Ventilator Waveforms: Interpretation Albert L. Rafanan, MD, FPCCP Pulmonary, Critical Care and Sleep Medicine Chong Hua Hospital, Cebu City Types of Waveforms • Scalars are waveform representations of pressure, flow or volume on the y axis vs time on the x axis • Loops are representations of pressure vs volume or flow vs volume Scalar Waveforms Loop Common problems that can be diagnosed by analyzing Ventilator waveforms Patient-ventilator Abnormal ventilatory Ventilatory circuit related Interactions Parameters/ problems E.g. flow starvation, lung mechanics E.g. auto cycling and Double triggering, E.g.. Overdistension, Secretion build up in the Wasted efforts Auto PEEP Ventilatory circuit COPD Active expiration Lung Mechanics • Use Scalar • Pressure Time Waveform with a square wave flow pattern Understanding the basic ventilator circuit diagram ventilator TheEssentially ventilator the makes circuit up diagram the first of part a mechanicallyTheof the patient’s circuit. ventilated own Its pump respiratory patient like action system can isbe ET Tube mdepictedakesbroken up thedownsimplistically 2nd intopart two of as theparts….. a piston circuit. airways thatTheseThe moves diaphragm two systemsin a reciprocating is alsoare connectedshown fashion as a by 2ndanpiston; endotrachealduring causing the respiratorytube air to which be cycle.drawn we can into considerthe lungs as during an extension contraction. of the patient’s airways. Chest wall Diaphragm Understanding airway pressures The respiratory system can be thought of as a mechanical system consisting of resistive (airways +ET tube) and elastic (lungs and chest wall) elements in series ET Tube THUS Paw = Flow X Resistance + Volume x 1/Compliance Paw Airway pressure airways ET tube + Airways Lungs + Chest wall (resistiveAirwaysLungs + element)+Chest ET tube wall (elastic element) (resistive(elastic element) PPL Pleural pressure TheResistive elastic pressure pressure varies varies with with airflow volume and and the diameter of ETT and airways. stiffness of lungs and chest wall. Chest wall Diaphragm Flow resistance Pel = Volume x 1/Compliance Palv Alveolar pressure Understanding basic respiratory mechanics Thus the equation of motion for the respiratory system Ventilator is Paw (t) = Pres (t) + Pel (t) Elungs RET tube ET Tube RawErs airways Rairways Echest wall The total ‘elastic’ resistance (E ) offered by the Let us nowThe understand total ‘airway’ how resistancethe respiratoryrs systems’ Thus respiratoryto move air systeminto the is lungs equal at to any the (Rgiven sumaw) oftime (t), inherent elastance and resistance to airflow Diaphragm thein elasticthe ventilator mechanically resistances has to ventilatedgenerateoffered by sufficient patient the determinesis equal to the the sum pressures of the generatedresistances within offered a pressure (P Lung) to E overcomeand the the combined bymechanically the endotrachealaw(t) lungsventilated tube (Rsystem. ) elastic (P )chestand resistance wall E (P ET) propertiestube andel the(t) patient’s airwayschest wall res(t) of the respiratory system( R airways ) Understanding the pressure-time waveform using a ‘square wave’ flow pattern Ppeak Pres pressure ventilator Pplat Pres R time ET tube Pres Rairways After this, the pressure rises in a linear fashion to finally reach P . Again at end inspiration, TheAt pressurethe beginning-timepeak waveformof the inspiratory is a reflection cycle, air flow is zero and the pressure drops by an Diaphragm theof ventilator the pressures has to generated generate withina pressure the P amount equal to P to reach the plateau res airwaysto overcome during theres each airway phase resistance. of the pressure P . The pressure returns to Note: No volumeplatventilatory is deliveredcycle. at this time. baseline during passive expiration. Pressure-time waveforms using a ‘square wave’ flow pattern Paw(peak) = Flow x Resistance + Volume x 1/ Compliance Scenario # 1 Paw(peak) pressure Pres Pplat Pres time flow This is a normal pressure-time waveform time With normal peak pressures ( Ppeak) ; plateau pressures (Pplat )and ‘Square wave’ airway resistance pressures (Pres) flow pattern Waveform showing high airways resistance Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP Scenario # 2 Normal Ppeak pressure e.g. ET tube Pres blockage Pplat Pres time flow The increase in the peak airway pressure is driven time Thisentirely is an by abnormal an increasepressurein the- timeairways waveform resistance pressure. Note the normal plateau pressure. ‘Square wave’ flow pattern Waveform showing increased airways resistance ‘Square wave’ flow pattern Ppeak Pplat Pres Waveform showing high inspiratory flow rates Paw(peak) = Flow x Resistance + Volume x 1/compliance + PEEP Scenario # 3 Paw(peak) Normal