During research into some features of the response of the pulmonary circulation

Transcription

1 70 J. Physiol. (I949) I09, I6:6I2.22I.3 AIR DISTRIUTION IN LUNGS DURING HYPERVENTILATION Y G. H. ARMITAGE AND W. MELVILLE ARNOTT From the Department of Medicine, Queen Elizabeth Hospital, University of irmingham (Received 24 Autgust 1948) During research into some features of the response of the pulmonary circulation to high cardiac output, curiosity arose as to the mechanism of the adaptive expansion of the pulmonary vascular bed during the high cardiac outputs of exercise, severe anaemia, etc. It seemed possible that this adaptation might be a simple consequence of the alveolar expansion brought about by hyperpnoea. There was uncertainty whether deep breaths are accommodated by: (1) a further distension proportionate to their ventilation during quiet breathing, of those alveoli used at rest; (2) the opening up of successive ranges of 'reserve alveoli', which are folded up parachute-wise during quiet breathing; or (3) a mixture of these two mechanisms. If the first hypothesis be correct and alveolar ventilation uniform, it should be possible to predict, after any depth of inspiration, the final alveolar concentration of an inert 'reference gas' by the following formula P1 V-+P2 V2 3 Vl+V2 where P1 =concentration of gas in alveolar air before inspiration; P2 =concentration of gas in inspired air; P3= concentration of gas in alveolar air after inspiration; V1=volume of air in lungs before inspiration; V2= volume of inspiration. (A correction must be added to the formula if the reference gas takes part during the period of the breath in any significant e'xchange with the blood). The sample taken at the end of a complete expiration (hereafter called an end-expiration sample) is probably the most representative specimen of alveolar air that can be obtained as it is least likely to be influenced by contamination with dead-space air.

2 ALVEOLAR VENTILATION IN HYPERPNOEA 71 Any systematic irregularity of alveolar ventilation would cause a discrepancy between the alveolar concentration thus predicted and that observed by analysis of an end-expiration alveolar sample. ut, if the second hypothesis be correct, then, as the depth of inspiration increases, a rising proportion of the air inspired must enter newly opened alveoli and thus neither mix with the contents of the normally open alveoli, nor contribute to the subsequent end-expiration sample on the reasonable assumption that the air expired last originates in alveoli which are always 'open'. The latter will therefore exhibit, with deeper breaths, a progressive deviation from the composition predicted on the basis of the first hypothesis. METHODS Certain initial experiments were made using oxygen as reference gas, with a correction for uptake on the basis of a previous M.R. determination. The control experiments, with inspiration from the residual to the resting respiratory level, gave satisfactory results; but with inspiration to maximum capacity discrepancies were encountered to the significance of which consideration is given in a separate paper (Armitage & Arnott, 1949); these clearly indicated the unsuitability of oxygen for this purpose. Instead hydrogen, helium and nitrogen were employed as reference gases; none of which involved any significant correction for transfer across the alveolar membrane within the time intervals concerned. Four healthy males, aged 39, 30, 20 and 22, all accustomed to the experimental procedures, served as subjects. The subdivisions of lung volume were measured using a enedict-knipping spirometer with a Shakspear katharometer, employing the constant-volume hydrogen (or helium) dilution method (McMichael, 1939). Experiments using hydrogen Inspiration of air into an air-hydrogen alveolar mixture. With the subject seated in circuit with the enedict-knipping spirometer (circuit A), an air-hydrogen mixture was breathed as in the lung-volume estimation procedure. When the katharometer reading indicated the attainment of hydrogen-equilibrium in the system, the subject detached himself at the end of a normal expiration and, turning his head to an adjacent mouthpiece leading from an air-filled enedict-roth spirometer (circuit ), took a measured inspiration of air. Thereafter, an end-expiration alveolar sample was taken in the usual way in a sampling-tube of 100 c.c. capacity. The sample was then passed through the katharometer and its hydrogen content determined with appropriate corrections for the C02 content. This was compared with the value predicted by the formula on the basis of the F.R.A. volume (separately determined), hydrogen concentration and the volume of air inspired (corrected to 370 C., with a constant deduction of for subject's dead space). A number of paired experiments were performed with normal and with maximal inspirations. They are tabulated in Table 1. Inspiration of hydrogen. These experiments were all performed on a single subject. At first they were carried out without preliminary oxygen breathing. The seated subject, quietly breathing air, applied himself to a mouthpiece (dead space ) and at the end of a normal expiration switched to a enedict-roth spirometer and took from it a measured ibpspiration of hydrogen of % purity. A subsequent alveolar sample was collected, and its hydrogen content estimated by the katharometer. The transient anoxia resulting, especially from the larger inspirations, caused no subjective inconvenience. However, it was appreciated that the effect of even a relatively small inspiration of hydrogen on oxygen and CO2 tension-gradients across the alveolar membrane-diminishing one and increasing the other-would be slightly to increase the alveolar air volume by raising the n.q.,

