Effects of rapid permissive hypercapnia on hemodynamics,gas exchange,and oxygen transport and consumption during mechanical ventilation for the acute respiratory distress syndrome

Objective To measure the effects of rapid permissive hypercapnia on hemodynamics and gas exchange in patients with acute respiratory distress syndrome (ARDS).Design Prospective study.Setting: 18-bed, medical intensive care unit, university hospital.Patients 11 mechanically ventilated ARDS patients.Intervention Patients were sedated and ventilated in the controlled mode. Hypercapnia was induced over a 30–60 min period by decreasing tidal volume until pH decreased to 7.2 and/or P₅₀ increased by 7.5 mmHg. Settings were then maintained for 2 h.Results Minute ventilation was reduced from 13.5±6.1 to 8.2±4.1l/min (mean±SD), PaCO₂ increased (40.3±6.6 to 59.3±7.2 mmHg), pH decreased (7.40±0.05 to 7.26±0.05), and P₅₀ increased (26.3±2.02 to 31.1±2.2 mmHg) (p<0.05). Systemic vascular resistance decreased (865±454 to 648±265 dyne·s·cm^–5, and cardiac index (CI) increased (4±2.4 to 4.7±2.4 l/min/m²) (p<0.05). Mean systemic arterial pressure was unchanged. Pulmonary vascular resistance was unmodified, and mean pulmonary artery pressure (MPAP) increased (29±5 to 32±6 mmHg,p<0.05). PaO₂ remained unchanged, while saturation decreased (93±3 to 90±3%,p<0.05), requiring an increase in FIO₂ from 0.56 to 0.64 in order to maintain an SaO₂>90%. PvO₂ increased (36.5±5.7 to 43.2±6.1 mmHg,p<0.05), while saturation was unmodified. The arteriovenous O₂ content difference was unaltered. Oxygen transport (DO₂) increased (545±240 to 621±274 ml/min/m²,p<0.05), while the O₂ consumption and extraction ratio did not change significantly. Venous admixture (Q_va/Q_t) increased (26.3±12.3 to 32.8±13.2,p<0.05).Conclusions These data indicate that acute hypercapnia increases DO₂ and O₂ off-loading capacity in ARDS patients with normal plasma lactate, without increasing O₂ extraction. Whether this would be beneficial in patients with elevated lactate levels, indicating tissue hypoxia, remains to be determined. Furthermore, even though hypercapnia was well tolerated, the increase in Q_va/Q_t, CI, and MPAP should prompt caution in patients with severe hypoxemia, as well as in those with depressed cardiac function and/or severe pulmonary hypertension.