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In-vitro evaluation of the neonatal tonometer


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.


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.


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.


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 1

overleaf shows the known

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Thorburn, K., Roberts, P., Hatherill, M. et al. In-vitro evaluation of the neonatal tonometer. Crit Care 3 (Suppl 1), P168 (2000).

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  • Linear Regression
  • Phosphate Buffer Saline
  • Emergency Medicine
  • Correction Factor
  • Saline Solution