We experienced unintentional hyperventilation during mechanical ventilation in infants under general anesthesia. It is very difficult to decide upon respiratory rate or tidal volume for adequate ventilation without respiratory gas monitoring. During pulmonary ventilation using a Mapleson D circuit, the utilization of high fresh gas flow dilutes the expired gas and causes an underestimation of end-tidal CO₂. We undertook to find a proper respiratory rate (RR) with a fixed tidal volume during controlled ventilation in infant general anesthesia.

We studied 50 infants weighing below 10 kg during general anesthesia. An uncuffed endotracheal tube was selected 3.5 or 4.0 mm (inner diameter). After intubation with midazolam, thiopental sodium and vecuronium, controlled ventilation was applied: total fresh gas flow 3 L/minute, peak inspiratory pressure 15-20 cmH₂O, and RR 19 or 20/minute. Arterial blood gas analysis was done 20 minutes later. We calculated the RR for a PaCO₂ of 36 mmHg according to the equation: RR (applied) = estimated RR × estimated PaCO₂/ideal PaCO₂. In addition, linear regression was performed to analyze the relation between age and RR.

The estimated regression equation for RR using Pearson's correlation coefficient was as follows: RR = 22.835 - 0.415 × age (months).

The equation (RR = 22.835 - 0.415 × age [months]) could be used as an index for safe ventilatory management without severe hyper- or hypo-carbia in infants during general anesthesia.