Department of Intensive Care, University Hospital, University of Crete, School of Medicine, Heraklion, Crete, Greece
Corresponding author: Intensive Care Unit, University Hospital, P.O. Box 1352, Heraklion, 71110, Crete, Greece. E-mail: georgop@med.uoc.gr
Keywords: ventilation; equipment, ventilators
The respiratory control system consists of a motor arm, which executes the act of breathing, a control centre in the medulla oblongata and a number of pathways that convey information to the control centre.2 69 On the basis of this information, the control centre activates spinal motor neurones serving respiratory muscles, with an intensity and rate that can vary substantially between breaths. The activity of spinal motor neurones is carried by peripheral nerves to the respiratory muscles, which contract and generate pressure (Pmus). According to the equation of motion for the respiratory system, Pmus is dissipated to overcome the resistance (Rrs) and elastance (Ers) of the respiratory system (inertia is assumed to be negligible), as follows:
Pmus=(RrsxV·)+(ErsxV)(1)
where V is volume relative to passive functional residual capacity (FRC) and V· is flow. Equation 1 determines volume in relation to time and, depending on the frequency of activation of the respiratory muscles, ventilation. Volume changes with time can affect Pmus via the forcelength and forcevelocity relationships of the respiratory muscles (mechanical feedback), and can modify the activity of spinal motor neurones and the control centre via afferents from receptors in the airways, chest wall or respiratory muscles (reflex feedback). Inputs from other sources (e.g. behavioural, temperature, postural) may also modify the function of the control centre. In addition, ventilation and gas exchange in the lung determine arterial blood gas composition (PaO2, PaCO2). These variables affect the activity of the control centre via peripheral and central chemoreceptors (chemical feedback). This system can be affected at any site by disease or treatment.
During mechanical ventilation, the pressure provided by the ventilator (Paw) is added to the muscle pressure.21 In mechanically ventilated patients the driving pressure for inspiratory flow (PT) is the sum of Pmus and Paw.21 54 According to the equation of motion, PT is dissipated to overcome the resistance (Rrs) and elastance (Ers) of the respiratory system, determining the volume time profile as follows:
PT=Pmus+Paw=(V·xRrs)+(VxErs)(2)
The change in volume affects the pattern of Pmus through mechanical, chemical, reflex and behavioural feedback systems, which can then alter the waveform of Paw (Fig. 1). During assisted mechanical ventilation there is interaction between the patient and ventilator. This interaction depends on (i) the response of the ventilator (i.e. Paw) to patient effort (i.e. Pmus) and (ii) the response of the patient to ventilator delivered breath (Fig. 1).21
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This depends on factors related to the ventilator and the patient.54 The ventilator-related factors are (i) the triggering variable, (ii) the variable that controls gas delivery, and (iii) the cycling off criterion. Patient-related factors are (i) the mechanics of the respiratory system and (ii) the characteristics of the Pmus waveform.
Ventilator-related factors
For given mechanical properties of the respiratory system and Pmus waveform, the response of the ventilator to patient effort is greatly influenced by the ventilator variables.
