THE INFLUENCE OF POSTURE ON THE VOLUME OF THE RESERVE AIR. BY WILLIAM H. WILSON. (From the Physiological Department of the Faculty of Medicine, Cairo.) THE purpose of this communication is to draw attention to certain facts xegarding the effect of posture on the volume of the lungs, as indicated by measurements of the reserve air, of which I can find no record in the literature of the subject, and to their physiological significance. Introduction. " Reserve Air " is the term originally applied by Hutchinson(l) in 1846 to that volume of air which can be expelled at will from the lungs after the end of a normal expiration. It is stated by this author to have an average volume in adult men of 100 cubic inches (1560 c.c.), the corresponding volumes of the tidal and complemental being respectively 20 and 110 cubic inches (312 and 1710 c.c.). The figures given in most English textbooks (e.g. Starling(2)) are 1500 for the reserve air, 500 for the tidal and 1500 for the complemental air. Boruttau(3) gives reserve air, 1600 c.c.; tidal air, 500; complemental air, 1600. It is of course now evident that the tidal air varies with the rate of production of C02 by the body at the time. Hutchinson(4) records that the vital capacity is less in the recumbent than in the erect posture: he gives the following figures in his own case: Standing 260 cubic inches, sitting 255, recumbent supine 230, and prone 220, a difference of about 600 c.c. Whether the diminution affected the reserve or the complemental air is of course not shown'. Observations which seem to show that considerable variations in the volume of the reserve air are found in different individuals, apart from differences in their respective vital capacities, have been made by Flack(5) in flying officers. Flack examined 69 pilots and found volumes for the reserve air varying from 26 to 55 p.c. of the vital capacity volume. The mean figures of all the observations were, reserve air, 1560, vital capacity 3950, the minimum being 1050, the maximum 2100 c.c. That the difference was not due to weakness of the expiratory muscles seems to be indicated by the fact that the "expiratory force" was normal in both subjects, the one giving the maximum, the other the minimum 1 In observations of the vital capacity made on others and myself no marked difference has been noted in different postures.
EFFECT OF POSTURE ON LUNGS figure. It is to be noted that the subjects were all examined under the same conditions. There is, in my opinion, little doubt that the variations noted by Flack are largely due to the difficulty of obtaining reliable measurements in inexperienced subjects. Flack's observations refer to different individuals, but I can find it nowhere indicated that in normal persons there are variations in the reserve air volume corresponding to different positions in the same individual. The method usually adopted in measurements of the pulmonary capacity is perhaps responsible for this. It is in fact the same as that originally described by Hutchinson(l) in his directions for the use of the spirometer. The directions were "...let the person to be examined loose his vest, stand perfectly erect, with the head thrown well back,..." The posture is that which for convenience I have called "standing at attention" and is well shown by diagram 26 in Hutchinson's original paper. As regards the measurement of the reserve air it is usually directed that after the end of a quiet breath a maximum expiration should be made into the spirometer, the volume so expelled being observed. Method. To demonstrate and record changes in the position of the tidal exchange relative to the two extremes of ventilation and the corresponding changes in the volume of the reserve air, I have employed a recording spirometer of the Hutchinson type. The spirometer is arranged for rebreathing, being provided with inlet and outlet tubes connected through valves to a Douglas mouth-piece. On the inlet passage is a chamber containing pumice and sulphuric acid, the purpose of which is to enlarge the total air space of the apparatus and to act as an air-filter, there being a certain danger of infection from one individual to another if some such device is not adopted. The movements of the spirometer chamber are transmitted to a lever recording on a kymograph. The volume corresponding to a given movement of the writing point having been determined, volumes may be accurately measured on the completed record. To conduct an observation, the mouth-piece is fixed in position and the subject instructed to breath through the nose. When the record is to begin the subject is told to put on the nose-clip. He is then allowed to breathe quietly for about 20 seconds, at the end of which time he is told to fill the chest to the fullest possible extent and then expire as deeply as possible; after which he continues breathing quietly until the end of the observation, when he is iatructed to inspire and expire again as deeply as possible. 55
56 W. H. WILSON. A line joining the points of maximum expiration at the beginning and end of the record gives the base line from which the volume of the reserve air is determined. From this line measurements are made to the lowest points of the record of breathing in any posture. If it be desired to compare the volumes of the reserve air in different postures successively in the same experiment, as in the example shown (Fig. 1), the subject must be instructed previously as to the procedure to be adopted. The following figures are reproduced from records obtained in the manner described. Fig. 1. Spirometer record of respiration in different postures. Vital capacity: 3500 c.c. Subject W. H. W. Posture Reserve air 1 1150 2 950 3 700 4 1550 5 % 1550 Direction of record: right to left. Time mark: 6 secs.
