Detrimental hemodynamic effects of assisted ventilation in hemorrhagic states

Our goal was to demonstrate explicitly that lower-frequency positive-pressure ventilation not only preserves adequate oxygenation and acid-base status in hemorrhagic states, but also that "normal" or higher respiratory rates significantly compromise hemodynamics, even with moderate degrees of hemorrhage.

Eight intubated pigs (ventilated with 12 mL/kg tidal volume, 28% FIO2, respiratory rate = 12 breaths/min) were hemorrhaged to <65 mm Hg of systolic blood pressure. Respiratory rates were then sequentially changed every 10 mins to 6, 20, 30, and 6 breaths/min.

With respiratory rates at 6 breaths/min, all subjects maintained pH of >7.25 and SaO2 of >99% while increasing systolic blood pressure (mean, 65-84 mm Hg; p < .05), time-averaged coronary perfusion pressure (50 +/- 2 to 60 +/- 4 mm Hg; p < .05), and cardiac output (2.4 to 2.8 L/min; p < .05). With respiratory rates of 20 and 30 breaths/min, systolic blood pressure (73 +/- 4 and 66 +/- 5 mm Hg, respectively), coronary perfusion pressure (47 +/- 3 and 42 +/- 4 mm Hg), and cardiac output (2.5 and 2.4 L/min) diminished. When returned to 6 breaths/min, systolic blood pressure (95 mm Hg), coronary perfusion pressure (71 + 6 mm Hg), and cardiac output (3.0 L/min) improved significantly (p < .05 for all comparisons).

After moderate hemorrhage, animals maintain adequate oxygenation and acid-base status with lower-frequency respiratory rates, whereas increasingly higher respiratory rates progressively and significantly impair hemodynamics. Current ventilatory protocols for trauma resuscitation should be re-examined and considered a possible cause of worsened clinical outcomes and unrecognized confounded study results.