The effect of PEEP on oxygenating capacity in acute respiratory failure associated with multiple trauma

Of the first 500 admissions to our ICU (1972-75), 23 patients with multiple trauma (MT) required the addition of PEEP to mechanical ventilation (MV) to improve arterial oxygenation. PEEP was used whenever P(A-a)DO₂/(FIO₂=1.0) on MV exceeded 350 torr. Group A (6 patients) had MT with or without fat embolism syndrome. Group B (8 patients) had MT complicated by gram negative sepsis, and Group C (9 patients) had MT with lung contusion. Oxygenating capacity (OC) prior to PEEP was markedly and equally reduced [P(A-a)DO₂/(FIO₂=1.0) 469-491 torr] in all groups. Mean maximal PEEP employed was 6.3, 7.0 and 6.6 cm H₂O for 1.8, 4.3 and 4.6 days in groups A, B and C, respectively. Both the initial (2-3 hr) and the total overall improvements in OC with PEEP were evaluated. The initial increases in PaO₂/FIO₂ were 152 ± 46 (SEM), 36.1 ± 12 and 59.2 ± 21 torr, and the decreases in P(A-a)DO₂/(FIO₂=1.0) were 191.8 ± 45, 48.8 ± 16 and 86.3 ± 25 torr in groups A, B and C, respectively. Only the differences between groups A and B were statistically significant. Similarly, the total improvement in OC due to PEEP was manifested by a mean increase in PaO₂/FIO₂ of 196.8 ± 36, 57.5 ± 21 and 107.0 ± 21 torr, and a mean decrease in P(A-a)DO₂/(FIO₂=1.0) of 197.0 ± 37, 61.5 ± 16 and 140 ± 18 torr in groups A, B and C, respectively. Overall OC improvement was significantly less in group B. Mortality on PEEP ventilation was 16.6%, 62.5% and 11.1% in groups A, B and C, respectively. The results point out the diversity of response to PEEP in acute respiratory failure associated with MT due, probably, to different pathophysiological processes. Gram negative septicemia seems to be less amenable to treatment with PEEP when it complicates MT. While the initial response in OC due to PEEP in patients with MT and lung contusion is moderate, the overall response is adequate and similar to that in uncomplicated MT.