PEEP, while it is never affected by the PEEP setting. - 7 -

The graphic monitor of B-Collapse also shows graphs for the Alveolar Pressure (Palv), a parameter that, unfortunately, cannot be shown in the monitors of real ventilators. In clinics Palv cannot be recorded continuously, but just observed occasionally by performing an occlusion manoeuvre by means of the Hold functions of the ventilator. Palv is the absolute value of the average pressure instantaneously ruling inside the alveoli. In a passive patient, it depends on the lung volume above the atmospheric FRC and on the elastic recoil of the respiratory system (expressed by the compliance). The baseline of Palv corresponds to the total intrapulmonary PEEP, i.e. to the sum of externally applied PEEP and intrinsic PEEP (PEEPi). Therefore you will see the Palv curve baseline rise above PEEP, whenever the patient-ventilator interaction generates PEEPi and dynamic pulmonary hyperinflation. The rise will correspond to the value of PEEPi shown in the display.

The Complete Static P-V curve shown by B-Collapse is plotted in the pressure range between zero (atmospheric pressure) and 60 cmH2O, and depends on the user settings for Crs, LIP and UIP. In B-Collapse the P-V curve is a straight line up to 35 cmH2O, with the slope corresponding to Crs. Above 35 cmH2O the curve shows an upper inflection, simulating respiratory system overdistention. The simulated Static P-V curve is similar to the curves obtained clinically with the continuous slow-inflation methods, as provided by the monitoring system of the most advanced ventilators.

The Tidal Static P-V curve is the part of the complete curve that is interested by the tidal breath, and moves along the complete curve depending on PEEPtot and Vt. In practice it is a plot of dynamic Palv superposed on the Complete Static P-V curve.

Cycle start and end, as shown in the real-time graphs, are defined by the point of zero crossing of flow between exhalation and inspiration.

Version history

V1 - B-Collapse.xls (2002) Designed for the ESICM PACT module on Mechanical Ventilation. B-Collapse is based on CurviLin v1.0 (2002). Variable expiratory resistance and PEEP-PEEPi interaction have been implemented. All controls (except the PEEP control) have been set at fixed values for simulation of a passively ventilated COPD patient. v1.0 (2002) - 8 -

CurviLin v1.0

Purpose

CurviLin is an Excel-based simulator of mechanical ventilation designed for calculation and graphic representation of the mechanics of a single respiratory cycle in passive Volume- Control Ventilation (VCV) in a steady state.

CurviLin relies on a respiratory system model with nonlinear compliance, and shows how the tidal breath moves along the Static Pressure-Volume Curve of the respiratory system depending on ventilator settings and patient-ventilator interaction.

CurviLin has been designed as an interactive educational tool. By trying various inputs, you can simulate the mechanics of different patients (for instance, those with obstructive or restrictive lung disease), and observe the mechanical response to different ventilator settings.

Models

The Ventilator of CurviLin behaves as an ideal ventilator, with no limits in flow delivering capability, and able to perfectly control the airway pressure and flow at the airway opening. The effects of circuit compliance (due to elasticity and gas compression) and circuit resistance are not taken into account, and thus the ventilator moves the airway pressure and flow more rapidly when cycling from inspiration to exhalation and vice versa. Hence the ventilator of CurviLin behaves in a similar way to those ventilators that are provided with an efficient system for circuit resistance compensation. Due to a number of features of the CurviLin ventilator, the peaks of flow may be higher than usually observed with real ventilators. The monitored data in displays and graphs are pertinent to the airway opening, and hence similar to the data shown by those ventilators that are provided with proximal sensing for airway pressure and flow at the airway opening.

In CurviLin the Respiratory System corresponds to a single-compartment model with nonlinear compliance. The Static Pressure-Volume (P-V) Curve of the respiratory system depends on the settings for:  LIP: Lower Inflection Point, expressed as static pressure  UIP: Upper Inflection Point, expressed as static pressure  Crs: compliance in the part of the curve between LIP and UIP (best compliance) The compliance below the LIP and above the UIP is equal to Crs divided by 3. At the same static pressure, the compliance is the same during inspiration and exhalation.

The respiratory system is basically described by one value of resistance (Rrs) for the entire cycle. Optionally, you can set an Additional Expiratory Resistance (Rexp,add). The Rexp,add control makes the expiratory resistance higher than the inspiratory resistance, and can be used:  To simulate the effects of expiratory bronchial collapse,  Or to simulate the expiratory resistance of the apparatus (in order to simulate the behaviour of common ventilators, not provided with expiratory circuit resistance compensation, it is suggested you set the Rexp,add control at a value of at least 5 cmH2O/l/s).

