ARDSNet Lower Tidal Volume Ventilatory Strategy May Generate Intrinsic Positive End-Expiratory Pressure in Patients with Acute Respiratory Distress Syndrome

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Abstract
The ARDSNet trial revealed that the use of a smaller tidal volume (VT) reduced mortality by 22%. However, three earlier studies that lowered VT did not find a decrease in mortality. We tested the hypothesis that the increased respiratory rate used in the ARDSNet lower VT strategy might have led to intrinsic positive end-expiratory pressure (PEEPi), raising total PEEP (PEEPtotal). Ten patients with acute respiratory distress syndrome (ARDS) were ventilated using the ARDSNet lower VT protocol. Respiratory rate was then reduced (10–15 breaths/minute) to obtain a VT of 12 ml/kg (ARDSNet traditional VT). PEEP on the ventilator (PEEPnominal: 10.1 ± 0.7 cm H2O), Fi O2 (0.7 ± 0.1), and minute ventilation (VE: 12.4 ± 1.7 L/minute) were set using the ARDSNet protocol and maintained constant during the two ventilatory strategies. Values of airway pressure at end-expiration of a regular breath (PEEPexternal) and 3–5 seconds after the onset of an end-expiratory occlusion (PEEPtotal) were measured. PEEPi was calculated by subtracting PEEPexternal from PEEPtotal. PEEPtotal and PEEPi were, respectively, 16.3 ± 2.9 and 5.8 ± 3.0 cm H2O during the lower VT strategy and 11.7 ± 0.9 and 1.4 ± 1.0 cm H2O during the traditional VT strategy (p < 0.01). The reduced mortality observed with the ARDSNet strategy may have been due to the protective effect of a higher PEEPtotal.

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