Prediction of the physiological dead space/tidal volume ratio during anaesthesia/IPPV from simple pre-operative tests

It can be shown that the assumption of an arbitrary value for VDphys/VT during IPPV can lead to unacceptable degrees of hypo- or hyperventilation. We investigated 33 adult patients scheduled for major non-thoracic surgery, to see if any simple tests could be used to predict VDphys/VT during anaesthesia/IPPV. Fifteen were smokers and 18 non- or ex-smokers. The tests were spirometry, and the single breath tests for CO2 and for N2 (SBT-CO2: SBT-N2). Patients were ventilated during anaesthesia with a Servo Ventilator 900 B, and SBT-CO2 was recorded from a CO2 Analyzer 930. During anaesthesia/IPPV, smokers had significantly greater VDphys/VT (0.40 +/- 0.10 vs. 0.31 +/- 0.07 [P less than 0.01]), and they had more steeply sloping phase IIIs of SBT-CO2 (P less than 0.01) than non- and ex-smokers. For smokers, VDphys/VT was correlated to age (r = 0.75, P less than 0.01), to the slope of phase III of both SBT-CO2 and SBT-N2, and to the ratio of FEV% to its predicted value. For non- and ex-smokers, only one variable, efficiency, describing the shape of SBT-CO2, was correlated to VDphys/VT (r = 0.53, P less than 0.05). Pre-operative prediction of VDphys/VT based on age, smoking history, and SBT-CO2 can reduce the uncertainty in estimating VDphys/VT and therefore ventilatory requirements. It appears to offer the greatest benefits amongst smokers, who show a large variation in VDphys/VT.