blood-pressure as determined on the two arms are less than the experimental King, 1937].

Transcription

1 THE DIFFERENCES BETWEEN BLOOD-PRESSURE MEASURE- MENTS OBTAINED SIMULTANEOUSLY ON THE TWO ARMS.' By NATHAN W. SHOcK and ERIC OGDEN. From the Division of Physiology of the Medical School and the Institute of Child Welfare, University of California, Berkeley. (Received for publication 6th December 1939.) INTRODUCTION. ALTHOUGH differences in the arterial blood-pressure at different points of the circulatory system are known to exist, it is generally believed that in normal persons at rest in the supine position, differences in blood-pressure as determined on the two arms are less than the experimental error of the measurement. Consistent major differences between blood-pressure measurements on the two arms have, in fact, long been recognised as indication of the possibility of a deformity in or around one of the larger arteries [Osler and McCrae, 1935; Hills, 1938; King, 1937]. Moreover, in studies of clinically normal persons small differences in pressure measurements have been reported by Hensen [1 900], Bing [1906], Fischer [1924], Geisbdck [1905], Hare [ ], Hirsch [1901], McWilliam and Kesson [ ], Jellinek [1900], Bodenstab [1925], Kilgore [1914], and more recently by Yoshino [1933], Kay and Gardner [1930], and Korns and Guinand [1933]. In a study of the reliability of measurement of blood-pressure determinations [Shock and Ogden, 1939], it has been found that in most normal persons a measurable difference exists between simultaneous measurements made on the two arms. The details of experimental procedure and examinations of some other aspects of the data have already been reported [Ogden and Shock, 1938; Shock and Ogden, 1939]. INCIDENCE AND MAGNITUDE OF DIFFERENCE. For each of 102 subjects the average values of blood-pressure for each arm were computed from 48 simultaneous pairs of measurements (16 by each of three observers working two at a time in rotation). 1 Grateful acknowledgment is hereby made to the Board of Research of the University of California for financial assistance in the collection of data, and to the Works Progress Administration (O.P Unit A-8) for statistical and clerical assistance. 155

2 156 Shock and Ogden The frequency distribution of the differences between these average values is presented in fig. 1, which shows that the average difference (j 20 Q14J ' o/ DIFFERENCE (Riqht - Left) mm 1-. FIG. 1.-Frequency distribution of mean difference between blood-pressure determinations made simultaneously on right and left arms in 102 adult males. systolic pressure differences; diastolic pressure differences. Vertical lines indicate mean of the distribution. (Systolic: 1-60 ± '36 mm. Hg; diastolic: *18 mm. Hg.) (R-L) is mm. for systolic pressure and for diastolic pressure.' The average value for right-arm measurements is higher in spite of one individual whose mean left-arm systolic pressure was > 40 (4 20 I' (4 Ia a /4 ABSOLUTE DIFFERENCE rm Hy. FIG. 2.-Frequency distribution of mean absolute difference (without respect to sign) between blood-pressure determinations made simultaneously on right and left arms in 102 adult males. systolic pressure differences; diastolic pressure differences. Vertical lines indicate mean of the distributions. (Systolic: 3-1 ± -23 mm. Hg; diastolic: 1-5 i -13 mm. Hg.) 12-5 mm. higher than his right. In fig. 2, which presents mean differences without respect to sign, it may be seen that the absolute mean 1 Throughout this paper standard error is used rather than P.E.

3 The Differences between Blood-Pressure Measurements difference in systolic pressure is 3*1 + *23 mm. Hg and that 44 per cent. of the subjects show differences greater than the mean value. Even without the special facilities used in this study a difference greater 2' ~20 N. a; /5 I 'A I / /0 -/5-20 CRITICAL RATIO FIG. 3.-Frequency distribution of critical ratios of differences between bloodpressure determinations made simultaneously on right and left arms in 102 adult males. critical ratios for systolic pressure differences; critical ratios for diastolic pressure differences. Vertical lines indicate the mean of the distributions. (Systolic: ; diastolic: ) 40: 4j20 -/< I \ lg 2$ CRITICAL RATIO FIGe. 4.-Frequency distribution of critical ratios of absolute difference between blood-pressure determinations made simultaneously on right and left arms in 102 adult males. critical ratios for systolic pressure differences; critical ratios for diastolic pressure differences. Vertical lines indicate the mean of the distributions. (Systolic: 7-9 ±.6; diastolic: ) Differences with critical ratios greater than 2-5 (see arrow) will occur by chance only 6 times in than 3 mm. should be measurable. The mean difference without respect to sign for the diastolic pressure is *13 mm., which indicates a significant difference in the average. In order to test the significance of these differences in individual subjects a distribution of the differences between simultaneous rightand left-arm measurements (R - L) was drawn up for each of the