Trigger variable
The trigger variable is usually pressure or flow.51 With pressure triggering, in order to trigger the ventilator and initiate the inspiratory flow, the patient must decrease the pressure in the ventilator circuit to a preset value, which will then open a demand valve. With flow triggering, the patient triggers the ventilator when the respiratory muscles generate a certain preset inspiratory flow.51 With pressure triggering, the inspiratory muscles contract isometrically, and with flow triggering contraction is isotonic. It is generally believed that triggering of the ventilator is better with flow than with pressure.1 3 23 49 The clinical significance is unclear in terms of the work of breathing and patientventilator interaction. Pressure sensors in current ventilators are much improved, reducing any difference between flow- and pressure-triggering systems.5 46 55 Recent studies in patients with different diseases show that the difference in the work of breathing between flow and pressure triggering is of minimal clinical significance.1 24 59
Recently, a new microprocessor-controlled positive pressure ventilatory assist system has been introduced (BiPAP Vision; Respironics, Pittsburg, PA, USA) with new algorithms to trigger the ventilator. They are designed to improve patientventilator interaction, with the flow waveform mainly used to trigger the ventilator. Triggering occurs either when patient effort generates inspiratory flow, causing 6 ml of volume to accumulate above baseline flow (volume method), or when the patient inspiratory effort distorts the expiratory flow waveform sufficiently, whichever occurs first. The latter method of triggering is referred to as the shape signal method. This method is based on the generation of a new flow signal (flow shape signal) by offsetting the signal from the actual flow by 0.25 litre s1 and delaying it for 300 ms. The intentional delay causes the flow shape signal to be slightly behind the patients flow rate. As a result, a sudden decrease in expiratory flow from an inspiratory effort will cross the shape signal and this creates a signal for ventilator triggering (Fig. 2). Similarly, the flow waveform can be used to terminate the mechanical breath (Fig. 2). We found that the flow waveform method of ventilator triggering was more sensitive to patient effort than the flow triggering with less ineffective effort from the patient.43 An active lung model showed that, at controlled levels of dynamic hyperinflation and inspiratory effort, the simulated patient effort required to trigger the ventilator was 50% less with the shape method than with flow triggering.43
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The converse of patientventilator asynchrony is when the patients inspiratory effort does not trigger the ventilator.22 57 Ineffective triggering is very common in ventilator-dependent patients when dynamic hyperinflation is present.13 47 65 Dynamic hyperinflation is caused by factors such as low elastic recoil, high ventilatory demands, increased expiratory resistance and short expiratory time.40 47 When dynamic hyperinflation is present, end-expiratory lung volume is greater than passive FRC determined by the set external PEEP (PEEPe). Consequently, elastic recoil pressure at end-expiration is higher than PEEPe. This difference in elastic recoil pressure, referred to as intrinsic PEEP (PEEPi), represents an elastic threshold load for the patient. With flow or pressure triggering, the patient must first generate a Pmus equivalent to PEEPi to be able to decrease alveolar pressure below PEEPe and trigger the ventilator. Therefore, part of Pmus is dissipated to counteract PEEPi (elastic threshold load) and this delays the onset of effective inspiratory effort and the triggering. At times triggering is so delayed that the ventilator cycles are almost completely out of phase with the patient, defeating the purpose of assisted ventilatory support (Fig. 3). In some circumstances (high PEEPi or low Pmus) the patient cannot decrease the Paw below PEEP and the inspiratory effort is ineffective (Fig. 3). When asynchrony occurs, the relationship between the patients spontaneous breathing and the machine frequency is easily affected by changes in ventilator settings or in the patients respiratory output (Fig. 4). Although ineffective triggering is usually associated with obstructive lung disease, it may also occur in patients with normal or restrictive lung disease, particularly when assistance is great. In addition, the expiratory circuit of the ventilator may impose significant resistance on expiratory flow. This can prevent the respiratory system reaching equilibrium at the end of expiration, even in patients with normal respiratory mechanics.
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Apart from asynchrony between the patient and ventilator and wasted effort, ineffective triggering may have serious consequences for inspiratory muscle function. Ineffective triggering often occurs during exhalation of the previous mechanical breath, and the inspiratory muscles are activated to contract when they would normally be lengthening as lung volume decreases. This type of muscle contraction is referred to as pliometric contraction and causes ultrastructural damage to muscle fibres and reduced strength.10 28 After a single maximal pliometric contraction of skeletal muscle the injury to the muscle fibres causes a marked force deficit, which may exceed 50%.28 Ineffective triggering during mechanical expiration could injure the inspiratory muscles and cause inspiratory muscle weakness and weaning failure. No study of mechanically ventilated patients has studied this possibility.
Ineffective inspiration and autotriggering affect the assessment of ventilatory output during mechanical ventilation. If ineffective efforts or autotriggering are present, ventilator frequency does not indicate the patients spontaneous breathing rate. Both features can alter patient respiratory effort by changes in neural feedback (Fig. 1).
Factors that alter pressure delivery
There are several modes of assisted mechanical ventilation.54 67 Depending on the variable that controls the delivered pressure, they can be classified into three categories: (i) assist volume control (AVC), in which the ventilator, once triggered, delivers a preset tidal volume with a preset flowtime profile; (ii) pressure support (PS), in which the ventilator delivers a preset pressure;54 and (iii) proportional assist ventilation (PAV),67 in which the ventilator delivers pressure which is proportional (the proportionality is preset) to instantaneous flow and volume and, thus, to Pmus. With AVC the mechanical inflation time is determined by the ventilator, whereas with PS it is influenced both by the patient and ventilator,54 and with PAV mechanical inflation time is controlled mainly by the patient.67 Modern ventilators can combine various modes and ventilate the patient simultaneously with more than one mode. Currently, PAV is under investigation and it is not universally available. We consider here the relationship of Paw and Pmus in PAV mode with some important aspects of patientventilator interaction.
