CONTRIBUTION OF EXPIRATORY MUSCLE PRESSURE TO DYNAMIC INTRINSIC POSITIVE END-EXPIRATORY PRESSURE

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To the Editor :

We read with interest the Editorial by Dr. Magdy Younes (1) concerning our study (2). Dr. Younes raised two questions that we feel deserve clarification. Dr. Younes notices a peculiar inconsistency in the data; i.e., corrected PEEPi,dyn for expiratory muscle activity using the approach of Appendini and coworkers (3) (PEEPi,dyn-Pga,zf decay) was given as 8.5 ± 3.1 cm H₂O in Table 2 of our study, whereas Pdi was reported as being 5.3 ± 0.6 cm H₂O between the beginning of the fall of Pcs to the point of zero flow. This inconsistency in the data was created by an inadvertent omission in the text, which should have read: "amounting on the average to 5.3 ± 0.6 cm H₂O (range 2.8-9.2 cm H₂O) from the beginning to the end of the trial" at the end of the RESULTs section. In fact, the approach of Appendini and coworkers, as well as the Pdi measured as the average of the values obtained at the middle and the end of the spontaneous breathing trial yielded 8.5 ± 3.1 cm H₂O, as the two values are mathematically bound to be identical (both are Pes-Pga over the same interval). The Pdi of 5.3 ± 0.6 cm H₂O (range 2.8-9.2 cm H₂O) was the average of the values measured at three points of the spontaneous breathing trial in each patient (i.e., at the beginning, middle, and end); it was only reported to indicate that actual PEEPi,dyn requiring inspiratory muscle contraction to be counterbalanced was present in every patient of our study throughout the trial and to give a measure of diaphragmatic contraction. Regarding Dr. Younes' concern about the limitations of the Campbell diagram when it is called to arbitrate between results that differ from each other by only a few cm H₂O, we generally agree and discuss this issue in the critique of methods of our study. However, construction of the Campbell diagram is the sole method available to obtain the reference PEEPi,dyn. Moreover, the Campbell diagram is called in our study to validate methods giving results that on an individual basis may differ by as much as 6-7 cm H₂O, when the reference PEEPi,dyn these methods attempt to approximate may be only 2-4 cm H₂O. Indeed, as shown in Figure 5 in our study, the approach of Lessard and coworkers (4) (PEEPi,dyn-Pga,total decay) that achieves the best performance differs from the reference PEEPi,dyn by less than 1 cm H₂O, regardless of the intensity of expiratory muscle contraction, whereas the approach of Appendini and coworkers (3) overestimates the reference PEEPi,dyn by 2-7 cm H₂O, when expiratory muscle contraction expressed by Pga,exp rise is powerful (Pga,exp rise > 6 cm H₂O).

Evangelismos Hospital, Athens, Greece

	Footnotes

Dr. Younes was given an opportunity to respond to this letter but declined to do so.

1. Younes M. Dynamic intrinsic PEEP (PEEPi,dyn): is it worth saving? Am J Respir Crit Care Med 2000; 162: 1608-1609 [Free Full Text].

2. Zakynthinos S, Vassilakopoulos T, Zakynthinos E, Mavrommatis A, Roussos C. Contribution of expiratory muscle pressure to dynamic intrinsic positive end-expiratory pressure: validation using the Campbell diagram. Am J Respir Crit Care Med 2000; 162: 1633-1640 [Abstract/Free Full Text].

3. Appendini L, Patessio A, Zanoboni S, Carone M, Gukov B, Donner CF, Rossi A. Physiologic effects of positive end-expiratory pressure and mask pressure support during exacerbations of chronic obstructive pulmonary disease. Am J Respir Crit Care Med 1994; 149: 1069-1076 [Abstract].

4. Lessard MR, Lofaso F, Brochard L. Expiratory muscle activity increases intrinsic positive end-expiratory pressure independently of dynamic hyperinflation in mechanically ventilated patients. Am J Respir Crit Care Med 1995; 151: 562-569 [Abstract].