flow may not be specific enough to be useful, and current practice guidelines recommend continuing to use fev1, vc, and fev1/vc as indicators of obstructive disease. more rarely, forced expiratory flow may be given at intervals defined by how much remains of total lung capacity. in such cases, it is usually designated as e.g. fef70%tlc, fef60%tlc and fef50%tlc. forced inspiratory flow 25–75% or 25–50% forced inspiratory flow 25–75% or 25–50% (fif 25–75% or 25–50%) is similar to fef 25–75% or 25–50% except the measurement is taken during inspiration. peak expiratory flow (pef) normal values for peak expiratory flow (pef), shown on eu scale. peak expiratory flow (pef) is the maximal flow (or speed) achieved during the maximally forced expiration initiated at full inspiration, measured in liters per minute or in liters per second. tidal volume (tv) tidal volume is the amount of air inhaled or exhaled normally at rest. total lung capacity (tlc) total lung capacity (tlc) is the maximum volume of air present in the lungs. diffusing capacity (dlco) diffusing capacity (or dlco) is the carbon monoxide uptake from a single inspiration in a standard time (usually 10 seconds). during the test the person inhales a test gas mixture that consisting of regular air that includes an inert tracer gas and co, less than one percent. since hemoglobin has a greater affinity to co than oxygen the breath-hold time can be only 10 seconds, which is a sufficient amount of time for this transfer of co to occur. since the inhaled amount of co is known, the exhaled co is subtracted to determine the amount transferred during the breath-hold time. the tracer gas is analyzed simultaneously with co to determine the distribution of the test gas mixture. this test will pick up diffusion impairments, for instance in pulmonary fibrosis. this must be corrected for anemia (a low hemoglobin concentration will reduce dlco) and pulmonary hemorrhage (excess rbc's in the interstitium or alveoli can absorb co and artificially increase the dlco capacity). atmospheric pressure and/or altitude will also affect measured dlco, and so a correction factor is needed to adjust for standard pressure. maximum voluntary ventilation (mvv) maximum voluntary ventilation (mvv) is a measure of the maximum amount of air that can be inhaled and exhaled within one minute. for the comfort of the patient this is done over a 15-second time period before being extrapolated to a value for one minute expressed as liters/minute. average values for males and females are 140–180 and 80–120 liters per minute respectively. static lung compliance (cst) when estimating static lung compliance, volume measurements by the spirometer needs to be complemented by pressure transducers in order to simultaneously measure the transpulmonary pressure. when having drawn a curve with the relations between changes in volume to changes in transpulmonary pressure, cst is the slope of the curve during any given volume, or, mathematically, δv/δp. static lung compliance is perhaps the most sensitive parameter for the detection of abnormal pulmonary mechanics. it is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition. in those with acute respiratory failure on mechanical ventilation, "the static compliance of the total respiratory system is conventionally obtained by dividing the tidal volume by the difference between the 'plateau' pressure measured at the airway opening (pao) during an occlusion at end-inspiration and positive end-expiratory pressure (peep) set by the ventilator". measurementapproximate value malefemale forced vital capacity (fvc)4.8 l3.7 l tidal volume (vt)500 ml390 ml total lung capacity (tlc)6.0 l4.7 l others forced expiratory time (fet) forced expiratory time (fet) measures the length of the expiration in seconds. slow vital capacity (svc) slow vital capacity (svc) is the maximum volume of air that can be exhaled slowly after slow maximum inhalation. maximal pressure (pmax and pi) spirometer - erv in cc (cm3) average age 20 male female 4320 3387 pmax is the asymptotically maximal pressure that can be developed by the respiratory muscles at any lung volume and pi is the maximum inspiratory pressure that can be developed at specific lung volumes. this measurement also requires pressure transducers in addition. it is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition. a derived parameter is the coefficient of retraction (cr) which is pmax/tlc . mean transit time (mtt) mean transit time is the area under the flow-volume curve divided by the forced vital capacity. maximal inspiratory pressure (mip) mip, also known as negative inspiratory force (nif), is the maximum pressure that can be generated against an occluded airway beginning at functional residual capacity (frc). it is a marker of respiratory muscle function and strength. represented by centimeters of water pressure (cmh2o) and measured with a manometer. maximum inspiratory pressure is an important and noninvasive index of diaphragm strength and an independent tool for diagnosing many illnesses. typical maximum inspiratory pressures in adult males can be estimated from the equation, mip = 142 - (1.03 x age) cmh2o, where age is in years. technologies used in spirometers volumetric spirometers water bell bellows wedge flow measuring spirometers fleisch-pneumotach lilly (screen) pneumotach turbine/stator rotor (normally incorrectly referred to as a turbine. actually a rotating vane which spins because of the air flow generated by the subject. the revolutions of the vane are counted as they break a light beam) pitot tube hot-wire anemometer ultrasound