Vital Signs Part 1: Blood Pressure

     The classic vital signs of blood pressure, pulse, respiration and temperature have been the backbone of EMS since its inception. Vital signs give EMS providers insight to what's going on inside our patients, and let us evaluate their responses to our interventions. This multipart series will take a fresh look at these vital signs and what they actually tell us in terms of changing our prehospital treatment, predicting the severity of presenting problems and even predicting survival. This article will examine blood pressure.

     "The cuff of Riva-Rocci is placed on the middle third of the upper arm; the pressure within the cuff is quickly raised up to complete cessation of circulation below the cuff. Then, letting the mercury of the manometer fall, one listens to the artery just below the cuff with a children's stethoscope. At first no sounds are heard. With the falling of the mercury in the manometer down to a certain height, the first short tones appear; their appearance indicates the passage of part of the pulse wave under the cuff. It follows that the manometric figure at which the first tone appears corresponds to the maximal pressure. With the further fall of the mercury in the manometer, one hears the systolic compression murmurs, which pass again into tones (second). Finally, all sounds disappear. The time of the cessation of sounds indicates the free passage of the pulse wave; in other words, at the moment of the disappearance of the sounds, the minimal blood pressure within the artery predominates over the pressure in the cuff. It follows that the manometric figures at this time correspond to the minimal blood pressure."

     With those 190 words, spoken in an address to the Imperial Military Academy in 1905, Russian physiologist Nikolai Korotkoff introduced the classic technique of obtaining systolic and diastolic blood pressure with the use of a sphygmomanometer and stethoscope.

     Korotkoff actually described five types of sounds, now named after him. The first is the snapping sound first heard at the systolic pressure. The second is the murmuring heard for most of the area between the systolic and diastolic pressures. The third and fourth, at pressures within 10 mmHg above the diastolic blood pressure, are described as "thumping" and "muting." The fifth sound is silence as the cuff pressure drops below the diastolic blood pressure. It is recorded as the last audible sound.

     Traditionally, the systolic blood pressure is taken to be the pressure at which the first Korotkoff sound is first heard, and the diastolic blood pressure is the pressure at which the fourth Korotkoff sound is just barely audible. There has been disagreement in the past as to whether the fourth or fifth Korotkoff sound should be used for recording diastolic pressure, but phase IV tends to be even higher than phase V when compared against the true intra-arterial diastolic pressure, and is more difficult to identify. There is now general consensus that the fifth phase should be used, except in situations in which the disappearance of sounds cannot reliably be determined because sounds are audible even after complete deflation of the cuff—for example, in pregnant women and patients with arteriovenous fistulas (e.g., for hemodialysis) or aortic insufficiency.¹ All providers must be using the same criteria, Korotkoff V, in establishing the diastolic pressure.

     Conventional sphygmomanometry is fraught with potential sources of error, which may arise in the subject, observer, sphygmomanometer or overall application of the technique. Measuring blood pressure using a cuff is known as indirect measurement; direct measurement involves placing an arterial catheter. Direct arterial measurement is the gold standard of sphygmomanometry. While indirect pressure findings correlate well with intra-arterial readings, Korotkoff phase I sounds do not appear until an average of 3 mmHg below the direct systolic pressure, and phase V sounds disappear an average of 9 mmHg higher than the direct diastolic pressure.²

     Sometimes the Korotkoff sounds may become inaudible after systole, then reappear, then disappear again at true diastole. This phenomenon is known as the auscultatory gap. If blood pressure is not palpated to estimate the systolic number before cuff inflation, this may mislead the responder into thinking the first sound they hear is the systolic number. The clinical significance of this gap, if any, is unknown. However, obtaining a wrong systolic blood pressure can be fatal, especially when many pharmacological decisions are based on it.

     Pulse pressure is the result of subtracting the diastolic number from the systolic—for example, in a blood pressure of 120/80, the pulse pressure is 40. If the pulse pressure is genuinely low—i.e., less than 25% of the systolic blood pressure—the cause may be low stroke volume, as in CHF or shock. This interpretation is reinforced if the resting heart rate is relatively rapid (e.g., 100–120), representing increased sympathetic nervous system activity. A small study demonstrated that reduced pulse pressure is a marker of reductions in stroke volume and may allow for early identification of volume loss because of hemorrhage, and therefore more accurate and timely triage.³ A sharply narrowed pulse pressure (e.g., a blood pressure of 88/80) is associated with cardiac tamponade. A widened pulse pressure, greater than 50% of the systolic blood pressure (e.g., 180/80), along with hypertension and bradycardia, describes Cushing's triad, indicative of increased intracranial pressure.

