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Detection of hypoventilation by capnography and its association with hypoxia in children undergoing sedation with ketamine

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Literature Review

Detection of hypoventilation by capnography and its association with hypoxia in children undergoing sedation with Ketamine

Langhan ML, Chen L, Marshall C, Santucci KA.  Pediatr Emerg Care. 2011 May; 27(5):394-7.

Reviewed by Daniel Tsze, MD and Joseph Cravero, MD

This article explores the relationship between hypopneic hypoventilation (as detected by capnography) and hypoxia in children undergoing sedation with ketamine in a pediatric emergency department (PED).  Capnography is not routinely used to measure end tidal carbon dioxide (ETCO2) in this context, but the authors suggest it has the potential to improve clinical outcomes for patients in this care setting.

Capnography can detect hypoventilation in one of two ways: by detecting a decrease in respiratory rate leading to a rise in ETCO2 (bradypneic hypoventilation); or detecting a decrease in tidal volume while maintaining a normal respiratory rate, but leading to lower than normal ETCO2 (hypopneic hypoventilation).  This paper focuses on the latter, and describes how often it occurs with ketamine sedations, and its relationship with pulse oximetry of less than 95%.

Of the 58 eligible subjects, 16 patients (27%) had what they defined as “persistent low ETCO­2”, which was a decrease in ETCO2 less than 30 mm Hg for greater than 30 seconds in duration.  When the number of patients who had “persisting hypoxemia” (pulse oximetry less than 95% for greater than 30 seconds) with persisting low ETCO2 was compared to those who had “persisting hypoxemia” without persisting low ETCO2, they found that patients in the first group were more likely to have persisting hypoxemia, with a relative risk of 6.6 (95% CI 1.4-30.7). In spite of the large disparity in numbers of patients with persisting hypoxia in these groups, the difference did not appear to be statistically significant.  In addition, only 4 of the 7 patients who had persisting hypoxemia (from both groups) required supplemental oxygen, two had airway repositioning maneuvers applied, and none required bag mask ventilation or any further intervention.

Comments:
Although this study describes 50% of patients demonstrating hypopneic hypoventilation (both persistent and non-persistent), only 27% were symptomatic for greater than 30 seconds.  In addition, the clinical significance of this observation is uncertain.  The authors state that an association with pulse oximetry less than 95% was chosen because of previous reports showing that this level of mild hypoxemia for greater than 5 seconds has preceded more serious adverse events.1 However, no adverse events were observed in this study, which likely related to the small sample size and frequency at which sedation-related adverse events occur. It should be noted that the addition of supplemental oxygen would eliminate coexisting hypoxia and further throw the significance of hypopneic hypoventilation into question. Furthermore this study does not help us understand the relationship between the ETCO2 and PaCO2 - which presumably is increased during hypopneic hypoventilation.

A potential benefit of capnography is that changes in ventilation will be detected prior to decreases in pulse oximetry.  This study supports this purported utility of capnography by showing that changes in ETCO­2, specifically lower levels consistent with hypopneic hypoventilation, preceded changes in pulse oximetry by several minutes.

The question of how CO2 monitoring may affect clinical outcomes is uncertain in the context of this particular study.  Although it would provide the opportunity for staff to respond more quickly with an intervention to improve a patient’s ventilatory efforts, and potentially decrease the number of episodes of oxygen desaturations2 whether such an intervention provides any significant clinical benefit or prevents clinically significant adverse events is still to be determined.

Ultimately, it is unclear from this study whether capnography can improve clinical outcomes in patients undergoing sedation.  The study does illustrate, however, that hypopneic hypoventilation occurs frequently in patients undergoing sedation with ketamine.  How this observation should be interpreted or used to guide additional intervention will require further study in the context of large sample sizes to accommodate the infrequent incidence of adverse events.  This study emphasizes the importance of the use of accepted, standardized definitions for “clinically significant adverse event”, so that the importance of the observations made can be interpreted appropriately in different studies and between various investigators.

References:

  1. Coté CJ et al. A single-blind study of combined pulse oximetry and capnography in Children. Anesthesiology. 1991;74:980-987.
  2. Lightdale JR, et al. Microstream capnography improves patient monitoring during moderate sedation: A randomized, controlled trial. Pediatrics. 2006;117:21170-e1178.
    Klein EJBrown JCKobayashi AOsincup DSeidel K.  Ann Emerg Med. 2011 Oct;58(4):323-9.

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