Ventilation, dynamic compliance and ventilatory response to CO ₂

Age dependent variations in minute ventilation (VE), tidal volume (Vr), respiratory rate and dynamic compliance (Cdyn) as well as ventilatory response to inhalation of carbon dioxide (CO2) were investigated in 20 spontaneously breathing intubated infants and children during halothane anaesthesia. Ages ranged from 6 days to 5 years. Seven patients were younger than 6 months of age. Ventilation volumes were measured by pneumotachography and end tidal carbon dioxide concentration by an in-line capnograph. Fluid-filled oesophageal catheters were used for pressure recordings. Measurements were made before surgery (with and without 2.22 and 3.71% of CO2 in inspired gas) and during surgery. Regression analysis of the relationship between VE and body weight revealed no direct proportionality. On a weight basis, VE was significantly higher in younger than in older patients. Tidal volume was directly proportional to body weight. The mean (SEM) value of tidal volume was 4.3 (0.2) ml/kg. Dynamic compliance showed a direct proportionality with weight. The mean (SEM) value of Cdyn was 10 (1.1) ml/kPa/kg. There was no ventilatory response in any patient to inhalation of 2.22% CO2. In the older group of children (greater than 6 months of age) VE increased by 34% during inhalation of 3.71% CO2 (p less than 0.025). In the younger patients (less than 6 months of age) no ventilatory response to inhalation of 3.71% of CO2 was found, indicating a more pronounced depression of ventilation in these infants.