A Method To Assess the Pressure Developed by Inspiratory Muscles in Patients with Acute Lung Injury Undergoing Pressure Support |
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ABSTRACT |
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ABSTRACT
INTRODUCTION
METHODS
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DISCUSSION
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We evaluated the end-inspiratory occlusion maneuver as a means to estimate the inspiratory effort during pressure support ventilation (PS). In nine nonobstructed acute lung injury (ALI) patients, we applied four levels of PS (0, 5, 10, 15 cm H2O) to modify the inspiratory effort. End inspiratory occlusions (2 to 3 s) were performed at the end of each experimental period by pushing the inspiratory hold button of the ventilator (Servo 900 C; Siemens, Berlin, Germany). We took the difference between the end-inspiratory occlusion plateau pressure and the airway pressure before the occlusion (PEEP + PS) as an estimate of the inspiratory effort and called it PMI (Pmusc,index). From the esophageal pressure tracing we obtained a reference measurement of the pressure developed by the inspiratory muscles at end inspiration (Pmusc,ei) and of the pressure-time product per breath (PTP/b) and per minute (PTP/min). In each patient, PMI was correlated with Pmusc,ei (p < 0.01) and PTP/b (p < 0.01). A PMI threshold of 6 cm H2O detected PTP/min < 125 cm H2O s/min with a sensitivity of 0.89 and a specificity of 0.89. We conclude that PMI is a good estimate of the pressure developed by the inspiratory muscles in ALI patients and may be used to titrate PS level. The major advantage of PMI is that it can be obtained from the ventilator display without any additional equipment.
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Pressure support (PS) ventilation is a mode of assisted ventilation designed to provide a synchronized inspiratory support to spontaneously breathing patients; it has been used both as a weaning technique and as a stand-alone ventilatory support mode in acute respiratory failure patients (1). Notwithstanding rather extensive clinical and experimental research, rationale and clinical guidelines for the use of PS are still rather undefined. In most instances, the level of PS is indeed empirically adjusted, aiming at some convenient degree of the respiratory muscles unloading.
It has been stated that "the debate about the best level of PS is on its way of becoming a classic in critical care medicine, alongside that on the best PEEP" (4). An individual adjustment of the pressure support is indeed essential in order to maintain enough inspiratory muscle activity while avoiding fatigue. A need has therefore emerged for simple, clinically applicable techniques for measuring the inspiratory effort in ventilated patients. Such measurements could guide the individual tailoring of ventilatory assistance.
The assessment of the respiratory drive, as measured by the occlusion pressure (P0.1), has been recently proposed as useful in setting the level of PS, since it indirectly reflects the patient's work of breathing (5).
The direct measurement of the respiratory muscle mechanical activity requires, however, an efficient esophageal pressure (Pes) line, and it is not entirely devoid of problems.
Alternative techniques to evaluate patient's inspiratory effort, like the least square fitting method (6), or the respiratory muscles electromyography (7), are still largely confined to investigational settings.
In this paper we present a specific application of the airway occlusion method. Using this method we put forward a simple estimate of the pressure developed by the inspiratory muscles at end inspiration (Pmusc,ei). We called this estimate PMI (Pmusc,index), and compared it with the reference measurement obtained through a Pes line. In principle, however, assessing PMI involves no apparatus besides the ventilator in use, nor does it involve a Pes line.
