Peak expiratory flow at increased barometric pressure: comparison of peak flow meters and volumetric spirometer

Increasing numbers of patients are receiving hyperbaric oxygen therapy as a n intensive care treatment, some of whom have pre-existing airway obstructi on. Spirometers are the ideal instruments for measuring airway obstruction, but peak flow meters are useful and versatile devices. The behaviour of bo th types of device was therefore studied in a hyperbaric unit under conditi ons of increased pressure. It is important to have a non-electrical indicat or of airway obstruction, to minimize the fire risk in the hyperoxic enviro nment. The hypothesis was tested that, assuming that dynamic resistance is unchanged, both the Wright's standard and mini-peak flow meters would over- read peak expiratory flow (PEF) under increased pressure when compared with a volumetric spirometer, as the latter is unaffected by air density. It wa s postulated that a correction factor could be derived so that PEF meters c ould be used in this setting. Seven normal subjects performed volume-depend ent spirometry to derive PEF, and manoeuvres using both standard and mini P EF meters at sea level, under hyperbaric conditions at 303, 253 and 152 kPa (3, 2.5 and 1.5 atmospheres respectively; 1 atmosphere absolute = 101.08 k Pa), and again at sea level. There was a progressive and significant declin e in PEF with increasing pressure as measured by the spirometer (69.46 +/- 0.8% baseline at 303 kPa compared with 101 kPa), while the PEF meters showe d a progressive increase in their readings (an increase of 7.86 +/- 1.69% a t 303 kPa with the mini PEF meter). Using these data points, a correction f actor was derived which allows appropriate values to be calculated from the Wright's meter readings under these conditions.