Evaluation of the Microstat™ sublingual PCO₂ monitor in ambulatory patients

Physicians often need to measure arterial PCO₂ in clinical practice. Arterial blood gas sampling is typically available only in hospitals and may be unpleasant for patients. Minimally invasive techniques for measuring PCO₂ offer the potential for overcoming these limitations. The MicroStat monitor non-invasively measures PCO₂ in the sublingual tissues, which should track arterial PCO₂ in hemodynamically stable patients. This was a prospective observational study. Patients undergoing routine cardiac catheterization were recruited. Following arterial cannulation, two sequential sublingual PCO₂ measurements were taken and a contemporaneous arterial sample was sent for blood gas analysis. For each subject we calculated the mean sublingual–arterial CO₂ gradient and the test–retest sublingual PCO₂ difference. Twenty-five patients were studied. Mean sublingual–arterial PCO₂ gradient was +6.8 mmHg (95 % limits of agreement −3.0 to 16.6 mmHg). Test–retest difference was 3.4 mmHg (95 % limits of agreement −1.1 to 7.9 mmHg), p = 0.11 (Wilcoxon test), repeatability was 11 mmHg. The MicroStat sublingual PCO₂ monitor over-estimates arterial PCO₂ with wide limits of agreement. Test–retest repeatability was poor. Use of sublingual PCO2 monitoring with the MicroStat monitor cannot currently replace blood gas sampling.