Can we assess asthma severity using expiratory capnography in a pediatric emergency department?

Abstract

Asthma is a common presenting problem in pediatric emergency departments (EDs). Accurate evaluation of asthma severity is important for acutely ill children, and pulmonary function tests are the current gold standard. Peak expiratory flow rates (PEFR) are possible in some children, but FEV1 (forced expiratory volume in one second) requires specialized equipment and a respiratory technician, a combination not available in most EDs. Both these manoeuvres require cooperation and are difficult to perform in young children. Clinical scores and oxygen saturations are simpler and more widely applicable, but are poor severity markers unless airflow is markedly decreased. 1 Similarly, transcutaneous or expired carbon dioxide (CO2) levels, or blood gas analysis are mainly useful in severe asthma. The ideal measure of airway obstruction should be simple, non-invasive, objective and valid in mild and severe asthma. It should also be effort-independent, therefore feasible in young and acutely ill asthmatic children. Capnography is a technique that provides both a waveform and a numerical value for the partial pressure of expired CO2 during each respiratory cycle (Fig. 1). Capnographs capture 2 key respiratory parameters: the rate of rise of [CO2] during the final phase of alveolar gas exhalation (alveolar plateau slope), and the Q angle, the angle between the initial rapid expiratory [CO2] rise and the alveolar plateau. This angle is normally very distinct, approaching 90°, but in asthma, airway obstruction and air trapping delay the mixing of alveolar and dead space gases, which results in a slower rise in [CO2] in expired air, an increased Q angle and a greater slope of alveolar plateau. 2