Capnography Primer for Oral and Maxillofacial Surgery: Review and Technical Considerations

Abstract

th century the histories of anesthesia and dentistry have been intertwined. However, office-based dental procedures which frequently involve analgesia, conscious sedation, and anesthesia vary from practice-to-practice with regard to respiratory monitoring. In modern oral and maxillofacial surgery, oxygenation is often emphasized over ventilation via usage of pulse oximetry (S p O 2 ). A recent Pubmed search using multiple appropriate Medical Subject Headings Terms (MeSH Terms) and following up on related citation trails results in a limited number of topic specific citations dating back to 1987. In this review, we summarize the literature that compares oximetry (S p O 2 and the less common P TC O 2 ) to ventilatory measurements, specifically capnometry (P ET CO 2 ) in the setting of oral and maxillofacial surgery and provide a comprehensive primer on the technological and respiratory considerations essential for a practitioner attempting to incorporate ventilatory monitoring. Anderson et al. [1] studied the combination of a capnograph (expired carbon dioxide (CO 2 ) monitor) and a transcutaneous oxygen monitor (P TC O 2 ) as a non-invasive system for monitoring of respiratory function in 10 ASA class I patients undergoing general anesthesia for removal of third molars. They concluded that the continuous display of the measured end tidal volume CO 2 (P ET CO 2 ), which the author’s state was measured via nasal prong sampling, proved to be a sensitive and accurate method for detecting apnea and airway obstruction. They further state that all episodes of apnea or obstruction were immediately detected as a dramatic decrease in the expired CO 2 level and that the P ET CO 2 values served as useful indicators of hypoventilation. Interestingly, during steady-state conditions of respiration, P TC O 2 correlated well with simultaneously measured partial pressure of oxygen in arterial blood (P a O 2 ) measured by blood gas technology. However, during any period when oxygenation was rapidly changing (step increase in FIO 2 , step decrease in FIO 2 , or apnea) the P TC O 2 lagged behind changes in P a O 2 even after a five-minute equilibration period, thereby not accurately reflecting the true state of oxygenation. They concluded that transcutaneous oxygen monitoring does not appear to be optimal as a respiratory monitor in the setting of ultralight general anesthesia where rapid, critical changes in oxygenation must be detected without delay [1]. Based on a randomly enrolled blind study of fifty-five patients, Bennet et al. [2] stated that in patients with nasal ventilatory exchange rates maintained throughout anesthesia, sampling of nasal P ET CO 2 was an effective way to monitor ventilation status or the respiratory system function. Respiratory depression or obstructive changes in ventilation were detected by capnography with a high sensitivity and low positive predictive value in detecting oxygen desaturation. They state that current technology did not show a clinically significant correlation between P ET CO 2 and oxygen saturation as estimated by pulse oximetry (S p O 2 ), however a combined increase in P ET CO 2 and decrease in respiratory rate suggested a trend of decreasing oxygen saturation [2].