Physiological deadspace during normocapnic ventilation under anaesthesia

Abstract

Respiratory physiological deadspace (V(D)phys) 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 (F(E)CO2) around 5.5 kPa with normocapnic confirmation using arterial blood gas analysis. V(D)phys was calculated using Enghoff's modification of Bohr's equation. V(D)phys and deadspace to tidal volume ratios (V(D)/V(T)) 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 V(D)phys/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). V(D)phys showed positive correlation with weight, height and body surface area (BSA) but V(D)phys/kg showed negative correlation with BMI. Multiple regression analysis produced a best fit equation for V(D)phys = 9.7 + 64.3 x BSA + 13.51 S where S = 1 for females and 2 for males.