[Capnometry in pediatric anesthesia. The effect of the measurement site and respiratory rate]

To evaluate the influences of site of measurement, respiratory rate, and tidal volume on end-tidal PCO2 measurement in children ventilated with a non-rebreathing system.

Paediatric surgical patients of a university hospital.

Thirty-one children scheduled for major abdominal or urogenital surgery; weight varying between 2.2 and 9.8 kg.

During a relative steady-state situation, end-tidal carbon dioxide partial pressure (PetCO2) was measured at the proximal and distal ends of the endotracheal tube by a sidestream analyser (Datex, Normocap) and between the proximal end of the tube and the Y-piece of the ventilator by a mainstream analyser (Hewlett Packard, HP14265A). PetCO2 was corrected for water vapor and calculated as partial pressure at a barometric pressure of 760 mmHg. At the same time, capillary blood was taken for blood gas analysis. The capillary-end-tidal PCO2 gradient [dPCO2(cap-et)] was computed to compare the three capnometric methods. Statistical analysis was performed with the Friedmann test. Correlations were calculated by means of the least-square fitting method and significance of the correlation was checked with the F-test.

dPCO2 (cap-et) did not differ significantly in children with more than 6 kg body weight. In patients less than 6 kg, however, the three capnometric methods revealed significantly different dPCO2 (cap-et) values (P less than 0.01): dPCO2 (cap-et) was 3.0 +/- 4.7 mmHg at the distal end of the endotracheal tube, 5.8 +/- 4.6 mmHg at the proximal end, and 8.7 +/- 4.6 mmHg between the proximal sidestream connector and the Y-piece of the ventilator. There was no correlation between tidal volume and dPCO2 (cap-et) (Fig. 1), however, a significant relation was found between respiratory rate and dPCO2 (cap-et) (Fig. 2) and between respiratory rate and the PCO2 difference between the distal and proximal ends of the endotracheal tube (Fig. 3).

Even in a non-rebreathing system, capnometry is influenced by the site of measurement. In small children with body weight below 6 kg, analysis of an endotracheal sample may provide the best PetCO2 values. In our opinion, dPCO2 (cap-et) in the present investigation was not caused by rebreathing or by pendelluft (a significant correlation between dPCO2 (cap-et) and tidal volume would then have been expected), but was mainly due to ventilation-perfusion mismatch. This may result from high respiratory rates causing inadequate ventilation of lung regions with long time-constants.