The features of each ventilator mode determine the relationship between Paw and Pmus. Figure 5 shows the response of the ventilator to respiratory effort of a patient ventilated with different modes of support.38 A carbon dioxide challenge was used to alter patient effort. With volume control, Paw decreased to almost zero with a greater patient inspiratory effort caused by hypercapnia and a set tidal volume. It follows that with this mode the ventilator overcomes patient effort. With PS, tidal volume and inspiratory flows are increased with increasing carbon dioxide, while Paw remains relatively constant. With PS there is no relationship between Pmus and Paw. With PAV carbon dioxide stimulation causes an increase both in patient effort and pressure provided by the ventilator. Here there is a positive relationship (the gain is preset) between Pmus and Paw. It is obvious from Fig. 5 that during mechanical ventilation the ventilatory output cannot be interpreted properly if the mode of support is not taken into account, as changes in ventilatory output may not reflect corresponding changes in patient effort.38
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The effects of asynchrony depend on the type of asynchrony present.21 If mechanical inspiration ends before neural inspiration, then ventilator assistance will cease while the inspiratory muscles continue to contract. This asynchrony may cause double triggering and overestimation of patient breathing frequency (Fig. 6). At the end of mechanical inspiration, Pmus continues to increase and, because inspiratory flow is zero or is reversed, the muscle tension is applied to overcome the elastic recoil of the respiratory system. If the respiratory system volume decreases rapidly, Pmus may be greater than elastic recoil. Airway pressure could decrease below PEEP and this will trigger the ventilator (Fig. 6). A short mechanical inflation time and low elastic recoil at end-inspiration can promote retriggering. When neural inspiration ends before the end of mechanical inspiration, ineffective triggering may occur because the ventilator delivers gas flow during neural expiration. This can reduce the time available for neural expiratory time (Figs 3 and 4), and the ventilator frequency underestimates the patients breathing frequency. In addition to ineffective or double triggering, expiratory asynchrony may cause discomfort to patients and unnecessary inspiratory and expiratory work. It may also activate various receptors (reflex feedback) and change the pattern of respiratory muscle activation (see below).
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Characteristics of the Pmus waveform
The pattern of Pmus waveform affects Paw in several ways, depending on factors related to both patient and ventilator. An extensive review of these factors is beyond the scope of this article, but some examples can explain how Pmus can affect ventilator function.21
The initial rate of Pmus increase interacts with the trigger function of the ventilator. If Pmus increases slowly, for example when respiratory drive is small (i.e. low PaCO2, sedation, sleep, high level of assist), the time between onset of the patients inspiratory effort and ventilator triggering increases, causing asynchrony (see Trigger variable above) (Fig. 7).33 If dynamic hyperinflation is also present, the prolonged triggering time, particularly if the neural inspiratory time is short and peak Pmus is small, can cause ineffective efforts. If inspiratory effort is great, for example with increased metabolic rate, high PaCO2, reduced sedation or reduced ventilation assistance, then both the rate of increase of Pmus and peak Pmus will increase. This will reduce the time delay and allow patientventilator synchrony (Fig. 7).33 On the other hand, if the patient inspiratory effort is vigorous and longer than mechanical inflation time the ventilator may be triggered more than once (double triggering) during the same inspiratory effort (Fig. 6). Changes in ventilation caused by such interactions may modify patient effort secondarily, through feedback loops (Fig. 1).
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The waveform of Pmus during mechanical ventilation is affected by mechanical, chemical, reflex and behavioural feedback (Fig. 1).
