Noninvasive monitoring of oxygen and carbon dioxide.

New technologies for the noninvasive assessment of oxygen and carbon dioxide are transforming clinical practice. Transcutaneous monitoring of PO2 (PtcO2) and PCO2 (PtCO2) provides an approximation of PaO2 and PaCO2 values in hemodynamically normal individuals, but both PtcO2 and PtcCO2 diverge from the corresponding arterial values when cardiac output is reduced, even in the absence of hypotension. Transcutaneous monitors also have relatively slow equilibration and response times. Pulse oximeters rapidly assess arterial O2 saturation, but give spurious results when dyshemoglobins (for example, carboxyhemoglobin, methemoglobin) are present in significant quantity. End-tidal CO2 (PetCO2) monitoring tracks breath-by-breath changes in ventilation, and PetCO2 approximates PaCO2 when significant physiologic dead-space is not present. Respiratory inductive plethysmography provides a semiquantitative assessment of tidal volume and the relative contribution of the thorax and abdomen to ventilation; among other uses, this technology may allow for the early detection of respiratory muscle fatigue prior to the onset of respiratory failure.