3 72 G. H. ARMITAGE AND W. MELVILLE ARNOTT thus there would be a tendency towards dilution of the alveolar hydrogen. Further experiments were therefore done, as before, but with preliminary oxygen-breathing, thus ensuring an adequate alveolo-capillary oxygen gradient throughout the experiment. Experiment8 u8ing nitrogen When the nitrogen content of the F.R.A. is sufficiently lowered by preliminary breathing of a high oxygen mixture, then a subsequent inspiration of pure nitrogen becomes in effect an inspiration of foreign gas and the alveolar distribution may be followed with the same facility as that of hydrogen. An additional advantage is an accuracy of analysis of 0.05% using the Sleigh modification of the Haldane gas analysis apparatus (Sleigh, 1937). The apparatus was arranged as in the first group of hydrogen experiments (inspiration of air into alveolar hydrogen air mixtures). Circuit A was completely filled-after four or five preliminary rinsings-with pure oxygen. The subject was switched into circuit at the point of maximum expiration so as to introduce the minimum quantity of nitrogen. He then breathed quietly for 4 or 5 min. with the volume kept constant by an inflow of oxygen. The subject then detached himself from the circuit at the F.R.A. level or above (the exact point could be read from the spirometer tracing) and, with or without a preliminary complete expiration and alveolar sampling, drew a measured inspiration-the maximum possible from the starting level-of nitrogen, from circuit. The subsequent expiration terminated with the collection of an alveolar air sample. When the nitrogen inspiration was taken from the resting respiratory level or above, a sample from the equilibrium mixture in the first spirometer was regarded as representative of the initial alveolar nitrogen content; otherwise, expiration and sampling preceded the nitrogen inspiration. Direct comparison revealed no disagreement between samples taken in these ways. -RESULTS Table 1 gives the results of diluting intrapulmonary mixtures of air/hydrogen with air. There is with both small and large inspirations a measure of agreement between observed and predicted values for hydrogen content as close as can reasonably be expected. TALE 1. Hydrogen Dilution experiments Predicted final alveolar H2 % = Equilbrium H2 % x F.R.A. -F.R.A. + volume inspired Volume air inspired Equi (1.) Alveolar H2 % librium ( dead space -, Subject H2 % to be deducted) Observed Predicted Differences W.M.A. (F.R.A. = ) X G.H.A. (F.R.A.= ) H.G.C. (F.R.A. =2-521.) The results of hydrogen inhalations are plotted graphically in Fig. 1. A statistical analysis was made by listing the extent to which.each observa-

4 ALVEOLAR VENTILATION IN HYPERPNOEA 73 tion differed from the predicted value. The mean of this unique sample of differences did not differ significantly from zero, the value of P being < 02 and > 0 1 (Fisher, 1944) /x- _~~~~~~~~~~~ x 2e0 _~~~~~~ x w ~ ~ x Fig. 1. Inspirations of H2 from resting respiratory level. Comparison of observed with predicted values for alveolar H2 concentration. 0 observed concentration with preliminary 02 breathing; x observed concentration with preliminary air breathing. Curve represents concentrations predicted from formula. Abscissa: percentage H2 in final alveolar sample. Ordinate: volume of H. inspired (1.). Subject: G.H.A. The fact that the hydrogen concentrations with small breaths tend to be greater than the predicted values suggests that the dead space may be exerting at these levels an influence which disappears with larger breaths. This aspect is more fully considered in a later section of the paper. TALE 2. Experiments with inspiration of N2 to maximum capacity from various initial levels. (Subjects breathing 02 beforehand) Volume (1.) Initial inspired Observed Predicted lung Initial (dead space final final volume alveolar of Inspired alveolar alveolar Subject (1.) N2 % deducted) N2 % N2 % N2 % Differences W.M.A W.M.A G.H.A * G.H.A G.H.A * *80 G.H.A * G.H.A G.H.A