Mechanical feedback
Mechanical feedback is related to the length (related to lung volume) and velocity of contraction (related to flow) of the respiratory muscles, and to the way chest wall geometry can influence Pmus.68 For a given neural activation of the inspiratory muscles, Pmus will be less when lung volume and flow are greater. Thus, for a given degree of muscle activation, Pmus will be less during mechanical ventilation than during spontaneous breathing if the pressure provided by the ventilator results in greater flow and volume. The influence and consequences of mechanical feedback during mechanical ventilation have not been studied. It is likely that the effects are relatively small because the values of operating volume and flow are small.68 Nevertheless, mechanical feedback should be taken into account if pressure measurements are used to infer changes in respiratory muscle activation. At high ventilatory demands, Pmus may underestimate the neural output to respiratory muscles: during hypercapnic hyperventilation mechanical feedback can reduce peak Pmus by up to 15%.20
Chemical feedback
Chemical feedback involves the response of the respiratory system to PaO2, PaCO2 and pH. It acts to reduce changes in blood gas tensions that would otherwise occur from changes in metabolic rate or gas exchange.2 69 In spontaneously breathing normal subjects chemical feedback determines respiratory motor output both during wakefulness and during sleep. In mechanically ventilated patients, there are two important questions: (i) Does mechanical ventilation affect the chemical drive to Pmus? (ii) Is the effectiveness of chemical drive modified by mechanical ventilation?
Contribution of chemical feedback to Pmus during mechanical ventilation
An important reason for using mechanical ventilation is to unload the respiratory muscles.57 Theoretically, the respiratory control system can respond to unloading in three ways. First, respiratory muscle activation is reduced and ventilation remains the same as before the unloading. Secondly, respiratory muscle activation remains the same and ventilation will increase as assistance increases. Thirdly, there may be an intermediate response, whereby ventilation is greater and respiratory muscle activity is reduced, indicating incomplete downregulation of respiratory muscle activity. It is generally believed that the respiratory system follows the third pathway. Several studies have shown that, with unloading, ventilation is increased and respiratory motor output is less.11 67 68 These results were interpreted as indicating that non-chemical feedback related to the load itself plays a role in determining the level of respiratory muscle activation.11 67 68 Thus, at first glance it seems that the contribution of chemical feedback in determining Pmus is reduced by mechanical ventilation. However, most of the studies of this type used an open loop system and chemical feedback was not rigorously controlled. The observed downregulation of respiratory muscle output could have been related to a reduction of chemical feedback because of the increased ventilation.
Milic-Emili and Tyler37 studied the ventilatory response to carbon dioxide in normal subjects breathing with different resistive loads. For a given PCO2, the work output of the inspiratory muscles did not change appreciably with the load. In patients with constant-flow synchronized intermittent mandatory ventilation (SIMV), with constant assistance, inspiratory effort was the same for spontaneous and mandatory breaths.30 33 35 Leung and colleagues33 compared the respiratory effort of patients ventilated with SIMV and patients ventilated with a combination of SIMV and PS. When PS was added, the inspiratory pressuretime product (an index of patient effort) was decreased both in mandatory and intervening breaths. This additional reduction during mandatory breaths was proportional to the decrease in respiratory drive (estimated using the change in oesophageal pressure before triggering, dP/dt) during intervening breaths. This suggests that inspiratory activity was preprogrammed and did not respond to the breath-by-breath changes in load seen during SIMV. Chemical feedback could be important. This is supported by a study in which the chemical stimulus was rigorously controlled. Unloading of the respiratory muscles by mechanical ventilation did not reduce respiratory muscle activation. For a given carbon dioxide stimulus, the waveforms of transdiaphragmatic pressure and total pressure of respiratory muscles were not affected by unloading.20 This study showed that neuromuscular output was tightly linked to carbon dioxide (i.e. to the chemical stimulus) and not to load reduction. In a study of patients with acute respiratory distress syndrome (ARDS) receiving PS, respiratory motor output was measured when changes in the level of support were applied for two breaths, which is unlikely to affect chemical stimuli.61 The respiratory drive was not affected by the change in support, although there was a small change in breathing frequency. The altered frequency meant that no change occurred in the pressuretime product per minute, which is an important index of respiratory effort. However if the changes in PS were applied for a longer period, respiratory drive and the pressuretime product were affected, which was presumably mediated by chemical stimuli.
Such studies suggest that mechanical ventilation does not significantly affect how chemical feedback controls respiratory muscle activity. As during spontaneous breathing, chemical feedback remains an important determinant of Pmus during mechanical ventilation.
