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Some Observations on Peak Expiratory Flow by R Thorax: first published as 10.1136/thx.17.1.39 on 1 March 1962. Downloaded from Thorax (1962), 17, 39. SOME OBSERVATIONS ON PEAK EXPIRATORY FLOW BY R. J. SHEPHARD From the War Office, Chemical Defence Experimental Establishment, Porton Down, Wilts (RECEIVED FOR PUBLICATION AUGUST 2, 1961) The peak flow meter (Wright and McKerrow, calibration of the orifices with water manometer and 1959) was originally conceived as a simple port- piezometer ring showed resistances of 2.0, 8.0, and able method of estimating forced expiratory 28.0 cm. H 0/1./sec. at a flow of 0.5 1./sec., and a volume (F.E.V.). However, the correlation with flow exponent of 1.80 to 1.85 in each case over the F.E.V. and with maximum voluntary ventilation normal range of peak flows. In view of the wide use of the Wright meter in field (M.V.V.) was not always very close, and the peak studies, the characteristics of the instrument were flow meter reading was therefore advocated as a examined in some detail. The resistance of the meter measure of "ventilatory capacity" in its own to transient flows was substantially as described by right. The present paper reports some normal Wright and McKerrow (1959) for steady flow condi- peak flow readings obtained with the Wright tions, amounting to 4 cm. H 0 with peak flows of meter: such values are influenced not only by 300 1./min., and rising to 7 cm. H,O with short puffs airway resistance but also by the pressure at 500 to 600 1./min. (Fig. 1). This exceeds the resist- developed by the thoracic musculature, and the ance of the screen pneumotachograph by more than results obtained depend largely on co-operation an order. At steady flow rates of 40-500 l./min., delivered through a rotameter, the peak flow meter from the patient. I have used peak flow read- over-read by 50 to 70 1./min. (Fig. 2); this character- ings, obtained before and after introduction of an istic, which was recognized by Wright and McKerrow, http://thorax.bmj.com/ external resistance, to calculate a value for airway is a consequence of the method of calibration used. resistance that is less dependent on subjective Oscillating flows from a sine-wave pump (frequency co-operation and muscular power. The frequency 1-3 c/sec.) gave discrepancies of peak flow readings response characteristics of the Wright peak flow that were always as large as, and often larger than, meter do not permit measurements with an those produced by steady flow. Wright's original external resistance; however, if peak flow instrument was calibrated "physiologically," a small readings are obtained by high frequency equip- group of subjects blowing alternately through a screen pneumotachograph and the peak flow meter. Using ment, such as a screen pneumotachograph, and the flow meter did this method (Fig. 3), present peak on September 26, 2021 by guest. Protected copyright. a small external resistance is used, the calculated not show a significant systematic error under normal airway resistance does not differ systematically operating conditions, though when the wave form from interrupter valve values. was artificially blunted by introduction of an external resistance, significant systematic discrepancies resulted (see Table I). A technical fault of the peak flow meter METHODS was that with repeated use sufficient water vapour PEAK FLOW MEASUREMENT.-The peak flow has condensed in the compartment housing to jam the been defined by Wright and McKerrow (1959) as the ratchet mechanism. This could probably be overcome highest flow rate sustained by a subject for at least by fitting a more air-tight seal between the ratchet and 10 m.sec. In the present experiments, this parameter vane compartments. After about 1,000 measurements was measured by a standard commercial peak flow meter bought recently (serial No. 557), and also by a TABLE I Y-shaped double-screen pneumotachograph of the PEAK FLOW METER READTNGS INTRODUCING type described previously (Shephard, 1957), but having EXTERNAL RESISTANCE a flow resistance of only 7 mm. H 0 at 1,000 1./min. I I~~~~~~~~~~~~~~~ Added External Mean Discrepancy flow. Resistance 4+S.E. Percentage (cm. H20/1. sec.) (1. min. A.T.P.S.) Error Readings were obtained alternately with the two -I- instruments, and after control values had been estab- 0 -22+12 -4-2 standard orifices of 8 mm., 6 mm., and 4 mm. 2 +49+8 +14-2 lished, 8 +124±9 +70 internal diameter were placed in turn in the mouth- 28 +70±6 +77 piece. and peak flow readings repeated. Static Thorax: first published as 10.1136/thx.17.1.39 on 1 March 1962. Downloaded from 40 R. J. SHEPHARD 7 0 1-6 E 0 cx~ 0 5 - 0 0 LL. 4 V 0 U-; 3 0 0ol 0 LU 0* 2 0 : * 0 .0 S co cv -~~~~~~~~~~~~I 100 200 300 400 500 600 NOMINAL TRANSIENT FLOW RATE (I./min.) http://thorax.bmj.com/ FIG. 1.