Tidal volume reduction in ARDS: Effect on cardiac output and arterial oxygenation

During continuous positive pressure ventilation (CPPV), mean airway pressure and lung volume will be influenced both by the tidal volume (VT) employed and the amount of positive end-expiratory pressure (PEEP). The effect of varying levels of CPPV on PaO₂ and cardiac output (Q) has been previously assessed by adjusting the level of PEEP at constant VT. This study examined the influence of a 200-ml reduction in VT, at a constant PEEP of 15 cm H₂O, on the PaO₂ and Q of 21 patients with adult respiratory distress syndrome (ARDS). The relationship between change in Q and change in total respiratory system compliance (Cst) after VT reduction was also examined. VT reduction from 14.1±0.8 ml/kg to 11.2±0.9 ml/kg yielded an increase in Q (+15±12 percent, p<0.01) without a significant change in PaO₂ (-6.3±15.0 mm Hg, p = 0.08). Cst increased with VT reduction (+3.1±1.8 ml/cm H₂O). There was only a modest correlation (r = +0.42, p = 0.06) between ΔQ percent and ΔCst following VT reduction. VT reduction at high level PEEP may yield a significant improvement in Q and net O₂ delivery, but the degree of hemodynamic improvement is variable and is not reliably predicted noninvasively by measurement of Cst.