Validity of transcutaneous oxygen carbon dioxide pressure measurement in the monitoring of mechanical ventilation in stable chronic respiratory failure

The accuracy and precision of transcutaneous pressure measurements of oxyge n (Ptc,O-2) and carbon dioxide (Ptc,CO2) in the monitoring of nocturnal ass isted ventilation in adult patients were evaluated. Transcutaneous measurements obtained with two analysers, Radiometer TINA-TC M3 (R) and Kontron MicroGas-7650 (K), were compared with arterial blood gas es analysed in blood samples withdrawn simultaneously in 10 patients, Senso rs were heated to 43 degrees C, Measurements of trascutaneous blood gases a nd arterial blood gases were collected six times at l-h intervals, The data obtained with both instruments were similar and did not significan tly change over the 5 h test period, Measurement of Ptc,O-2 underestimated arterial oxygen tension (Pa,O-2) and this underestimation increased with th e level of Pa,O-2 (p<0.01). Measurements of Ptc,CO2 overestimated arterial carbon dioxide tension (Pa,CO2) and this overestimation increased with the level of Pa,CO2 (p<0.05). These errors suggested an instrumental bias. Math ematical correction of this bias neutralized the error in accuracy and impr oved the precision (sD of the differences transcutaneous blood gases - arte rial blood gases). An additional correction, suppressing the between-subjec t scattering, improved the actual precision: precision was reduced from 1.9 to 0.8 kPa (14.4 to 5.7 mmHg) (R) and from 1.7 to 0.5 kPa (13.1 to 3.7 mmH g) (K) for oxygen, and from 1.0 kPa (7.8 mmHg) (R) and 0.7 kPa (5.6 mmHg) ( K) to 0.4 kPa (3.2 mmHg) for carbon dioxide (R and K). In conclusion, with these two successive corrections, transcutaneous oxygen and carbon dioxide provide a reliable estimation of blood gases to monitor nocturnal ventilation in adults with chronic respiratory failure.