A device that measures the rate at which an individual can expel air from the LUNGS. This is an indication of the reserve in the capacity of the lungs. Narrowed airways (bronchospasm) slow the rate at which air can be expelled; the peak flow meter can assess the severity of the condition. ASTHMA causes bronchospasm and the device can measure the effectiveness of treatment with BRONCHODILATOR drugs; this should be done regularly to monitor the progress of the disease.