Updating sphygmomanometry

EDITORIALS

Updating Sphygmomanometry LEONARD HAROLD MYRON

STEINFELD, ALEXANDER, L. COHEN,

MD, FACC PhD

PhD

York, New York Hoboken, New Jersey New

The measurement of systemic blood pressure using a sphygmomanometer and stethoscope is well entrenched in medical practice. As a noninvasive tool for the rapid determination of intraarterial pressure, the technique has proved accurate and reliable most of the time-that is, if one has no interest in the obese, the very young, those with shock or those whose upper limbs are unusually conically shaped and thus adapt poorly to standard cuffs. With the exploding technology of the past two decades one would have expected a more precise technique to have evolved for the noninvasive assessment of systemic blood pressure. But except for a few abortive flourishes in automation, blood pressure measurements are obtained in much the same manner as in grandfather’s day. Today, needless to say, the errors inherent in the technique are no less nettlesome. In the generally accepted procedure of sphygmomanometry, after inflation of the pneumatic bladder, the intrabladder pressure, as measured by either an aneroid or mercury manometer, is assumed to be identical to pressure applied to the artery wall by the compressed tissues. After deflation of the bladder, the onset of tapping sounds (Korotkoff soundsl) heard through the stethoscope is taken as a sign of peak systolic brachial pressure and the cessation or muffling of the sounds is the sign of end-diastole. Too often, unfortunately, these signs fail to signal true systole and diastole. On closer examination of the procedure, we find that this error stems from two principal sources: (1) inaccurate transmission of bladder pressure to the artery wall due to the design of the cuff-bladder assembly and, (2) lack of understanding of the relation of Korotkoff sounds to the systolic and’ diastolic end-points, resulting in their improper detection. Although faulty instrumentation and clumsy technique cannot be ignored as frequent From the Division of Pediatric Cardiology, Mount Sinai Hospital, New York, N. Y. and the Stevens Institute of Technology, Hoboken, N. J. Manuscript accepted June 14,1973. Address for reprints: Leonard Steinfeld, MD, Division of Pediatric Cardiology, Mount Sinai Hospital, 1176 Fifth Ave., New York, N. Y. 10029.

January

causes of error, their elimination is simpler than elimination of the problems of cuff and sound, which requires a better understanding as a prerequisite. Von Recklinghausen2 in 1901 is reputed to have been the first to propose changing the design of the sphygmomanometer cuff to improve accuracy in blood pressure measurement. However, after 70 years of attention focused on the problems surrounding the choice of the ideal sphygmomanometer cuff, a satisfactory and universally acceptable product has not been developed. The principal reason for this failure is the lack of information on the conditions necessary for the accurate transmission of bladder pressure to the artery wall; consequently, a blood pressure cuff that can reliably satisfy these conditions has not been designed. The 1967 American Heart Association recommendations for human blood pressure determinations proposed a pneumatic bag 20 percent wider than the diameter of the limb on which it is to be used. With respect to length, it was suggested that the bag should go halfway around the limb, but be placed directly over the compressible artery. These recommendations have not been updated although there is now convincing evidence that a revised set of standards for the pneumatic bag are in order. Recommendations for occlusive cuff and pneumatic bag: From a series of experiments performed in our laboratory on an upper arm-brachial artery hydromechanical analog,4 and from analytic studies,s we have concluded that intrabladder pressure is transmitted with least decay to the underlying compressible artery when the length of the pneumatic bag is such that it completely encircles the model arm. The conclusions drawn from these studies have substantial support in data derived from human studies in a number of laboratories, including our own. The results of studies carried out by King,s Karnovinr and others8-lo support the view that a completely encircling pneumatic bladder is preferable to any abbreviated version. The compromise that we indulge in today of employing a short inner bladder and an outer sleeve of so-called stiff fabric seems to be less of a compromise than an open invitation to error.

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EDITORIALS

POLYOLEFIN

PRESSURE APPLIED TO AN ARTERY UNDER AN OCCLUDING CUFF pP 1.5 yr d Arm Circ 18 cm

CUFF

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RECOMMENDATION

POLYOLEFIN

CUFF

9 x 21

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1

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I

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RATIO OF DISTANCE ALONG ARM MEASURED FROM CENTER BLADDER TO ARM RADIUS

With respect to determining the optimal width of the pneumatic bag, it has been well documented in clinical studies that very narrow bladders yield inordinately high systolic and diastolic values at all age levels. However, no physically sound explanation has been previously presented for this phenomenon. Recently, analytic studies5 have determined that constraint of longitudinal arm tissue motion is associated with accurate pressure transmission to the artery wall. These studies have also predicted the minimal ratio of bladder width to arm circumference for accurate pressure transmission to at least a point on the artery under the center of the cuff. The American Heart Association recommendation of a bladder width 20 percent greater than arm diameter (1.2/n X arm circumference) accords very well with these analytic results as a minimal requirement (Fig. 1). However, in a number of clinical experiences, for example, with the upper arm of small infants, the long

January

1974

The American

Journal

AXILLARY ARTERY PRESSUUE

OF

FIGURE 1. Pressure applied to an artery under an occluding cuff. With the application of bladders with a width to arm diameter ratio (w/d) ranging from 0.2 to 2.0, it is found that at w/d = 1.2 the arterial wall pressure reaches the bladder pressure value at one point on the artery under the center of the cuff (z = 0). As w/d is increased, a longer length of the artery is exposed to the full cuff pressure, but no segment of artery is ever subjected to a pressure greater than the bladder pressure. Analytic studies, therefore, negate the view that wide bladders cause underestimation of arterial pressure. d = diameter of arm; r = radius of arm; w = width of bladder; z = distance along arm measured from center of bladder.