Effectiveness of chemical feedback during mechanical ventilation
Although mechanical ventilation does not affect chemical feedback, the ability of this feedback to change the chemical stimuli may be altered. This issue is fundamental in understanding patientventilator interaction. The effectiveness of chemical feedback may differ substantially between wakefulness and sleep or sedation, so these two aspects will be discussed separately.
Wakefulness
Several studies have examined the ventilatory response to carbon dioxide in mechanically ventilated normal conscious subjects.18 20 38 As in spontaneous breathing, increases in PaCO2 caused increased respiratory effort (Pmus) with initially no change in respiratory rate. Respiratory rate increased if PaCO2 increased considerably. This response was found with the three modes of support, with no fundamental difference in response to carbon dioxide for these modes of ventilatory support.38
The above studies used carbon dioxide as a changing chemical stimulus but similar principles should apply if other chemical stimuli (PaO2, pH) are altered. The steady-state ventilatory response to these stimuli is qualitatively similar, affecting the intensity of respiratory effort.2 45 We studied the effectiveness of chemical feedback in normal humans mechanically ventilated with the three main modes of support: PAV, AVC and PS.38 The subjects were ventilated with the maximum tolerable level of assist, which was 80% reduction of patient resistance and elastance with PAV, 10 cm H2O pressure with PS and 1.2 l tidal volume with AVC. The response of the respiratory system to carbon dioxide challenge was observed. Compared with spontaneous breathing, before carbon dioxide challenge, hypocapnia was caused by AVC and PS but not by PAV. However, the intensity of respiratory effort, expressed by the pressuretime product of respiratory muscles (PTP), was reduced by all modes of support. The reduction was greater with AVC and PS than with PAV, and tidal volume with these two modes was considerably greater. Breathing frequency was very similar with all modes; the subjects continued to trigger the ventilator despite being hypocapnic. We can explain these results by considering the respiratory system in terms of respiratory loop gain (change in ventilation for a given change in carbon dioxide stimulus), respiratory controller gain (change in respiratory effort for a given change in carbon dioxide stimulus) and controlled system gain (change in ventilation for a given change in respiratory effort) (Fig. 8). End-tidal PCO2 (PE'CO2), PTP per minute (PTPminute) and ventilation (V·E) were used, respectively, as indices of carbon dioxide stimulus (input), respiratory efforts (motor arm activity) and output. At zero FICO2 respiratory loop gain to carbon dioxide (V·E/PE'CO2) was less with PAV than with PS and AVC (Fig. 9).38 The respiratory loop gain to carbon dioxide is the product of respiratory controller gain (PTPminute/PE'CO2) and respiratory controlled gain (V·E/PTPminute). Thus, the reduced respiratory loop gain with PAV could be caused by either a small respiratory controller gain or a small controlled system gain. The respiratory controller gain did not differ between the various modes of support, indicating that, at least at low PE'CO2, the sensitivity of the respiratory muscles to carbon dioxide was not appreciably affected by the mode of support (Fig. 9).38 On the other hand, with PAV, the controlled system gain was 5- to 6-fold less than with AVC and PS, approaching the value observed during spontaneous breathing (Fig. 9). Therefore, neuroventilatory coupling was preserved with PAV, but not with PS and AVC. This suggests that, before carbon dioxide was given, the ventilatory mode effect on controlled system gain was the main determinant of hypocapnia. For a similar respiratory controller gain, controlled system gain was less with PAV (Fig. 9). The low controlled system gain forced the respiratory loop gain to be reduced, thus preventing a reduction in PaCO2. The mode of ventilatory support affected the response of the respiratory loop gain to carbon dioxide. With PAV, the respiratory loop gain increased as carbon dioxide increased, whereas loop gain remained constant with PS and decreased with AVC (Fig. 9). The increase in respiratory loop gain with carbon dioxide challenge observed with PAV came entirely from the increase in respiratory controller gain, and the controlled system gain remained constant (Fig. 9). On the other hand, with PS and AVC there was a significant decrease in controlled system gain, while respiratory controller gain responded as it did with PAV, increasing with increasing carbon dioxide stimulus (Fig. 9). Therefore, with PS and AVC there was a negative feedback between respiratory effort and controlled system gain; controlled system gain decreased with increasing respiratory effort. In contrast, with PAV the controlled system gain was independent of respiratory effort; neuroventilatory coupling remained even at high drive. Figure 9 clearly shows that the capacity of chemical feedback to compensate for changes in chemical stimuli depends on the effect of ventilator mode on controlled system gain.