Moreover, in a group of acute lung injury (ALI) patients undergoing pressure support ventilation, we tested whether we could expand the use of PMI from the estimate of Pmusc,ei, to the evaluation of the total inspiratory muscle workload. To do so we compared PMI with the pressure-time product, an estimate of the respiratory muscles' metabolic work, derived from the Pes measurement. Finally, we verified whether PMI could be clinically useful in identifying higher than desirable levels of inspiratory effort.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Principle
When the inspiratory hold button of a Servo Ventilator 900 C (Siemens, Berlin, Germany) is pressed during PS, both the inspiratory and the expiratory valves of the ventilator are held closed from the end of inspiration to the release of the button. Therefore, by pushing the inspiratory hold button, the operator performs an end-inspiratory occlusion maneuver. Following a variable amount of time (usually less than 0.5 s) the patient totally ceases his inspiratory effort. He then appears to relax his inspiratory muscles, as indicated on the airway pressure tracing, by a so-called occlusion plateau, which may be higher or lower than the airway pressure value before the occlusion (P,aw). This occlusion plateau pressure should approximate the relaxed elastic recoil pressure of the respiratory system (Pel,rsi) (8).
The difference between the relaxed occlusion plateau (Pel,rsi) and the alveolar pressure at the time of occlusion, measures the pressure developed by the respiratory muscles at end inspiration (Pmusc,ei).
Since alveolar pressure equals P,aw minus the resistive pressure
drop (P,res) the following equation applies: Pmusc,ei = Pel,rsi 
P,aw + P,res.
P,res in turn equals respiratory resistance (R,rs) times flow: P,res = Flow * R,rs.
During pressure support ventilation, however, end inspiratory flow is always relatively small, due to the ventilator design. Indeed, a Siemens Servo 900 C switches from inspiration to expiration either when the inspiratory flow has decayed to less than 25% of the peak inspiratory value (flow criterion), or when P,aw exceeds of at least 3 cm H2O the set PS + PEEP level (pressure criterion). The end inspiratory flow therefore, though not necessarily nil, is always relatively low, being between 0 (if the pressure criterion is at work) and 25% of the peak inspiratory value (if the flow criterion is at work). P,res should therefore always amount to a rather small figure, particularly in nonobstructed patients.
We reasoned that the difference between airway pressure before the occlusion (P,aw) and the end inspiratory elastic recoil plateau pressure (Pel,rsi) could approximate Pmusc,ei closely enough to be a clinically useful estimate of the end inspiratory effort. We called this difference Pmusc,index (PMI).
Subjects
We studied nine patients with ALI (11) admitted to the ICU with various primary diagnoses (Table 1), who were ventilated in PS mode, and who were able to withstand a trial at PS 0 cm H2O for at least 30 min. The patients were clinically and hemodynamically stable, had no neurological injury, nor a history of asthma or chronic obstructive pulmonary disease and no active air leaks. The level of sedation (attained by diazepam or fentanyl intravenously) was such as to allow the patient arousal and coordinated motor responses to verbal commands.
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The institutional ethical committee approved the investigation protocol, and all subjects or their relatives gave informed consent.
Protocol
All patients were ventilated in PS mode by a Siemens Servo 900 C
ventilator (Table 1). PEEP and FIO2 were not modified throughout the
study period. The ventilator inspiratory trigger sensitivity was set at
2 cm H2O. In each patient, four levels of pressure support ventilation
were applied in random sequence: 0, 5, 10, 15 cm H2O. Each level lasted a minimum of 20 min.
At the end of each experimental period, three end inspiratory and three end expiratory occlusions were obtained by pressing for 2 to 3 s the appropriate button on the ventilator's panel. We allowed at least 1 min of nonobstructed breathing between each occlusion. Airway pressure, flow, and esophageal pressure were measured by a CP100 Monitor (Bicore, Irvine, CA) (12).
The pneumotachograph was positioned between the Y connector of the ventilator circuit and the endotracheal tube. Tidal volume (TV) was obtained by integration of the airway flow signal. Respiratory rate (RR) was computed by counting the breaths in the minute preceding the start of the occlusion maneuvers. Proper positioning of the esophageal catheter was verified by an end expiratory occlusion as described by Baydur and colleagues (13).
All signals were visually inspected on the acquisition system's screen for adequacy and recorded in digital form on a personal computer for subsequent analysis.
