CORRELATION OF paCO₂ WITH EXPIRED CO₂ (P_ECO₂) MEASURED VIA MASS SPECTROMETRY IN NEONATAL CARDIOPULMONARY DISEASE

Abstract

Measurement of arterial carbon dioxide tension (paCO₂) is an invasive technique for determining the adequacy of ventilation and, therefore, is associated with possible morbidity. Consequently, a non-invasive means for estimating paCO₂ is desirable. A Chemetron mass spectrometer (MS) with a response time of 0.68 sec to 63% of a step change in CO₂ and analytic accuracy for CO₂ of ± 2% was used to measure PECO₂ of neonates on IPPV/CPAP. Gas was continually sampled from 1.0 cm beyond the plastic adapter of an oral ET tube via a catheter with inner diameter 0.38 mm. 9 neonates (mean BW 2182g, range 870-3440; mean GA 33.8 wk, range 26-40) with cardiopulmonary diseases (5 HMD, 2 aspiration, 1 each PDA, CHD, BPD, pneumothorax) were studied at an initial mean age of 82 hrs (6/9 < 72 hrs), 75 paired comparisons of PECO₂ and paCO₂ had a correlation of 0.69 (p < 0.001) and a mean gradient (paCO₂-PECO₂) of 8.3 torr (range +23 to -3). 61.3% of the gradients were ≤ 10 torr. We conclude that the MS is a useful monitoring device but that the correlation between paCO₂ and PECO₂ was adversely affected by the relatively long response time of the total system. We speculate that reduction of this parameter would enhance the usefulness of the MS by increasing the correlation and decreasing the gradient.

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