Pulmonary ventilation, CO2response and inspiratory drive in spontaneously breathing young infants during halothane anaesthesia

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Abstract
Pulmonary ventilation, CO2 response and inspiratory drive were studied during halothane anesthesia prior to surgery in 13 spontaneously breathing infants less than 6 months of age. Pneumotachography and capnography were used. Airway and oesophageal pressures were measured and occlusion tests were performed at functional residual capacity. Measurements were made before and during 8 min of 4% CO2 stimulation. Inspiratory drive increased significantly (P < 0.001) at CO2 stimulation. This resulted in increased minute ventilation (P < 0.001) and tidal volume (P < 0.001) while respiratory rate was unchanged. As .**GRAPHIC**. VT ratios were the same, the net effect was increased alveolar ventilation (P < 0.001). CO2 elimination was unpredictable in these young infants and decreased during CO2 stimulation (P < 0.05), while mean end-tidal CO2 concentration only increased from 5.2 to 6.3% (P < 0.001). The ventilatory response to 4% CO2 could therefore be deemed to be adequate during the short period (8 min) of CO2 breathing. However, this was achieved at the cost of increased work as witnessed by the increased ratio between minute ventilation and CO2 elimination (P < 0.01). Stabilisation of end-tidal CO2 concentrations during CO2 inhalation took only 10 s while the maximal increase in ventilation volumes was not achieved until after 150 s. It is concluded that young spontaneously breathing infants anesthetized with halothane (MAC 1.3) have an increased respiratory drive with greater tidal volumes during CO2 stimulations. Respiratory timing, dynamic compliance and total pulmonary resistance were, however, uninfluenced by 4% CO2 stimulation. Increased monitoring of CO2 output in anesthetized infants is suggested.