Spot check analysis of gas exchange: invasive versus noninvasive methods.

BACKGROUND

Correct measurement of PO₂ and PCO₂ is essential to establish appropriate therapy such as long-term oxygen therapy (LTOT) in patients suffering from respiratory failure.

OBJECTIVES

We aimed to compare common invasive and noninvasive methods for assessing blood gas components for spot check analysis.

METHODS

Arterial (PaO₂, PaCO₂) and capillary blood gas (PCBGO2, PCBGCO2) measurements were taken consecutively in a randomized order and were compared with noninvasive measurements obtained from the transcutaneous monitoring of PO₂ and PCO₂ (PtcOv, PtcCO₂, sensor-temperature 44°C). Capillary samples were taken from both arterialized earlobes, where samples of right earlobes were defined as a reference value. Pain assessment of all measurements was evaluated by each subject using the 100-mm visual analogue scale.

RESULTS

83 patients and 17 healthy subjects were included. The mean difference between PaO₂ and PtcO₂ was 11.9 ± 15.0 mm Hg, with lower limits of agreement (LLA) of -17.4 mm Hg (95% confidence interval (CI) -22.5 to -12.3 mm Hg), and upper limits of agreement (ULA) of 41.1 mm Hg (95% CI 36.0-46.2 mm Hg). The comparison of PaO₂ with PCBGO2 showed a mean difference of 5.6 ± 7.2 mm Hg (LLA -11.0; ULA 19.6 mm Hg). The mean difference between PaCO₂ and PtcCO₂ was 1.1 ± 4.9 mm Hg (LLA -8.6; ULA 10.8 mm Hg) and that between PaCO₂ and PCBGCO₂ was 0.7 ± 2.0 mm Hg (LLA -3.3; ULA 4.8 mm Hg). The analysis of capillary blood gases (36.2 ± 22.3 mm) was rated as more painful than the analysis of arterial blood gases (26.1 ± 20.6 mm), while transcutaneous measurement was rated as the least painful method (1.9 ± 7.4 mm; all p < 0.0001).

CONCLUSIONS

The comparison of different methods for blood gas measurements showed substantial differences between capillary and arterial PO₂ and between transcutaneous and arterial PO₂. Therefore, arterial PO₂ analysis is the essential method evaluating indication for LTOT. Nevertheless, comparative analysis further indicated capillary PCO₂ as an adequate surrogate for arterial PCO₂.