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

Purpose: 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. Methods: 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 (Vover dot i) 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 (Ti-occ/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 (Ti-occ) were obtained. We studied ten infants <48 wk post-conceptional age (PCA) and ten infants >48 wk. PCA Flow (Vover dot), 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. Results: In both groups Vover dot i, 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 Ti-occ/Ti decreased In infants <48 wk PCA, Ti-occ/Ti did not change. Conclusions: The different response in the timing parameter Ti-occ/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.