Hypoventilation and apnea precede hypoxia, therefore, end-tidal carbon dioxide should be affected before oxygen saturation.([FOOTNOTE=Cacho, G., Perez-Calle, J. L., Barbado, A., Lledo, J. L., Ojea, R., & Fernandez-Rodriguez, C. M. Capnography is superior to pulse oximetry for the detection of respiratory depression during colonoscopy. Rev Esp Enferm Dig. 2010;102(2):86-89.],[ANCHOR=],[LINK=]) Likewise, continuous monitoring of EtCO2 has been demonstrated to show greater sensitivity for detecting episodes of respiratory compromise.([FOOTNOTE=Overdyk, F. J., Carter, R., Maddox, R. R., Callura, J., Herrin, A. E., & Henriquez, C. Continuous oximetry/capnometry monitoring reveals frequent desaturation and bradypnea during patient-controlled analgesia. Anesth Analg. 2007;105(2):412-418.],[ANCHOR=],[LINK=])

Overdyk et al. continuously monitored postoperative patients (n=178) receiving patient-controlled analgesia on the ward who were assigned to receive morphine or meperdine by their surgeons with EtCO2 and SpO2 in order to determine the sensitivity of each parameter to episodes of respiratory compromise.2