Value of nocturnal monitoring of transcutaneous O2 and CO2 pressures in adults with respiratory failure.

We have evaluated the reliability of the transcutaneous (t.c.) method of measurement of arterial PO2 and PCO2 in adult man. In 33 simultaneous measurements of 9 normals and 12 patients with a wide range of hypoxemia, we found: t.c. PCO2 = 3.62 + 1.29 PaCO2 +/- 7.3 (r = 0.96) and t.c. PO2 = 11.14 + 0.86 PaO2 +/- 9.89 (r = 0.92). Recalculating t.c. PCO2 to 37 degrees C we can obtain: t.c. PCO2 = 2.7 + 0.97 X PaCO2, stating that there is no significant difference between t.c. PCO2 and PaCO2. The t.c. apparatus detects 10 and 90% O2 pressure changes with a delay of time of about 15 s and 1 min, respectively; the t.c. method is therefore not suitable for detecting changes in PaO2 caused by sleep apnea of short duration. On the contrary the t.c. method provided a useful monitoring of arterial PO2 and PCO2 changes during the night in chronic obstructive pulmonary disease (COPD) and non-COPD patients. A nocturnal monitoring of t.c. PO2 and PCO2 seems: (a) absolutely necessary in non-COPD hypoxemics, especially if total lung capacity (TLC) and/or residual volume (RV) are significantly reduced; (b) not absolutely necessary in COPD hypoxemics, provided they have an enlarged TLC and/or a very expanded RV; (c) advisable in intermediate situations, e.g., in COPD hypoxemics with an associated restrictive disorder caused by heart failure, congestion of pulmonary bed, parenchymal or rib cage disease, in order to establish the optimal concentration of oxygen for each patient and to avoid severe nocturnal hypoxemia without producing a dangerous rise in PaCO2.