END-TIDAL CARBON-DIOXIDE MEASUREMENT DURING PEDIATRIC ANESTHESIA - THE INFLUENCE OF SITE OF MEASUREMENT AND RESPIRATORY RATE

Objective. 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. Setting. Paediatric surgical patients of a university hospital. Patients. Thirty-one children scheduled for major abdominal or urogenital surgery; weight varying between 2.2 and 9.8 kg. Interventions and methods. 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. Results. 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 < 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). Conclusions. 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 (capet) 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.