Thorax: first published as 10.1136/thx.23.1.33 on 1 January 1968. Downloaded from Thorax (1968), 23, 33. Factors determining maximum inspiratory flow and maximum expiratory flow of the lung J. JORDANOGLOU AND N. B. PRIDE From the M.R.C. Clinical Pulmonary Physiology Research Unit, King's College Hospital Medical School, Denmark Hill, London, S.E.5 The factors determining maximum expiratory flow and maximum inspiratory flow of the lung are reviewed with particular reference to a model which compares the lung on forced expiration to a Starling resistor. The theoretical significance of the slope of the expiratory maximum flow- volume curve is discussed. A method of comparing maximum expiratory flow with maximum inspiratory flow at similar lung volumes is suggested; this may be applied either to a maximum flow-volume curve or to a forced expiratory and inspiratory spirogram. During the last 15 to 20 years a number of tests FACTORS DETERMINING MAXIMUM FLOW AT A GIVEN based on the forced vital capacity manceuvre LUNG VOLUME have come into widespread use for the assessment of the ventilatory function of the lungs. Origin- ISO-VOLUME PRESSURE-FLOW CURVES These ally the forced vital capacity manoeuvre was intro- curves were introduced by Fry and Hyatt (1960), measurement of the who measured oesophageal pressure and the duced as a substitute for copyright. maximum breathing capacity, and to a large simultaneous flow during a series of vital capacity extent the use of such tests as the forced manceuvres made with varying effort. From expiratory volume in one second (F.E.V.,.0) has these records they constructed plots of the relation continued to be empirical and descriptive. In between oesophageal pressure and flow at selected 1958 Hyatt, Schilder, and Fry examined the lung volumes. For the present purpose it is more factors determining maximum expiratory flow and useful to plot the actual driving pressure-alveolar http://thorax.bmj.com/ demonstrated that this flow was extremely depen- pressure-against flow. Alveolar pressure equals, dent on lung volume. As a result of their analysis to a close approximation, the sum of the pleural they introduced a new test of ventilatory function, (or oesophageal) pressure and the pressure of the plotting of expiratory flow against lung volume elastic recoil of the lung (Pel) at the volume con- during a forced vital capacity manceuvre. The sidered. An alveolar pressure-flow plot for a advantages in plotting this maximum flow-volume normal subject studied at 50% vital capacity is (M.F.-V.) curve were that a large part of it was shown in Figure 1. While inspiratory flow goes on relatively uninfluenced by the subject's effort or increasing until the subject reaches his most nega- on September 27, 2021 by guest. Protected by the resistance of the upper airways. This test tive value of alveolar pressure (Palv,min), on appears to have been used very little outside the expiration flow at first increases with increasing United States of America. More recently, further alveolar pressure, but, when a critical level of studies have been made which help to elucidate alveolar pressure is generated (Palv'), maximum the physiological factors responsible for the shape expiratory flow (M.E.F.) is reached. With further of the M.F.-V. curve and reinforce the claims increases in alveolar pressure expiratory flow re- made for it by the original authors. mains at the maximum level or is even slightly In the present paper the factors determining reduced. Similar relationships are found in disease maximum flow are reviewed and the theoretical except that in some subjects with emphysema significance of two measurements derived from expiratory flow is considerably below the maxi- the M.F.-V. curve is discussed. mum level when alveolar pressure is high. Since In the following paper (this journal, p. 38) a airways resistance equals the ratio of alveolar comparison of the M.F.-V. curves obtained in pressure over flow it is represented on this curve healthy subjects and in subjects with emphysema, as the reciprocal of the slope of the line from the severe asthma, and fibrosis of the lungs is origin to the appropriate point on the pressure- presented. flow curve. 