pressure Pres e.g. high flow rates Pplat Pres time flow The increase in the peak airway pressure is caused Normal (low) time flow rate by highThis inspiratoryis an abnormal flow pressurerate and- timeairways waveform resistance. Note the shortened inspiratory time and high flow ‘Square wave’ flow pattern Waveform showing decreased lung compliance Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP Scenario # 4 Paw(peak) Normal pressure P res e.g. ARDS Pplat Pres time flow The increase in the peak airway pressure is driven by the decrease in the lung compliance. time ThisIncreased is an abnormal airways pressureresistance-time is often waveform also a part of this scenario. ‘Square wave’ flow pattern Common problems that can be diagnosed by analyzing Ventilator waveforms Patient-ventilator Abnormal ventilatory Ventilatory circuit related Interactions Parameters/ problems E.g. flow starvation, lung mechanics E.g. auto cycling and Double triggering, E.g.. Overdistension, Secretion build up in the Wasted efforts Auto PEEP Ventilatory circuit COPD Active expiration Recognizing Lung Overdistension Flow-time waveform • Flow-time waveform has both an inspiratory and an expiratory arm. • The shape of the expiratory arm is determined by: – the elastic recoil of the lungs – the airways resistance – and any respiratory muscle effort made by the patient during expiration (due to patient-ventilator interaction/dys=synchrony) • It should always be looked at as part of any waveform analysis and can be diagnostic of various conditions like COPD, auto- PEEP, wasted efforts, overdistention etc. Recognizing lung overdistension Suspect this when: There are high peak and plateau Pressures… Accompanied by high expiratory flow rates PEARL: Think of low lung compliance (e.g. ARDS), excessive tidal volumes, right mainstem intubation etc The Stress Index • In AC volume ventilation using a constant flow waveform observe the pressure time scalar. flow • Normal, linear change in airway pressure Stress index =1 Paw • Upward concavity indicates decreased compliance and lung overdistension Stress index > 1 • Downward concavity indicates increased compliance and potential alveolar recruitment Stress index < 1 Note: Patient effort must be absent time The Stress Index Pressure-volume loop Compliance (C) is markedly reduced in the injured lung on the right as compared to the normal lung Normal on the left lung Upper inflection point (UIP) above this pressure, additional alveolar recruitment requires disproportionate increases in applied ARDS airway pressure Lower inflection point (LIP) Can be thought of as the minimum baseline pressure (PEEP) needed for optimal alveolar recruitment Overdistension Peak inspiratory pressure Upper inflection point Note: During normal ventilation the LIP cannot be assessed due to the effect of the inspiratory flow which shifts the curve to the right Recognizing Auto-PEEP Dynamic Hyperinflation (Gas Trapping) • Dynamic hyperexpansion, defined as premature termination of exhalation, often occurs when respiratory rate, inspiratory time, or both have been increased. • By not permitting exhalation to finish, an increase in mean airway pressure results. • Gas trapping may occur leading to an elevation in PCO2. • Careful attention must be paid to dynamic hyperexpansion in patients with obstructive lung disease whose long time constants and slow emptying can result in progressive air trapping, hypercarbia, and eventually decreased cardiac output. Expiratory flow continues and fails to return to the baseline prior to the new inspiratory cycle Detecting Auto-PEEP Recognize Auto-PEEP when Expiratory flow continues and fails to return to the baseline prior to the new inspiratory cycle The development of auto- PEEP over several breaths in a simulation Notice how the expiratory flow fails to return to the baseline indicating air trapping (AutoPEEP) Also notice how air trapping causes an increase in airway pressure due to increasing end expiratory pressure and end inspiratory lung volume. Understanding how flow rates affect I/E ratios and the development of auto PEEP Decreasing the flow rate Increase the inspiratory time and consequently decrease the expiratory time (decreased I/E ratio) Thus allowing incomplete emptying of the lung and the development of air trapping and auto-PEEP Lluis Blanch MD, PhD et al: Respiratory Care Jan 2005 Vol 50 No 1 Understanding how inspiratory time affect I/E ratios and the development of auto-PEEP • In a similar fashion, an increase in inspiratory time can also cause a decrease in the I: E ratio and favor the development of auto-PEEP by not allowing enough time for complete lung emptying between breaths. Recognizing Expiratory Flow Limitation (e.g. COPD, asthma) Recognizing prolonged expiration