1337 EFFECTS OF CHANGES IN CIRCULATION ON TRANSCUTANEOUS PCO₂ (TcPCO₂)

Abstract

We compared TcPCO₂ with arterial CO₂ (PaCO₂) & continuous measures of heart rate (HR) & blood pressure (BP) (T°) in 14 infants (mean GA=32.7 weeks, mean wt.=2011 gms & mean postnatal age=1.76 days).

The Novametrix heated electrode was mounted on the chest after calibration (mean calibration time=16.5 min.). The equilibration time (mean=12.15 min.) was directly correlated with GA (r= .74 p <.005). The correlation coefficient for TcPCO₂ vs PaCO₂ (n=29) was 0.76. The slope of the regression line was 1.15±0.18 with an intercept of 19.18±8.56.

HR & BP, indirect measures of tissue perfusion, were related to TcPCO₂ using time series & cross correlation analysis. Mean BP, accumulated HR & TcPCO₂ were calculated for 3 min. intervals in 4 infants. The relationship between changes in variables was examined following correction for biologic & methodologic lags. The change in TcPCO₂ between successive 3 min. intervals was strongly correlated with the changes in HR (Δ TcPCO₂ vs Δ HR), range r= -.58 to -.76 & occasionally with the change in BP range r= -.09 to -.46.

These results indicate that much of the variability in TcPCO₂ can be accounted for by changes in circulation. Variation in tissue perfusion occurring independently of changes in PaCO₂ could make estimates of PaCO₂ from TcPCO₂ highly misleading.

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