Physiological dead space tidal volume ratio during face mask, laryngeal mask, and cuffed oropharyngeal airway spontaneous ventilation

Objective: To compare the physiological dead space/tidal volume ratio and arterial to end-tidal carbon dioxide tension (ETCO2) difference during spontaneous ventilation through a face mash, a laryngeal mask (LMA), or a cuffed oropharyngeal airway. Design: Prospective, randomized, cross-over study. Setting: Inpatient anesthesia at a university department of orthopedic surgery. Patients: 20 ASA physical status I and II patients, without respiratory disease, who underwent ankle and foot surgery. Interventions: After a peripheral nerve block was performed, propofol anesthesia was induced and then maintained with a continuous intravenous (IV) infusion (4 to 6 mg/kg/h). A face mash, a cuffed oropharyngeal airway, or an LMA were placed in each patient in a random sequence. After IS minutes of spontaneous breathing through each of the airways, ventilatory variables, as well as arterial, end-tidal, and mixed expired CO2 purtial pressure, were measured, and physiological dead space/tidal volume ratio runs calculated. Measurements and Main Results: Expired minute volume and respiratory rate (RR) were lower with LMA (5.6 +/- 1.2 L/min and 18 +/- 3 breaths/min) and the cuffed oropharyngeal airway (5.7 +/- 1 L/minand 18 +/- 3 breaths/min) than the face mash (7.1 +/- 0.9 L/min and 21 +/- 3 brenths/min) (p = 0.0002 and P = 0.013, respectively). Physiological dead space/tidal volume ratio and arterial to end tidal CO2 tension difference were similar with the cuffed oropharyngeal airway (3 +/- 0.4 mmHg and 4.4 +/- 1.4 mmHg) and LMA (3 +/- 0.6 mmNg and 3.7 +/- 1 mmHg) and lowe, than with the face mash (4 +/- 0.5 mmHg and 6.7 +/- 2 mmHg) (p = 0.0001 and p = 0.001, respectively). Conclusion: Because of the increased dead space/tidal volume ratio, breathing through a face mash required higher RX and expired minute volume than either the cuffed oropharyngeal airway or LMA, which, in contrast, showed similar effects on the quality of ventilation in spontaneously breathing anesthetized patients. (C) 1998 by Elsevier Science Inc.