In CurviLin the patient is only passive, and always perfectly relaxed.

Note: The respiratory system model does not include any function to simulate interaction between external and intrinsic PEEP. External PEEP has just an additive effect on intrinsic PEEP. - 9 -

General description

The File CurviLin.xls is made up of three main Worksheets, each one designed with the same controls and a different graphic display. Each worksheet has a control panel and a graphic monitor.

The Control Panel is divided into two sections, for:  Patient's Passive Mechanics (light blue)  Ventilator (light green).

The control panel includes:  Controls, typed in black, and operated by arrows  Displays, typed in blue, and showing "monitored" data that result from control settings and patient-ventilator interaction.

Note: You do not need, and should not, directly enter numeric data in the worksheet cells. You should just operate the arrows.

The Graphic Monitor includes: - Real-time graphs for - Paw airway opening pressure - Flow airway opening flow - Volume volume change (spirogram) - Palv alveolar pressure. - or X-Y graphs for - Flow-Volume loop - Paw-Volume loop - Palv-Volume loop - Dynamic Paw-Volume loop on Complete Static P-V curve - Tidal P-V curve on Complete Static P-V curve.

All data on displays and graphs are referred to a single breath in steady state.

Each worksheet is linked to the other ones. Any change in the control settings of a given worksheet is automatically mirrored on the other worksheets. - 10 -

Controls

Patient's Passive Mechanics The controls for passive mechanics are:  Crs: respiratory system compliance (best compliance on the static P-V curve)  Rrs: respiratory system resistance  Rexp,add: additional expiratory resistance.  LIP: lower inflection point of the static P-V curve, expressed as static pressure (range: Off, 5-15 cmH2O)  UIP: upper inflection point of the static P-V curve, expressed as static pressure (range: 25-40 cmH2O, Off).

Ventilator

The ventilation mode is VCV, with the following controls: - Freq.: frequency - Vt: tidal volume - Ti%: inspiratory time expressed as inspiratory duty cycle in % - PEEP: positive end-expiratory pressure applied by the ventilator.

Displays

The displays, typed in blue in the control panels, show "monitored" data that result from control settings and patient-ventilator interaction.  MinVol: minute ventilation  Ti: inspiratory time, expressed in seconds  Te: expiratory time, expressed in seconds  PEEP: external PEEP  PEEPi: intrinsic PEEP  PEEPtot: total intrapulmonary PEEP  Vol,PEEP: FRC increase due to PEEP  Vol,PEEPi: dynamically trapped volume associated with PEEPi and added on top of Vol,PEEP  Vol,ee: total end-expiratory volume above atmospheric FRC, i.e. Vol,PEEP plus Vol,PEEPi  Cqs: quasi-static compliance.

Note: Due to the nonlinear compliance of the model, the displayed valued of Cqs (actual quasi-static compliance) can be lower than the control setting of Crs, that defines the best compliance on the static P-V curve.

The displays include parameters that ventilator monitors cannot provide, or usually do not provide, like:  PEEPi, expressing dynamic pulmonary hyperinflation in terms of pressure  PEEPtot, expressing total end-expiratory inflation in terms of static end-expiratory pressure  Vol,PEEP, expressing the increase in FRC due to PEEP  Vol,PEEPi, expressing dynamic pulmonary hyperinflation in terms of volume  Vol,ee, expressing total end-expiratory inflation in terms of volume above the atmospheric FRC.

Graphic monitor

Basic signals Volume. The zero of the Volume signal always corresponds to the Functional Residual Capacity (FRC) at atmospheric pressure. The Volume baseline is affected by both PEEP and intrinsic PEEP. The instantaneous Volume rises during inspiration, and decreases during exhalation. - 11 -

Paw. The Paw zero corresponds to the atmospheric pressure. The Paw baseline corresponds to PEEP.

Palv. The Palv zero corresponds to the atmospheric pressure. The Palv baseline is the sum of PEEP and intrinsic PEEP.

Real-time graphs The upper panel combines Volume (in litres) and Flow (in l/min) on the same value scale. The lower panel combines Paw and Palv on the same scale (in cmH2O).

Dynamic loops The upper panel represents the Flow-Volume loop. The lower panel combines the Paw-Volume and Palv-Volume loops.

