partial pressure is to be applied to the dissociation curve of fully oxygenated

Transcription

1 6I2. I27. I THE DETERMINATION OF THE CARBON DIOXIDE CONTENT OF THE MIXED VENOUS BLOOD. Part I. The effect of oxygenation and the critical oxygen tension. BY M. C. G. ISRAELS (Platt Physiological Scholar) AND F. W. LAMB. (From the Physiology Department, University of Manchester.) THE method of Henderson and Prince(9) for determining the C02 partial pressure in the mixed venous blood by a series of repeated rebreathings from a bag containing a suitable gas mixture has been modified in various details by subsequent workers (e.g. Meakins and Davies; Field, Bock, etc.). In any such method the maintenance of the oxygen partial pressure in the rebreathing bag at a level that ensures full oxygenation of the blood entering the lungs when the gas is inhaled at each rebreathing is a condition of outstanding importance, if the CO. partial pressure is to be applied to the dissociation curve of fully oxygenated blood for the purpose of ascertaining the C02 content. If the venous blood is not fully oxygenated, the C02 partial pressure will be lower and the C02 content read from the dissociation curve will be less than the actual value. Such methods are based on the observations of Douglas and Haldane(4) who verified in vivo the effect of oxygenation on the C02 combining capacity of the blood which was established in vitro by Christiansen, Douglas and Haldane(2). Douglas and Hal dane give data from which it can be shown that the same value for the arterio-venous C02 difference in volumes per 100 c.c. blood is obtained whether it is computed on the basis of true venous or oxygenated venous bloods. The errors caused in the values of C02 partial pressure and content caused by insufficient oxygenation may not be large if the unsaturation is not great; but when these data are used in the calculation of the minute volume of the heart's output the influence of such errors becomes greatly enhanced. It is of interest to see what effect an error of known magnitude in the determinations required for the measurement of the minute volume by the Fick principle will have on the final result. PH. LXVII. 4

2 50 M. C. G. ISRAPLS AND F. W. LAMB. The variables to be considered are: (1) The C02 pressure difference between arterial and venous bloods, the venous blood being supposed to be fully oxygenated without change of its contained C02. (2) The arterio-venous CO2 difference measured in volumes per 100 c.c. blood. (3) The C02 output per minute. (4) The minute volume. If p = the arterio-venous C02 pressure difference in mm., v = the arterio-venous difference in volumes p.c., c = the C02 output per minute in c.c., m = the minute volume of the heart in c.c., then, from Fick's principle, mv= looc... (1). Now there are to be considered absolute and relative errors. The absolute errors are directly proportional to the magnitude of the minute volume. Relative errors are, however, more complex and must be considered further. Relative errors in the determination of the arterio-venous C02 pressure difference. The first is that in reading the volumes p.c. corresponding to a given C02 pressure. This can be neglected if the C02 dissociation curve of the subject be accurately known. The well-known curve determined by Christiansen, Douglas and Haldane(2) shows that between 38 mm. and 60 mm. of C02 the relation between volumes p.c. and C02 pressure is practically a straight line. This line approximates closely to the equation 2. Y -5 (X+ 100) (2). When considering differences only, this can simply be written 5Ay = 2Ax, or, using the special symbols given above, 5v= 2p (3). This gives the relation that, between the limits specified, a difference of 1 mm. in the arterio-venous C02 pressure corresponds to a difference of 0-4 volumes p.c. C02. In order to consider the error in the minute volume caused by an error of a known magnitude in the determination of the arterio-venous

3 CARBON DIOXIDE IN BLOOD. C02 pressure difference, take the CO2 output to be a constant = k/100; then from equation (1), mv = k... (4). Let n mm. = the error made in determining the arterio-venous C02 pressure difference. From equation (3) this corresponds to an error of 2n volumes of C02 p.c. The uncorrected minute voluime (M) will be given by M (v+ ) k...(5). = The percentage error (P) in the minute volume caused by such a change is given by p= M-m.100. Substituting from equations (4) and (5) this becomes [ 5V+= - v] k n 5v + 2n Therefore P (5v + 2n) + 200n = 0... (6). Or, expressing this in the usual coordinates, x (5y + 2n) + 200n= 0... (7). The curves for various positive and negative values of n corresponding to this equation are given in Fig. 1. Note that a positive error in the arterio-venous pressure difference causes a negative error in the minute volume, and vice versa. In deriving these equations, k, the constant C02 output, has been eliminated. Hence, the above equations hold for any value of C02 output, and, consequently, for any magnitude of minute volume, so long as x expresses relative (percentage) errors. For example, at an arterio-venous difference of 4 volumes p.c., an error of + 1 mm. causes a decrease in the minute volume of 9*1 p.c., and an error of - 1 mm. causes an increase of 11 1 p.c. over the correct value. It is also possible to draw a curve to show the relation of the error (n) made in determining the arterio-venous C02 pressure difference to the percentage error (P) so caused in the minute volume, at any one value of the arterio-venous C02 difference (v). A rearrangement of equation (6) gives 1 +5v 1 0 P +0n+ loo=

