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THE PRINCIPLE OF THE SPHYGMOMANOMETER
THE INSTRUMENTAL ESTIMATION OF BLOOD-PRESSURE
The basis of modern sphygmomanometry is founded upon circular pneumatic compression, without which the modern sphygmomanometer could not have been developed, and the immense value of the results of blood-pressure study might still be unknown to clinical medicine.
The direct method of the physiologist is not applicable, as it would require a direct communication between the blood-vessel and the tube leading to the manometer, and is therefore not practical for the consulting room even if tolerated by the patient.
Circular Compression.—It remained for Riva-Rocci? and Hill and Barnard? each working independently of the other, to substitute the arm-encircling cuff for the uncertain and inaccurate pelote of the early investigators.
By means of the encircling arm-band (either full or partial) the pressure produced within the hollow inflatable rubber portion is exerted equally from every direction against the artery. Physiologic experiment has shown this method to be accurate, as the tissues intervening between the arm-band and the artery offer a negligible
1 Gazz. Med. di Torino, 1896, Nos. 50 and 51. 2 Leonard Hill and H. Barnard, Brit. Med. Jour., 1897, Vol. 2, p. 904.
resistance, so that the readings obtained, indirectly through the tissues, by the modern method, agree very closely with those obtained by the direct method.
Character of Cuff.—It has already been said that the width of the tubular cuff influences to a significant degree the readings obtained. Janeway has found in high-pressure cases that there exists a difference of as much as 60 mm. between the 12-cm. and the 5-cm. arm-band. This is
Fig. 4.-A. Schematic section of arm, showing narrow arm-band (a) with retaining device, (6) before inflation artery (c). B. Same, showing change in form of compression band (a), after inflation, artery (c) compressed. Note great change in form and increase in circumference of compression bag. This change occurred only at the expense of a measurable amount of pressure.
easily explained by noting the change which occurs within a narrow (2 in.) and a wide (412 in.) cuff during inflation under a rigid retaining device-reference to the accompanying illustration will aid the explanation. Fig. 4 shows a narrow armlet which allows insufficient material to indent the tissues and compress the vessel without requiring additional pressure to expand the rubber bag, this amount being registered on the scale of the sphygmomanometer in addition to that required to compress the vessel. Chamberlain' has determined that this amount of error on an arm of average size is 8 mm. or more.
1 Von Recklinghausen, Arch. f. Exp. Pathol. u. Pharmakol., 1901, xlvi, 78; Gumprecht, Zeitsch. f. klin. Med., 1900, xxxv; and Hensen, Deutsch. Arch. f. klin. Med., 1900, xlvii, p. 4.
- T. C. Janeway, "Clinical Study of Blood-pressure," 1907, D. Appleton & Co.
Fig. 5 shows wide arm-band (A) before compression and (B) after compression, where the change in form of the rubber portion is insufficient to exert pressure beside that required to indent the tissues and compress the vessel.
B Fig. 5.-A. Schematic section of arm showing wide arm-band (a) with retaining device (b) and artery (c) before inflation. B. Same, showing change in form of compression bag (a) after inflation artery (c) compressed, note slight change in form of compression bag, insufficient to exert any additional force than that required to compress artery.
Influence of the Vessel Wall.—Upon this subject aụthorities differ. The early experiments of v. Bascha show that the resistance of a normal radial artery to closure scarcely amounts to 1 mm.
Russell takes the opposite stand when he says, “I cannot but think that those who have thought that the
Chamberlain, Philippine Jour. of Sci., vi, Sec. B, December, 1911. 2 Berlin. klin. Wochen., xxiv, 1887.
3 Wm. Russell, “Arterial Hypertension, Arteriosclerosis and Blood-pressure,” J. B. Lippincott Co., 1908, p. 52.
arterial wall is negligible, have not had the data necessary to a correct opinion."
Hoover' recently attempted to prove Russell's contention by submerging an arm or leg in an ice-filled trough and demonstrating the changes in arterial pressure caused thereby. In this work Hoover apparently failed to recognize the well-known effect of peripheral constriction upon the proximal portion of the vessel (i.e., central to the constriction) which, in his experiments is undoubtedly the cause of the rise in pressure; it therefore cannot be ascribed to change in the condition of the arterial wall.
Janeway? and later Janeway and Park3 believe the element of error to be greater than was formerly supposed, as they found that the resistance offered by the arterial wall in adults with normal pressures to be more than 1 or 2 mm. Hg. while they were able to demonstrate that this rarely equalled more than 10 mm. Hg. Practically this difference may be discounted as in adults the factor is a comparatively fixed one, and being ever present may be clinically ignored, and also because it is no more than the spontaneous variation occurring in any individual from time to time (often within a few minutes). In children the factor is entirely negligible.
Arteriosclerosis, according to Janeway and Park,4 even when marked, increases but moderately the normal resistance to compression. By actual experiment they found that it did not exceed 17 mm. Hg.
C. F. Hoover, Jour. A. M. A., Vol. lv. No. 10, Sept. 3, 1910.
From the data at hand, we may conclude that the vessel wall, as a factor, need not be considered from a clinical standpoint, as any resistance which could be offered by a vessel even markedly sclerosed, would be insignificant when compared to the alterations in pressure often occurring within the vessel in disease. I submit as further proof the many high-pressure cases met here but little change can be demonstrated in the superficial vessels, while on the other hand, cases occur whose superficial vessels, as far as they can be digitally traced, are absolutely rigid and yet blood-pressure by palpation may never register over 110 mm.
Influence of Other Intervening Structures.—Vital tissue is perfectly elastic; therefore any pressure applied to the surface of a living body will be transmitted directly to the underlying structure without loss of force.
It would seem, therefore, safe to assume that both the vessel wall and the intervening tissues, as definite factors which modify blood-pressure readings, can be absolutely eliminated, at least clinically, because all pressures are read through them and so, as they are always included in the estimation, they can be ignored.
THE ESTIMATION OF BLOOD-PRESSURE
No one will deny at this time the great importance of accurate blood-pressure estimation, and the signal value that clinical medicine has derived from the modern sphygmomanometer. Neither will anyone be deluded into believing that he or anyone else is able, by tactile sense alone, to determine the relative height of blood-pressure with anything like the same degree of accuracy as is attained