P-V curves The upper panel represents the Dynamic Paw-Volume loop, plotted on the Complete Static P- V curve from pressure zero to 60 cmH2O. The lower panel represents the Tidal Static P-V curve, plotted on the Complete Static P-V curve from pressure zero to 60 cmH2O.

In ventilator monitors, the Volume signal is reset at zero at the start of every breath. In other words the Volume signal baseline corresponds to the end-expiratory lung volume of the previous breath, taken as zero in the volume scale. On the contrary, in CurviLin the zero of the Volume signal corresponds always to the Functional Residual Capacity (FRC) at atmospheric pressure. Therefore, unlike the monitor of any real ventilator, CurviLin shows a rise above zero in the spirogram and in the X-Volume - 12 - loops, whenever a PEEP is applied and/or the patient-ventilator interaction generates intrinsic PEEP and dynamic pulmonary hyperinflation. This rise corresponds to the value of the parameter displayed as Vol,ee , that describes the total increase of the end-expiratory volume above the atmospheric FRC.

Note: The Volume plot of CurviLin is different from the one of Virtual-MV. In CurviLin the Volume zero corresponds to the atmospheric FRC and hence the Volume baseline is affected by the PEEP setting. On the contrary, in Virtual-MV the Volume zero corresponds to the FRC as modified by PEEP, and hence the Volume baseline is not affected by the PEEP setting. In both CurviLin and Virtual-MV the Volume baseline is affected by intrinsic PEEP.

The graphic monitor of CurviLin also shows graphs for the Alveolar Pressure (Palv), a parameter that, unfortunately, cannot be shown in the monitors of real ventilators. In clinics Palv cannot be recorded continuously, but just observed occasionally by performing an occlusion manoeuvre by means of the Hold functions of the ventilator. Palv is the absolute value of the average pressure instantaneously ruling inside the alveoli. In a passive patient, it depends on the lung volume above the atmospheric FRC and on the elastic recoil of the respiratory system (expressed by the compliance). The baseline of Palv corresponds to the total intrapulmonary PEEP, i.e. to the sum of externally applied PEEP and intrinsic PEEP (PEEPi). Therefore you will see the Palv curve baseline rise above PEEP, whenever the patient-ventilator interaction generates PEEPi and dynamic pulmonary hyperinflation. The rise will correspond to the value of PEEPi shown in the display.

The Complete Static P-V curve shown by CurviLin is plotted in the pressure range between zero (atmospheric pressure) and 60 cmH2O, and depends on the user settings for Crs, LIP and UIP. When LIP is of at least 5 cmH2O and UIP is equal to 40 cmH2O or lower, the curve shows a lower and an upper inflection corresponding to LIP and UIP, respectively. When LIP and UIP are Off, the P-V curve becomes a straight line with the slope corresponding to Crs. The simulated Static P-V curve is similar to the curves obtained clinically with the continuous slow-inflation methods, as provided by the monitoring system of the most advanced ventilators.

The Tidal Static P-V curve is the part of the complete curve that is interested by the tidal breath, and moves along the complete curve depending on PEEPtot and Vt. In practice it is a plot of dynamic Palv superposed on the Complete Static P-V curve.

Cycle start and end, as shown in the real-time graphs, are defined by the point of zero crossing of flow between exhalation and inspiration.

Version history

V0 - PVplus.xls (2000) The progenitor. It allowed plotting of a static P-V curve with user's settings for LIP, UIP and the compliance of the three sections of the curve (low, intermediate and high). User's settings for Vt and PEEPtot allowed plotting of the Tidal Static P-V curve on the Complete Static P-V curve.

V1 - CurviLin.xls (2002) Designed for the ESICM PACT module on Mechanical Ventilation. CurviLin is based on Virtual-MV v4.0 (2000-2002), worksheets VCV. The calculation spreadsheet has been modified, to make it compatible with a respiratory system model with nonlinear compliance. The volume reference has been changed from PEEP-modified FRC to atmospheric FRC. Static P-V curve plots have been added. The temporal definition for exhalation has been doubled. v1.0 (2002) - 13 -

EasyTrigger v1.0

Purpose

EasyTrigger is an Excel-based simulator of mechanical ventilation focused on the Inspiratory Trigger, designed to show the differences between Pressure-Trigger and Flow-Trigger.

EasyTrigger allows the graphic simulation of the last part of an exhalation, followed at a given time by an inspiratory effort of the patient, possibly able to activate the inspiratory trigger of the ventilator, and thus to start a pressure-controlled (or pressure-supported) inspiration.

