The Effect of Expiratory Resistance on the Blood Gas Tensions of Anesthetized Patients

Abstract

The effect of resistance to exhalation on arterial P02, (O2 pressure) Pco2 (CO2 pressure) and pH was studied in 12 anesthetized patients receiving positive pressure ventilation. After a control period, expiratory resistance was imposed for 30 minutes while tidal volume and respiratory rate were held constant. The effect of expiratory resistance was evaluated at tidal volumes of 10 ml/kg and 7 ml/kg respectively. Expiratory resistance resulted in no significant changes in Paco2 (arterial CO2 pressure). During the period of expiratory resistance, Pao2 (arterial O2 pressure) was also unchanged; but when resistance was removed, Pao2 increased 49 mm of mercury at a tidal volume of 10 ml/kg (P <0.001) and 19 mm of mercury at 7 ml/kg (P = 0.05). The rise of Pao2 suggests that expiratory resistance reversed collapse of air spaces. The failure of Pao2 to rise until resistance was released may be explained either by redistribution of pulmonary blood flow or by changes in cardiac output.