Prediction of the Physiological Dead Space /Tidal Volume Ratio during Anaesthesia/IPPV from Simple Pre-operative Tests

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Abstract
Assumption of an arbitrary value for VDphys/VT [physiological dead space-tidal volume ratio] during IPPV [intermittent positive pressure ventilation] can lead to unacceptable degrees of hypo- or hyperventilation. Adult patients (33) scheduled for major non-thoracic surgery were investigated to see if any simple tests could be used to predict VDphys/VT during anesthesia/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 anesthesia with a Servo Ventilator 900B, and SBT-CO2 was recorded from a CO2 Analyzer 930. During anesthesia/IPPV, smokers had significantly greater VDphys/VT (0.40 .+-. 0.10 vs. 0.31 .+-. 0.07 [P < 0.01]), and they had more steeply sloping phase IIIs of SBT-CO2 (P < 0.01) than non- and ex-smokers. For smokers, VDphys/VT was correlated to age (r = 0.75, P < 0.01), to the slope of phase III of SBT-CO2 and SBT-N2, and to the ratio of FEV% [percent forced expiratory volume] to its predicted value. For non- and ex-smokers, only 1 variable, efficiency, describing the shape of SBT-CO2, was correlated to VDphys/VT (r = 0.53, P < 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 among smokers, who show a large variation in VDphys/VT.