EasyTrigger has been designed as an interactive educational tool. By trying various controls you can change a patient's spontaneous activity, as well as trigger type and sensitivity. EasyTrigger shows the different working principles and effects of the two kinds of trigger (pressure-trigger and flow-trigger).

Models

The Ventilator of EasyTrigger behaves as an ideal ventilator, with no limits in flow delivering capability, and able to perfectly control the airway pressure and flow at the airway opening. When the Pressure-Trigger is active, the ventilator works with no expiratory base-flow. Before reaching the trigger threshold, the patient's inspiratory effort determines the closure of the expiratory valve of the ventilator, while the inspiratory valve is still closed. Therefore the pressure-trigger is associated with an occlusion period before the trigger responds: the airway pressure drops with the inspiratory effort, while the flow remains at zero. When the Flow-Trigger is active, the ventilator works with an ideal expiratory base-flow able to perfectly match the call for inspiratory flow from the patient, even before the trigger threshold is reached. Therefore with the flow-trigger the airway pressure does not drop below the set PEEP level, while the patient's inspiratory effort reverses the flow from exhalation to inspiration without any interruption. The EasyTrigger ventilator has a very fast response, with a maximum electromechanical delay of 10 milliseconds once the trigger threshold is reached. Therefore the response delay shown in graphs and displays is only pertinent to the active and passive mechanics of the patient, and to the type and sensitivity settings of the trigger. Real ventilators respond with an additional delay, ventilator-specific and normally longer than 10 milliseconds. The EasyTrigger ventilator responds with a pressure-controlled (or pressure-supported) inspiration. The effects of circuit compliance (due to elasticity and gas compression) and circuit resistance are not taken into account, and thus the ventilator moves the airway pressure and flow more rapidly when responding to the trigger. Hence the EasyTrigger ventilator behaves similarly to those ventilators that are provided with an efficient system for circuit resistance compensation. Due to a number of features of the EasyTrigger ventilator, the inspiratory peak flow may be higher than usually observed with real ventilators. The monitored data in displays and graphs are pertinent to the airway opening, and hence similar to the data shown by those ventilators that are provided with proximal sensing for airway pressure and flow at the airway opening.

In EasyTrigger the basic model of the Respiratory System is the single-compartment linear model, i.e. the respiratory system is basically described by one value of resistance (Rrs) and one value of compliance (Crs) for the entire cycle. An optional deviation from the basic model can be obtained by setting an Additional Expiratory Resistance (Rexp,add). The Rexp,add control makes the expiratory resistance higher than the inspiratory resistance, and can be used:  To simulate the effects of expiratory bronchial collapse,  Or to simulate the expiratory resistance of the apparatus (in order to simulate the behaviour of common ventilators, not provided with expiratory circuit resistance compensation, it is suggested to set the Rexp,add control at a value of at least 5 cmH2O/l/s). Note: The respiratory system model does not include any function to simulate alveolar recruitment or interaction between external and intrinsic PEEP. Therefore any change in - 14 -

PEEP will just result in a shift of the entire curve of airway pressure, with an unchanged shape. The other curves and all the displayed parameters will remain unchanged.

In EasyTrigger, Spontaneous Breathing is simulated with a sinus wave for the global action of the inspiratory muscles, while the expiratory muscles are considered as always relaxed. The duration of the inspiratory effort depends on the spontaneous frequency, with a normal duty cycle of 0.4. The inspiratory effort can be started at will after a complete exhalation or before the previous exhalation is completed.

General description

The File EasyTrigger.xls is made up of two Worksheets, with the first one designed for simulation of the pressure-trigger and the other one for the flow-trigger. Each worksheet has a control panel and a graphic monitor.

The Control Panel is divided into three sections, for:  Ventilator (light green)  Patient's Passive Mechanics (light blue)  Patient's Activity (yellow).

The control panel includes:  Controls, typed in black or red (main controls), and operated by arrows  Displays, typed in blue, and showing "monitored" data that result from control settings and patient-ventilator interaction. Note: You do not need, and should not, enter directly numeric data in the worksheet cells. You should operate just on arrows and buttons. - 15 -

Controls

Patient's Passive Mechanics The controls for passive mechanics are:  Crs: respiratory system compliance  Rrs: respiratory system resistance  Rexp,add: additional expiratory resistance.

