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Control of Breathing in Mechanically Ventilated Patients

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Control of Breathing in Mechanically Ventilated Patients Eur Respir J, 1996, 9, 2151–2160 Copyright ERS Journals Ltd 1996 DOI: 10.1183/09031936.96.09102151 European Respiratory Journal Printed in UK - all rights reserved ISSN 0903 - 1936 SERIES "CLINICAL PHYSIOLOGY IN RESPIRATORY INTENSIVE CARE" Edited by A. Rossi and C. Roussos Control of breathing in mechanically ventilated patients D. Georgopoulos, C. Roussos Control of breathing in mechanically ventilated patients. D. Georgopoulos, C. Roussos. Pulmonary and Critical Care Dept, General ©ERS Journals Ltd 1996. Hospital "G. Papanicolaou", University of ABSTRACT: During mechanical ventilation, the respiratory system is under the Thessaloniki, Thessaloniki, Greece and influence of two pumps, the ventilator pump and the patient's own respiratory Critical Care Dept, "Evangelismos" Hospital, muscles. Depending on the mode of mechanical ventilatory support, ventilation University of Athens, Athens, Greece. may be totally controlled by the ventilator or may be determined by the interac- Correspondence: D. Georgopoulos tion between patient respiratory effort and ventilator function. In either case, com- General Hospital "G. Papanicolaou" pared to spontaneous breathing, the breathing pattern is altered and this may Pulmonary Dept influence: 1) force-length and force-velocity relationships of respiratory muscles Respiratory Failure Unit (mechanical feedback); 2) chemical stimuli (chemical feedback); 3) the activity of Exochi 57010 various receptors located in the respiratory tract, lung and chest wall (reflex feed- Thessaloniki back); and 4) behavioural response (behavioural feedback). Changes in these feed- Greece back systems may modify the function of the ventilator, in a way that is dependent Keywords: Behavioural feedback on the mode of mechanical ventilatory support, ventilator settings, mechanics of chemical feedback the respiratory system and the sleep/awake stage. mechanical feedback Thus, the response of ventilator to patient effort, and that of patient effort to mechanical ventilation ventilator-delivered breath are inevitably the two components of control of breath- reflex feedback ing during mechanical ventilation; the ventilatory output is the final expression of the interaction between these two components. As a result of this interaction, the Received: May 20 1996 various aspects of control of breathing of the respiratory system may be masked Accepted after revision July 7 1996 or modulated by mechanical ventilation, depending on several factors related both to patient and ventilator. This should be taken into consideration in the manage- ment of mechanically ventilated patients. Eur Respir J., 1996, 9, 2151–2160. The act of breathing is a complex process [1, 2]. Brief- the mechanical properties of the respiratory system (elas- ly, the medullary respiratory controller (central control- tance and resistance) (fig. 1). Depending on the mode ler) accepts information from chemical (peripheral and of mechanical ventilatory support, volume-time profile central chemoreceptors) and nonchemical sources. Based and ventilator timing may be totally controlled by the on this information, the central controller activates spinal f R,pt motor neurons serving respiratory muscles, with an inten- Mode sity and rate that may vary substantially between breaths. Mode The activity of spinal motor neurons is conveyed to res- Pmus + Paw = V '×Rrs + V×Ers Ventilator timing Neural timing piratory muscles, which contract and generate pressure (Pmus). Pmus is dissipated to overcome the resistance and Volume-time profile elastance of the respiratory system (inertia is negligible) VT f 'R,vent and this combination determines the volume-time profile and, depending on breath timing, ventilation. Volume- time profile and breath timing via force-length and force- Mechanical Chemical Reflex Behavioural velocity relationships of respiratory muscles affect Pmus, whereas they modify the activity of spinal motor neu- Feedback rons and the medullary respiratory controller via affer- Fig. 1. – Schematic representation of the interaction between patient ent nerves from various receptors. On the other hand, respiratory effort and ventilator-delivered breath. Pmus: pressure gen- ventilation and gas exchange properties of the lung deter- erated by respiratory muscles (inspiratory muscles generate positive mine arterial blood gas values, which in turn, via peri- pressure and expiratory muscles negative); Paw: airway pressure; V: pheral and central chemoreceptors, affect the activity of instantaneous volume above passive functional residual capacity (FRC); V': instantaneous flow (inspiratory flow is positive); Rrs: resistance of the medullary respiratory controller, closing