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.