Gas exchange during thoracotomy in children. A study using the single‐breath test for CO2

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Abstract
Gas exchange during thoracotoniy was studied in 13 children aged 6 months to 14 years (median age 5 years), anaesthetized for repair of coarctation of the aorta or closure of a patent ductus arteriosus. All received halothane in equal parts of N2O/O2 supplemented with fentanyl. CO2) single‐breath tests were obtained with a computerised on‐line system based on the Servo ventilator. From signals for airway flow pressure, CO2 concentration and timing, the computer calculated the airway deadspace (VDaw) and the static compliance and resistance of the respiratory system. Given a value for Paco2, the computer also calculated the physiological aud alveolar deadspaces. Measurements were taken at six stages during the procedure, starting with the supine position before surgery. After turning to the lateral position, airway deadspace increased by 19%, thus increasing the physiological deadspace fraction. When the pleura was opened, both Voaw and Pao2 were reduced. When the upper lung was retracted, compliance was reduced and also Pao2 ‐ the minimum value noted was 17.3 kPa. Hypoxic Pao2 values were possibly avoided because both ventilation and perfusion were reduced in the retracted lung. The alveolar dradspace fraction increasrd during these intra‐operative stages. Although the net effect of the changes in airway and alveolar dradspace during surgery was a significant increase in physiological deadspace fraction (from 0.23 to 0.28), gas exchange could be maintained at the cost of only moderate increases in peak airway pressure: the mean increase was from 2.4 to 2.8 kPa (24 to 29 cmH2O). After manual hyperinflation of the lung and wound suture, deadspace variables returned to their original values, but compliance was reduced and resistance increased compared to preoperatively. Children who initially had high Pao2s showed a slight deterioration in oxygenation by the end of surgery. The mainly younger children whose initial Pao2s were low (presumably because of atelectasis) improved, perhaps as a result of the manual hyperinflation.