The pattern of breathing in acute severe asthma

The pattern of breathing was studied in 8 patients with acute severe asthma on admission to hospital and during recovery to determine how chest wall motion varied with the degree of air-flow obstruction (AO), the relationship between degree of AO and respiratory timing and ventilation, and whether the pattern suggested respiratory muscle fatigue when asthma was most severe. Pattern was assessed by simultaneous measurement of respired volumes (pneumotachygraph) and anteroposterior (AP) motion of lower rib cage and abdomen (magnetometers). There was a phase lag of AP rib cage relative to AP abdominal motion that was greatest in those with lowest FEV1 and progressively decreased during recovery. Fractional inspiratory time was decreased in severe asthma. Mean inspiratory flow was increased in moderately severe asthma but decreased when FEV1 was less than 25% predicted. Breathing pattern was no more variable during severe asthma than during recovery. We conclude that during severe AO the magnitude of phase lag of AP rib cage relative to AP abdominal motion reflects severity of asthma; respiratory drive is increased but is not associated with increased ventilation below an FEV1 of 25% predicted, and analysis of the breathing pattern provides no clear evidence of respiratory muscle fatigue.