There is some debate as to the utility of capnography for procedural sedation. While it is clear that capnography decreases the incidence of hypoxia, some studies suggest that it may not reduce patient-centered outcomes such as adverse respiratory events, neurologic injury, aspiration, or death compared to standard monitoring.35,37,38 However, pulse oximetry alone can suffer response delay, while EtCO2 can rapidly detect hypoventilation.39
Potential Uses/Applications
Respiratory Distress
Capnography can provide dynamic monitoring in patients with acute respiratory distress. Measuring EtCO2 with each breath provides instantaneous feedback on the clinical status of the patient and has numerous specific uses.1,3,4
Determining the etiology of respiratory distress in either the obtunded patient or those with multiple comorbidities can be a challenge. Vital sign abnormalities and physical exam findings can overlap in numerous conditions, which may only further obscure the diagnosis. Since different etiologies for respiratory disease require different management modalities, anything that can help clue in to the specific cause can be beneficial. As discussed above, obstructive diseases such as COPD or asthma demonstrate a “shark-fin appearance” on capnogram due to both V/Q heterogeneity and a prolonged expiratory phase due to airway constriction, which will contrast to the typical box-waveform in other conditions (Figure 2).1,2,6 Some studies have been able differentiate COPD from congestive heart failure (CHF) by waveform analysis alone, though this was primarily done via computer algorithms.40 Seeing the shark-fin (or the lack thereof) can help guide management of respiratory distress in conjunction with the remainder of the initial assessment.
Monitoring capnography can help with management and disposition in those with COPD or asthma. During exacerbations, EtCO2 levels may initially drop as the patient hyperventilates to compensate.1 It is not until ventilation becomes less effective that EtCO2 levels begin to rise. This may occur before hypoxia sets in and can prompt the clinician to escalate ventilation strategies. In addition, the normalization of the “shark-fin” obstructive pattern towards the more typical box-form wave may indicate effective treatment, though more data is needed before it can be recommended.41 One of the advantages of this technique would be that it is independent of patient effort, unlike peak-flow monitoring.
EtCO2 can be beneficial even before patients get to the ED. In one study, prehospital patients presenting with asthma or COPD who were found to have EtCO2 of >50 mm Hg or <28 mm Hg, representing the upper and lower limits in the study, had greater rates of intubation, critical care admission, and mortality.42 The patients in this cohort with higher EtCO2 were likely tiring after prolonged hyperventilation and therefore would be more likely to need ventilatory support. Those on the lower end were likely hyperventilating and had not yet tired out. It is important to note that while arrival EtCO2 levels may aid in determining the more critically ill, post-treatment levels were not found to have a statistical difference in determining disposition in patients with asthma or COPD.43