From the analysis of the airway pressure, airflow, and esophageal pressure tracings of the occluded breaths we obtained the following parameters (Figure 1): (1) Pel,rsi was measured as the airway pressure plateau, usually reached within 1 s from the end inspiratory occlusion, indicating the near relaxation of the inspiratory muscles (8). The minimum acceptable length for a plateau was 0.25 s and its adequacy was judged by inspection. Occlusions not reaching an identifiable plateau were discarded. (2) Pel,rse was measured as the airway pressure plateau obtained by an end expiratory occlusion. Occlusions not reaching an identifiable plateau (i.e., shorter than 0.25 s) were discarded. (3) Pes,ee was identified as the esophageal pressure at end expiration, i.e., the value of esophageal pressure at which a rapid change in slope indicates the start of an active inspiratory effort (14). (4) Pes,ei was identified as the esophageal pressure measured when the inspiratory flow reached the 0 value at the time of end inspiratory occlusion. (5) Pel,cw was identified as the esophageal pressure plateau corresponding to the airway pressure plateau following the end inspiratory occlusion. (6) Ti, duration of inspiratory time, was calculated from airflow tracings.
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Computation of Total Respiratory System Compliance (Cpl,rs)
Cpl,rs was computed as TV/(Pel,rsi Pel,rse).
Computation of Lung Flow Resistance (R,l)
R,l was computed by the isovolume method (15) as: R,l = (Tp,vt50%i Tp,vt50%e)/(
,vt50%i + ,vt50%e) where Tp,vt50%i, ,vt50%i and Tp,vt50%e,
,vt50%e are the dynamic transpulmonary pressure and the corresponding flow at 50% of the tidal volume during inspiration and expiration, respectively.
Measurement of P0.1
The airway pressure drop in the first 100 ms of an occluded inspiration (P0.1) is a useful index of the neuromuscular inspiratory drive (16). We measured P0.1 by depressing the end expiratory pause button of the ventilator, thus obtaining a quasi-isovolumic inspiratory effort. We discarded by inspection the initial 10 to 20 ms, so as to avoid uncertainties in recognizing the actual start of the inspiratory effort. The airway pressure drop of the following 100 ms was taken as the P0.1 value.
Estimate of the End-inspiratory Effort
We then computed PMI (Pmusc,index) as an estimate of end inspiratory effort: PMI = Pel,rsi (PEEP + PS) thus indicating the difference between the elastic recoil pressure of the respiratory system and
the total pressure applied to the airways by the ventilator.
We validated PMI by comparing it with: (1) the direct measurement of Pmusc,ei; (2) the pressure time product per breath (PTP/b), a global index of inspiratory effort.
Validation of PMI against Pmusc,ei. The pressure generated by the
inspiratory muscles at end inspiration can be directly measured from
the esophageal pressure tracing as Pmusc,ei = Pel,cw Pes,ei (Figure
1). We compared PMI with Pmusc,ei.
Validation of PMI against PTP/b. PTP/b is an accepted estimate of patient's inspiratory effort (17); we measured it by integrating the area inscribed by the line connecting Pes,ee to a value equal to Pel,cw, translated to the end of inspiration, and the inspiratory esophageal pressure tracing (Figure 1). In so doing we chose to include in the PTP/b computation the isometric inspiratory effort and to measure Pel,cw instead of extrapolating it from normal reference values.
We identified the PMI threshold that discriminated best between PTP/min < 125 and PTP/min > 125 cm H2O s/min, hence differentiating desirable from excessively high levels of inspiratory effort (18). PTP/min was calculated as the product PTP/b · RR.
Statistics
Unless otherwise indicated, data are reported as mean ± SD. Linear regression analyses were used to compare PMI with the corresponding values of Pmusc,ei and PTP/b.