Patient's Activity The controls for patient's activity are:  Pmus,max: maximum pressure generated by the inspiratory muscles during inspiration  Freq,sp: frequency of the spontaneous inspiratory efforts  Vol,start: Volume corresponding to the starting point of the inspiratory effort (expressed as volume above the PEEP-modified FRC, taken as zero volume).

Note: By setting the Vol,start at zero, you simulate an inspiratory effort starting after a complete exhalation to the equilibrium point that corresponds to the set PEEP level. By setting a Vol,start higher than zero, you simulate an anticipated inspiratory effort, that first actively brakes and offsets the exhalation, and then generates the airway pressure drop or inspiratory flow necessary to activate the trigger.

Ventilator The ventilator controls are:  Pinsp: inspiratory pressure applied above PEEP in PCV and PSV  PEEP: positive end-expiratory pressure applied by the ventilator

 P,trig: pressure-trigger sensitivity, expressed as pressure drop below PEEP, in cmH2O  Flow,trig: flow-trigger sensitivity, expressed as inspiratory flow, in l/min.

Graphs Time zero corresponds to the start of the patient's inspiratory effort. A Zoom control allows you to change the time scale for better focusing on the transition period between exhalation and inspiration.

Displays

The displays, typed in blue in the control panels, show "monitored" data that result from control settings and patient-ventilator interaction.  RCi: respiratory system inspiratory time constant  RCe: respiratory system expiratory time constant - 16 -

 PEEPi: intrinsic PEEP  Vol,PEEPi: dynamically trapped volume associated with PEEPi

 P0.1: occlusion pressure at 0.1 s  Delay,trap: delay in trigger response solely due to dynamically trapped volume and PEEPi  Delay,trig: delay in trigger response solely due to interaction between patient's effort and trigger sensitivity.

The displays are located in the more pertinent section of the control panel.

The displays include parameters that ventilator monitors cannot provide, or usually do not provide, such as:  RCi, i.e. the product of Inspiratory Resistance and Crs, expressing how fast the respiratory system is during inspiration  RCe, i.e. the product of Expiratory Resistance and Crs, expressing how fast the respiratory system is during exhalation  PEEPi, expressing dynamic pulmonary hyperinflation in terms of static pressure  Vol,PEEPi , expressing dynamic pulmonary hyperinflation in terms of volume (the value is slightly lower than the Vol,start setting, due to continuing exhalation during the initial inspiratory effort of the patient)

 P0.1: expresses the inspiratory effort applied by the patient for trigger activation, and corresponds to the value that would be observed, should an inspiratory occlusion manoeuvre be performed.  Delay,trap: expresses the time needed for the inspiratory effort to offset expiratory flow and PEEPi  Delay,trig: expresses the time needed for the inspiratory effort to reach the trigger sensitivity threshold, after full PEEPi compensation.

Graphic monitor

Basic signals Volume. The zero of the Volume signal always corresponds to the Functional Residual Capacity (FRC) modified by the external PEEP applied by the ventilator. The Volume baseline is not affected by PEEP, but it is affected by intrinsic PEEP. The instantaneous Volume rises during inspiration, and decreases during exhalation.

Paw. The Paw zero corresponds to the atmospheric pressure.

Pmus. The Pmus zero corresponds to the atmospheric pressure. Negative values express a net inspiratory action by the patient, while a value of zero expresses full relaxation of the respiratory muscles.

Real-time graphs The upper panel combines Volume (in litres) and Flow (in l/min) on the same value scale. The lower panel combines Paw and Pmus on the same scale. The Time scale has a maximum range and resolution of - 1 s (100 Hz) with Zoom 100% - 0.5 s (200 Hz) with Zoom 200% - 0.33 s (300 Hz) with Zoom 300%. Time zero always corresponds to the start of the inspiratory effort.

The graphic monitor is similar to the one of modern ventilators, but its performance has been made much higher, in order to provide data for an easier and more complete understanding of all the phenomena. - 17 -

In ventilator monitors, the Volume signal is reset at zero at the start of every breath. In other words the Volume signal baseline corresponds to the end-expiratory lung volume of the previous breath, taken as zero in the volume scale. On the contrary, in EasyTrigger the zero of the Volume signal corresponds always to the Functional Residual Capacity (FRC) modified by the external PEEP applied by the ventilator. Therefore, unlike the monitor of any real ventilator, EasyTrigger shows a rise above zero in the spirogram, whenever the patient-ventilator interaction generates intrinsic PEEP and dynamic pulmonary hyperinflation. This rise corresponds to the value of the parameter displayed as Vol,PEEPi , that describes the dynamically trapped volume.