EFFECT OF POSTURE ON LUNGS Fig. 1 illustrates the effect of a series of postural changes in the course of a single observation. The numbers mark the changes of posture, which were as follows: 1. Standing in an easy attitude with one hand resting on table. 2. Sitting in chair. 3. Recumbent on back, legs extended. 4. Sitting erect in chair with shoulders well back. 5. Standing at attention, head up, hands at side. The records reproduced in Fig. 2, A and B, are from the same subject taken about 18 months later than Fig. 1. They illustrate the effect of the recumbent posture in B as compared to the erect posture in A, the posture in both cases being maintained unchanged during the period of the observation. The vital capacity is the same in each, namely 3430 c.c.; while, however, the reserve air volume in the erect posture, A, is 1430 c.c., in the recumbent posture, B, it is reduced to 240 c.c. It is to be noted that while in the earlier observations, of which Fig. 1 is a record, the subject lay on three chairs with the head and shoulders supported and the legs extended in a rather strained position, in later observations, of which the records shown in Fig. 2 are examples, the subject lay on a couch with the head only supported and the knees slightly drawn up. The muscular relaxation obtained with the more comfortable arrangement was more complete; a fact which may explain the considerable difference in the reserve air in observation 2 A as compared to 1. Note. The measurements are not corrected for temperature. As, however, the air temperature in the laboratory (in Cairo) was about 860 F. in each case no important difference would result. The records shown in Figs. 1, 2 A, and 2 B are from observations on myself. As will be seen from the following table experiments on four other healthy subjects aged respectively 35, 30, 27 and 39 years, gave a similar result, although not with a minimum for the reserve air so low as that seen in Fig. 2 A. Reserve air Vital A a capacity Erect Recumbent A. H. 3900 1600 650 R. W. 4400 1700 700 A. J. 5500 2400 800 R. H. 4160 1460 590 Result. Figs. 1, 2 A, and 2 B, demonstrate sufficiently clearly that the air content of the lungs may be more than a litre less in the recumbent 57
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EFFECT OF POSTURE ON LUNGS " 59 than in the erect posture, and that with every change in the position of the body the base-line of normal respiration shifts to a different level. By the expression "the base-line of respiration" is meant the level reached at the end of a normal expiration as indicated by a line joining these points on a record. That this level is unaffected except by change of posture is shown very well in Fig. 3, the record of an observation in which the subject remained sitting, leaning slightly against the back of a chair, throughout. Fig. 3. Spirometer record. Subject sitting in chair. Expiration, below; inspiration, above. Vital capacity: 3400. Reserve air: 1000 c.c. Complemental air: beginning, 1770; end, 1330. Duration of experiment: 2 mins. 10 secs. Time mark: 10 sees. It wil be seen that the volume of the reserve air remains constant, the line joining the points of maximum expiration being parallel with the line joining the terminations of individual expirations, although, owing to hyperpncea due to the accumulation of 002, the depth of inspiration is 450 c.c. greater at the end than at the commencement. The fact that in dyspncea, even when the inspiratory hyperpnoea almost reaches the extent of a maximal inspiratory effort, the volume
60 W. H. WILSON. of the reserve air is unaffected except by posture', can be readily demonstrated in a rebreathing observation with the recording spirometer. If any considerable acceleration in the rate of breathing accompanies the hyperpncea there is a slight shifting of the base-line upwards in the direction of inspiration with a corresponding increase in the reserve air. The maintenance of the base-line in a fixed position, however considerable the volume of the tidal air, is related to the essentially passive character of expiration as compared to the almost entirely active character of inspiration. The rise of this base-line in rapid breathing may perhaps be regarded as an indication that the elastic recoil of expiration requires a certain time for the expulsion of air against the frictional resistance of the air-passages, the inspiration thus commencing at a higher level than would normally be the case. The base-line of respiration in any posture is evidently determined by the point at which the various passive forces acting on the ribs and diaphragm are for that posture in equilibrium. The act of inspiration disturbs this balance of forces; at its cessation equilibrium is restored. In connection with the observations recorded above it is of interest to consider the postural changes in the level of the diaphragm. Postural changes with the trunk vertical, as in sitting or standing, must affect primarily the position of the ribs, and secondarily, through