The effects of the different levels of PS within patients were tested by two way ANOVA. The efficacy of a PMI threshold to detect excessive values of PTP/min was tested retrospectively in terms of sensitivity (true positive/true positive + false negative) and specificity (true negative/true negative + false positive). True positive was defined as occuring when PMI resulted below the threshold value and PTP/min was < 125 cm H2O s/min, true negative when PMI was above the threshold and PTP/min resulted > 125 cm H2O s/min. Conversely, false positive was defined when PMI was below the threshold and PTP/min was above 125 cm H2O s/min; false negative was defined when PMI resulted higher than the threshold and PTP/min was < 125 cm H2O s/ min. The selected threshold value was the one that resulted in the fewest false classifications (19).
Probability values lower than 0.05 were taken as significant.
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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In all investigated patients the identification of a near relaxation end inspiratory plateau was successful (Figure 1). Similar considerations apply to the end expiratory occlusions; the difference between Pel,rse and PEEP was in all instances smaller than 1 cm H2O.
In Patient 6, however, a total of eight occlusions were needed to be able to collect the required three near relaxation plateaus at PS 0 cm H2O.
Effects of PS level changes upon selected respiratory parameters. In Table 2 the effects of the increasing levels of PS upon some respiratory parameters are summarized. RR and TV significantly changed in opposite directions while the level of pressure support was changed; VE was rather constant, with minor and statistically not significant changes. P0.1 significantly decreased with increasing pressure support, thus indicating a decreased ventilatory drive. Both Pmusc,ei and PTP/b decreased significantly and substantially by increasing the pressure support level. It is worth noting how PMI follows the behavior of Pmusc,ei. At the highest PS levels, when Pmusc,ei reached its lowest values, the average PMI was negative, indicating that the airway pressure was higher during inspiration than during near relaxation.
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Evaluation of PMI as an estimate of Pmusc,ei. The regression of all PMI values over Pmusc,ei (108 measurements in
nine patients) showed a very good agreement between the two
measurements (Figure 2). In each individual patient the linear
regression between PMI and Pmusc,ei was significant (p < 0.01) and close to the identity line (average intercept 1.0 ± 1.3 cm H2O and average slope 1.17 ± 0.23).
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Evaluation of PMI as an indicator of PTP/b. Since Pmusc,ei
was in turn very strictly related to PTP/b (Pmusc,ei = 1.16 · PTP/b 0.13; r = 0.86, n = 108, p < 0.01), a significant regression was also present between PMI and PTP/b (Figure 3). In
individual patients the linear regression between PMI and
PTP/b was significant (p < 0.01). Individual regression lines
had rather similar slopes (1.66 ± 0.28) while the intercept, i.e.,
the PMI value when PTP/b is nil, resulted more scattered
(3.1 ± 2.5 cm H2O). The intercept resulted negatively correlated to lung flow resistance: intercept = 0.7 R,l + 2.8 cm
H2O (r = 0.75, n = 9, p < 0.05), i.e., patients with higher R,l
showed lower values of intercept. In individual patients PTP/b
showed no significant correlation with RR, TV nor VE.
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A retrospectively identified threshold value of PMI = 6 cm H2O predicted PTPmin < 125 cm H2O s/min with a sensitivity of 0.89 and a specificity of 0.89 (Figure 4).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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General
The main result of this study is that the difference between airway pressure at the end of inspiration (i.e., PEEP + PS) and the elastic recoil pressure of the respiratory system is a very good estimate of the pressure developed by the inspiratory muscles at end inspiration. This measurement can be performed by simply depressing the inspiratory hold button on the Servo Ventilator; in principle it does not even require a recording of the pressure tracing, but can just be taken from the analog airway pressure display.
Moreover, at least in our group of nonobstructed acute
lung injury patients, we could take PMI as a good indicator of
changes of PTP/b in response to modifications of the PS level.
A PMI threshold of 6 cm H2O detected, with adequate clinical
accuracy, levels of inspiratory effort (PTP/min 
125 cm H2O
s/min) currently considered as reasonable clinical targets (18).