     A 40-year-old study that compared systolic blood pressure measurements taken by auscultation and palpation found them within 8 mmHg.⁴ While palpation has been commonly limited to the measurement of systolic blood pressure, one study reported that diastolic pressures could be accurately palpated using the brachial artery to identify the sharp phase IV Korotkoff sound. However, the value of this technique in clinical practice, and its accuracy when used by healthcare workers, has yet to be demonstrated.⁵

     The length and width of the inflatable cuff (bladder) used during the measurement of blood pressure may be a source of error. Much research has focused on cuff width (the dimension across the bladder). The standard width of currently available cuffs is approximately 12 cm, with both larger and smaller sizes available. Studies have shown that using a cuff that's too narrow results in an overestimation of blood pressure, and using a cuff that's too wide leads to underestimation. There is strong evidence that using too narrow or too short a bladder (undercuffing) will cause overestimation of blood pressure, so-called cuff hypertension, and there is growing evidence that using too wide or too long a bladder (overcuffing) may cause underestimation. Undercuffing has the effect in clinical practice of overdiagnosing hypertension, and overcuffing leads to diagnosis of hypertensive subjects as normotensive.⁶

     While most texts and instructional aids demonstrate blood pressure measurements taken on bare arms with no clothing, that's often not possible in the prehospital arena, and the evidence for it is not clear. Studies in 1993 and 2003 looked at the effect of using sleeved arms, as well as compared blood pressures taken on a bare arm, below a rolled-up sleeve and over a sleeve. The results showed no statistically significant differences. While an author did recommend measuring blood pressure in known hypertensive patients in bare arms, this recommendation was meant for physicians planning long-term blood pressure control options.^7–9

     Comparisons of blood pressures measured in sitting subjects with arms supported horizontally versus resting at their sides found average differences of 11 mmHg systolic and 12 mmHg diastolic. When the arm was placed above or below the level of the heart, blood pressure measurements changed by as much as 20 mmHg. As a result, it is recommended that blood pressures be taken on seated subjects with their arms supported horizontally at approximately heart level, when possible.¹⁰ Without support, muscles contract to support the arm, raising blood pressure and heart rate. Diastolic blood pressure may increase by as much as 10% by having the arm extended and unsupported during blood pressure measurement. This effect is greater in hypertensive patients and those taking beta blockers. Supporting the arm is best achieved by holding the subject's arm at the elbow.²

     In which arm to measure blood pressure remains controversial. Some studies (not all) have demonstrated significant differences in simultaneous blood pressures of both arms. Emergency physicians Adam Singer and Judd Hollander noted that 19% of studied ED patients had an interarm difference of more than 20 mmHg without evidence of substantial intrinsic arterial disease. However, these patients did appear to have more coronary artery disease.¹¹ According to the American Heart Association, an interarm difference of 10 mmHg or less is normal, and if one arm has a higher blood pressure than the other, use that arm to determine if the patient has hypertension.¹²

     Two studies have found that using the stethoscope's bell resulted in readings higher than those taken using the diaphragm. The most recent indicated that both sides of the acoustic stethoscope give similar results in the measurement of office blood pressures, and either side can be used reliably.¹³ Whichever side you use, place it on the patient's bare skin.

     Uncalibrated and inaccurate sphygmomanometers are another cause of blood pressure measurement error, responsible for both false positives and false negatives. One study found 50% of tested aneroid devices giving false readings in excess of 10 mmHg.¹⁴ Sphygmomanometers should be calibrated regularly by accredited organizations or technicians.¹⁵ Manometer accuracy can vary greatly from one manufacturer to another. Four surveys conducted in hospitals in the past 10 years found inaccuracies ranging from 1%–44%.¹ The few studies that have been conducted with aneroid devices focused on the accuracy of the pressure registering system as opposed to the degree of observer error, which is likely to be higher with the small dials used in many of the devices.

     Techniques used by healthcare workers to measure blood pressure have been shown to differ from recommended practice. One study found that 57% of nursing students failed to comply with AHA guidelines in areas such as cuff placement, estimation of systolic pressure by palpation, calculation of proper inflation pressure and proper stethoscope placement. In fact, no individual student complied with every step of the recommended procedure, and 95% failed to comply with some specific steps unique to the AHA standards that experts consider critical to accurate measurement.¹⁶ Another study of 172 healthcare workers concluded that nurses and physicians evaluated blood pressures in inadequate, incorrect and inaccurate ways, and that only 3% of general practitioners and 2% of nurses obtained reliable results.¹⁷ Two studies evaluating the impact of education programs on blood pressure measurement found that they increased agreement between different blood pressure readings and significantly reduced differences in operator technique.^18,19 A study in Australia noted that providers who deflated cuffs more rapidly than is standard overestimated systolic blood pressures by an average of 9 mmHg. This was critical, as the study was evaluating the professionals' ability to detect preeclampsia by blood pressure.²⁰

     Advanced Trauma Life Support guidelines for assessing systolic blood pressure based on palpation of pulses released in 1985 suggested that the presence of a carotid pulse indicated a systolic blood pressure (SBP) greater than 60 mmHg, a femoral pulse indicated one greater than 70 mmHg, and a palpable radial pulse meant an SBP greater than 80 mmHg. This rule was removed from current ATLS guidelines because research showed it to be inaccurate, generally overestimating the patient's systolic blood pressure and underestimating the degree of hypovolemia.²¹ Despite what prehospital professionals may have heard in class or on the streets, there is no association between peripheral pulses and blood pressure in the literature.