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BLADDER PRESSURE

of CARDIOLOGY

8o

0 a4 FIGURE 2. Blood pressure determinations. A, w/d = 1.22 indicates the first Doppler signal. B, w/d = 1.57. Arrow indicates the first Doppler signal. d = arm diameter; w = bladder width. Arrow

thin upper arm of the tall adolescent, and the broad tapered upper arm of the very obese, bladder widths 1.2 X arm diameter yield systolic and diastolic values considerably higher than those of simultaneously recorded intravascular pressures. In our patient sampling, a bladder width that complied with the Heart Association recommendation produced a narrow cuff effect, indicating that these minimal requirements should be exceeded to ensure accurate measurement (Fig. 2). In both human and analog experiments in which wider bladder widths were tested, peak systolic values failed to show the marked discrepancies noted with narrow bladders. In the majority of instances, the correspondence between direct and indirect peak systolic pressures was identical or nearly so. In human investigations, unusually wide bladders, extending from the axilla to a level 1 or 2 cm above the antecubital crease, failed to elicit previously reported low peak

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EDITORIALS

140.000~

. .

. 130.000;

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m 120.000~ . . .

: ii 3 ii if G : 5 z

FIGURE 3. Comparison of direct and indirect use of peak systolic pressure measurements obtained with polyolefin cuffs. Indirect peak systolic pressure measurements were obtained using polyolefin cuffs with a width to arm diameter ratio ranging from 1.22 to 2.36. Arterial peak systolic pressures were measured with a catheter tip at the Subclavian-axillary artery junction. The center diagonal is the line of identity, and the light parallel lines are +3 mm Hg. The seven instances in which indirect oressures were noted to exceed the 3 mm error were’measurements obtained using polyolefin cuffs with a width to arm diameter ratio of approximately 1:2 (American Heart Association recommendation).

.

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systolic values3 when direct and indirect pressures were compared. These clinical results are in agreement with the analytic prediction of the nonexistence of the so-called “wide cuff effect.“5 Indications of the production of Korotkoff sounds in our studies, however, were sensed with a Doppler ultrasonic transducer rather than with a stethoscope. Poor signal pickup with a stethoscope may actually account for the low values previously reported with the application of wide cuffs to the upper arm. We have come to realize that th,e one or two different bladder-cuff sizes used for blood pressure measurement in the adult and the few models available for infants and children are inadequate to achieve the levels of accuracy and reliability possible with this technique. The goal now would appear to be a pneumatic bladder that completely encircles the limb and that has a minimal width to length ratio of approximately 1:3, with an option for greater width to length ratios when circumstances demand. For the conically shaped obese arm or leg, the rectangular configuration should be replaced by a bladder of

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trapezoidal design so that, when applied to the limb, it would conform more closely to its natural contours. In effect, we are suggesting an individualized pneumatic bladder prescription which might be accomplished with the aid of a sphygmomanometer bladder kit containing a variety of bladder sizes and types. If this seems unnecessarily tedious, it is no more so than for the ophthalmologist, for example, who refracts eyes with a battery of lenses at his disposal rather than with a few choice pieces.

Relation of Korotkoff sounds to systolic and diastolic pressure levels: Unfortunately, the matter

of the choice of a proper occlusive cuff is only a portion of the solution to the problem of improving the accuracy of sphygmomanometry. The relation of the Korotkoff sounds to the systolic and diastolic pressure levels must be appreciated. These sounds have at various times been attributed to a water hammer effect,11 fluid shock wave propagation,12 turbulent motion of the blood13 and oscillations of the arterial wa11.14Js It is obvious that the major component of the sounds cannot be associated with all of these

