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Transcutaneous (tcCO2) measurement versus arterial blood gas CO2 measurement in critically ill adult patients

PaCO2 reflects the adequacy of alveolar ventilation in the intensive care unit patient. Arterial blood-gas sampling is considered the 'gold standard'. There is, however, a significant morbidity associated with arterial line placement.

We compared the efficacy of transcutaneous monitoring of CO2 (tcCO2) with traditional PaCO2 from arterial blood-gas samples.


We studied consecutive ICU admissions during an 8-day period. We included all new patients aged 18 and over who were expected to survive for 24 hours. A TOSCA transcutaneous monitor (tcCO2) was attached to the earlobe. The probe was moved every 4 hours and 4-hourly routine arterial blood-gas samples were taken and PaCO2 values recorded.


We studied eight patients (four males and four females). The mean age was 56 years. The mean admission APACHEII and SAPS scores were 19.5 and 31.3, respectively. The mean hospital stay was 31 days. ICU survival was 75%. Two patients were ventilated using a high-frequency oscillator. A total of 64 comparisons were made.

We detected a significant difference between blood-gas PaCO2 and TOSCA tcCO2 of 0.43 kPa (paired t test P = 0.0012). Figure 1 shows the bias and limits of agreements. A regression line was fitted and the slope and intercept were highly significant. It is possible to predict PaCO2 from tcCO2 using the following regression equation: PaCO2 = 2.7 + 0.5 × tcCO2.

Figure 1
figure 1

Bland Altman plot of TOSCA data.


Although there is a difference between PaCO2 and tcCO2, it is possible to accurately predict the PaCO2 level using tcCO2 with moderate precision. tcCO2 may be an adequate alternative to arterial blood gas sampling.


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Weaver, C., Mecklenburgh, J., Saayman, A. et al. Transcutaneous (tcCO2) measurement versus arterial blood gas CO2 measurement in critically ill adult patients. Crit Care 8 (Suppl 1), P55 (2004).

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