![]() ![]() The Cap-ONE (Nihon Kohden, Tokyo, Japan) uses mainstream capnography for its adult and pediatric masks, whereas the OxyMask and OxyKid devices (SouthMedic, Barrie, Canada) use sidestream capnography. There are 2 approaches to designing O 2 masks that incorporate capnography. 9 Whereas 38% of the subjects had a P aCO 2 − P ETCO 2 difference within 5 mm Hg, an equal percentage of patients had a difference ≥10 mm Hg. For example, a clinically reasonable correlation was found between P aCO 2 and P ETCO 2 (r = 0.82), but also there was substantial variability in performance. 3 Nonetheless, studies of oral guided nasal cannula capnography in subjects with respiratory distress produced results of uncertain reliability. This modification was found to accurately reflect P aCO 2 in subjects recovering from general anesthesia. To address this problem, nasal cannulas designed for capnography now incorporate an oral guided design that samples expired gas from the center of the mouth. 3 This problem also occurs in respiratory failure because the elevated work of breathing associated with high levels of minute ventilation (V̇ E) 7, 8 obviates nasal breathing due to the additional resistance. Under these circumstances the partial pressure of end-tidal carbon dioxide (P ETCO 2) does not reliably reflect the partial pressure of arterial carbon dioxide (P aCO 2). This became readily apparent in obese patients (particularly those with obstructive sleep apnea) because they tend to be mouth breathers. However, utilizing capnography in non-intubated patients with respiratory insufficiency presents technical challenges that may interfere with measurement accuracy, including: (1) dilution of expired gas with varying supplemental O 2 flows, (2) oxygen mask design, (3) the orientation of capnographic cuvette chamber or sample line in relation to expired gas flow, and (4) variability of the respiratory pattern, including the contributions of nasal versus mouth breathing.Įmblematic of the technical issues for monitoring critically ill patients has been the limitations imposed by sidestream CO 2 sampling with a standard nasal cannula. 1Ĭritically ill, non-intubated patients represent another cohort that may benefit from capnography. 1, 2, 6 In one study, hypoventilation preceded oxygen desaturation in approximately 75% of cases by an average of 1.7 min. 5 Studies using capnography in these patients have found that hypoventilation is common and that abnormal capnometry/capnography generally precedes oxygen desaturation. 3 Respiratory depression and airway obstruction in these patients occurs frequently 4 and represents the leading cause of adverse events. Respiratory pattern assessment and its integration with indices of oxygenation and ventilation adequacy are crucial aspects of patient monitoring during procedural sedation 1, 2 as well as recovery from general anesthesia.
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