Validation of morning dip of peak expiratory flow as an indicator of the severity of nocturnal asthma.

Overnight falls in peak expiratory flow (PEF) (with the morning dip of the index) may be considered the hallmark of nocturnal asthma. To validate the morning dip a quantitative marker of the degree of nocturnal bronchoconstriction, the dip was measured in 11 subjects (six with a history consistent with nocturnal asthma) undergoing all-night monitoring of lower respiratory resistance by a double-catheter method. In six subjects, marked and recurrent increases in resistance were recorded, along with morning dips higher than 20 percent; however, on the following morning, only two of them reported having suffered significant breathlessness and wheeze. Peak and average values for resistance, as well as the duration for which resistance was increased, were closely correlated with the magnitude of morning dips. Therefore, unlike the subjective report, PEF may be considered a reliable quantitative indicator of nocturnal bronchoconstriction.