-Flow resistance of Wright peak flow meter (transient flows). the vane also showed such distortion that it had to previous experience with the peak flow meter. Sixteen be replaced. patients with emphysema, most of whom were severely OTHER EXPERIMENTAL TECHINIQUES. Pulmonary disabled, were also tested. All these were diagnosed airway resistance was determined by interrupter valve initially by Guy's Hospital Chest Clinic and showed (Clements, Sharp, Johnson, and both the standard physiological criteria of emphysema. Elam, 1959), on September 26, 2021 by guest. Protected copyright. flow and interruption pressures being measuired during including a decrease of vital capacity and F.E.V. the middle third of expiration (Shephard, 1959). The (the latter not responding to isoprenaline spray), and mean of ten successive estimates was taken. an increase in the slope of the single breath oxygen Vital capacity and forced expiratory volume (1 sec.) inhalation test. were measured by means of a high frequency spiro- THEORY meter (Bernstein and Mendel. 1951). The highest of CALCULATION OF AIRWAY RESISTANCE FROM three successive readings was taken. PEAK FLOW. It is assumed that the chest muscu- SUBJECTS. All normal subjects were clinically and lature can generate a constant pressure P, and radiographically free of chest disease. Thirty males that this is proportional to a power funct.on of and eight females were drawn from the research and flow F. Under normal circumstances, pulmonary laboratory staff of a teaching hospital, and 58 males resistance is and were volunteers from the fighting Services. All were Ri purely internal, of at least average physical fitness, but none was P -- Ri (F 1)5 (I undergoing regular athletic training. The average If the internal resistance Ri is supplemented by tobacco consumption of the hospital staff (four an external resistance Re, peak flow is reduced cigarettes/day) was low compared with that of the from to F2, and Service men (13.5 cigarettes/day), and the Service Fl volunteers had certain characteristic personality trends P=Ri (F2)n + Re (F2)" (2) (Kemp and Shephard, in preparation); however, there Thus, is no evidence that the latter influenced peak flow (F)n - Ri + Re readings. None of the normal subjects had had (F2)n Ri (3) Thorax: first published as 10.1136/thx.17.1.39 on 1 March 1962. Downloaded from SOME OBSERVATIONS ON PEAK EXPIRATORY FLOW 41 *- ROTAMETER FLOW X- SINE-WAVE FLOW 600r- x . x x 5001-. 0 X X - 0* t/0 z 400H /- x L..z / X 4 x w I- 300 I- <r a X C. 200 X ) http://thorax.bmj.com/ 0 0 0 / 100_- / /2 on September 26, 2021 by guest. Protected copyright. I4I II:_I I I_ 51c)Q 0 100 200 300 400 ROTAMETER FLOW (U/MNATPS) FIG. 2.-Steady flow and sine-wave flowcalibration of peak flow meter. and if Re is known, Ri can be calculated from The highest peak flow reading recorded in any Fl and F 2 The validity of the assumptions subject exceeded the mean peak flow for the same underlying this calculation is discussed on page subject by an average of 55.2 1. /min. A.T.P.S. 47. (S.D.+26.9 l./min., range 13-108 1./min.). The RESULTS mean peak flow on any one day was quite 1. PEAK FLOW METER READINGS IN NORMAL reproducible (coefficient of variation 4.5%, S.D. SUBJECTS of C.V.+2.8%, range 2.2-13.7%); the difference (a) Technique of Measurement.-Repetition of between mean values on successive days was the peak flow test at half-minute intervals gave somewhat larger (8.8%, S.D. 6.3%, range 0.2- increasing values for the first two to five puffs, 29.4%). a plateau for the next ten puffs, and then usually The effect of varying the initial chest position a gradual decline. Grouped results for the first (normal inspiration in place of full inspiration) l5 Service subjects are shown in Fig. 4. was tested in 22 subjects. As would be antici- D Thorax: first published as 10.1136/thx.17.1.39 on 1 March 1962. Downloaded from 42 R. J. SHEPHARD 0 - NO EXTERNAL RESISTANCE 0 EXTERNAL RESISTANCE 2cmH2O/f/SEC. x -EXT2ERNALRESISTANCE 8cmH2/t/SEC. a - EXTERNAL RESISTANCE 28 cmH2o/t/SEC. 800r 700- 0 - 600[ 0 a: 0 *~~~~~~~~~~~~oo 0 E 500 U z 6 x~ ~~~~~~~~~0 0 400[ C LU 0 s~~~~~~~ S ce o~~~~~~Cc)/ o IL /IxYso/0I I IIIII 0~ 300- LL LU2 ***o/ http://thorax.bmj.com/ 0~ 200 U Ux tL 100 ItI 0 00 200 300 400 So0 600 700 800 900 on September 26, 2021 by guest.
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