Time-between control charts for monitoring asthma attacks

The monitoring of peak expiratory flow rate (PEFR) is crucial for effective management of asthma. Daily PEFR monitoring is recommended, yet the data are rarely used by patients to help them understand their progress or by clinicians to modify treatment plans. Time-between control charts, which have been shown to be specially suited for monitoring rare events, can be used to monitor asthma attacks.

Each patient is asked to record his or her PEFR value once a day, and these data are used to construct the control chart. PEFR data for three previously reported cases are presented and used to illustrate the control chart methodology. If duration of consecutive attacks is plotted and the observed duration exceeds the upper control limit (UCL), the patient is getting worse. If length of consecutive symptom-free days is plotted and the observed duration exceeds the UCL, the patient is getting better. In both circumstances, the clinician and the patient explore what brought about the prolonged recovery or periods of deterioration. The object is to increase time until the next attack.

Using time-between control charts in monitoring asthma attacks has the advantage of providing a visual display of data that, unlike eyeballing of trends, clarifies when patients should seek additional clinical advice. The control limit allows clinicians and patients to ignore random variations and focus on real changes in underlying patterns of asthma attacks.