The SICU and MICU patients were evaluated separately for all monitored values. In Table 4, the following two observations were made: (1) SICU patients demonstrated significant improvement in Sa02, pH, PIP, and MAP after switching from CV to HFPV; and (2) MICU patients showed significant improvement in Pa02/Fl02, PIP, and MAP after converting to HFPV. None of the interactions between SICU or MICU patients or changes of respiratory or hemodynamic parameters from CV to HFPV time periods were statistically significant. This finding indicated that the patterns of change of these parameters in the two subgroups were similar. The same was found for the two subgroups defined by pressure or volume-control ventilation.
In Table 5, three values for each parameter were compared: (1) the last value on CV (pre-HFPV), (2) the first value after 1 h of HFPV (1-h post), and (3) the last recorded value within the 48-h monitored period of HFPV (48-h post). The Pa02/Fl02, Sa02, pH, PIP, and MAP were significantly improved on comparison of the pre-HFPV values with either the 1-h post or the 48-h post values. Of these parameters, the changes in Pa02/Fl02, PIP, and MAP were deemed to be clinically significant. There was no difference between the 1-h post and the 48-h post values. Eleven surgical and 8 medical patients died, for an overall mortality rate of 59%. Here

Discussion
This study demonstrates an improvement in respiratory function of trauma and medical patients with ARDS after institution of HFPV, with no adverse effects in circulatory function. The benefit in oxygenation was shown when the mean values and the rates of hourly change of the Pa02/Fl02 ratio on CV were compared with those after initiation of HFPV, The main mechanism responsible for these results seems to be the ability of the percussive ventilator to reduce the PIP while, at the same time, increase the mean airway pressure. The HFPV provides for the delivery of accumulative/diminishing subtidal stroke volumes until time interruption or a prescheduled plateau occurs. Therefore, instead of literally pushing the pulmonary structure out of the way (during mechanical Vt delivery by an uninterrupted in-trapulmonary pressure/flow gradient), time is allowed for lung volume increase (pulmonary conformance) by fracturing the inspiratory flow gradient through pulsation of the proximal-distal inspiratory pressure/flow gradient. Breaking up the inspiratory pressure gradient with the pulsed (subtidal) delivery of Vt provides a more uniform intrapulmonary gas exchange that requires a lower lung volume for an equivalent blood gas interface. The net result is improved gas exchange, with an associated reduction in the potential for barotrauma due to a lower mechanically induced airway pressure during tidal delivery.
Table 4—Respiratory and Hemodynamic Parameters During 48 h of CV and 48 h of HFPV in 32 Patients With ARDS, Stratified by Type of ICU

Parameters Surgical ICU Patients (n = 20) Medical ICU Patients (n = 12)
CV HFPV p Value CV HFPV p Value
Pa02/Fi02 145 ± 10 32±78 0.141 106 ± 11 147 ± 22 0.040
PaC02, mm Hg 42.9 ± 1.8 38.9 ± 1.9 0.052 41.7 ± 3.4 40.3 ± 2.9 0.605
Sa0£, % 96.1 ± 0.4 97.5 ± 0.4 0.008 93.5 ± 0.4 94.7 ± 0.8 0.120
ph 7.31 ± 0.02 7.37 ± 0.01 0.022 7.36 ± 0.01 7.36 ± 0.03 0.998
PIP, cm H2O 40.3 ± 2.6 33.2 ± 2.2 0.017 38.1 ± 1.7 31.3 ± 3.8 0.051
MAP, cm H2O 18.7 ± 1.4 27.6 ± 1.8 < 0.001 19.9 ± 2.2 27.4 ± 2.4 0.019
Qs/Qt, % 33.9 ± 1.4 31.4 ± 2.4 0.293 48.3 ± 3.1 43.8 ± 4.1 0.315
Do2I, mL/min/m2 592 ± 44 596 ± 30 0.913 696 ± 115 759 ± 174 0.689
‘Vo2I, mL/min/m2 165 ± 9 158 ± 7 0.181 159 ± 24 147 ± 18 0.500
O2ER, % 30.0 ± 2.2 27.1 ± 1.0 0.283 25.2 ± 3.6 23.6 ± 4.4 0.471
MBP, mm Hg 83.7 ± 3.7 80.9 ± 3.1 0.384 80.2 ± 3.7 82.8 ± 4.4 0.421

Table 5—Respiratory and Hemodynamic Parameters of 32 Critically Injured Patients With ARDS at Three Time Points

Parameters Pre-HFPV 1-h Post 48-h Post Pre- vs 1-h Post Pre- vs 48-h Post 1-h Post vs 48-h Post
Pao2/Fio2 111 ± 14 163 ± 18 193 ± 21 0.006 < 0.001 0.296
Paco2, mm Hg 43.8 ± 1.8 38.1 ± 2.0 40.9 ± 1.3 0.005 0.129 0.044
Sao2, % 95.2 ± 0.5 96.8 ± 0.6 96.6 ± 0.6 0.045 0.017 0.976
pH 7.32 ± 0.02 7.37 ± 0.02 7.36 ± 0.01 0.010 0.034 0.482
PIP, cm H2O 42.4 ± 1.9 33.2 ± 2.1 32.5 ± 2.5 < 0.001 < 0.001 0.560
MAP, cm H2O 21.0 ± 1.5 28.0 ± 1.5 27.0 ± 1.7 0.001 0.003 0.293
Qs/Qt, % 41.7 ± 1.9 34.8 ± 2.3 33.6 ± 2.4 0.118 0.112 0.801
Do2I, mL/min/m2 624 ± 47 587 ± 50 630 ± 48 0.228 0.479 0.413
Vo2I, mL/min/m2 155 ± 91 153 ± 8 150 ± 8 0.211 0.369 0.852
O2ER, % 26.4 ± 1.6 28.0 ± 1.7 25.3 ± 1.7 0.722 0.805 0.535
MBP, mm Hg 76.2 ± 3.4 84.0 ± 4.1 79.2 ± 4.1 0.050 0.700 0.269