5 74 G. H. ARMITAGE AND W. MELVILLE ARNOTT Inhalations of nitrogen give data which are tabulated in Table 2,' together with the deviation of each from the predicted value. The overall result of these experiments is to confirm the hydrogen observations in suggesting that no significant operation of 'reserve alveoli' is involved in the taking of a. deep breath. Fractional analysis of ex&pired. air The foregoing experimental results seemed clearly against the occurrence of important alterations in alveolar ventilation pattern in deep as compared with quiet breathing. This conclusion conflicted with no established opinion. ut a second more controversial consequence was inherent in the same results, for these appeared to suggest that alveolar ventilation was uniform. This was contrary to the conceptions of Krogh & Lindhard (1917) on the one hand, and Sonne (1934) and Roelsen (1938 and 1939) on the other; for they, by the application of fractional sampling techniques, had independently shown that endexpiration alveolar samples almost always contained proportionately the least admixture of the preceding inspiration. These workers of otherwise divergent views were united in accepting this as evidence of relative under-ventilation of the deeper alveolar air; and unless the present findings could be reconciled with theirs any conclusion from them would be suspect. The remaining section represents an attempt at such reconciliation. Single expirations are passed through a fractional sampler which collects five to six successive small aliquots, the bulk of the expiration passing into a recording spirometer. The collection of each sample actuates a signal on the spirometer tracing enabling the correlation, of the sample with the phase of expiration (Armitage, Arnott & Pincock, 1949). Preliminary experiments were made in which serial samples were taken from complete expirations following inspirations of hydrogen or helium. In general, the results obtained confirmed the Scandinavian reports in so far as a gradient was found indicating a higher concentration of the inspired gas in the earlier samples than in the later. However, it still appeared true that the end-expiration samples were of the predicted composition; in other words, the observed gradient appeared to represent diminishing surpluses of inspired gas rather than a spread aroundr a mean concentration found somewhere in the earlier part of the expiration, which is the concept implied in Roelsen's (1938) interpretation of similar findings. The conclusion that a gradient of gas concentrations is indicative of unequal ventilation is supportable only if it can be shown that the later samples of an expiration have concentrations deviating as much in one direction from the level predictable on the assumption of uniform mixing as do the earlier samples in the other direction. In other words, in using these gas-dilution methods the basic requirement should be borne in mind that all the gas inspired must be

6 ALVEOLAR VENTILATION IN HYPERPNOEA 75 accounted for in the expired and residual air; if higher values are found in some fractions then there must be correspondingly lower values in others so that the mean of the concentrations in all fractions equals the value that would result from uniform mixig. It follows, therefore, that if a diminishing concentration of an inspired reference gas is not due to terminal contributions from imperfectly ventilated alveoli, then the earlier specimens must contain a surplus of reference gas derived from the dead space which is filled by the final portion of the inspiration. Through this massive concentration must pass the earlier fractions of the expiration, resulting in a heavy 'contamination' with a resulting content bearing no simple relationship to the concentration in the alveoli whence they came. In the use of the 'uniform mixing' formula to predict end-expiration concentrations this difficulty was overcome by deducting an assumed volume for the dead space from the volume inspired and therefore from the total volume of which the inspiration forms a part. This procedure is quite legitimate as regards an end-expiration sample as by the time it passes through the dead space the latter is washed clear of the inspired gas and contains a mixture very similar to that of the final sample. The really important point is the relationship between the concentrations observed in the later samples and those predicted on the basis of uniform mixing. In the one example in which Roelsen (1938) makes this comparison the concentration in the final fraction is in fact some 2% lower than predicted, an observation at variance with the results reported earlier in this paper. A close scrutiny of his results shows that the lung volume used in calculating the perfect mixing value is derived from the volume of hydrogen inspired divided by the alveolar concentration of hydrogen obtained by averaging the second to the fifth of the six fractional samples. Such a procedure makes it inevitable that the concentration in the final alveolar sample will be less than the predicted mean. It is clear that in order to make any useful deductions from the relationship of the 'uniform mixing' and final alveolar concentrations of a diluting gas it is essential that the estimation of the lung volume into which the inspiration is received must be made by an independent method which is quite free from any possible error inherent in the fractional sampling technique. Our procedure avoids this fallacy as it depends upon the initial estimation of hydrogen or helium concentration in the lungs by breathing to equilibrium in a closed circuit. A method of demonstrating as well as correcting for the contaminating effect of the dead space is to wash in a known volume of diluent gas with a known volume of lung gas mixture ('equilibrium mixture') so as to ensure that the early fractions expired will not pass through pure diluent gas but through a mixture of closely similar composition to that in the lungs. Of course-taking an oversimplified illustration-contamination of 500 c.c. of alveolar air con-

