Noninvasive measurement of pulmonary gas exchange during general anesthesia

Expired gas flow volume (VE), carbon dioxide excretion (Vco2) and oxygen consumption (Vo2) were measured continuously for 2-minute periods at 15-minute intervals during at least 75 minutes of general anesthesia and surgery in clinical patients. Analog tape-recorded outputs from an infrared CO2 analyzer, from a rapid polarographic O2 analyzer, and from a pneumotachograph were subsequently processed by a general purpose digital computer. Values for VE, VCO2, and VO2 in a group of 50 normal paralyzed endotracheally intubated women with balanced N2O-O2-fentanyl anesthesia for lower abdominal surgery compare favorably with the few published reports of similar measurements. The measured response to anesthesia and surgery in most patients included a progressive increase in O2 uptake and a concurrent but not necessarily simultaneous decrease in CO2 output with a consequent decreased respiratory gas exchange ratio (RE).