Clinical relevance of the flow rate response to low density gas breathing in asthmatics.

The maximal expiratory flow rate response to low density gas breathing was examined at the mid-vital capacity point in 31 asthmatic patients. Clinical features and long-term follow-up date were documented to assess the clinical relevance of the various responses. Many of the patients with chronic asthma had not responded adequately to outpatient therapy, and they had been admitted to hospital for evaluation and management. With increased steroid doses and intensive bronchodilator therapy, all showed considerable improvement and were studied when this improvement occurred. The remainder of the patients were studied after recovery from acute asthmatic attacks or during maintenance management as outpatients. All patients with a forced expiratory volume in 1 second is greater than 75 per cent of the predicted value and mid-expiratory flow rate is greated than 50 per cent of the predicted value at the time of study showed a good response to helium. Those patients with more severe obstruction could be divided into 2 groups, responders and nonresponders. A qualitatively similar response to normal subjects (density-dependent flow rates) was a feature of those patients who in general showed further improvement in ventilatory function on follow-up. A qualitatively similar response to that seen in patients with chronic irreversible obstruction (density-independent flow rates) was a feature of those patients who in general showed no further improvement in ventilatory function on long-term follow-up. There were, however, exceptions to both groups. We conclude that in asthmatics with more than mild air flow obstruction, the assessment of helium response can be of value in identifying those patients who have, in addition to asthma, chronic irreversible obstruction due to concommitant chronic bronchitis and/or emphysema. Clinical assessment and measurement of single-breath diffusion of carbon monoxide provide additional support for the latter diagnoses and separate the few exceptions from the bulk of the nonresponders. Responders and nonresponders can be fairly reliably identified from the simply recorded exhaled flow volume curve, thus obviating the need for a volume displacement plethysmograph.