an increase or decrease of the lower costal circumference, the position of the diaphragm. In the recumbent posture, whatever may be the position of the ribs, the fact of primary importance is the increase in the subdiaphragmatic pressure from + 5 in the erect to + 16 (Keith(6)) in the recumbent position of the body. I have to thank Prof. Elliot Smith for giving me the necessary facilities, and Mr Melville, of the X-Ray Department of the Anatomical Institute at University College, London, for making skiagrams of the thorax under various conditions of respiration corresponding to the spirometer records shown; and also Dr Sankey (Radiologist to the Radeliffe Infirmary, Oxford) for taking three similar skiagrams of the thorax in the recumbent supine posture. I was myself the subject. Fig. 4 is a reproduction on reduced scale of a tracing on one sheet from five films (I, II, III, IV and V). The broad dark lines show well the 1 Whether this is equally true of the hyperpncea of muscular exertion is not indicated by the experiments detailed in this paper. If, however, there be a diminution, apart from postural changes accompanying the exercise, it implies that an active factor takes part in expiration under such conditions which is not present during rest or in the hyperpnoea produced by a moderate excess of C02 in the air breathed.
EFFECT OF POSTURE ON LUNGS 61 relative position of the dome of the diaphragm in the different postures at the end of a normal expiration, the level rising about half an inch in Fig. 4. Tracing from skiagrams of thorax showing level of diaphragm under different conditions. I. End of normal expiration, subject standing erect. II. End of normal expiration, subject standing easy. III. End of normal expiration, subject recumbent supine. IV. Dome of diaphragm, maximum expiration, subject standing. IV C. The same corrected for level of tube (D). V. Maximum inspiration (uncorrected). A. 4th rib. N. Nipple. B. Point four inches below nipple. C. Approximate line of costal margin. D. Level of tube focus. The broken lines on the left show outline of heart. passing from the first to the second and a further inch in the recumbent posture (III). The position of these lines needs no correction, the focus of the tube being approximately level with the highest point of the diaphragm. The upper broken line (IV) shows the actual position of the shadow of the diaphragm, the lower continuous line (IV c) the corrected position, in maximal expiration. The lowest line (V) gives the level of
62 W. H. WILSON. the diaphragm in the fullest possible inspiration. (Corrected, this would be slightly higher.) Skiagrams VI and VII, extreme expiration and inspiration in the recumbent posture, are not shown on the tracing as they showed no great difference from IV and V (standing). Note. It is clear that the level of the dome of the diaphragm as shown in the skiagrams is considerably higher than would be shown by the usual clinical method. The maximum range of movement of the dome of the diaphragm appears from these films to be approximately 31 in. (9 cm.) and the postural range 1 in. (3 7 cm.). The level of the upper limit of the diaphragm at the points of maximum expiration and inspiration as shown in Fig. 4 appears to agree with the results of X-ray photographs from a healthy man given by Haldane, Meakins and Priestley (7) in the figure on p. 448 of their paper on "Shallow Breathing." My observations do not, however, show the marked difference in the levels in the recumbent as compared to the standing posture of the subject noticed by these authors. The extreme displacement of the lung into the upper segment of the thorax by a deep voluntary expiration, and its relatively small volume in the recumbent posture, are well seen in the skiagrams. In the case of a deep voluntary expiration the measurement of the right lung must be less than six inches from the base to the highest point of the apex. The position of the ribs, except in the recumbent position III and in the films showing maximal expiration, is not shown. The skiagrams demonstrate the same fact as the records of ventilation. REMARKS. The conclusion which may be drawn from the above is that the usually accepted volume for the reserve air is only true of an unusually erect posture. This may be the normal posture of an exceptionally well set up vigorous man belonging, for example, to Dreyer and Hanson's Class A(8) but it is certainly not the posture of the ordinary individual. The change to the recumbent posture, it is seen, may result in a diminution of more than a litre in the volume of the reserve air. The residual air is stated to vary in different individuals from 1200 to 1600 c.c. assuming, as is probable, that 1400 c.c. corresponds to an average vital capacity of 3500, the result, as regards the volume of air which remains in the lungs and is ventilated by the ebb and flow of the tidal air, is that while in the erect posture this would amount to 2900 c.c., it might in the recumbent posture be as little as 1650.
