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© 2001 International Anesthesia Research Society
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Abstract |
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With pressure support ventilation (PSV), each PSV breath is flow-cycled, and the breath termination criterion (TC) is usually nonadjustable. When TC does not match the interaction between the patient’s inspiratory-expiratory efforts to the opening and closing of the inspiratory and expiratory valves, patient-ventilator asynchrony may occur, and the work of breathing (WOB) may increase. Therefore, we studied the effect of TC on breathing patterns and WOB during PSV in eight patients with acute respiratory distress syndrome or acute lung injury. We studied five levels of TC during PSV—1%, 5%, 20%, 35%, and 45% of the peak inspiratory flow. With increasing levels of TC, the tidal volume decreased and respiratory frequency increased, along with a decrease in duty cycle. WOB markedly increased with increasing levels of TC from 0.31 ± 0.12 J/L with TC 1% to 0.51 ± 0.11 J/L with TC 45%. Premature termination with double breathing occurred in one patient with TC 35% and four patients with TC 45%. Delayed termination with a duty cycle of >0.5 occurred in two patients with TC 1%. In conclusion, the proper adjustment of TC improves patient-ventilator synchrony and decreases WOB during PSV.
Implications: Although termination criterion (TC) is usually nonadjustable, it influences the effectiveness of pressure support ventilation for mechanical ventilation. The proper adjustment of TC is crucial to improve patient-ventilator synchrony and decrease work of breathing. TC 5% of the peak inspiratory flow may be the optimal value for patients with acute respiratory distress syndrome or acute lung injury.
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Introduction |
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Pressure support ventilation (PSV) has been widely used as partial ventilatory support for better patient-ventilator synchrony as compared with synchronized intermittent mandatory ventilation or assist-control ventilation (1–5). The efficacy of PSV depends on a breath-by-breath interaction between the patient’s demand for spontaneous flow and the ventilator’s flow. The patient interacts with the delivered flow and has a certain amount of control of the tidal volume (VT), respiratory frequency, and ventilatory timing. Each PSV breath is flow-cycled and the breath termination criterion (TC) is usually nonadjustable. TC is either a fixed terminal flow (usually 5 L/min), or a percentage of the peak inspiratory flow (usually 5% or 25%). When TC does not match the interaction between the patient’s inspiratory-expiratory efforts to the opening and closing of the inspiratory and expiratory valves, patient-ventilator asynchrony may occur, and the patient’s inspiratory effort may increase. MacIntyre and Ho (6) have reported that changing TC from 25% to 50% had minimal effects on breathing patterns or synchrony. However, they did not study lower levels of TC, and there are no systematic clinical investigations of TC during PSV. We therefore evaluated the effect of TC on breathing patterns and work of breathing (WOB) during PSV.
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Methods |
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After IRB approval, informed consent was obtained from the patients’ families. Eight patients, two men and six women, with a mean age of 69 yr (range 64 to 77 yr) were studied ( Table 1). All patients fulfilled either the acute respiratory distress syndrome (ARDS) or acute lung injury (ALI) criteria according to the report of the American-European consensus conference on ARDS (7). They required ventilatory support. Midazolam or propofol was administered for sedation. The Ramsay sedation score was level 4 (8). At study entry, arterial blood gases were analyzed with a blood gas analyzer ( Bayer 860 ; Bayer Diagnostics, Sudbury, England). The static compliance of the respiratory system was obtained by dividing VT by the difference between the end-inspiratory pause pressure and the end-expiratory pressure during controlled ventilation.
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All patients were ventilated on a PSV mode. The PSV level was determined by an attending physician to partially unload the respiratory muscles without respiratory distress based on observation of the patient’s breathing. Five levels of TC during PSV, 1%, 5%, 20%, 35%, and 45% of the peak inspiratory flow, were studied in random order. We used the Puritan-Bennett Ventilator 840 (Mallinckrodt, CA) in which TC was adjustable from 1% to 45% of the peak inspiratory flow. Each TC level lasted 10–15 min and measurements were taken for 3 min at the end of each period. The trigger sensitivity was set at -0.7 cm H2O. Flow acceleration percent, which determines how quickly the ventilator drives pressure to the target PSV level, was set at 80%. This value was the fastest flow rate without overshoot of airway pressure. Other variables, such as positive end expiratory pressure (PEEP) and fraction of inspired oxygen, were not changed during the investigation.
