Intrinsic positive end-expiratory pressure in mechanically ventilated patients with and without tidal expiratory flow limitation.

OBJECTIVE

To assess static intrinsic positive end-expiratory pressure (PEEPi,st) and expiratory flow limitation (FL) in 32 consecutive mechanically ventilated patients with acute respiratory failure (ARF), using a commercial ventilator with an incorporated device that allows the application of a negative expiratory pressure (NEP).

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary intensive care unit of a university hospital.

PATIENTS

Thirty-two consecutive ventilated patients with ARF of various etiologies.

INTERVENTIONS

Evaluation of respiratory mechanics, PEEPi,st, and FL from pressure, flow, and volume traces provided by the ventilator.

MEASUREMENTS

Peak airway pressure, PEEPi,st, dynamic elastance, and interrupter resistance were measured in relaxed patients in a supine position. Comparison of tidal flow-volume curves before and during the application of an NEP of 5 cm H2O was used to assess tidal expiratory FL.

RESULTS

Twelve of 32 patients studied exhibited tidal expiratory FL, which was detected by the absence of increase in expiratory flow despite application of an NEP over the entire or part of the baseline expiratory flow-volume curve. All patients exhibited PEEPi,st, which amounted to 1.2 +/- 0.9 cm H2O (mean +/- SD) in the 20 non-FL patients and 7.1 +/- 2.8 cm H2O in the 12 FL patients (p < 0.00001). The majority of patients with ARF resulting from underlying lung disease (11 of 13) had FL and a PEEPi,st > 4 cm H2O, whereas in patients with ARF of extrapulmonary origin, PEEPi,st was always < 4 cm H2O and only one grossly obese patient exhibited FL. Based on multiple regression analysis, in non-FL patients, PEEPi,st correlated significantly only with minute ventilation, whereas in FL patients PEEPi,st correlated significantly with peak airway pressure.

CONCLUSIONS

Because all the patients exhibited PEEPi,st and 12 of 32 patients (38%) also had FL, the authors conclude that the assessment of these variables at the bedside could provide useful information concerning respiratory mechanics in mechanically ventilated patients.