the loop. the respiratory system; Ers: elastance of the respiratory system; Ventilator In a mechanically-ventilated patient, the breath deliv- timing: duration of inspiratory and expiratory flow (mechanical inspi- ered by the ventilator has two components, one related ratory and expiratory time); Neural timing: neural (patient) inspira- to the volume-time profile and the other to ventilator tim- tory and expiratory time; VT: tidal volume. fRvent: ventilator (respirator) frequency; fR,pt: patient spontaneous breathing frequency. Depending ing [3, 4]. Volume-time profile, according to the equa- on the mode of ventilatory support, Pmus and neural timing may or may tion of motion [5], is determined by the combined action not affect Paw and ventilator timing, respectively. Note that fR,vent may of Pmus, pressure provided by the ventilator (Paw) and not reflect fR,pt. See text for further details. 2152 D. GEORGOPOULOS, C. ROUSSOS ventilator or may be determined by the interaction bet- (proportional assist ventilation) [3, 4]. With volume-con- ween patient respiratory effort and ventilator function [3, trol modes, the volume-time profile and duration of 4]. In either case, compared to spontaneous breathing, inspiratory flow are predetermined by the ventilator set- the pattern of breathing and ventilation are changed. tings. Thus, changes in Pmus and neural inspiratory time These changes may alter: 1) force-length and force-velo- cannot modify tidal volume (VT) delivered by the ven- city relationships of respiratory muscles (mechanical tilator. Any change in Pmus causes Paw to change in feedback) [6, 7]; 2) chemical stimuli (chemical feed- the opposite direction, because total pressure (Paw+Pmus) back) [8]; and 3) the activity of various receptors locat- is not changed. Therefore, with volume-control modes, ed in the respiratory tract, lung and chest wall (reflex the ventilator antagonizes the intensity of patient effort feedback) [9, 10]. Furthermore, changes in volume-time (fig. 1). Furthermore, the time at which inspiratory flow profile and breathing pattern are readily perceived in is terminated is independent of neural inspiratory dura- awake subjects and may evoke behavioural ventilatory tion. It follows that, with volume-control neither the responses (behavioural feedback) [11, 12]. As a result of intensity of patient effort nor neural inspiratory time mechanical, chemical, reflex and behavioural feedback, are expressed by the output of the ventilator. Pmus and patient neural timing (neural inspiratory and With pressure-control, the ventilator once triggered expiratory duration) are altered and these alterations, causes Paw to increase rapidly to a preset level, remain- depending on the mode of mechanical ventilatory sup- ing at that level until a preset cycling-off criterion (the var- port [3, 4], may or may not influence Paw and ventilator iable that terminates gas delivery) is reached [3, 4, 14]. timing (fig. 1). Thus, the ventilatory output is the final Because Paw is constant, the volume-time profile is under expression of the interaction between patient effort and the influence of Pmus, and any change in the intensity ventilator. It follows that the response of ventilator to of patient effort is expressed by a change in inspiratory patient effort, and that of patient effort to ventilator- flow rate (fig. 1). The cycling-off criterion may be a set delivered breath are inevitably the two components that time or flow. With time-cycling, neural inspiratory time control breathing during mechanical ventilation. An is ignored by the ventilator and the tidal volume is deter- understanding of these two components is essential for mined by Pmus waveform (inspiratory and expiratory) the physician dealing with the issue of control of breath- and mechanical properties of the respiratory system [5] ing in mechanically-ventilated patients. (fig. 1). With flow-cycling, gas delivery is terminated when inspiratory flow reaches a fixed level (usually 0.1 L·s-1) or a value which is proportional to peak inspira- Response of ventilator to patient effort tory flow (usually 25%). This method is called pressure- support (PS) and is widely-used [3, 14]. Theoretically, with PS the patient retains considerable control of the Basic principles of positive pressure ventilators inspiratory volume-time profile and inspiratory flow dura- tion; any change in the intensity and rate of patient effort should be expressed by VT and ventilator timing. Never- Positive pressure ventilators can be characterized by theless, in the face of high ventilatory demands, many various variables, which control the initiation of the me- ventilators are not able to maintain constant Paw, and chanical breath, gas delivery and mechanical inspira- Paw deviates from the target level [15]. Furthermore, it tory time [3]. The response of the ventilator
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