Adequacy of the Postocclusion Plateau Pressure as a Measure of Pel,rsi
We have previously shown that during PS the airway pressure plateau that follows an inspiratory occlusion is a good measurement of Pel,rsi (10). Obtaining a plateau following an inspiratory occlusion during PS is relatively easy; this is probably related to the presence of artificial airways, which eliminates the problem of glottis closure, and to the fact that all patients in study were under variable degrees of sedation. Doubts might be raised as to whether Pel,rsi really reflects a relaxed condition, and whether one can exclude any activation of the expiratory muscles during inspiration (18).
Recent work has indeed proven substantial expiratory muscles activation during both inspiration (18) and expiration (20), mainly in COPD patients.
In our ALI patients, however, any activation of the expiratory muscles at end inspiration is very unlikely. No significant difference on Cpl,rs could be shown between the various PS levels: this fact strongly militates against any expiratory muscle activity at end inspiration. No significant expiratory muscle activity is possible during inspiration at PS 0, since this will immediately cause a switch to expiration. At PS levels higher than 0, any expiratory muscle activity will cause a decrease in the measured Cpl,rs (i.e., a falsely high Pel,rsi), which was never the case, at least in the investigated patients.
We cannot however extrapolate our conclusions to chronically obstructed respiratory patients, nor to patients with longer inspiratory times: expiratory activity has indeed been shown in various occasions (18, 20).
We performed our measurements in a selected, relatively homogenous group of patients, and with a specific ventilator (Siemens 900 C). When different ventilators, i.e., when different criteria to cycle from inspiration to expiration are used, the relationship between the computed PMI and Pmusc,ei and PTP/b might be different and, once again, caution must be used when extrapolating our data to different patients and to different ventilators.
PMI as an Estimate of Pmusc,ei
As indicated in the METHODS section, there are good theoretical reasons for the noncoincidence of PMI and Pmusc,ei.
First, PMI does not include the resistive pressure drop. Although we only tested this method in patients with no evidence of increased respiratory resistance, a measurable resistive pressure drop must have been at play. Although the end
inspiratory flow is quite low, or even nil during PS, we can suggest the use of PMI in nonobstructed patients only. The reader
will notice that when Pmusc,ei is small, then PMI has negative
values (Figure 2). Since PMI = Pmusc,ei P,res, when P,res
exceeds Pmusc,ei (i.e., when Pmusc,ei is very small) PMI is
negative. In this case the occlusion plateau pressure will be
lower than the end-inspiratory airway pressure.
Second, since our aim was to assess the validity of PMI as a very simple clinical measurement, we took PEEP + PS level as the prevailing airway pressure at end inspiration, rather than its direct measurement. The identification of the exact time of occlusion, and of the end inspiratory flow and pressure, would have been rather difficult, since we did not use a dedicated rapid airway occluding valve, but the comparatively slow valve system of the Servo Ventilator.
PMI is essentially based upon the attainment of an end inspiratory pressure plateau (Pel,rsi). We showed that PMI and Pmusc,ei, though not coincident, were very strictly related, and that PMI was a reliable estimate of Pmusc,ei.
PMI as an Estimate of PTP/b
Considerable uncertainty exists on how to compute PTP/b during assisted ventilation. By selecting the start of inspiration at the esophageal pressure drop rather than at the start of inspiratory flow, we chose to include in the computation the isometric muscle contraction. This portion of the respiratory work has been shown to be an important proportion of the total inspiratory "work" (21, 22). As for the portion of PTP/b needed to expand the chest wall, we used the actually measured Pel,cw rather than extrapolating it from normal Pes-volume curve. This was suggested by the recent observation that in acute lung injury patients chest wall compliance may be substantially decreased with respect to normal subjects (23). It has been shown that once the respiratory muscles are activated to trigger the ventilator, their contraction goes on throughout inspiration, even during supported breathing (14). The relationship between Pmusc,ei and PTP/b, which measures the integral of the pressure developed by the inspiratory muscles over the entire span of inspiration, was therefore an expected one. The same considerations apply to the PMI versus PTP/b relationships.
