Should Capnographic Monitoring Be Standard Practice During Emergency Department Procedural Sedation and Analgesia? Pro and Con

The Pro Arguments 

Capnography is simple, noninvasive, and easy to interpret.3 The current study by Deitch et al1 provides compelling evidence that its use can decrease hypoxemia, and accordingly it improves the existing standard of care. What emergency physician would not want forewarning of ventilatory compromise, especially in young children? Abnormal capnography could signal clinicians to reevaluate their patients, to be prepared to provide airway support, and to avoid administering additional doses of sedatives until the concern is resolved.

Few would dispute that hypoxemia should be minimized whenever possible. Transient hypoxemia may not be dangerous in and of itself; however, it is a harbinger of serious morbidity and even mortality. Patients oversedated to the point of desaturation are likely to be unresponsive and have impaired protective airway reflexes. The ultimate study of any monitoring device would instead evaluate serious complications such as hypoxic brain injury, aspiration, or death; however the exceptional rarity of such events in modern sedation practice makes them virtually impossible to study. Indeed, pulse oximetry became a standard of care not because of proof that it reduced adverse outcomes,13 but rather because it exhibited compelling theoretical benefit without harm. The current study by Deitch et al1 elevates capnography to similar status.

Capnography should also enhance sedation safety by permitting clinicians to administer higher levels of supplemental oxygen as it effectively supersedes pulse oximetry as the preferred early warning device. Such “hyperoxygenation” permits patients to safely tolerate short periods of respiratory depression or apnea without need for positive-pressure-assisted ventilation and its potential for gastric insufflation and aspiration,10 which is particularly helpful for young children who desaturate more quickly than older children or adults.10

The Con Arguments 

Despite the pro viewpoint above that capnography is beneficial and, at the very least, not harmful, it is premature to embrace it as a standard of care. The safety benefit purported in this1 and similar11, 12 studies is decreased hypoxemia, according to thresholds ranging from 90% to 95%, lasting from 5 to 15 seconds. In the clinical context, many of these events are self-limiting or resolve with minimal interventions such as airway repositioning or supplemental oxygen. For instance, in this study, 41% (18/44) of all hypoxemic events did not lead to any intervention. More meaningful, clinically relevant outcomes would have been hypoxic brain injury, aspiration, death, or at least assisted ventilation or adverse events warranting interruption, delay, or abandonment of the procedure. Hypoxemia may be a substantially less difficult outcome to evaluate in a clinical trial; however, it cannot be automatically assumed that decreases in this surrogate marker extrapolate into similar decreases in complications that unequivocally matter.

Capnography may not only be unnecessary but also a hindrance because of spurious readings. Although the current technology is far superior to that of older devices, all recent studies report artifacts and false positives, typically caused by patient movement, nasal cannula displacement, or patient verbalizations or crying. The latter can be a particular problem in uncooperative children. For instance, Lightdale et al11 observed that all of their 163 children exhibited loss of waveform at some point during their procedures, with most (96/163) attributed to patient verbalization. What is unclear from the current evidence are the practical implications of these “nuisance” alarms.

Adoption of capnography as the standard of care could substantially affect quality improvement programs. Would all false-positive readings need to be tracked as performance indicators, and if not, then how do we select which waveform trends warrant reporting? And in those rare cases in which complications occur, followed by legal review, what new challenges to defending proper care are created when it is observed that an emergency physician failed to take immediate action despite objective evidence of respiratory depression, even if subclinical and without associated hypoxemia?

In the current study, 27% of capnographic abnormalities did not lead to hypoxemia and were thus falsely positive. However, this may underestimate the actual incidence of false alarms because the authors had to exclude an additional 12% of patients from this study because of data loss. Such exclusion was appropriate for research integrity; however, treating clinicians still had to assess whether these aberrancies were true respiratory depression or just technical glitches. Thus, the rate of nuisance alarms may have been as high as 39% in this study.

A specific challenge to capnography in pediatric emergency medicine is that anxious or frightened children are unlikely to tolerate the cannula before achieving moderate if not deep sedation, which necessitates starting the sedation without such monitoring, and then, at a most inconvenient time as the procedure is about to begin, fitting the cannula and verifying proper function of the equipment. Initiation of capnography once sedation is already under way also means that no baseline etco2 can be established, hindering optimal interpretation of subsequent changes. Overcompensation in response to the unknown baseline will likely lead to an even higher rate of “nuisance” false-positive warnings.

Instead of being routine, capnography can be selectively used in situations in which it is more likely to be helpful. For instance, radiology sedations are increasingly being provided by sedation services run by pediatric emergency physicians.14, 15 Such imaging studies may be of substantial duration, and direct visualization of the patient may not be feasible. Capnography can substitute for interactive monitoring in such situations.