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 h emodynamics and gas exchange in patients with acute respiratory distre ss syndrome (ARDS). Design: Prospective study. Setting: 18-bed, medica l intensive care unit, university hospital. Patients: 11 mechanically ventilated ARDS patients. Intervention: Patients were sedated and vent ilated in the controlled mode. Hypercapnia was induced over a 30-60 mi n period by decreasing tidal volume until pH decreased to 7.2 and/or P -50 increased by 7.5 mmHg. Settings were then maintained for 2 h. Resu lts: Minute ventilation was reduced from 13.5 +/- 6.1 to 8.2 +/- 4.11/ 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 P-50 increased (2 6.3 +/- 2.02 to 31.1 +/- 2.2 mmHg) (p < 0.05). Systemic vascular resis tance decreased (865 +/- 454 to 648 +/- 265 dyne . s . cm(-5), and car diac index (CI) increased (4 +/- 2.4 to 4.7 +/- 2.4 l/min/m(2)) (p < 0 .05). Mean systemic arterial pressure was unchanged. Pulmonary vascula r resistance was unmodified, and mean pulmonary artery pressure (MPAP) increased (29 +/- 5 to 32 +/- 6 mmHg, p < 0.05). PaO2 remained unchan ged, while saturation decreased (93 +/- 3 to 90 +/- 3%, p < 0.05), req uiring an increase in FIO2 from 0.56 to 0.64 in order to maintain an S aO(2) > 90%. PvO(2) increased (36.5 +/- 5.7 to 43.2 +/- 6.1 mmHg, p < 0.05), while saturation was unmodified. The arteriovenous O-2 content difference was unaltered. Oxygen transport (DO2) increased (545 +/- 24 0 to 621 +/- 274 ml/min/m(2), p < 0.05), while the O-2 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). Conclusi ons: These data indicate that acute hypercapnia increases DO2 and O-2 off-loading capacity in ARDS patients with normal plasma lactate, with out increasing O-2 extraction. Whether this would be beneficial in pat ients with elevated lactate levels, indicating tissue hypoxia, remains to be determined. Furthermore, even though hypercapnia was well toler ated, the increase in Q(va)/Q(t), CI, and MPAP should prompt caution i n patients with severe hypoxemia, as well as in those with depressed c ardiac function and/or severe pulmonary hypertension.