Ventilatory responses to inspiratory mechanical loads in spontaneously breathing children during halothane anaesthesia.

The effect of the addition of two different resistive loads (producing 23 and 36 cmH2O (2.26 and 3.53 kPa) 1(-1) s-1, respectively, at 71 min-1) on minute volume, tidal volume, respiratory rate, duration of inspiration and inspiratory drive was studied in six intubated children during nitrous oxide, oxygen and halothane anaesthesia. With both resistive loads, tidal volume was initially reduced in all patients except one, the reduction being greater in older children. Tidal volume returned to baseline values within 3-5 min in most cases, and a transient increase was seen on removal of the load. Changes in inspiratory drive were also most marked in older children. End-tidal carbon dioxide concentration rose by 0.5% when the higher resistance was used. Respiratory rate and duration of inspiration were unchanged, suggesting the absence of a respiratory off-switch-reflex mechanism directly mediated by stretch receptors within the airways or lungs. Ventilatory compensation occurring after 3-5 min may have resulted from chemical stimulation and/or from reflexes from joint receptors and respiratory muscle spindles.