Introduction Low Maximum Inspiratory Flow is characteristic of patients with muscle weakness, Laryngeal dysfunction, or Extra-thoracic airway obstruction. We have been exploring the use of an Inspiratory Flow Metre (In Check Dial- Clement Clarke International) to help in the identification of those patients who need more detailed investigation.
Methods We have compared the Inspiratory Flow rate (I) with the flow metre against Maximum Inspiratory flows taken from a flow volume loop (FVI) taken as part of routine lung function testing in 100 sequential subjects attending the Cardio Respiratory department for lung function testing.
Results We have found major variability in the FVI on flow volume traces despite attempts to obtain traces with maximum volume and effort. Only 36% of subjects had variability between attempts of <1 l/s, with 64% showing variability between attempts of >1 l/s, 24% of >2 l/s, and 3% >3 l/s. For measurements using the Inspiratory Flow metre If I>2 l/s all of the 38% of subjects showed FVI of 2 l/s or more. With I of <2 l/s there was agreement between the two methods ±0.3 l/s in 26%, and a further 14% with FVI of <2 l/s. 40% of subjects with Inspiratory Flow (I) of <2 l/s had FVI of <2 l/s. But in 22% of subjects I <2 l/s but FVI >2 l/s. FVI-I showed mean difference for these subjects of 2.4 l/s (range 0.9–4 l/s). In total 78% of subjects showed concordance of Maximum Inspiratory Flow to >2 l/s or >2 l/s between the two measurements and for 22% the inspiratory flow metre reading of <2 l/s did not reflect maximum Inspiratory Flow.
Conclusion There are major variations in the Maximum Inspiratory Flow measured with a flow volume loop but for a simpler measurement with an Inspiratory Flow metre if Maximum flow is >2 l/min then it is unlikely that Inspiratory flow is compromised. A simple clinic based measurement can be useful to exclude limitation of Inspiratory Flow but if abnormal further investigation is needed.