Materials
Patients who developed ARDS and were placed on HFPV from June 1994 to December 1995 were included in this study. ARDS was considered present if (1) the Pao2/fraction of inspired oxygen (Fio2) ratio was < 200, (2) bilateral diffuse lung infiltrates were present on a plain chest radiograph, and (3) wedge pressure was < 18 mm Hg. Patients placed on HFPV before they were placed on CV for at least 48 h were excluded. Similarly, patients who had fewer than three values in respiratory and hemodynamic data during the 24 h preceding or after the institution of HFPV were excluded due to the inability to evaluate the acute changes associated with the new mode of ventilation.
In total, 32 patients met the inclusion criteria. Twenty were being cared for in the surgical ICU (SICU) and 12 in the medical ICU (MICU). All 20 surgical patients had severe injuries (12 penetrating, 8 blunt) as the reason for their admission. Nontrauma patients were managed during the period of the study by a different surgical team that was not familiar with the VDR device. The admitting diagnoses among the 12 medical patients included pneumonia (3), tuberculosis (2), meningitis (1), cirrhosis (3), pancreatitis (1), hypertension with renal failure (1), and malignancy with aspiration (1). The mean ± SE Pao2/Flo2 for this population before HFPV was 130 ± 8.

Mechanical Ventilation
In the absence of improvement in respiratory function, the mode of mechanical ventilation was converted to HFPV after patients remained on CV for a mean period of 4 days (range, 2 to 6 days). Volume- and pressure-control ventilation were delivered by one of two types of ventilators (Servo Ventilator 900C; Siemens Medical System; Iselin, NJ or 7200 Micropressor Ventilator; Puritan-Bennett; Wilmington, MA). Although there is no universal agreement on the classifications of high-frequency ventilators, the percussive ventilator that we used (Percussion-aire; Bird Space Technologies; Sandpoint, ID) can be described as a time-cycled and pressure-limited ventilator with an additional piston (phasitron) mechanism positioned at the end of the endotracheal tube.