4 158 Shock and Ogden 102 subjects. From these distributions of 48 differences the standard deviation of the mean difference and the standard deviation of the actual distribution were computed for each subject. The ratio between this mean difference and its standard error could be readily computed, giving a critical ratio for each subject.1 The frequency distributions of these 102 ratios are presented in figs. 3 and 4, which show that in 83 per cent. of the subjects the critical ratio was greater than 2-5 for systolic pressure and greater than 2 S in 52 per cent. of the subjects for diastolic pressure. Critical ratios as high as 38 were obtained in some subjects for systolic pressures if the direction of the difference is ignored. The average C.R. was for differences in svstolic pressures but only for differences in diastolic pressures. STABILITY OF THE DIFFERENCE. That the difference does not consistently increase or decrease during an experiment is shown in fig. 5. Each point on this graph is the mean 22C C, 2 i 6 6 l 2 Ii 6 'B 20i ' 26t 38 2 '4 36 ' '2 ' 4 46 '4St '50 Fia. ;.- Mean difference between systolic blood-pressure mearsurements made simultaneously on two arms. The dotted lines indicate ± hr of the distribution of differences for 102 adult males. This figure shows the absence of any systematic relationship between the3 serial positions of pairs of readings and their mean differences. See text, p value of one difference observation on each subject made at corresponding times during the experiment. Thus the first point is the average difference observed on the first pair of simultaneous observations on all subjects; the second point is the average difference on all subjects as observed 30 seconds later and so forth. Although the 1 The results were checked by computing each critical ratio by the following formula as well: C.R. =MnR -MnL 'gmnl + gmna,-2rrlamnmamonl

5 The Differences between Blood-Pressure Measurements 159 observers were changed after each 8 readings, the changes in mean difference values at these times are statistically insignificant; moreover, observations made by the three observers of this experiment have been shown to be without systematic difference [Shock and Ogden, 1939]. The average blood-pressure values were steadily falling during the first half of the experiment [Ogden and Shock, 1938], but the figure shows that this fall did not influence systematically the difference between the two arms. That the difference is independent of the time within the experiment and therefore of the temporal decrement of blood-pressure already discussed [Ogden and Shock, 1938] is further shown by the fact that the mean difference during the first half of the experiment was , and for the second Although the mean difference for the group was stable during the experiment, it appeared that the mean difference for all observations made on one individual during half an hour was not stable from month to month. This instability appeared when 40 of the subjects were retested after an interval of about a month. The correlations of the results of these two tests with respect to the differences of the mean measurements on the two arms of each subject and to the critical ratios of the mean differences are presented in Table I. TABLE I.-CORRELATION OF RIGHT-LEFT ARM DIFFERENCES ON FIRST AND SECOND TESTS. N = 40. Systolic. Diastolic. Mn. r.* Mn. r.* Mn. diff. R - L Test I Test II Critial ratio Test I Crtclrto- -Test II * r =Pearsonian product-moment correlation. The close similarity between the correlations obtained by these two methods (systolic r=0-315 and r=0-309) is interesting, and the low values for the correlations indicate that although differences were observed with approximately equal frequency in the two tests, the largest and most measurable differences were not shown by the same individuals in Test I. as in Test II. Accordingly a table was prepared (Table II.) separating those individuals who never showed a significant difference from those

6 160 Shock and Ogden whose arms differed in Test I. only, in Test II. only, and in both tests. TABLE II. PER CENT. OF SUBJECTS SHOWN-ING SIGNIFICANT RIGHT-LEFT ARM DIFFERENCES WHEN RETESTED. N 40. (Critical ratio of difference greater than 2 5.) Systolic. Diastolic. N. Per cent. N. Per cent. Test I * Test II Test I. or II. or both Both Test I. and Test II Neither Test I. nor Test II * When computed for the 102 subjects taking the test for the first time, the proportion showing a significant difference was 83 per cenit. This table shows tlhat, whereas there are only 40 per cent. of the subjects in whom the difference in systolic pressure appears in both tests and therefore might be due to a permanent anatomical characteristic, in 55 per cent. of them the phenomenon is liable, appearing only once in two tests; in only 5 per cent. (two cases) was there no significant difference in either of the two tests. It is apparent also that in any subject a significant difference is more commonly found in systolic pressure measurements than in diastolic. Within the limitation imposed by a small group and only two experiments, we may conclude that it is very unusual to find someone whlose systolic blood-pressure readings on the twvo sides never differ from eaclh other. Discussio N. The comminion liability of the arimi-to-arim differences revealed by these tests suggests that their major cause be souglht in functional rather than aniatomical peculiarities. It is possible that the failure of the differences in blood-pressures of the two arms to distribute themselves about zero (mean systolic difference R- L mimn. Hg) imay be an indication of the expected hydrostatic pressure drop along the aorta between the origins of the blood supply to the two arms. Ilhe larger differences, however, which are characteristically neither stable nor favouring one arm, are more likely to be due to clhanges in the physical properties of the tissues

