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Example of monitor with capnography displayed.

Vital signs monitor with capnography information obscured.

Supplemental Oxygen and Respiratory Monitoring. Capnography (ETCO₂) is the purest measure of hypoventilation. The lower tracing in this graph shows a simultaneous elevation in ETCO₂ as hypoventilation commences. The top 2 tracings show pulse oximeter readings for patients supplemented with oxygen and those breathing room air. Note the top tracing shows no warning of hypoventilation, but the tracing for patients breathing room air declines in concert with the capnographic reading (adapted from Fu ES et al ⁸ ).

Lateral View of the Patient After Successful Orotracheal Intubation.

The patient's thyromental distance was 58 mm. Successful intubation was confirmed by capnography and the patient's thoracic motions.

Oral placement of a nasopharyngeal airway, buccal to a bite-block. Note that the supplemental oxygen line in place only, no capnography sample line, adjustable flange is placed as a reference at the labial commissure.

Capnography monitor demonstrating respiratory waveform (upper panel) and rate (89 breaths/min), oxygen saturation (SpO₂; 98%), end-tidal carbon dioxide (ETCO₂; 11), and pulse rate (129 beats/min). This illustrates hyperventilation with resultant low ETCO₂; SpO₂ is normal and alone would not serve as an index of respiratory disturbance.

Participants Under General Anesthesia with Bilateral Nasopharygeal Airways (NPAs) in Place

Supplemental oxygen is supplied through the left NPA (red arrow), while the capnography sample line is inserted in the right NPA (yellow arrow). The oxygen analyzer probe (pink) is fixed within the silicon intraoral bite block on the left side of the mouth.

Oxygen-Hemoglobin Dissociation Curve. There is a nonlinear relationship between the percentage of total hemoglobin saturated with oxygen (SaO₂) and PaO₂, as demonstrated by the oxygen-hemoglobin dissociation curve illustrated in Figure 1. Hemoglobin saturations of 95% and higher sustain PaO₂ at or above 80 mm Hg, preventing hypoxemia. At 90% saturation, the curve becomes steep, and within a relatively narrow period, the percent hemoglobin saturation and PaO₂ decline dramatically. The relationship between SaO₂ and PaO₂, although in different units, approximates a value of 30 during this rapid decline. Intracellular oxygen tension or that of mixed venous blood is normally 40 mm Hg, so that hemoglobin saturation below 70% would indicate that normal cellular function is compromised.