The clinical exam is inherently subjective from one provider to another, but it has even been shown that individual physicians are not consistent with their own assessment of wheezing on auscultation.1 Peak expiratory flow rate, or peak flow, is another modality that has been extensively used in EDs to quantify the extent of a patient’s asthma exacerbation and response to treatment. The downside to peak flow is that it is highly dependent on the patient’s ability to properly perform the test and to consistently reproduce the test. Some investigators have explored the use of end-tidal capnography as a means of monitoring a patient’s response to treatment because it eliminates the patient cooperation variable to obtain an accurate assessment.
Capnometers were initially used in submarines in World War II but now are found in nearly every ED in the country.2 A capnogram in a normal, healthy adult consists of a few separate phases (see Figure 1). Phase I occurs during the beginning of exhalation where the majority of the air is dead space, and hence, very little carbon dioxide is released, resulting in a relatively flat portion of the curve. As the patient continues to exhale, the mixed air has an increased carbon dioxide concentration, and subsequently, the capnography curve begins a steep incline; this is called Phase II. The plateau Phase (Phase III) has a decreased slope, with only a small increase in CO2 concentration, and peaks at the end-tidal point. This part of the capnogram represents the expiration of alveolar air and remains nearly horizontal because of the homogeneity of alveolar ventilation within healthy lungs.3 Once the patient begins inhalation (Phase 0), the end-tidal drops precipitously and ends the waveform. In a healthy patient, the angle formed (α) from Phase II to Phase III is approximately 110 degrees, and the angle formed between Phase III and 0 (β) is typically about 90 degrees.4
When asthmatics present with a severe attack, their waveform changes in such a manner that it begins to resemble a shark fin. The bronchoconstriction of the small airways during an asthma exacerbation causes a decrease in alveolar ventilation in different regions of the lung.
When asthmatics present with a severe attack, their waveform changes in such a manner
that it begins to resemble a shark fin (see Figure 2). The bronchoconstriction of the small airways during an asthma exacerbation causes a decrease in alveolar ventilation in different regions of the lung. Each portion of the lung is associated with its own ventilation-perfusion ratio (V:Q) that subsequently determines its respective PaCO2. During expiration in an asthma exacerbation, the areas of the lung with less bronchoconstriction have a lower PaCO2 and will preferentially be expired first. On the other hand, the regions of the lung with a greater degree of obstruction will have a higher PaCO2 and will have delayed emptying. Some authors refer to these differences in expiration of CO2 during an asthma exacerbation as desynchronization.3,5 The desynchronization of alveolar emptying causes changes within the capnogram waveform; the slope of Phase II decreases, the slope of Phase III increases as the more highly obstructed alveoli expire their retained CO2 in a delayed fashion, and these changes in the slope result in an increased α angle (see Figure 2).3 The focus of using a capnogram as an assessment tool in asthmatics is not on the numeric end-tidal value but more so to evaluate the changes in the waveform morphology.