Physiological deadspace during normocapnic ventilation under anaesthesia

Respiratory physiological deadspace (VDphys) during normocapnic ventilation under anaesthesia was studied in 253 patients scheduled for elective non-thoracic surgery. Subjects were ventilated with SERVO 900B ventilator using CO2 analyser 930 (Siemens-Elema Sweden) to adjust minute volume sufficient to maintain end-tidal carbon dioxide fraction (FECO2) around 5.5kPa with normocapnic confirmation using arterial blood gas analysis. VDphys was calculated using Enghoff's modification of Bohr's equation. VDphys and deadspace to tidal volume ratios (VD/VT) in male patients were 142.6 +/- 28.4 ml and 0.37 +/- 0.068 respectively and were significantly higher than that in females (119.4 +/- 35.6 ml and 0.36 +/- 0.067). Males had significantly higher VDphys/mass (2.5 +/- 0.68 ml.kg) compared with females (2.2 +/- 0.54 ml/kg, P < 0.001) but significantly lower body mass index (BMI) (20.67 +/- 3.2 in males and 22.47 +/- 4.1 in females, P < 0.001). VDphys showed positive correlation with weight, height and body surface area (BSA) but VDphys/kg showed negative correlation with BMI. Multiple regression analysis produced a best fit equation for VDphys = 9.7 + 64.3 x BSA + 13.51S where S = 1 for females and 2 for males.