7 76 G. H. ARMITAGE AND W. MELVILLE ARNOTT taining say 20% reference gas by passage through a dead space of 200 c.c. of air which it washes out by 50% will only reduce the reference gas concentration to 16%, whereas expiration through the same dead space filled with pure reference gas will raise its concentration to 33 %. Therefore the procedure of completing the test inspiration of reference gas with air cannot be expected to alter the predicted concentration to the same extent as expiration through pure reference gas will elevate it, unless alveolar concentrations are of the order of 50%. Although this 'washing-in' procedure is crude it is capable of determining in a qualitative way the influence of the dead space on the successive fractions of an expiration. METHODS The arrangement of apparatus was very similar to that employed in the first section of this report with the addition of the fractional sampler and a three-way valve, so that switching from the closed to the open circuit could be effected without change of mouthpieces as in the earlier experiments. Helium instead of hydrogen was used as the reference gas. Inspiration of air into lungs filled with a helium/air mixture has the very substantial advantage of enabling an exact measurement to be made of the lung volume into which the air inspiration is taken. Furthermore, this arrangement is much more economical of the expensive gas helium than inspirations of pure helium into lungs filled with air. The helium content of these samples was measured in the katharometer after removal of CO2 by bubbling through caustic soda and lime water. A portion of the sample was used for determination of the C02 content by the Haldane method. The helium percentage was corrected for the removal of the C02. In some experiments the inspiration of air from spirometer was washed in by a quantity of air/helium mixture from spirometer A. RESULTS The results of one experiment are reported in detail, each stage in the calculation being marked by a reference letter to simplify tabulation of the results of the group (Table 3). Subject: W.M.A. Large inspiration of air into lungs filed with air/helium. Air not washed in with air/helium mixture. Helium percentage in lungs in equilibrium with spirometer 'A' = 6-45 % (a). Volume of mixture in lungs at instant of switching to spirometer '' = (b). Volume of air inspired (less for anatomical dead space and less for dead space of mouthpiece and 3-way valve) = (c). Volume of air/helium mixture used to wash in air inspiration = Nil. (d). Total volume in lungs at end of inspiration= (e). Predicted helium concentration assuming uniform mixing 65 8 =4-17 (f)

8 ALVEOLAR VENTILATION IN HYPERPNOEA Fractional analysis of expiration. Sample Litres expiration (A) % He () If 1 1. of 4-17 % air/helium mixture passes through dead space air and emerges as 1 1. of gas containing 3-50 % helium, then the amount of air/helium mixture which has been exchanged for air to effect this alteration is l 0~~ 1 00 x = Making this calculation for each of the five fractions and summing the results the total surplus of air in the expiration becomes This surplus is derived from a total dead space of made up of the dead space quoted above plus the additional dead space between the 3-way valve and the fractional sampling point. The calculation is very approximate as the figure for helium concentration is from the end of each fraction and this introduces a considerable error particularly in the first sample. There does not seem to be any simple method of interpolating the mean concentration of this first sample owing to ignorance of the mean velocity of the gas stream and in any case the impossibility of applying any mathematical treatment to flow through a tube of such complex internal configuration as the anatomical dead space. (a) (b) (c) G.H.A. 'Control' W.M.A W.M.A ' W.M.A G.H.A G.H.A C.T.G.F W.M.A G.H.A G.H.A G.H.A i-1 TALE 3 (d) (e) (f) A A A A A A A A A A A 'Surplus' ' -

9 78 G. H. ARMITAGE AND W. MELVILLE ARNOTT As the main theme of the paper is a study of mixing in a large inspiration, in Table 3 are emphasized the critical observations which show close agreement between observed values and those predicted on the basis of uniform mixing, irrespective of the size of inspiration and irrespective of whether or not the inspiration of diluting gas (air) is washed in by the same mixture (air/helium) as fills the lungs prior to the test inspiration. It is clear from Table 3 that there is a slight diminishing gradient of diluting gas concentration (air) to the final sample when no washing-in is done, but the fact that the final concentration does not fall below the uniform mixing value suggests that this gradient is due to a surplus of air which can be derived only during expiration through the air-filled dead space. Calculation of the excess air represented by the gradient (although only very approximate) gives values compatible with such an origin. In this respect the control experiment in which no air is inspired gives a very slight gradient with an air surplus of only 58 c.c. which seems dependent on the exposure of this expiration to.contamination by air during passage through the fractional sampler dead space of 80 c.c. between the 3-way valve and the fractional sampling point. The last four washing-in experiments show, in general,.an obliteration of the gradient, although there is no reversal for the reason stated in the introduction to the final section; furthermore, although the expiration now passes through the main dead space filled with air/helium there is still the sampler dead space of 80 c.c. filled with air. DISCUSSION Much work has been done on the intrapulmonary mixing of gases, a very full review of which has recently been provided by Rauwerda (1946). One of the earliest contributions was that of Grehant (1864) who studied the distribution in expired air of inspirations of a known volume of hydrogen; he found the hydrogen content of expired air to be independent of the depth of expiration and concluded that there was uniform ventilation of the lungs.. Siebeck (1911) found essentially similar results except that uniformity was only reached after the expulsion of approximately 400 c.c. of. air. Krogh & Lindhard (1917), while investigating the size of the dead space, used inspirations of pure hydrogen and found that the last sample taken during expiration invariably contained least hydrogen and concluded that there was a definite but slight inequality in the mixing. Sonne (1934) and Roelsen (1938, 1939), using fractional sampling of expired air after inspirations of hydrogen, also found that the concentration fell steadily to the level of the end-expiration sample. On the other hand, Darling, Cournand & Richards (1944), on the basis of a study of the progressive washing out of lung nitrogen by inhalations of oxygen, conclude that alveolar ventilation is uniform in the majority of normal subjects. There seems little room for argument that our results exclude the operation of any 'reserve alveolar' mechanism with large as opposed to normal breaths.