     The interobserver reliability of blood pressure measurement by auscultation is low. This means that different people auscultating blood pressures on the same subject at the same time will get statistically significant differences. Anecdotally, I've been told that auscultating a blood pressure is as much a hearing test for the clinician as it is a physiological measurement of the patient. One study of 140 emergency room patients found significant differences between vital signs recorded by different persons, and concluded that the reproducibility of vital sign measurements may be limited, and vital signs should be interpreted with caution.²² Automated blood pressure devices could ultimately be a solution to this, but another study found that automated determinations were consistently higher than those taken manually, particularly in hypotensive patients. That study concluded that automated devices should not be used for field or hospital triage decisions, and that manual determinations should be used until the SBP is consistently greater than or equal to 110 mmHg.²³ As well, using automated devices doesn't eliminate human error, such as using the wrong-sized cuff or putting the cuff on improperly.

     Each patient poses their own problems with measuring blood pressure accurately. Children present the most variety in size; therefore, cuff size is important. In addition, Korotkoff sounds are not reliably audible in children under one year and many under five years. Patients with arrhythmias, especially atrial fibrillations and slow bradycardias, present special challenges in determining exact BPs. For the obese, large cuffs may be required, and a thigh cuff may be needed if arm circumference exceeds 41 cm.²⁴

     There is debate about the usefulness of auscultating forearm blood pressures in the obese. One study identified the blood pressures in the arms and forearms of average-sized volunteers within 20 mmHg of each other, while another of obese patients found a difference of only 5 mmHg, but called that result significantly different.^25,26 It's in the prehospital provider's best interests to routinely carry the larger-sized cuff with their equipment to the patient. The forearm method is, at this point, a guess at best, and should not be used to guide patient care decisions.

     While prehospital providers may feel that blood pressure is simply an indicator for pharmacological therapy or even just part of the routine, literature suggests that prehospital blood pressure can predict the need for hospital services. Several studies describe the incidence of trauma patients who are hypotensive in the field but normotensive in the emergency department. This population has a higher incidence of requiring emergent procedures and surgeries, and the finding may be a predictor of overall mortality.^27–29

• The last audible sound should be recorded as the diastolic pressure.
• Support the patient's arm at the elbow.
• All providers at all levels should review their techniques in accordance with this article and current science to ensure we're all measuring vital signs the same way.
• Both the bell and the diaphragm of the stethoscope, when placed on bare skin, give similar results.
• There is no association between pulses and estimation of blood pressures.
• It is in the prehospital provider's best interests to routinely carry the larger-sized cuff with their equipment to the patient.
• Agencies should calibrate their sphygmomanometers in accordance with manufacturers' recommendations.

     1. Pickering TG, Hall JE, Appel LJ, Falkner BE, Graves J, Hill MN, Jones DW, Kurtz T, Sheps SG, Roccella EJ. Recommendations for blood pressure measurement in humans and experimental animals, part 1: Blood pressure measurement in humans. Circ 111:697–716, 2005.

     2. McAlister FA, Straus S. Measurement of blood presssure: An evidence based review. BMJ 322(7,291):908–11, Apr 14, 2001.

     3. Convertino VA, Cooke WH, Holcomb JB. Arterial pulse pressure and its association with reduced stroke volume during progressive central hypovolemia. J Trauma 61(3):629–34, Sep 2006.

     4. Putt AM. A comparison of blood pressure readings by auscultation and palpation. Nurs Res 15:311, 1966.

     5. Vaidya S, Vaidya SJ. Diastolic blood pressure can be reliably recorded by palpation. Arch Intern Med 156(14):1,586, July 22, 1996.

     6. Beevers G, Lip GY, O'Brien E. ABC of hypertension: Blood pressure measurement. Part I—sphygmomanometry: Factors common to all techniques. BMJ 322(7,292):981–5, Apr 21, 2001.

     7. Holleman DR Jr., Westman EC, McCrory DC, Simel DL. The effect of sleeved arms on oscillometric blood pressure measurement. J Gen Intern Med 8(6):325–6, June 1993.

     8. Kahan E, Yaphe J, Knaani-Levinz H, Weingarten MA. Comparison of blood pressure measurements on the bare arm, below a rolled-up sleeve, or over a sleeve. Fam Pract 20(6):730–2, Dec 2003.

