Background: The aim of the present trial was to study the relationship between end-tidal pCO2 (p(et)CO2) and transcutaneous pCO2 (ptcCO2) after in-vivo calibration in ventilated newborns.
Patients: 61 end-tidal and transcutaneous pCO2 measurements were simultaneously performed in 30 ventilated preterm and term newborn infants (weight at birth 1862.6 +/- 981.9 g).
Method: End-tidal pCO2 was measured in mainstream mode at the end of the endotracheal tubus (Novametrix 7000 Medical Systems Inc., USA, dead volume of the chamber 0.6 ml). Transcutaneous pCO2 was measured by means of a Servomed (SMK 365 Hellige, FRG) analyser.
Results: The statistical analysis demonstrated a good correlation (r = 0.72, p < 0.001) between ptcCO2 (mean +/- SD, 44.3 +/- 11.2 mmHg) and p(et)CO2 (32.4 +/- 10.4 mmHg). A considerable difference between transcutaneous and end-tidal pCO2 values was observed (p(tc-et)CO2 = +11.9 +/- 8.7 mmHg). This phenomenon was probably caused by ventilation-perfusion disturbances in the studied critically ill neonates. The statistical analysis revealed that the absolute magnitude of the P(tc-et)CO2 difference was independent from disease, episodes of spontaneous respiration or of respiratory frequency.
Conclusions: Capnographic determination of P(et)CO2 provides informations about alveolar ventilation-perfusion-disturbances. Capnography enables the on-line control of end-tidal pCO2 in neonates with respiratory failure. It cannot replace transcutaneous pCO2 measurements or blood gas analysis but it can reduce its frequency in clinically stable patients. The analysis of the capnogram can be used to optimise artificial ventilation. A quantitative evaluation of the capnogram by calculation of Murányi's-CO2-Index was possible only in 28% of the ventilated newborns which limits its value in such patients.