4 52 M. C. G. ISRABLS AND F. W. LAMB. Or, expressing the variables in the usual coordinates, 1 5v 1 1 x *y =o These curves for various values of v are given in Fig. 2. Arterio-venous C02 difference. Vols. per 100 c.c....(8) Percentage error in minute volume Fig. 1. The relation between the percentage error in the minute volume of the heart and the arterio-venous CO2 volume difference, for different values of the error made in determining the arterio-venous C02 pressure difference (n in mm. Hg). Curves drawn to equation (7) in text. Relative errors in the determination of the C02 output. These are directly proportional. An error of 1 p.c. in measuring the C02 output will cause a 1 p.c. error in the value for the minute volume. The foregoing shows that it is of paramount importance in the determination of the minute volume that great care should be taken to eliminate or reduce as far as possible all sources of error in measuring the difference in C02 partial pressures of the arterial and venous bloods; especially at low arterio-venous differences, such as exist at rest. However, the relative error rapidly decreases when the arterio-venous difference increases, as it does during exercise: this is fortunate as the

5 CARBON DIOXIDE IN BLOOD. minute volume in exercise is greatly increased, making all errors tell, absolutely, to a greater extent. Error in determination of arterio-venous CO. pressure difference, mm * Percentage error in minute volume *! ~ ~~ ~~ :5 ~~~~~2-5 Fig. 2. The relation between the error in the determination of the arterio-venous CO2 pressure difference and the percentage error in the minute volume of the heart, for different values of the arterio-venous C02 volume difference (v in volumes p.c.). Curves drawn to equation (8) in text. The curves also show clearly that it is better to err on the side of making the arterio-venous C02 pressure difference too large rather than too small. This renders more important the proper oxygenation of the venous blood, as failure to carry this out properly leads to a too small arterio-venous C02 pressure difference. In view of the foregoing the question at once arises as to what is the minimal pressure of oxygen permissible in the rebreathing bag to ensure full oxygenation of the blood during the rebreathing. Henderson and Prince(9) and Meakins and Davies(12) used expired air in the rebreathing bags. Field, Bock, Gildea and Lathrop5 and Brocklehurst, Henderson and Haggard(l) used mixtures of oxygen and C02; the former authors expressing themselves of the opinion that 150 mm. (19.7 p.c.) of oxygen, the final oxygen tension in their method, was sufficient fully to oxygenate the blood, and they give evidence to show that the blood taken from an artery during the last 5 sec. of a rebreathing period was 99 p.c. oxygenated.

6 54 M. C. G. ISRAILS AND F. W. LAMB. Dautrebande(3) utilising the method of Meakins and Davies pointed out the error this method would lay itself open to if the blood were not fully oxygenated. He showed that after several rebreathings from a bag initially filled with air, following the attainment of equilibrium, the CO2 partial pressure in the lung-bag system began to fall; and that by now adding oxygen to the gas in the rebreathing bag, the C02 partial pressure could be brought back to its former level. From experiments on this question he concluded that the critical point below which the oxygen tension must not be allowed to fall was 85 mm. (11.2 p.c.); and in an experiment on himself, showed that with oxygen tensions from 333 mm. (43.4 p.c.) to 94 mm. (12.4 p.c.) the final C02 tension was approximately constant. Results on three subjects are reported. Dautrebande also found that in some subjects it was a matter of indifference, as far as the final C02 tension was concerned, whether air or a mixture containing a higher percentage of oxygen were used. He showed that with such subjects the oxygen remaining in the rebreathing bag at the end of the experiment was more than 11 p.c. He states finally that as the critical point varies from subject to subject, it is best to fill the bag with a mixture rich in oxygen, so that the oxygen tension shall be mm. (14-15 p.c.) at the end of the experiment. The experiments undertaken by the writers on this point fall under two headings according to the method used. In the first place it was found that with methods such as that of Henderson and Prince, the oxygen in the rebreathing bag fell very rapidly and that it was consequently impossible to carry out a sufficient number of rebreathings to allow of proper C02 equilibrium being reached and, at the same time, to conserve a high final percentage of oxygen in the rebreathing mixture. A modification of the Henderson-Prince method was therefore devised, in which the subject washed out his lungs with a mixture of suitable oxygen and C02 content before rebreathing. The apparatus is shown in Fig. 3. It enables the subject of an experiment to breathe into or out of any of three circuits at will without changing his position or -emoving his mouth from the mouthpiece. It consists of a brass slider (S) in which there are three circular apertures, provided with rims for the attachment of tubing. The slider fits into a holder which has one central aperture into which a glass tube is fitted, this acting as the mouthpiece (M). The slider is moved along until.the aperture in it, to which is attached the required circuit, is opposite the mouthpiece. Correct alignment is facilitated by a spring catch (C), a projection on which fits into holes in the slider under each aperture. The screws (P)