7 The Differences between Blood-Pressure Measurements 161 in the arm being measured. Thus an increase in rigidity or decrease in viscosity in the arm might very well allow a pulsation to become audible, the energy of whose sound might otherwise be dissipated. Such changes might be of vasomotor, muscular, or cedematous origin. The more obvious physiological methods for altering conditions in the two arms differentially have been repeatedly used [Hoover, 1910; Margaria, 1926, 1927; Mennyey, 1928; Yoshino, 1933] without a consistently predictable change in blood-pressure readings. For instance, Yoshino [1933] reports a lowering of blood-pressure in the exercised arm following unilateral arm exercise, but Margaria [1926, 1927] reports a marked rise in the exercised arm. Similar contradictions in results appear when temperature changes are used to induce differential vasomotor alterations in the two arms. Mennyey [1928] reported a rise in blood-pressure in the arm subjected to temperature rise; Hoover [1910] reported a rise in pressure in the arm subjected to cold. Such inconsistencies leave the field open to the speculation that such changes may so commonly be produced psychogenically as to mask experimental attempts to show them, except by the statistical analysis of a large number of physiological experiments in which a serious attempt had been made to randomise or exclude the possible psychological causes of the change. Within our knowledge such a series has not yet been published. SUMMARY. Seventy per cent. of a group of normal young adult males showed a significant difference between blood-pressure measurements made simultaneously on the two arms. The distribution of the magnitude of these differences is presented. The same proportion of significant differences was found in 40 of the subjects when retested. The correlation between the magnitude of the differences for each individual in the two tests was low, indicating that the cause of the difference in blood-pressure can rarely be a fixed structural characteristic of the individual. Since these differences are not easily reproducible by variation of standard physiological conditions, it is suggested that they may be of psychogenic origin. REFERENCES. BING, H. J. (1906). Berliner klin. Wschr. 43, BODENSTAB, W. H. (1925). J. Lancet, 45, 360. FISCHER, P. (1924). Klin. Wschr. 3, Part 1, 784. GEISB6CK, F. (1905). Deutsch. Arch. klin. Med. 83, 363. HARE, D. C. ( ). Proc. Roy. Soc. Med. 19, Section 1, 36.

8 162 The Differences between Blood-Pressure Measurements HENSEN, H. (1900). Deutsch. Arch. klin. Med. 67, 436. HILLS, R. G. (1938). Bull. Johns Hopkins Hosp. 62, 475. HIRSCH, K. (1901). Deutsch. Arch. klin. Med. 70, 219. HOOVER, C. F. (1910). J. Amer. med. Assoc. 55, 815. JELLINEK, S. (1900). Ztsch. klin. Med. 39, 447. KAY, WV. E., and GARDNER, K. D. (1930). Calif. West. Med. 33, 578. KILGORE, E. S. (1914). Calif. State Med. 12, 97. KING, J. T. (1937). Ann. int. Med. 10, KORNS, H. M., and GUINAND, P. H. (1933). J. clin. Invest. 12, 143. MARGARIA, R. (1926). Boll. Soc. Ital. Biol. sper. 1, 107. MARGARIA, R. (1927). Arch. Sci. biol. 10, 309. MCWILLIAM, J. A., and KESSON, J. E. ( ). Heart, 4, 279. MENNYEY, G. (1928). Arch. Sci. biol. 11, 334. OGDEN, E., and SHOCK, N. W. (1938). Quart. J. exp. Physiol. 28, 341. OSLER, W., and MCCRAE, T. (1935). The Principles and Practice of Medicine Designed for Use of Practitioners and Students of Medicine, p New York, D. Appleton-Century Co. SHOCK, N. W., and OGDEN, E. (1939). Quart. J. exp. Physiol. 29, 49. YOSHINO, G. (1933). Acta Schol. Med. Univ. Kyoto, 16, 191.