Airway pressure and flow were measured at the proximal end of the endtracheal tube by using a differential pressure transducer ( VarFlex Flow Transducer ; Bicore, Irvine, CA). VT and minute ventilation were obtained by integrating the flow signal. From the flow signal, the respiratory frequency (f), inspiratory time (TI), and total respiratory cycle duration, were measured. The duty cycle, which was defined as the Ti/total respiratory cycle duration ratio, was calculated (9).
To assess the WOB by the patient, esophageal pressure was measured by using an esophageal balloon. The position of the esophageal balloon was confirmed by comparing the negative deflection of esophageal pressure and airway pressure during inspiration against an occluded airway (10). The inspiratory WOB was estimated by measuring the area enclosed between the esophageal pressure-volume loop during inspiration and the relaxation curve of the chest wall, by using the Campbell technique ( Pulmonary Monitor CP-100 ; Bicore) (2,11). Chest-wall compliance was assumed to be 200 mL/cm H2O. The mechanical WOB by the patient was expressed as work per liter of ventilation. Trigger failure was detected by comparison of esophageal pressure and airway pressure tracings (12). Dynamic auto-PEEP was determined as the change in esophageal pressure from the onset of respiratory muscle effort to the initiation of inspiratory flow.
All data were presented as mean ± SD. Statistical analysis was performed by analysis of variance for repeated measures followed by Bonferroni/Dunn test. The 
level was set at 0.05.
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Results |
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Patients’ characteristics are described in Table 1. VT decreased significantly from 7.8 ± 1.2 mL/kg with TC 1% to 6.5 ± 1.1 mL/kg with TC 45% ( Table 2, Fig. 1). With increasing levels of TC, f increased steadily from 20.1 ± 6.0/min with TC 1% to 26.0 ± 6.9/min with TC 45%. Minute ventilation did not change significantly. The duty cycle markedly decreased associated with a decrease in inspiratory time. WOB increased significantly with increasing levels of TC from 0.31 ± 0.12 J/L with TC 1% to 0.51 ± 0.11 J/L with TC 45%. Figure 2 shows the typical tracing of flow, volume, airway pressure, and esophageal pressure curves with TC 5% and TC 35%. Premature termination with double breathing occurred in Case 5 with TC 35% and in Cases 1, 2, 4, and 5 with TC 45% ( Fig. 3). Delayed termination with a duty cycle of >0.5 sometimes occurred in Cases 1 and 7 with TC 1%. Inspiratory time increased >2 s in Case 5 with TC 1%. With any level of TC, there were no ineffective efforts that failed to trigger the ventilator. The patient’s f determined from esophageal pressure tracings was the same as f determined from the flow/airway pressure tracings. Auto-PEEP was negligible and did not change significantly.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The principal observations in this study were that VT decreased and f increased with increasing levels of TC during PSV, along with a decrease in duty cycle. A higher TC also resulted in increased WOB. Premature termination with double breathing often occurred with a higher TC. In contrast, delayed termination sometimes occurred with TC 1%. Proper adjustment of TC is crucial to improve patient-ventilator synchrony and decrease WOB.
One of the potential advantages of PSV is better synchrony between the patient and the ventilator associated with a decrease in WOB (1–5). However, the efficacy of PSV depends on a breath-by-breath interaction between the patient’s demand for spontaneous flow and the ventilator’s flow. Each PSV breath is pressure-regulated and flow-cycled. Several points of flow delivery mechanism, including breath triggering, initial flow rate, and breath TC, influence patient-ventilator interactions (12–14). In this study, TC was considered and other factors were not changed.