7 CARBON DIOXIDE IN BLOOD. act as stops, since, as they cannot be moved past C, they prevent the slider being moved too far. 55?177.' A Fig. 3. Right: slider apparatus for changing breathing circuit. Left: rebreathing bag and connectionis to slider. The holder is made of sufficient length to close off the two apertures not in use. The opposing surfaces of the holder and slider are well greased with vaseline, to allow of easy movement and to render the apparatus airtight. The small dead-space of the apparatus gives it a great advantage over a series of taps for respiratory work. The apparatus is connected up as follows: the right-hand tube (R) is connected by means of a one-way tap to a D o u g las bag which acts as a reservoir and is filled with a suitable gas mixture. The centre aperture (A) is left open. The left-hand tube (L) is connected to a rebreathing bag (B). This is a 41 litre rubber bag provided with a wide rubber tube about 35 cm. long (D): between the bag and the slider is a one-way tap,

8 56 M. C. G. ISRARLS AND F. W. LAMB. and also, as near to the slider as possible, side tubes (T) through which samples can be taken; these are closed with glass rods when not in use. The rebreathing bag is filled with 4 litres of gas mixture. In making a determination, the taps are first opened. The subject being comfortably settled, applies his mouth to the mouthpiece and makes a deep expiration through A to the outer air. The slider is then moved to R; a deep breath of the gas mixture is inhaled from the reservoir and expelled through A. The subject now inhales deeply from the rebreathing bag, through L, holds his breath for a few seconds and expires forcibly into the bag, the slider being moved back to the A position immediately on completion of the expiration. The whole procedure, from the first expiration to air to the last into the bag, can be accomplished in 15 sec. with ease and, in any case, should not be longer than 20 sec. The procedure, with intervening periods of rest of not less than 1 minute's duration, is repeated six times. At the fifth and sixth rebreathings samples of the last of the air expired into the bag are taken through one of the side tubes into vacuous gas samplers. Although this apparatus has been described as used for determining the CO2 tension of mixed venous blood, it would be of use in any form of experiment where rapid changing from one respiratory circuit to another is required. The employment of mixtures of 94 p.c. oxygen and 6 p.c. C02 both in the rebreathing bag and in that for washing out the lungs made it possible to carry out six rebreathings without the oxygen in the rebreathing bag falling below 60 p.c. By using suitable mixtures of oxygen, nitrogen and C02, the last-named being always in the proportion of 6 p.c., the final oxygen tension in the rebreathing bag could be approximately controlled. In practice, mixtures containing 94 p.c., 50 p.c. and 25 p.c. oxygen were used for the lung washing. The mixtures utilised in the rebreathing bag were either 6 p.c. C02 in 94 p.c. or 50 p.c. oxygen, or in air, a fresh sample of a particular mixture being used for each determination; or else starting with 94 p.c. oxygen, samples were taken after the fifth and sixth rebreathings, and then, after allowing the subject a short rest, the same mixture, now containing only about 60 p.c. oxygen, was used for a further period of six rebreathings in conjunction with a suitable washing mixture, in this case 50 p.c. oxygen: this procedure was repeated until the oxygen in the rebreathing bag became reduced to about 15 p.c. In this manner the effect of oxygen tensions at intervals from 60 p.c. of an atmosphere downwards could be tested. Further experiments were carried out which omitted the intermediate