The graphic monitor of EasyTrigger shows also the graphs of Muscular pressure (Pmus), a parameter that unfortunately cannot be shown in the monitor of real ventilators. In clinics Pmus cannot be recorded continuously, but just observed occasionally by performing an occlusion manoeuvre by means of the Hold functions of the ventilator. Pmus is an ideal entity, rather than a real physical parameter. Pmus is the force instantaneously applied by the whole complex of the respiratory muscles, expressed as a pressure. A net inspiratory action by the respiratory muscles results in a negative Pmus, and a net expiratory action in a positive Pmus. A value of zero corresponds to full relaxation.

In order to allow a better visualisation of the transition between exhalation and inspiration, the graphic monitor of EasyTrigger has been provided with a temporal resolution much higher than that used in clinical monitors. Altering the Zoom control changes the temporal resolution for both display and calculation. For this reason the displayed data slightly change with the Zoom setting. The maximum precision is achieved with a Zoom of 300%.

Version history

V1 - EasyTrigger.xls (2002) Designed for the ESICM PACT module on Mechanical Ventilation. Easy trigger is based on CicloRespPlusLink (2000-2001), an old version of Virtual-MV. Errors in the post-trigger phase have been corrected. The temporal resolution of calculation has been increased. The user's interface has been re-designed. The graphic display has been improved. The range of a several controls has been limited. Extended instructions have been integrated. v1.0 (2002) - 18 -

Virtual-MV v4.0

Purpose

Virtual-MV is an Excel-based simulator of mechanical ventilation designed for calculation and graphic representation of the mechanics of a single respiratory cycle in steady state.

Virtual-MV allows the simulation of three kinds of breath, with  Volume Control (VCV),  Pressure Control (PCV),  Pressure Support (PSV), in a passive or actively breathing patient.

Virtual-MV has been designed as an interactive educational tool. By trying various inputs, you can simulate the mechanics of different patients (for instance, with obstructive or restrictive lung disease), and observe the mechanical response to different ventilator settings.

Virtual-MV is designed to reproduce the results of an ideal ventilator on an ideal patient. It should not be used for clinical purposes, for instance to predict the results of real ventilators on real patients.

Models

The Ventilator of Virtual-MV behaves as an ideal ventilator, with no limits in flow delivering capability, and able to perfectly control the airway pressure and flow at the airway opening. The effects of circuit compliance (due to elasticity and gas compression) and circuit resistance are not taken into account, and thus the ventilator moves the airway pressure and flow more rapidly when cycling from inspiration to exhalation and vice versa. Hence the ventilator of Virtual-MV behaves in a similar way to those ventilators that are provided with an efficient system for circuit resistance compensation. Due to a number of features of the Virtual-MV ventilator, the peaks of flow may be higher than usually observed with real ventilators. The monitored data in displays and graphs are pertinent to the airway opening, and hence similar to the data shown by those ventilators that are provided with proximal sensing for airway pressure and flow at the airway opening. The ventilator is pressure-triggered with trigger sensitivity set at -2 cmH2O. The ventilator- dependent triggering delay is very short.

In Virtual-MV the basic model of the Respiratory System is the single-compartment linear model, i.e. the respiratory system is basically described by one value of resistance (Rrs) and one value of compliance (Crs) for the entire cycle. An optional deviation from the basic model can be obtained by setting an Additional Expiratory Resistance (Rexp,add). The Rexp,add control makes the expiratory resistance higher than the inspiratory resistance, and can be used:  To simulate the effects of expiratory bronchial collapse,  Or to simulate the expiratory resistance of the apparatus (in order to simulate the behaviour of common ventilators, not provided with expiratory circuit resistance compensation, it is suggested to set the Rexp,add control at a value of at least 5 cmH2O/l/s). Note: The respiratory system model does not include any function to simulate alveolar recruitment or interaction between external and intrinsic PEEP. Therefore any change in PEEP will just result in a shift of the entire curves of airway pressure and alveolar pressure, with an unchanged shape. The other curves and all the displayed parameters will remain unchanged.

In Virtual-MV, Spontaneous Breathing is simulated with a sinus wave for the global action of the inspiratory muscles, while the expiratory muscles are considered as always relaxed. The duration of the inspiratory effort depends on the spontaneous frequency, with a normal duty cycle of 0.4 and a maximum duration of 2 s. - 19 -

Note: The effects of a given spontaneous activity are calculated and shown only when the inspiratory muscles action, set by the control Pmus,max, reaches a level that is sufficient to activate the ventilator trigger.

