Pattern of ventilation during halothane anaesthesia in infants less than two months of age

To examine the breathing pattern of infants aged less than two months in order to understand better the effect of halothane on ventilation in infants.

The inspiratory flow waveform, the CO2 waveform and occluded inspiratory pressure waveform were recorded at different inspired concentrations of halothane using a washout of halothane in two groups of infants undergoing elective herniorrhaphy. Data were analyzed for minute ventilation (Vi) and tidal volume (VT), parameters of timing of the breath [Total time (Ttot), Inspiratory time (Ti), and the ratio of the occluded to unoccluded inspiratory time (TiOCC/Ti)], parameters of Amplitude of the neural output [mean inspiratory flow (VT/Ti)] and parameters of the Shape of the inspiratory breath profile [the inspiratory flow centroid (Ci/Ti), the inspiratory duty cycle (Ti/Ttot)]. The airway was occluded at end expiration and the slope of the initial 100 msec of occlusion (dP/dt) together with the maximal negative pressure (PMAX) and occluded inspiratory time (TiOCC) were obtained. We studied ten infants < 48 wk post-conceptional age (PCA) and ten infants > 48 wk. PCA Flow (V), pressure (Pao) and carbon dioxide tension (PCO2) were recorded at three concentrations of inspired halothane (FiH): 0%, 1% and 2% which corresponded to an end-tidal halothane concentration of about 0.2%, 0.8% and 1.2% respectively.

In both groups Vi, VT and VT/Ti decreased whereas dP/dt, did not, suggesting that the respiratory pump was impaired. The parameters of breath Shape did not change. Importantly the parameters of Timing showed different tendencies. In infants > 48 wk PCA TiOCC/Ti decreased. In infants < 48 wk PCA, TiOCC/Ti did not change.

The different response in the timing parameter TiOCC/Ti is consistent with a different effect of halothane on parameters of ventilatory timing in infants < 48 wk PCA and this may represent a maturational effect.