9 CARBON DIOXIDE IN BLOOD. oxygen pressures and consisted only of estimating the C02 tension of the mixed venous blood with 94 p.c. oxygen and with air in the rebreathing bag; in the latter case the lung washing was dispensed with as it was unnecessary. The former determination gave a final oxygen tension of about 60 p.c. and the latter about 15 p.c. of an atmosphere. The results of some of these experiments are given in Table I below: TABLE I. Final oxygen Venous C02 tension tension Date p.c. atmosphere mm. Subject 4. iv M. C. G. I iv to * iv (approx.) 45-6,, 34 45X B vi (approx.) VP 28. vi (approx.) to 27. iv * C. E. B. From the results of these experiments it would seem that the oxygen tension in the air inspired from the rebreathing bag can be allowed to fall to about 26 p.c. without causing a lowering of the C02 tension of the mixed venous blood due to oxygen unsaturation of the blood; but that such an effect will be brought about if the oxygen tension falls as low as 15 p.c. of an atmosphere. The differences between the C0g pressures as determined with high or low oxygen pressures in the rebreathing bag, though small, were always in the same direction. In the above experiments only two subjects were used. In order to examine more subjects and at the same time apply these findings to those methods which follow the recommendation of Laurens(lo) in not holding the breath, but breathing to and from the bag during the rebreathing period, a second set of experiments were carried out. The C02 tension of the mixed venous blood was determined by a method similar to that of Meakins and Davies(n2). The apparatus consists simply of a rubber rebreathing bag as used before, with a twoway tap at the end of the 35 cm. tube and with a further short length of 57

10 58 M. C. G. ISRAfLS AND F. W. LAMB. rubber tube beyond this fitted with side tubes for sampling, and a glass mouthpiece. The subject rests in a chair for at least 15 mi. Then, with the nose clipped, he applies his mouth to the mouthpiece and expires as deeply as possible to air via the two-way tap. The tap is now turned to connect the mouthpiece with the rebreathing bag, which contains 4 litres of the required mixture. The subject breathes deeply three times in and out of the bag in about 12 sec., finishing with a deep expiration into the bag, when the tap is again turned to cut off the rebreathing bag. This procedure, with intermediate rest periods, is repeated six times in all. Samples of the last of the air expired into the bag at the end of the fifth and sixth rebreathings are taken into vacuous gas samplers through the side tubes provided. A sample of the gas in the bag can be taken through a convenient tube. It was found that, starting with 94 p.c. oxygen in the rebreathing bag, the procedure reduced this finally to about 26 p.c. The experiments with each subject were therefore two determinations of the C02 tension of the mixed venous blood within a few minutes of each other; one starting with a mixture of 94 p.c. oxygen and 6 p.c. C02 in the rebreathing bag, and one with air and 6 p.c. C02 in the bag; the experiments being performed in varying sequence. Some representative results are given in Table II below: TABLE II. Final oxygen Venous C02 tension tension Date P.C. atmosphere mm. Subject 13. vii M. C. G. I vii C. E. B. 14* ix P. J. C ix B5 17. ix F. S. W ix * ix B. F ix * The results show that the venous C02 tension is higher in the subjects examined when the final oxygen tension in the rebreathing bagis 26 p.c.

11 CARBON DIOXIDE IN BLOOD. than when it is allowed to fall to 14 p.c.; the difference varies from *0 mm., but the actual magnitude of the difference is of less importance than the fact that the change is always in the same direction. As a consequence it will be seen that, in order to avoid any effects of insufficient oxygenation of the venous blood in methods of this type, it is necessary to start the experiment with a gas mixture in the rebreathing bag consisting only of oxygen and C02. Further evidence can be provided by an examination of the known data of the blood with regard to this effect of oxygenation. When air is inspired, it is diluted by the air remaining in the lungs and respiratory passages. Haldane(6) gives a table showing the relation between oxygen pressure in the inspired air and that in the alveolar air: some of these figures are reproduced in columns 1 and 2 of Table III. In column 3 are given the figures for percentage oxygen saturation of the blood at these alveolar oxygen tensions. Now from the C02 dissociation curve the difference in tension exerted in the fully oxygenated and fully reduced blood by a given C02 content can be determined. It may be assumed that if these tensions = o and r respectively, then the C02 tension at any intermediate value of oxygen saturation, say p p.c., will = r + jjo (o - r). The C02 tensions thus calculated corresponding to representative values for venous blood, at the degrees of oxygenation given in column 3, are shown in columns 4 and 5. The oxygen dissociation curve used is that given by Haldane (7): it is for blood in the presence of 40 mm. C02, consequently the effective oxygen pressures will be a little lower than the figures given in column 3, according to the actual C02 tension of the venous blood. The C02 dissociation curve used is taken from the same source (8). TABLE III. Oxygen pressure Pressure exerted by P.C. atmosphere a C00 content of p.c. oxygen Inspired Alveolar saturation 55 vols vols. air air of the blood P.C. (mm.) P.C. (mm.) X1 The table shows that the differences are of a similar order to those determined experimentally and emphasises the necessity of using an 59

