Oxygen uptake during weaning from mechanical ventilation

Total body oxygen uptake (V̇(O₂)) increases during the transition from machine-assisted ventilation to spontaneous breathing. Since the volume of oxygen consumed by the respiratory muscles must contribute to the increase in V̇(O)2 (ΔV̇(O₂)), we explored whether ΔV̇(O₂) and/or measurements of respiratory power output (Ẇ(RESP)) provide clinically useful information in the evaluation of disease state and weaning decisions in patients with respiratory failure. We determined the metabolic, ventilatory, and hemodynamic responses of ten patients during weaning from controlled mechanical ventilation, and compared ΔV̇(O₂) and Ẇ(RESP) of patients without overt heart-lung disease (group 1) to that of patients with significant cardiopulmonary dysfunction and ventilator-dependent respiratory failure (group 2). We reasoned that for ΔV̇(O₂) to be clinically useful, individual values must either clearly differ between groups, must be higher in patients with heart-lung disease, and/or correlate with weaning outcome and independent measurements of respiratory work. The V̇(O₂) increased in nine of ten patients. The differences between the groups in the values of ΔV̇(O₂) (27 ml/min and 49 ml/min) and respiratory power (9.38 J/min and 11.99 J/min) were not significant. ΔV̇(O₂) and Ẇ(RESP) were not correlated (r = 0.2), and neither predicted weaning outcome. We conclude that the sensitivity and specificity of ΔV̇(O₂), and Ẇ(RESP) appear insufficient for evaluation of disease state and weaning decisions in individual patients.