10 ALVEOLAR VENTILATION IN HYPERPNOEA 79 While the incidental evidence gleaned from fractional analysis confirms above-quoted reports in that it shows a gradient of diluting gas falling to a minimal value towards the end of expiration, nevertheless, it is considered that this is not necessarily evidence of unequal ajveolar ventilation but is an effect of the dead space. Darling et al. (1944) may have been thinking along similar lines when they point out the curious anomaly in Roelsen's results that when he used hydrogen, the most miscible gas of all, 'these experiments showed a greater degree of imperfect mixture than those using air (possibly) indicating... that hydrogen diffused into stagnant air spaces not emptied by a forced expiration'. In effect this means that the washing out of the dead space takes a great deal more gas than the variously estimated c.c. While a volume of this order removes most of the dead-space gas it is maintained that a small portion remains which exerts a diminishing influence on subsequent samples-an influence which, although well nigh imperceptible when the difference is between 21 and 15 % oxygen, is obvious when the dead space contains pure hydrogen. We are engaged in applying these techniques to the ventilating problems of cardio-respiratory disease. The observations of Cournand, aldwin, Darling & Richards (1941) on the much diminished rate at which the inhalation of oxygen can replace nitrogen in emphysematous lungs make it likely that a significant difference between observation and prediction will be found in that disease. SUMMARY 1. ID, order to determine if voluntary large breaths are accommodated in the same alveoli used during quiet breathing the concentrations of hydrogen/ nitrogen/helium and air in alveolar gas following inspirations of varying volumes of these gases were studied. 2. The observed concentrations did not differ significantly from those predicted on the assumption that the large breaths entered the same alveoli. 3. Fractional sampling revealed a gradient of inspired gas concentration falling to the level predictable on the basis of uniform mixing as expiration ended. 4. The magnitude of the surplus of inspired gas represented by this gradient is compatible with the assumption that it is derived during expiration from the dead space air (that is, a large expiration is needed to wash out the dead space). We gratefully acknowledge the help and loan of a katharometer from Dr G. A. Shakspear, the receipt of an expenses grant from Messrs Imperial Chemical Industries Ltd., the assistance of Dr A. G. W. Whitfield, himself engaged on cognate problems, and the never-failing skill and industry of A. 0. Pincock, Senior Technician to the Department.

11 80 G. H. ARMITAGE AND W. MELVILLE ARNOTT REFERENCES Armitage, G, H. & Arnott, W. M. (1949). J. Physiol. 109, 64. Armitage, G. H., Arnott, W. M. & Pincock, A. C. (1949). J. Phy8iol. 108, 27P. Cournand, A., aldwin, E. De F., Darling, R. C. & Richards, D. W., Jr. (1941). J. clin. Invest. 20, 681. Darling, R. C., Cournand, A. & Richards, D. W. (1944). J. din. Invest. 23, 55. Fisher, R. A. (1944). Statistical Methods for Research Workers, 9th ed. chap v. Edinburgh: Oliver and oyd. Gr6hant, N. (1864). J. Anat., Paris, 1, 523. Krogh, A. & Lindhard, J. (1917). J. Physiol. 51, 59. McMichael, J. (1939). Clin. Sci. 4, 167. Rauwerda, P. E. (1946). Unequal Ventilation of Different Parts of the Lung. M.D. Thesis, Groningen. Roelsen, E. (1938). Acta med. Scand. 95, 452. Roelsen, E. (1939). Acta ned. Scand.98, 141. Siebeck, R. (1911). Z. iol. 60, 267. Sleigh, S. W. (1937). J. Soc. chem. Ind., Lond., 56, 430 (Trans.). Sonne, C. (1934). Z. ges. exp. Med. 94, 13.