Capnography

Capnography measures exhaled carbon dioxide and is most useful when applied directly to patient care. This is in circumstances of detecting misplacement of the tracheal tube, dysfunction of respiratory apparatuses, detection of abnormal lung function, successful cardiopulmonary resuscitation, and trending of deadspace changes. The least reliable application is to reflect alveolar ventilation (PaCO2). This application is most common during general anesthesia and weaning from mechanical ventilation. Provided the patient has a stable cardiac status, stable body temperature, absence of lung disease, and normal capnogram, PETCO2 monitoring may assist in estimating PaCO2. The use of capnography in patients with severe respiratory failure should be applied with careful reflection. The increased V/Q mismatch that is consistent with a widened P(a-ET) gradient, as well as worsening hypercapnea with increased peripheral carbon dioxide production, can lead to erroneous PETCO2 values. Capnography may be least useful in the sickest patients.