In-vitro evaluation of the neonatal tonometer

Objectives

A Neonatal Tonometer (5 French) using saline capnometry has been developed. We compared these tonometers in-vitro, using 0.9% saline (NS) and phosphate buffered saline (PBS) as the CO₂ vehicle, along with a Tonocap (14 F) against a set of known PCO₂'s in a saline solution.

Method

A scaled equilibration chamber containing 0.9% Saline was maintained at 37°C and the dissolved CO₂ was kept at constant pCO₂'s of 2.5, 5, 7.5, 10 kPa using a Paratrend 7 probe (Bio-medical Sensors). Two Neonatal Gastric tonometers (Tonometrics) were positioned in the chamber along with a Tonocap monitor (Datex). NS was the CO₂ vehicle in the first tonometer. PBS (pH 6.0) in the second, and recirculating gas tonometry in the Tonocap. 20 consecutive measurements were taken, each after 60 min equilibration periods, from each of the tonometers at PCO₂'s of 2.5, 5. 7.5, 10 kPa and processed in the IL BGE blood gas analyser. Data was analysed by linear regression and Bland-Altman plots.

Results

The Figure overleaf shows the known pCO₂ against the mean pCO₂ (95% CI) for Tonocap (Tcp), PBS and NS. The calculated Tonometer pCO₂ (TpCO₂) is derived from the linear regression equation.

Conclusion

Recirculating gas tonometry is undoubtedly the best mode of tonometry. Whilst we await its development for neonates either NS or PBS may be used. We suggest that correction factors specific to each unit's blood gas analyser should be calculated before appropriate comparison can be made between the arterial pCO₂ and the Neonatal tonometer's pCO₂.

Figure

overleaf shows the known