33 D Thorax: first published as 10.1136/thx.23.1.33 on 1 January 1968. Downloaded from 34 J. Jordanoglou and N. B. Pride Exp flow ((L/min.) If iso-volume pressure-flow curves are con- 6001 structed for different lung volumes it is found that they are of a similar shape over almost all the Palv vital capacity in patients with airways obstruction 400] PaIV' max. and over the lower 70% of the vital capacity in healthy subjects. Although of similar contour, 200 /,'~~~~ the values of Palv' and M.E.F. become progres- sively smaller with declining lung volumes. At / volumes greater than 70% of the vital capacity in * rs . I 1 -40 +40 +BO health expiratory flow continues to rise with Palv cm H20 increases in alveolar pressure, so that at these / 200 I volumes expiratory flow is effort dependent. / DETERMINANTS OF PALV' AND M.E.F.: THE 'STARLING .400 RESISTOR' ANALoGY Two broadly similar attempts Pqlv to analyse the factors determining M.E.F. and min. 600 Palv' in health and in various types of airways Insp.flow (I./min., obstruction have been published recently (Mead, FIG. 1. Diagrammatic iso-volume pressure-flow curve for Turner, Mazklem, and Little, 1967; Pride, a normal subject at 50% vital capacity. Flow on vertical Permutt, Riley, and Bromberger-Barnea, 1967). axis, alveolar pressure on horizontal axis. The slopes of the These analyses propose that the lung can be con- nterrupted lines equal the reciprocal of the airways sidered to act as a 'Starling resistor' when M.E.F. resistance at driving pressures of Palv' and Palv,min. is reached during a forced expiration. Mouth pressure is assumed to equal atmospheric pressure. copyright. From these relationships simple equations pre- dicting the values of maximum inspiratory flow and maximum expiratory flow at 50% of the vital capacity (M.I.F.50%' M.E.F.50%) can be written. 0 http://thorax.bmj.com/ M.I.F.5,oo5=-Palv,minM..F50 Raw,i (1) where Raw,i is inspiratory airways resistance at a driving pressure of Palv,min Palv' M.E.F.5o% :-" Raw,e (2) FIG. 2. Model of lung as a Starling resistor. The alveoli are represented as a common sphere emptying by a single where Raw,e is expiratory airways resistance at a airway. Part of the airway (indicated by the wavy lines) is driving pressure of Palv'. regarded as potentially collapsible. The box represents the on September 27, 2021 by guest. Protected Thus while M.I.F.50 is critically dependent on thorax. the effort emnployed by the subject, M.E.F50 is only effort dependent to the extent that an alveolar pressure of at least +20 cm. H O must In the model (Fig. 2) developed by Permutt be developed, which compares with the average of and Riley part of the airway is regarded as + 82 cm. H20 which can be generated by a healthy being collapsible and the analogue of the thin- subject on maximum effort (Hyatt, 1961). walled tube in the resistor used to control blood Combining equations (1) and (2), pressure in a Starling heart-lung preparation. This collapsible airway is surrounded by pleural pres- M.E.F.5o% _ Palv' (Raw,i N sure (Ppl). During expiration the airway pressure M.I.F.5(.% - Palv,min J Raw,e) (3) will drop from (Ppl + Pel) in the alveoli to zero at Equation (3) shows that conclusions about the the mouth. At a certain level of alveolar pressure relative change in inspiratory and expiratory resis- (which corresponds to Palv' in Figure 1) pleural tance can be drawn from measurements of maxi- pressure will exceed the pressure within the col- mum flow only if there are no changes in the ratio lapsible airway by a sufficient amount for the air- Palv'/Palv,min. way to narrow cri#.Ically and limit flow. The Thorax: first published as 10.1136/thx.23.1.33 on 1 January 1968. Downloaded from Factors determining maximum inspiratory flow and maximum expiratory flow of the lung 35 collapsible airway will not actually close; if it did the equation shows that M.E.F. at these lung the airway pressure on the alveolar side of the volumes is independent of upper airways resis- point of closure would rise to equal alveolar pres- tance, as was pointed out by Fry and Hyatt (1960). sure and the airway would reopen. Since the Finally, it is possible from this analogue to collapsible airway can neither close completely derive the relationship between M.E.F. and vital nor remain wide open, a narrow orifice forms capacity (V.). close to its downstream end (marked by the solid Since Pel=V/CL, where CL is the static lung circle in Fig. 2) and this acts to limit flow. When compliance, equation (5) may be rewritten this point of critical narrowing develops, it divides the airway into two functional segments-the S M.E.F. V. - Ptm' (6) segment from alveoli to the point of critical C-RRs Rs narrowing, and the D segment from this point Ptm'/Rs is probably a constant fairly indepen- downstream to the mouth. Expiratory flow from dent of lung volume at least at volumes greater such a system will reach a maximum when the than 25% vital capacity. Hence at lung volumes resistive pressure drop down the S segment equals between 25% and 70% of the vital capacity the (Pel-PPtm'), where Ptm' is the value of transmural AM.E.F./AV.