Effect of Anaesthesia on Respiratory Function after Major Lower Extremity Surgery A Comparison between Bupivacaine Spinal Analgesia with Low-Dose Morphine and General Anaesthesia

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Abstract
Postoperative pulmonary function was studied in 16 patients undergoing total hip or knee arthroplasty. Their mean age was 65 yr. half of them received spinal analgesia (22.5 mg bupivacaine + 0.3 mg morphine) and the other 1/2 underweent general anesthesia with halothane-N2O. Four h postoperative, the forced expirogram was maintained in the spinal analgesia group, compared with preanesthesia measurements. Functional residual capacity (FRC) measured by multiple breath N2 washout was reduced by 0.5 1, as was closing capacity (CC) measured by the bolus technique. The gas distribution index (N2 washout delay) was unaltered. The alveolar-arterial O2 tension difference (PA-aO2) was not significantly altered, but arterial O2 tension (PaO2) was reduced by 1.7 kPa, and arterial CO2 tension (PaCO2) was increased. No respiratory measurements could be made in the general anesthesia group 4 h postoperatively, but arterial blood gases were unaltered compared with preanesthesia values. Eighteen h postoperatively, force vital capacity (FVC) was reduced in the spinal analgesia group, FRC and CC remained diminished and the gas distribution index was increased, indicating less efficient gas mixing. Simultaneously, PA-aO2 was increased, and the PaO2 remained reduced despite increased alveolar ventilation (lowered PaCO2). In the general anesthesia group FVC, FRC and CC were also reduced, but the gas distribution index remained at the awake level and blood gases were unaltered. The slight hypoventilation in the spinal analgesia group early after surgery may have contributed to impaired gas distribution and ventilation-perfusion matching later postoperatively.