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EDITORIALS

phenomena. It is necessary to determine which component of the Korotkoff sound production is of interest in blood pressure determination and how these sounds relate to the physiologic events occurring under the cuff. Korotkoff sounds reproduced in an arm analog have been found to be virtually identical to their in vivo counterparts.4J6 This analog has been used as a tool to investigate the probable origins of the Korotkoff sound production. Simultaneous recordings were made of the audible sounds, the oscillation of the arterial wall and the arterial blood flow velocity. This was accomplished by sensing with a crystal microphone and a Doppler ultrasound transducer in both analog and human studies. Frequency analyses of these signals have shown that the arterial wall oscillations cause the major component of the Korotkoff sounds. The first arterial wall oscillations begin at the moment intrabladder pressure drops below peak intravascular pressure and terminates when intrabladder pressure dips below end-diastolic pressure. The vibrating arterial wall normally gives rise to sounds detectable with a stethoscope. Unfortunately, these sounds are often not discernible in human subjects because their frequency or amplitude, or both, lie below the range of human audibility. This becomes most apparent in attempts to determine blood pressure in small infants, in some obese adults and in subjects with shock. Consequently, one would prefer to sense the arterial oscillations at their source rather than listen to the sounds produced by those oscillations. A suitable filtered and properly positioned Doppler ultrasonic transducer is far more sensitive in directly detecting arterial wall oscillations than is a stethoscope in indirectly sensing these oscillations by picking up Korotkoff sounds. At present, in human studies employing the ultrasonic transducer in place of a stethoscope, extremely accurate measurements of peak syst.olic pressures are ob-

tainable. The determination of end-diastolic pressure, however, is more difficult since appropriate conditioning of the ultrasonic signal is necessary to isolate the arterial wall oscillations at end-diastole from the blood flow signal which is also sensed by the Doppler system. Instrumentation now available is capable of this discrimination. Recommended method for sphygmomanometry: In conclusion, we recommend that indirect blood pressure measurement by sphygmomanometry be performed with a properly designed and sized cuffbladder assembly as described and an appropriate arterial oscillation sensor. A variety of such systems have been under investigation and evaluation over the past several years by many investigators. For the past 2 years, in our laboratory, we have been developing and clinically testing an integral pneumatic cuff-bladder assembly made of a heat-sealable polyolefin film. This assembly has been designed to satisfy the conditions necessary for accurate bladder pressure transmission to the artery wall. It has been made available by the manufacturer in all desired lengths, widths and configurations and can be produced sufficiently inexpensively to be disposable. A relatively inexpensive, commercially available Doppler ultrasound system has been used in our studies and has been found to be adequate to our needs. In the tests performed to date on a variety of hypotensive, normal and hypertensive patients, ranging in size from a 1,000 g premature infant to a 280 lb adult, it has been found possible to make indirect blood pressure determinations accurately and reliably in situations in which the stethoscope-commercially available cuff system is completely inadequate (Fig. 31. This method is valuable in situations in which it was previously impossible to determine,blood pressure indirectly. However, it is most valuable in cases in which use of the traditional method yields inaccurate measurements.

References 1. Korotkoff NS: On the question of methods of studying blood pressure. Bull Imp Mil Med Acad 2:365, 1905 2. von Recklinghausen H: Uber Blutdruckmessung beim Menschen. Arch Exp Pathol Pharmakol46:78,1901 3. Kirkendall WM, Burton AC, Epstein FH, et al: Recommendations for human blood pressure determination by sphygmomanometers: report of a subcommittee of the postgraduate education committee, American Heart Association. Circulation 36:980-988, 1967 4. Balas C, Alexander H, Cohen ML, et al: A mechanical blood pressure analog. Proc Annual Meeting, Assoc Advancement Med Instrumentation, April 1972 5. Alexander H, Cohen ML, Steinfeld L: The measurement of blood pressure in neonates: the criteria in the choice of an occluding cuff. Proc 25th Annual Conference Engineering Med Biol, October 1972, p 109 6. King GE: Errors in clinical measurement of blood pressure in obesity. Clin Sci 32:223-237, 1967 7. Konronen MJ: Effect of sphygmomanometer cuff size on blood pressure measurement. Bull World Health Org 27:805-808, 1962 8. Orma E, Punsar S, Korvonen MJ: Cuff hypertension. Duo Decim 76:460, 1960

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9. Shekelle RB, Ostfeld AM: Sphygmomanometer cuff size and blood pressure measurement. Bull World Health Org 33:284-286,1965 10. Simpson J, Jamieson G, Dickhaus D, et al: Effect of size of cuff bladder pn accuracy of measurement of indirect blood pressure. Am Heart J 70:208-215. 1965 11. Erlanger J: Studies in blood pressure estimation by indirect methods, II. The mechanisms of the compression sounds of Korotkoff. Am J Physiol 40:82-l 25, 1916 12. Beam RM: Finite amplitude waves in fluid-filled elastic tubes: wave distortion, shock waves, and Korotkoff sounds. NASA TN D-4803, Sept 1968 13. McCutcheon EP, Rushmer RF: Korotkoff sounds: an experimental critique. Circ Res 20:149, 1967 14. Anllker M, Raman KR: Korotkoff sounds at diastole-a phenomenon of dynamic instability of fluid-filled shells. Int J Solids Structures 2:467-491, 1966 15. Raman KR: Korotkoff sounds at systole-phenomenon of dynamic instability. J Sci Eng Res ll:227, 1967 16. Cohen ML, Alexander H, Steinfeld L: A hydro-mechanical analog for examining the origins of Korotkoff sounds. Submitted for presentation at Xth Int Conf Med Bio Eng, August 1973

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