Measurement of minute ventilation in ventilator-dependent patients

Objectives a) To determine the variation in methods used to measure minute ventilation (VE) in patients who receive mechanical ventilation; b) to determine the effect of supplemental oxygen on VE, respiratory rate (RR), and tidal volume (VT) measurements. Design Telephone survey of hospitals, and a randomized control trial. Setting Medical and surgical ICUs in a university hospital. Patients Thirty-three patients who had required mechanical ventilation because of the inability to sustain adequate spontaneous ventilation. All patients were considered ready to undergo a weaning trial by their physicians. Interventions Spontaneous VE, RR, VT, and SaO2 were measured both in the presence and absence of supplemental oxygen; measurements were obtained in a randomized manner. Measurements and main results a) In a telephone survey of hospitals throughout the country, we found that the measurement of VE is variably obtained during room air breathing or in the presence of supplemental oxygen. b) Measurements of VE increased from 11.0 +/- 0.8 L/min while patients received supplemental oxygen to 13.5 +/- 1.1 L/min while patients breathed room air (p less than .001). Of 15 patients who had a VE less than 10 L/min while receiving supplemental oxygen, seven developed a value greater than 10 L/min while breathing room air; thus, a weaning trial might have been inappropriately deferred in these patients. c) Mean SaO2 decreased from 95.0 +/- 0.6% while breathing supplemental oxygen to 90.2 +/- 1.1% while breathing room air (p less than .001). Conclusions Measurements of VE in patients being considered for a weaning trial can result in significant oxygen desaturation if obtained during room air breathing, and the values obtained can significantly overestimate the patient's true ventilatory requirements, since most patients receive supplemental oxygen during a weaning trial. Standardized methods of measuring VE in critically ill patients need to be developed.