General description

The File Virtual-MV.xls is made up of several Worksheets, each one designed for a given ventilation mode and graphic display. Each worksheet has a control panel and a graphic monitor.

The Control Panel is divided into three sections, for:  Ventilator (light green)  Patient's Passive Mechanics (light blue)  Patient's Activity (yellow).

The control panel includes:  Controls, typed in black, and operated by arrows or buttons  Displays, typed in blue, and showing "monitored" data that result from control settings and patient-ventilator interaction.  Note: You do not need, and should not, directly enter numeric data in the worksheet cells. You should just operate the arrows and buttons.

The Graphic Monitor includes: - Real-time graphs for - Paw airway opening pressure - Flow airway opening flow - Volume volume change (spirogram) - Pmus muscular pressure - Palv alveolar pressure - or X-Y graphs for - Flow-Volume loop - Paw-Volume loop - Pmus-Volume loop - Palv-Volume loop.

All data on displays and graphs are referred to a single breath in a steady state.

Each worksheet is linked to the other ones. Any change in the control settings of a given worksheet is automatically mirrored on the other worksheets. - 20 -

Controls

Patient's Passive Mechanics The controls for passive mechanics are:  Crs: respiratory system compliance  Rrs: respiratory system resistance  Rexp,add: additional expiratory resistance.

Patient's Activity The controls for patient's activity are:  Pmus,max: maximum pressure generated by the inspiratory muscles during tidal breathing  Freq,sp: frequency of the spontaneous inspiratory efforts.

Ventilator The ventilator controls depend on the mode simulated in each worksheet, and are:  PEEP: positive end-expiratory pressure applied by the ventilator  Freq.: ventilator (minimum) frequency  Ti%: inspiratory time expressed as inspiratory duty cycle in %  Vt: tidal volume applied in VCV  Pinsp: inspiratory pressure applied above PEEP in PCV and PSV  ETS: expiratory trigger sensitivity, expressed as % of inspiratory peak flow. The inspiratory trigger sensitivity has a fixed setting of -2 cmH2O. In VCV buttons are available for setting the inspiratory Flow Wave, constant or decelerated. In VCV and PCV buttons are available for generating an Inspiratory Hold and an Expiratory Hold of 4 seconds. In Virtual-MV the Hold functions are not compatible with patient's activity. Therefore any patient's activity is zeroed when a Hold button is activated.

Displays

The displays, typed in blue in the control panels, show "monitored" data that result from control settings and patient-ventilator interaction.  MinVol: minute ventilation  Vt: tidal volume  Ti: inspiratory time, expressed in seconds  Te: expiratory time, expressed in seconds  RCi: respiratory system inspiratory time constant  RCe: respiratory system expiratory time constant  PEEPi: intrinsic PEEP - 21 -

 Vol,PEEPi: dynamically trapped volume associated with PEEPi  Te/RCe: ratio between Te and RCe.

In the each worksheet, the displays are located in the more pertinent section of the control panel.

The displays include parameters that ventilator monitors cannot provide, or usually do not provide, such as:  PEEPi, expressing dynamic pulmonary hyperinflation in terms of static pressure  Vol,PEEPi , expressing dynamic pulmonary hyperinflation in terms of volume  RCi, i.e. the product of Inspiratory Resistance and Crs, expressing how fast the respiratory system is during inspiration  RCe, i.e. the product of Expiratory Resistance and Crs,expressing how fast the respiratory system is during exhalation  Te/RCe, expressing the likelihood of dynamic pulmonary hyperinflation (values above 5 are associated with no dynamic hyperinflation, while values below 3 can be associated with relevant dynamic hyperinflation).

When the Hold buttons are activated, special displays appear for data read from the real-time curves, to be used for training in manual calculation of Quasi-static Compliance, Maximum Inspiratory Resistance and Intrinsic PEEP:  Vt: tidal volume  Flow,end-i: end-inspiratory flow  Ppeak: peak inspiratory pressure  Ppause: static end-inspiratory pressure  PEEP: externally applied PEEP  PEEPtot: static end-expiratory pressure.

Graphic monitor

Basic signals Volume. The zero of the Volume signal always corresponds to the Functional Residual Capacity (FRC) modified by the external PEEP applied by the ventilator. The Volume baseline is not affected by PEEP, but it is affected by intrinsic PEEP. The instantaneous Volume rises during inspiration, and decreases during exhalation.