12 60 M. C. G. ISRAALS AND F. W. LAMB. oxygen C02 mixture for rebreathing experiments of the type under consideration. It should also be noted that Meakins and Davies(ni) found that with normal subjects breathing ordinary air (20.93 p.c. oxygen) the arterial blood was 95 p.c. saturated with oxygen, and that breathing a mixture containing only 12-6 p.c. oxygen the saturation was reduced to 81-4 p.c. The deeper breathing in rebreathing experiments probably allows of better equilibrium being reached and therefore slightly higher saturation with oxygen ensues. No attempt was made to assign experimentally any value to the "critical point," i.e. that tension below which the oxygen in the rebreathing bag must not be allowed to fall. Owing to the shape of the oxygen dissociation curve the reduction of C02 tension (assuming the C02 content to remain constant) with falling oxygen tensions starts very gradually, so gradually as to be undetectable by any methods which are applicable to the human subject. The experimental data show that this "critical point" may be lower than 26 p.c. of an atmosphere (198 mm.) but is certainly higher than 14 p.c. (106 mm.). This, and the data in Table III, incline the writers to take 25 p.c. atmosphere (190 mm.) as a representative value for the critical point; so long as the final oxygen tension in the lung-bag system is not less than 25 p.c. of an atmosphere, the blood may be assumed, for all practical purposes, to be fully oxygenated. The results obtained differ considerably from those of D a ut r e b and e already quoted in indicating a much higher critical oxygen tension, 190 mm. as against 85 mm., or 120 mm. It is difficult to see why Dautrebande obtained such low results; reference to Table III shows that even at his safety point, 120 mm. (15.8 p.c.), there is sufficient oxygen unsaturation to cause a definite fall in the C02 tension of the mixed venous blood, and in this table it has been assumed that complete equilibrium between the lung and blood gases is attained. The results of Meakins and D a vies (11) given above point to a similar conclusion. SUMMARY. 1. Attention is drawn to the effects of insufficient oxygenation of the lung blood in rebreathing experiments for determining the C02 content of the mixed venous blood, and to their import in the application of the values obtained to the calculation of the minute volume of the heart's output.

13 CARBON DIOXIDE IN BLOOD. 2. An analysis is made of the absolute and relative errors in the derived values. 3. A method for carrying out rebreathing experiments without a very rapid lowering of the oxygen tension in the rebreathing bag, and a method for rapid changing from one respiratory circuit to another are described. 4. As a result of these, and a second series of experiments, it has been found that to ensure full oxygenation of the mixed venous blood the oxygen tension in the lung-bag system should not be allowed to fall below 25 p.c. of an atmosphere. 5. The necessity of beginning any series of rebreathing with a mixture of oxygen and C02 is demonstrated. The use of air and C02, or still more so, expired air, leads to errors in the values calculated for the minute volume of the heart which range up to 50 p.c. in the resting state. 6. Data from the known properties of blood are given to support this view. REFERENCES. 1. Brocklehurst, Henderson, and Haggard. Am. J. Physiol. 82. p Christiansen, Douglas and Haldane. This Journ. 48. p Dautrebande. Arch. Internat. de m6d. Exp6r. 2. p Douglasand Haldane. ThisJourn. 56. p Field, Bock, Gildea and Lathrop. J. Clin. Invest. 1. p Haldane, J. S. "Respiration," New Haven, p Haldane, J. S. Ibid. p Haldane, J. S. Ibid. p Henderson and Prince. J. Biol. Chem. 32. p Laurens. Am. J. Physiol. 46. p Meakins and Davies. J. Path. and Bact. 23. p Meakins and Davies. Heart, 9. p