Clinical physiologic correlates in asthma.

Although asthma is a disease of airways, it affects all aspects of lung function, and in acute severe episodes even cardiac performance is influenced. The typical exacerbation is characterized by symptoms of wheezing, dyspnea, and cough associated with the signs of tachycardia, tachypnea, hyperinflation of the thorax, and stridulous breathing. Usually, the pulse rate is 100 bpm or more, and the respiratory frequency varies between 25 to 28 breaths per minute. Use of accessory muscles and pulsus paradoxicus occur in 30% to 40% of episodes. From a functional standpoint, the FEV1, peak flow, and residual volume tend to average approximately 30%, 20%, and 40% of expected values, respectively. Neither the presenting signs, symptoms, or functional abnormalities can be used to predict a relapse or the need for hospitalization because these variables do not necessarily determine the subsequent response to therapy. Furthermore, these signs and symptoms imperfectly reflect the physiologic abnormalities, and their loss can not be relied on as indicating a return to functional normalcy.