Pleural Pressure and Derived Variables

Recording of changes in pleural pressure (Ppl) represents a basic means of quantifying respiratory muscle activity. Recording the tidal swings in esophageal pressure (Pes) (as an estimate of Ppl) and a knowledge of the passive properties of the chest wall over lung volume allow the quantification of the pressure developed by the muscles, also referred to as "muscular pressure" (Pmus). This measurement has facilitated the gathering of important information in the field of mechanical ventilation, both in quantification of the effort performed by patients receiving ventilator assistance and for comparison of different modes of assisted ventilation (see Esophageal, Gastric, and Transdiaphragmatic Pressures in Section 2 of this Statement).

Two main variables can be derived: work of breathing (WOB) and pressure-time product (PTP) (see Assessment of the Function of the Active Chest Wall: Campbell Diagram in Section 6 and Pressure-Time Product in Section 4 of this Statement) (76). For both calculations, knowledge of the compliance curve of the chest wall is necessary; several methods have been used for this measurement in mechanically ventilated patients. some investigators employ an assumed chest wall compliance value, amounting to 4% of the predicted vital capacity per cm H2O (47, 77). Assuming that the chest wall compliance is linear over the range of lung volumes studied, a straight line can be traced over lung volume to delineate the pressure reference. This method has the major disadvantage of not measuring the true chest wall compliance, which has been shown to be substantially modified in some intubated patients with acute respiratory failure (43, 44, 78). A preferable approach is to measure the pressure-volume (PV) relationship, using Pes recordings over the range of lung volumes studied, during passive inflation under controlled mechanical ventilation. This makes it necessary to heavily sedate the patient and/or to abolish spontaneous activity by hyperventilating the patient. Again, for simplicity, this curve can be assumed to be linear, or it can also be superimposed graphically over each studied breath, assuming that the end-expiratory lung volume can be estimated (79). The latter approach allows quantification of active expiratory effort.

WOB is measured from pressure-volume loops using the Campbell diagram method (80) (see Assessment of the Function of the Active Chest Wall: Campbell Diagram in Section 6 of this Statement). The measurement of the WOB in ICU patients raises no unique concerns, although recognition of some limitations of the measurement has led to the use of alternative methods of measuring respiratory energy expenditure. Measurement of WOB does not allow quantification of isometric efforts, such as an effort against a closed airway or inefficient or "wasted" inspiratory efforts that fail to trigger a ventilator (8183); in addition, measurement of WOB is probably inadequate for the comparison of settings where part of the change in Vt is achieved by the ventilator. in such a case, an increase in WOB may be recorded in the absence of a true change in muscular effort. Such concerns about WOB have been recognized for a long time (76) and have prompted the use of other indices to quantify respiratory muscle effort in the ICU, especially PTP (51, 76).

Calculation of the PTP necessitates the integration of the area under the Ppl curve versus time. It also requires, however, a solution to the problem of determining the beginning of the inspiratory effort and of using PEEPi to reference the true beginning of inspiration to the chest wall relaxation line (52). Different methods have been proposed to perform this calculation (which is applicable as well to the transdiaphragmatic pressure [Pdi]), and the termination of inspiratory effort has been based on the inspiratory-expiratory flow transition point (52) or once the pressure has returned back to its baseline level (84). PTP can be expressed on a per-breath or a perminute basis; today, it is probably one of the most useful tools for quantifying respiratory muscle effort in mechanically ventilated patients (18, 52, 80, 85-93) (see Pressure-Time Index of Inspiratory Muscles in Section 5 and Pressure-Time Product in Section 4 of this Statement). PTP can be separated into a number of components: the effort made to counterbalance intrinsic PEEP; the effort to trigger the ventilator; and the effort to inflate the chest in the posttrigger phase (83, 87, 93) (Figure 3). However, the correct interpretation of this measurement when end-expiratory lung volume (FRC) undergoes concurrent change remains a question of controversy that has yet to be solved and deserves further investigation.

The presence of a phase lag between the onset of the decline in inspiratory pressure decay and the time at which the flow reaches zero indicates the presence of positive alveolar pressure at end-expiration. Because this positive pressure can result from either elastic recoil pressure generated by hyperinflation or from activity of abdominal and/or thoracic expiratory muscles, interpretation of this pressure is extremely difficult. Different methods have been proposed to solve this problem, using the Pdi (94) or the expiratory swings in gastric pressure (Pga) (95) to estimate the part related to expiratory muscle activity (see subsequent text). When Pga is not available, one can calculate a range of values between two extremes or bounds: at one extreme, it is assumed that all of the pressure drop before reaching zero flow is related to dynamic hyperinflation, and at the other extreme, it is assumed that all of this pressure drop is related to the relaxation of the expiratory muscles (79).

Measurements of Ppl and derived variables are very helpful in research. In the clinical setting, however, it has gained relatively little acceptance despite the availability of commercial systems. Some investigators have suggested that Ppl may help in understanding the reasons for the inability to tolerate discontinuation from mechanical ventilation (96-98). Although Ppl has potential implications for setting the ventilator, clear-cut data are not available on which to base individual targets for the employment of these measurements. In addition, a number of artifacts, including interference by cardiac contractions, make automatic calculations unreliable, especially regarding the detection of the end and the onset of both inspira-tory and expiratory efforts.

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