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In the previous issue of Critical Care, Al-Rawas and colleagues [1] presented a method for assessing elastic and resistive properties of the respiratory system in patients with respiratory failure. The method assumes a first-order resistance-compliance (RC) model of breathing mechanics.  Although the procedure for estimating expiratory time constant (τE) is correct and efficient, some clarifications need to be made about the use of equations 3 to 5 in mechanically ventilated patients, in whom an intrinsic positive end-expiratory pressure (PEEPi) can be present [2,3].  Equations 3 to 5 are correct only if the term indicated with the acronym PEEP accounts not only for external PEEP but also for PEEPi. Neglecting PEEPi can lead to significant errors, as demonstrated by the following model-to-model analysis, in which data were generated by using a first-order RC model. Table ​Table11 shows the true model values and their corresponding estimates, which were obtained with the method of Al-Rawas and colleagues and which neglect PEEPi.    Table 1  True model values and corresponding estimates    The estimate of τE is correct for both patients because, in the first-order RC model, the relationship between lung volume changes and expiratory flow does not depend on PEEP (intrinsic or external or both). The estimates of total resistance (Rtot), respiratory system compliance (Crs), and plateau pressure (Pplt) for patient A (in whom PEEPi is 0) are identical to the true values. For patient B (in whom PEEPi is equal to about 3 cm H2O), the percentages of error of the estimations of Rtot, Crs, and Pplt are 17%, −14.5%, and −18.2%, respectively. This confirms that these estimates necessitate the measurement of PEEPi and this requires, for example, the end-expiratory occlusion technique [4,5].

Comments on the use of expiratory time constant for determinations of plateau pressure, respiratory system compliance, and total resistance