Peak Flow Meter is a small handheld device that measures a patient’s ‘Peak Expiratory Flow’ or ‘PEF’, a measure of how quickly a patient can forcefully exhale after full inhalation. It is important that patients consult their Clinicians/Specialist Nurses about what their personal best PEF is or should be, to provide a point of reference for tracking. This is the Medi Peak Flow instructions for use Low Range.