Ideally, the PSV flow should terminate when the patient’s inspiratory effort has ceased (13). However, there is no automatic linkage between the end of the patient’s effort and the end of the ventilator cycle with PSV. TC is either a fixed terminal flow or a percentage of the peak inspiratory flow. As a backup cycling mechanism, most ventilators provide an alternate cycle criterion of either pressure or time. When TC does not match the interaction between the patient’s inspiratory-expiratory efforts to the opening and closing of the inspiratory and expiratory valves, patient-ventilator asynchrony occurs, and the WOB increases. When the breath is too early to stop with a higher TC, premature termination occurs while the inspiratory effort is continuing. Double-triggering within the same cycle also occurs because the inspiratory effort persists after the ventilator has cycled off. Esophageal pressure shows a negative deflection even after PSV is terminated, as shown in Figure 2. WOB also increases because of the sudden loss of ventilatory support during continued muscle contraction.
However, TC 1% is too late to stop the inspiration and results in extremely prolonged inspiration. Delayed termination leaves positive airway pressure present while the patient is trying to exhale. Mechanical inflation may persist into neural expiration. Patients may activate their expiratory muscles to counteract the neural-mechanical dyssynchrony. Although we did not evaluate expiratory WOB, no abdominal muscle activity was observed clinically during the initial phase of expiration with lower levels of TC. Delayed termination may also cause auto-PEEP because of a reduction of expiratory time. In that case, it is important to monitor the flow and/or airway pressure to avoid the inverse ratio ventilation. In this study, auto-PEEP was negligible with any level of TC.
Our findings contrast with those of MacIntyre and Ho (6). They reported that changing flow criteria from 25% to 50% of the peak inspiratory flow did not appear to show much difference. However, their conclusion was not based on measurements of patient’s effort but essentially on optimizing VT. Furthermore, they did not study the effect with TC below 25%.
Breath termination is influenced by the peak inspiratory flow rate. TC of the ventilator used in our study was a percentage of the flow. An extremely early peak flow will tend to terminate the breath rapidly and potentially sooner than the patient’s own inspiratory time (14). However, a faster flow shows the efficacy in reducing the WOB (15–17). Thus, a fast pressure wave shape is desirable from the perspective of WOB. In this study, the adjustable flow acceleration percent, the rate of pressure increase, was set at 80%. This value was the fastest flow rate without overshoot of airway pressure. With this setting, TC 5% seems to be the optimal value for patients with ARDS or ALI.
This study has some limitations. The first limitation is that we might underestimate the WOB. WOB using the Campbell technique measures mechanical external work by the respiratory muscles. This does not reflect inspiratory efforts exactly. Furthermore, chest-wall compliance was not directly measured in this study. Chest-wall mechanics were assumed to be normal. Because the patient’s respiratory conditions were stable, it is unlikely that chest-wall compliance, and thus the WOB against the chest wall, would have changed significantly during the brief study period. Assuming a constant value for chest-wall compliance adds a systemic error that should not change over the course of ventilatory support (18). The level of PSV was kept constant for the individual patients. Thus, the comparison of WOB across different TC can be accepted. The second limitation is the evaluation of patient-ventilator synchrony. Although we measured the WOB and evaluated patient-ventilator synchrony, the WOB may not be the best variable to measure the patient’s response to the adjustment of TC. There is always a delay between the time the patient wants to exhale and when the ventilator starts the expiration. In addition to measuring WOB, it may be helpful for the adjustment of TC to measure the time delay. The problem is in clinically measuring when the patient terminates his inspiratory effort. The comparison of esophageal pressure and airway pressure curves may be helpful to solve the problem. Furthermore, it may be useful in a group of less-sedated patients to evaluate their perceived comfort with a dyspnea scale. Another limitation of the study is that the number of subjects evaluated was small and their pulmonary mechanics were similar. Moreover, the results cannot be applied to patients with chronic obstructive pulmonary disease, whose compliance is extremely high compared with that of patients with ARDS or ALI. Their inspiration is apt to be prolonged because of slow deceleration of inspiratory flow. The optimal value of TC in such patients may be different from that inpatients with ARDS or ALI. Further studies are needed to evaluate whether manipulation of the TC control is necessary throughout the patient’s course on the mechanical ventilation. It would be appropriate to recommend that manufacturers preset the value to a certain number, or at least limit the scope of the control to eliminate synchronization problems.
In conclusion, the proper adjustment of TC improved patient-ventilator synchrony and decreased WOB during PSV in the ALI/ARDS patients studied.
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References |
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