End-tidal CO2 and tissue pH in the monitoring of acid-base changes: a composite technique for continuous, minimally invasive monitoring.

The infrared CO2 analyzer continuously monitors the CO2 tension in exhaled air at end-tidal expiration. In experimental animals, we found a consistent relationship between PaCO2 and end-tidal CO2 (ET.CO2) in the normal steady state, and in acid-base disturbances (respiratory acidosis and alkalosis, and hypoperfusion acidosis). Paired data analyses of PaCO2 (X) and ET.CO2 (Y) yielded correlation coefficients of r = 0.98 (Y = 0.96X + 4.43) during progressive hypercarbia (PaCO2: 32----110 torr), and r = 0.93 (Y = 0.89X + 0.93) during hyperventilation hypocapnia (PaCO2: 41----14 torr). The relationship between PaCO2 and ET.CO2 was seen during hypovolemic shock if pulmonary perfusion was maintained uniform in all areas of lung. The ability of the ET.CO2 sensor to predict instantaneously the PaCO2 makes it attractive enough to be used in conjunction with the subcutaneous tissue pH(pHe) sensor in the management of acid-base disturbances. After hypercarbia (FiCO2 0.15 X 40 min; PaCO2/ET.CO2: 100/101 torr), when the dogs were returned to room air, abruptly both the ET.CO2 and pHe sensors were sensitive to the changes in Fi.CO2. But the response of the ET.CO2 was swifter. The advent of transcutaneous gas monitors has shown that intermittent blood gas analyses, however frequent, are inadequate for the monitoring of the rapidly altering blood gas status in the acutely ill. The ability of the pHe sensor to identify whole-body acidosis and alkalosis combined with the speed and ease of the ET.CO2 monitor in pinpointing hypercarbic and hypocarbic states makes this two-parameter system suitable for the continuous, noninvasive monitoring of the critically ill.