EFFECT OF POSTURE ON LUNGS It is clear that such considerable postural changes in the air content of the lungs should be taken into account in considering the change in composition produced by the inspiration of a given volume of air. It will be seen, if the above figures be accepted, that in the recumbent position as compared to the erect there would be a diminution of 40 p.c. in the volume of the lungs, which must be accompanied by a corresponding diminution in the alveolar surface. The work of Haldane, Meakins and Priestley(7) showed that in different parts of the lung the ratio between alveolar ventilation and the supply of venous blood varies. Certain parts are more ventilated and others less ventilated than the average, the result being, owing to the shape of the oxyhaemoglobin dissociation curve, that the mixed arterial blood is less fully saturated with oxygen than if the same amount of air had been evenly distributed over the alveoli. They found that not only was the effect exaggerated with shallow breathing, even when the rate was so great that the total lung ventilation was increased and the mean alveolar oxygen pressure raised, but that in the recumbent posture the exaggerated effect was much more easily produced. The observations detailed above, showing that in the recumbent position the reserve air is much reduced, are evidently related to the latter effect. It was pointed out by Keith(s) that when the lungs expand in inspiration they do not expand evenly as a whole but part by part, somewhat similarly to the opening of a lady's fan. It appears that when the reserve air is diminished, as in the recumbent position, this peculiarity is accentuated, and accounts for the "orthopncea " so often observed in severe heart or lung disease, since the blood is less fully oxygenated in the recumbent position of the body. It would be outside the scope of this communication to consider further the clinical bearing of the facts demonstrated, beyond indicating that the large normal variations in the level of the base-line of respiration must be related to the posture in which respiration is most efficiently carried out in persons suffering from pulmonary or cardiac affections. CONCLUSIONS. 1. The volume of the reserve air varies with the posture of the body in a man of average vital capacity from a minimum of as little as 250 c.c. in the recumbent supine, to a maximum of 1550 c.c. in the erect position. 2. The diminution in the reserve air in the recumbent position is related to the orthopncea observed in certain cases of chest disease. 63
64 W. H. WILSON. I have to thank my friend Dr Ali Hassan for much valuable help, and Dr J. S. Haldane, F.R.S., for his kindness in reading through the draft of this paper and for his advice thereon. REFERENCES. 1. Hutchinson. Med. Chir. Trans. 29. p. 176. 1846. 2. Starling. "Principles of Human Physiology." 4th ed. p. 895. 1926. 3. Boruttau. Nagel's Handb. d. Physiol. 1. p. 21. 1909. 4. Hutchinson. Article "Thorax," Todd's Encycl. of Anat. and Physiol. 4. p. 1067. 1849-1852. 5. Flack. Med. Res. Council Repts. Special Report Series No. 53. pp. 101-104. 1920. 6. Keith. Hunterian Lecture "Man's Posture." B.M.J. p. 588. 1923. 7. Haldane, Meakins and Priestley. This Journ. 52. p. 448. 1919. 8. Dreyer and Hanson. "The Assessment of Physical Fitness." Cassell and Co., London, pp. 76, 84, 106. 1920. 9. Keith. "Further Advances in Physiology," p. 182. 1909.