Noninvasive monitoring of cardiorespiratory parameters

Noninvasive methods for monitoring pH, pCO2 and pO2 have not achieved the original aim of providing a noninvasive, continuous assessment of arterial blood gases under all clinical conditions. Fingertip and ear oximeters provide measurement of oxygen saturation in arterial blood under most conditions, but accuracy is lost when intense peripheral vasoconstriction is present. Transcutaneous and conjunctival O2 and CO2 sensors also function as accurate trend monitors of PaO2 and PaCO2 in the hemodynamically stable patient. Measurement of pH by placement of a sensor into the subcutaneous tissue provides a continuous assessment of arterial pH in neonates with normal CO, but the use of these sensors in adult patients remains largely unproven. Noninvasive assessment of the adequacy of peripheral perfusion, cardiac function, and intravascular volume status can be performed using transcutaneous or conjunctival O2 and CO2 sensors. Early detection and correction of hypovolemia and diminished CO can be accomplished by monitoring tissue PO2 and PCO2. In the emergency department setting, in which the evaluation and initial treatment of critically ill patients often is performed by measurement of a limited set of physiologic parameters, the ability to evaluate tissue perfusion on a continuous basis is an important clinical advance.