A significant correlation between PMI and PTP/b was found both for the entire set of data and in each individual patient (Figure 3). It is worth noting that the scatter of the intercepts of the individual regressions (Figure 3) was related to pulmonary flow resistance. It is therefore possible to conclude that PMI provides a reliable estimate of PTP/b changes in response to PS level modifications. The estimate of the absolute value of PTP/b from PMI is not as good, due to the interindividual differences in the flow-resistive component of PTP/b.
Clinical Implications
One might question the real need for instrumental monitoring of the inspiratory effort. In the clinical setting the simple observation of RR and TV might be claimed as sufficient to tailor the PS level to the individual patient. Our study supports the need for more refined parameters of inspiratory effort. We selected a group of patients able to withstand a trial of approximately 30 min at PS 0, basically a CPAP on a demand flow system. In this group of patients, as in other studies, RR and TV showed only relatively minor changes while changing the level of pressure support (Table 2) (24, 25).
Moreover, when pooling all data, neither RR, nor TV was a reliable predictor of PTP/b. Undoubtedly, more severe cases would not have coped with very low levels of PS, and would have probably increased their respiratory rate to very high values (rapid shallow breathing). Under these conditions, clinical common sense would have alerted the therapist that the selected level of pressure support was not only inappropriate, but also unbearable by the patient. On the other hand, the measurements of PTP/b, Pmusc,ei and PMI showed a major change between the various levels of PS, even when applied to our group of moderately diseased patients, giving an unequivocal indication of the physiological effects of respiratory therapy.
The identification of a PMI threshold capable of detecting excessive inspiratory effort appears of some clinical relevance (Figure 4). Our retrospective analysis showed that a PMI value of 6 cm H2O could separate desirable from excessive inspiratory effort, with good sensitivity and specificity. The two false positive data (PMI < 6 cm H2O and PTP/min > 125 cm H2O s/min) belong to patients with the highest pulmonary flow resistance of the study's group. This underscores once again the potential limitation in the application of PMI as absolute inspiratory effort estimate in subjects with substantial resistive workload. PMI underestimates the inspiratory effort since it does not take into account the pressure developed by the inspiratory muscles to overcome the resistance but just the pressure needed to overcome the elastance of the respiratory system.
It is interesting to notice how a PMI threshold of 6 cm H2O translates a normal inspiratory effort. It measures the pressure developed at end inspiration by the inspiratory muscles of a subject with a normal Cpl,rs (100 ml/cm H2O), which inspires a normal TV (600 ml) with a PS of 0 cm H2O.
P0.1 has been shown to be very sensitive to changes in PS settings, and it has been suggested as a very useful parameter to set the PS level, when the work of breathing by the patient is taken as the target (5). At variance, we took the pressure-time product as the reference target, and we suggest PMI, which directly reflects Pmusc,ei, as an effective option to select the proper PS level.
Conclusions
We propose an application of the end-inspiratory airway occlusion method to pressure support ventilation in order to evaluate the pressure developed by the inspiratory muscles with no apparatus besides the ventilator in use.
PMI, the difference between end inspiratory airway pressure (PEEP + PS) and the elastic recoil pressure of the respiratory system (Pel,rsi), appeared to be a useful and sensitive indicator of the inspiratory effort in a selected group of nonobstructed acute lung injury patients.
Despite some gross oversimplification, we propose PMI as a possible aid to the tailoring of PS in the individual patient.
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Footnotes |
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Correspondence and requests for reprints should be addressed to Antonio Pesenti, M.D., Istituto di Anestesiologia e Rianimazione, Via Donizetti 109 Monza (MI), 20052 Italy.
(Received in original form February 7, 1996 and in revised form April 10, 1997).
Acknowledgments: The authors thank M. E. Sparacino, M.D., for her continuous support, Synesis snc for the artwork, and Ms. E. Moioli for her precious secretarial assistance.
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References |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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