Paw. The Paw zero corresponds to the atmospheric pressure.

Pmus. The Pmus zero corresponds to the atmospheric pressure. Negative values express a net inspiratory action by the patient, while a value of zero expresses full relaxation of the respiratory muscles.

Palv. The Palv zero corresponds to the atmospheric pressure. The Palv baseline is the sum of PEEP and intrinsic PEEP. The values of Palv depend instantaneously on the lung volume above the atmospheric FRC and on Pmus.

Real-time graphs The upper panel combines Volume (in litres and Flow (in l/min) on the same value scale. The lower panel combines Paw, Palv and Pmus on the same scale.

X-Y graphs The upper panel represents the Flow-Volume loop. The lower panel combines the Paw-Volume, Palv-Volume and Pmus-Volume loops. - 22 -

In ventilator monitors, the Volume signal is reset at zero at the start of every breath. In other words the Volume signal baseline corresponds to the end-expiratory lung volume of the previous breath, taken as zero in the volume scale. On the contrary, in Virtual-MV the zero of the Volume signal always corresponds to the Functional Residual Capacity (FRC) modified by the external PEEP applied by the ventilator. Therefore, unlike the monitor of any real ventilator, Virtual-MV shows a rise above zero in the spirogram and in the X-Volume loops, whenever the patient-ventilator interaction generates intrinsic PEEP and dynamic pulmonary hyperinflation. This rise corresponds to the value of the parameter displayed as Vol,PEEPi , that describes the dynamically trapped volume.

The graphic monitor of Virtual-MV also shows graphs for two parameters that, unfortunately, cannot be shown in the monitors of real ventilators:  Pmus  Palv. In clinics Pmusc and Palv cannot be recorded continuously, but just observed occasionally by performing an occlusion manoeuvre by means of the Hold functions of the ventilator.

The Muscular Pressure (Pmus) is an ideal entity, rather than a really measurable parameter. Pmus is the force instantaneously applied by the whole complex of the respiratory muscles, expressed as a pressure. A net inspiratory action by the respiratory muscles results in a negative Pmus, and a net expiratory action in a positive Pmus. A value of zero corresponds to full relaxation.

The Alveolar Pressure (Palv) is the absolute value of the average pressure instantaneously ruling inside the alveoli. It depends on the lung volume above the atmospheric FRC and on the elastic recoil of the respiratory system (expressed by Crs), as well as on Pmus. The baseline of Palv corresponds to the total intrapulmonary PEEP, i.e. to the sum of externally applied PEEP and intrinsic PEEP (PEEPi). Therefore you will see the Palv curve baseline - 23 -

rising above PEEP, whenever the patient-ventilator interaction generates PEEPi and dynamic pulmonary hyperinflation. The rise will correspond to the value of PEEPi shown in the display.

Cycle start and end, as shown in the real-time graphs, are defined by the point of zero crossing of flow between exhalation and inspiration. This means that, during simulation of an actively breathing patient, the action of the inspiratory muscles is shown partly at the start of the breath, and partly at the end.

Version history

V1 - CicloResp.xls (2000) The progenitor, contained independent worksheets for simulation of VCV, PCV and PSV, with real-time and loop graphs.

V2 - CicloRespPlus.xls (2000) Implementation of Campbell diagram for PSV (worksheet WOB PSV). Implementation of worksheets for simulation of Pressure-Trigger, Flow-Trigger, and asynchronous BiPAP ventilation. Short instructions provided with document ReadMeCRP.doc.

V3 - CicloRespPlusLink.xls (2001) Linked all worksheets, by mirroring control settings. Short instructions provided with document ReadMeCRPL.doc.

V4 - Virtual-MV.xls (2002) Re-designed for the ESICM PACT module on Mechanical Ventilation. The user's interface has been re-designed. The temporal resolution for calculation and plotting has been doubled. The worksheets for VCV and PCV have been improved: - Extended control panel for Patient's Activity, - Improved calculation and display for actively breathing patient, - Addition of Hold functions for passive patient, - Addition of cycling points at zero flow. The worksheets for PSV have been entirely re-designed. The worksheets for WOB in PSV, Pressure-Trigger, Flow-Trigger, and BiPAP ventilation have been removed (not completely reliable, up to now). Extended instructions have been integrated.

v4.0 (2000-2002)