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Reflex feedback
Other reflexes are important in controlling breathing.2 69 These reflexes are related to lung volume or flow and mediated by receptors located in the respiratory tract, lung and chest wall.2 6 52 69 Mechanical ventilation may stimulate these receptors by altering flow and volume in comparison with spontaneous breathing.21 Changes in volume and flow may also elicit Pmus responses caused by other reflexes.21 65 Such reflex responses have been largely ignored in mechanically ventilated patients, but under certain circumstances they may be important in patient management.
We studied mechanically ventilated patients with ARDS and measured the reflex response of Pmus to a ventilator breath when the ventilator setting was changed.32 We altered (i) VT at constant inspiratory flow, (ii) PS level, and (iii) inspiratory flow at constant VT for two breaths, and measured the response of the Pmus waveform. Because the patients were sedated and the changes were applied for two breaths only, behavioural and chemical influences were small, and any changes in Pmus would be caused by other reflex feedback. Changing ventilator settings altered the neural respiratory timing immediately (in one breath), whereas respiratory drive remained constant. By decreasing VT and PS and increasing inspiratory flow, breathing frequency was increased. Depending on the ventilation, the changes in breathing frequency were caused by changes in either the inspiratory or the expiratory direction. If inspiratory flow was increased, breathing frequency changed mainly by decreasing the neural inspiratory time. Decreasing VT and PS increased frequency by decreasing the neural expiratory time.32 This reflex response is qualitatively similar to responses in normal humans during wakefulness and sleep.8 14 17 56 It is interesting to note that neural expiratory time strongly depended on the time that mechanical inflation extended into neural expiration; neural expiratory time increased in proportion to the increase in the delay between ventilator cycling off and the end of neural inspiratory time (Fig. 11).32 This shows that expiratory asynchrony causes a reflex timing response, and the dependency of neural expiratory time on expiratory asynchrony was subsequently confirmed.66 The response seems to be relatively weak in patients with obstructive lung disease. The most obvious reflex mechanism is the HeringBreuer reflex.
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Behavioural feedback
Behavioural influences on breathing in mechanically ventilated patients are unpredictable, depending on the individual patient and the environment. Changing the ventilator settings to achieve a particular goal (e.g. reduction of dynamic hyperinflation) could be ineffective in awake patients because of behavioural feedback. In normal subjects ventilated with AVC mode, if inspiratory flow is more or less than the spontaneous value, then breathing discomfort, estimated using a visual analogue scale, is increased.34 Increased dyspnoea may cause rapid shallow breathing and disturb patientventilator synchrony. Jubran and colleagues31 observed active expiratory effort in patients with chronic obstructive pulmonary disease (COPD) when PS was increased. In patients with flow limitation, active expiratory efforts cause breathing discomfort.41 Thus, increasing assistance for patients with COPD may cause behavioural feedback that makes them fight with the ventilator. Behavioural efforts are affected by changes in sedation, the sleepwake state and other aspects of the patient environment.
Composite response of Pmus to Paw
The final response of respiratory efforts to ventilatory assistance is complex and influenced by several factors. Pmus can depend on (i) flow and volume, (ii) PaO2, PaCO2 and pH, (iii) sensitivity to these stimuli, (iv) the disease state, (v) the level of consciousness, and (vi) the type and strength of different reflexes. Unpredictable behavioural effects further complicate the situation. All these factors can influence the ventilatory outcome intended when the ventilator settings are changed.
Conclusion
In conclusion, during assisted mechanical ventilation there is an important interaction between the patient and ventilator. During mechanical ventilation the respiratory system is affected by two pumps, the ventilator (i.e. Paw), controlled by the physician, and the patients own respiratory muscle pump (Pmus), controlled by the patient. Patientventilator interaction is mainly an expression of the function of these two controllers, which should be in harmony if the result is to be appropriate for the patient. Harmony depends on the physician, who should realize that the respiratory system is not passive but reacts, sometimes vigorously, to pressure from the ventilator, depending on factors related both to the ventilator and the patient.
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