EFFECT OF ALTITUDE ON SPIROMETRIC PARAMETERS AND THE PERFORMANCE OF PEAK FLOW METERS

Background - Portable peak flow meters are used in clinical practice f or measurement of peak expiratory flow (PEF) at many different altitud es throughout the world. Some PEF meters are affected by gas density. This study was undertaken to establish which type of meter is best for use above sea level and to determine changes in spirometric measureme nts at altitude. Methods - The variable orifice mini-Wright peak flow meter was compared with the fixed orifice Micro Medical Microplus turb ine microspirometer at sea level and at Everest Base Camp (5300 m). Fi fty one members of the 1994 British Mount Everest Medical Expedition w ere studied (age range, 19-55). Results - Mean forced vital capacity ( FVC) fell by 5% and PEF rose by 25.5%. However, PEF recorded with the mini-Wright peak flow meter underestimated PEF by 31%, giving readings 6.6% below sea level values. FVC was lowest in the mornings and did n ot improve significantly with acclimatisation. Lower PEF values were o bserved on morning readings and were associated with higher acute moun tain sickness scores, although the latter may reflect decreased effort in those with acute mountain sickness. There was no change in forced expiratory volume in one second (FEV(1)) at altitude when measured wit h the turbine microspirometer. Conclusions - The cause of the fall in FVC at 5300 m is unknown but may be attributed to changes in lung bloo d volume, interstitial lung oedema, or early airways closure. Variable orifice peak flow meters grossly underestimate PEF at altitude and fi xed orifice devices are therefore preferable where accurate PEF measur ements are required above sea level.