     9. Liebl M, Holzgreve H, Schulz M, Crispin A, Bogner J. The effect of clothes on sphygmomanometric and oscillometric blood pressure measurement. Blood Press 13(5):279–82, 2004.

     10. Webster J, Newnham D, Petrie JC, Lovell HG. Influence of arm position on measurement of blood pressure. Br Med J Clin Res Ed 288(6,430):1,574–5, 1984.

     11. Singer AJ, Hollander JE. Blood pressure: Assessment of interarm differences. Arch Intern Med 156:2,005–08, 1996.

     12. www.americanheart.org/presenter.jhtml?identifier=3027042.

     13. Kantola I, Vesalainen R, Kangassalo K, Kariluoto A. Bell or diaphragm in the measurement of blood pressure? J Hypertens 23(3):499–503, Mar 2005.

     14. Waugh JJ, Gupta M, Rushbrook J, Halligan A, Shennan AH. Hidden errors of aneroid sphygmomanometers. Blood Press Monit 7(6):309–12, Dec 2002.

     15. Turner MJ, Irwig L, Bune AJ, Kam PC, Baker AB. Lack of sphygmomanometer calibration causes over- and under-detection of hypertension: A computer simulation study. J Hypertens 24(10):1,931–8, Oct 2006.

     16. Bogan B, Kritzer S, Deane D. Nursing student compliance to standards for blood pressure measurement. J Nurs Educ 32(2):90–2, Feb 1993.

     17. Villegas I, Arias IC, Botero A, Escobar A. Evaluation of the technique used by health-care workers for taking blood pressure. Hypertens 26(6 Pt 2):1,204–06, Dec 1995.

     18. Sherwitz L, Evans L, Hennrikus D, Valbona C. Procedures and discrepencies of blood pressure measurements in two community health centres. Med Care 22:727–38, 1982.

     19. Graves PS, Walson PD, Rainey LK, Pinyerd BJ. Nursing evaluation of pediatric blood pressure measurement. Qual Rev Bull 10(7):221–25, 1984.

     20. Reinders LW, Mos CN, Thornton C, Ogle R, Makris A, Child A, Hennessy A. Time poor: Rushing decreases the accuracy and reliability of blood pressure measurement technique in pregnancy. Hypertens Preg 25(2):81–91, 2006.

     21. Deakin CD, Low JL. Accuracy of the advanced trauma life support guidelines for predicting systolic blood pressure using carotid, femoral, and radial pulses: Observational study. BMJ 321(7,262):673–4, Sep 16, 2000.

     22. Edmonds ZV, Mower WR, Lovato LM, Lomeli R. The reliability of vital sign measurements. Ann Emerg Med 39(3):233–37, Mar 2002.

     23. Davis JW, Davis IC, Bennink LD, Bilello JF, Kaups KL, Parks SN. Are automated blood pressure measurements accurate in trauma patients. J Trauma 55(5):860–3, Nov 2003.

     24. Maxwell MH, Schroth PC, Waks AU, Karam M, Dornfeld LP. Error in blood pressure measurements due to incorrect cuff size in obese patients. Lancet 2:33–36, 1982.

     25. Vinyoles E, Pujol E, de la Figuera M, Tajada C, Montero P, Garcia D. Measuring blood pressure in the forearm of obese patients: Concordance with arm measurement. Med Clin (Barc) 124(6):213–4, Feb 19, 2005.

     26. Singer AJ, Kahn SR, Thode HC Jr., Hollander JE. Comparison of forearm and upper arm blood pressures. Preh Emerg Care 3(2):123–6, Apr–Jun 1999.

     27. Lipsky AM, Gausche-Hill M, Henneman PL, et al. Prehospital hypotension is a predictor of the need for an emergent, therapeutic operation in trauma patients with normal systolic blood pressure in the emergency department. J Trauma 61(5):1,228–33, Nov 2006.

     28. Shapiro NI, Kociszewski C, Harrison T, Chang Y, Wedel SK, Thomas SH. Isolated prehospital hypotension after traumatic injuries: A predictor of mortality? J Emerg Med 25(2):175–9, Aug 2003.

     29. Franklin GA, Boaz PW, Spain DA, Lukan JK, Carrillo EH, Richardson JD. Prehospital hypotension as a valid indicator of trauma team activation. J Trauma 48(6):1,034–7, discussion 1037–9, Jun 2000.

     Rob Curran, DC, EMT, is a human anatomy and physiology instructor at CUNY-Brooklyn College, adjunct faculty in the Physical Therapy, Physician Assistant and Nursing programs at SUNY-Downstate, and New York State Coordinator for the President's Council on Physical Fitness and Sports.