Redefinition of Normal Sinus Heart Rate

special report Redefinition of Normal Sinus Heart Rate* David H . Spodick , M.D ., D.Se. , F.G.G .P.

The labels "tachycardia" and "bradycardia" carry

bpm . At the top of the range, most physicians probably would not be pleased if their own resting heart rates exceeded 90 bpm. Based on deductive reasoning, this is, in the final analysis, a practical matter stemming from several considerations: (1) It can be difficult to define "normal ," a concept carrying philosophic as well as practical implications. (2) Mean values, as here, were readily determined. For bioIowe norms, however, as Marshall' has pointed out, "it is not clear at what point a mean value becomes an abnormal one." (3) Gaussian distributions are easy to use because of their symmetry, but such perfect symmetry does not conform to the natural world. As a corollary, the prevalent 2-S0 boundary is a useful assumption, but it is derived from the Gaussian curve and therefore cannot be universally applicable.

(Che,t 1993; 104:939-41)

intuitive as well as deductive implications. This essay embodies a proposal, based on original work published elsewhere, I to revise the operative (ie, working) definition of the expected ("normal") heart rates in resting individuals from the traditional 60 to 100 beats per minute (bprn) to 50 to 90 bpm. The results in 500 normal subjects, summarized in the Figure 1, give illustrative support to the proposal, and closely conform to epidemiologic results of other investigators in even larger numbers of normal subjects between 20 and 80 years old. 2•3 Concordant as they are, however, all three investigations are supportive without being absolutely fundamental to the argument in the sense that its major thrust addresses physicians' intuitions, particularly those that develop from everyday experience. At the low end of the range, we are aware of the large numbers of normal persons, especially those who are physically fit, who have resting heart rates well below 60 and even 50

NORMAL SINUS HEART RATE : CLINICAL INTUITION AND SCIENTIFIC JUDGMENT

Determining heart rate is fundamental to evaluating patients' status from physical fitness to the entire range of acute and chronic cardiac and noncardiac diagnoses. While any resting daytime heart rate within a range accepted as normal will tell us little, unless it had previously been significantly faster or slower, devia-

·From the Cardiology Division. St. Vincent Hospital . and the Department of Medicine. University of Massachusetts Med ical SelIO()!. Worl'ester. Reprint requests : Dr. Spodick , St Vincent Hospital . Worcester, MA

01604

HR

b /mln 100

MALES

FEMALES

90

80 70 60 50

I

I I

... I

I

9 I I

6 I

I I I

o

II I I I

o t I

11 I

I

b

x

AGE - Y •• r

FIGURE 1. Mean heart rate s (+ SD) by quinquennium in 500 normal subjects. Means are indicated by dashed line s. CHEST I 104 I 3 I SEPTEMBER. 1993

939

tions from the accepted range, judged as either too fast or too slow, can be a dependable sign-often the first-of acute or chronic disease. Fortunately, heart rate is determined as accurately by physical examination as by instrumental recording except, perhaps, at extremely fast rates and with irregular rhythms. The majority of patients with and without heart disease will be in sinus rhythm; therefore, the acceptable limits of heart rate in sinus rhythm-the normal sinus heart rate range-is a datum of elementary clinical importance. It has long been standard teaching that the limits of the normal sinus heart rate are 60 and 100 bpm, conditioning physicians to think "bradycardia" and "tachycardia" only outside of them . However, it appears that such limits were established by "general acceptance:' and a review of the literature 1 disclosed no formal study for their establishment many decades ago. Conceptually, "normal" is subject to wide definition and interpretation. For example, it could be considered "representative of its kind:' yielding the familiar trio average, mode (habitual), or median for any particular class. It is often represented by a Gaussian distribution (ie, a metrical variate with a particular probability density function"), We treat many statistics by assuming a Gaussian distribution (often actually termed "normal distribution curve"), although the very symmetry of Gaussian distribution does not conform to the biological universe, in which virtually nothing is perfectly straight, smooth, round, or regular. Totally apart from theoretical considerations, with their own very important implications, is the practical, or operational, use of normal (accepted) figures, the world in which the physician as clinician or investigator-by no means mutually exclusive roles-must function. Because the classic, generally agreed-upon limits of normal sinus heart rate seemed to be too high on both ends of the scale, we studied a group of 500 consecutively acquired normal patients (367 women and 133 men aged 50 to 80) who reported routinely for annual evaluations.' All of their heart rates were measured by routine electrocardiography while resting in a familiar environment during identical midafternoon hours . We were aware of circadian, particularly sleeping, rate variability, but afternoon is when most physicians encounter most normal patients and those with nonemergent conditions. Their heart rates were examined to investigate the clinical impression that the tachycardia threshold at 100 bpm and the bradycardia threshold at 60 bpm were each rather too high for operational (practical) consideration. Figure 1 shows the distribution by sex, age, and heart rate for each 5 years of age. For this sample, mean heart rate by quinquennium was between 67 940

and 76 bpm, with a fairly wide standard deviation by subgroup (probably due to the relatively small size of the subgroups). For the total sample, mean heart rate was 73 bpm for women and 69 bpm for men. Linear regression of heart rates on patients' ages showed no clinically important dependence of heart rate on age. 1 Two standard deviations yielded rounded extremes of 51 and 95 bpm for women and 46 and 93 bpm for men . The results are remarkably similar to those for resting heart rate in normal subjects between 20 and 80 years in one study of comparable subjects," as well as in the very large Framingham Heart Study for the age deciles 35 through 44 to 75 through 84 years for patients who had a mean heart rate of 74 to 76 bpm, with a slight to absent age trend. The absence of an age trend has also been reported in 24-h monitoring of subjects between 17 and 70 years of age. 6 Our impression of a lower tachycardia threshold is supported by others: in establishing criteria for tachycardia for patients with sepsis, both Knaus and colleagues" and Bone" recently adopted 90 bpm, apparently by intuitive consensus. Moreover, l-year mortality after myocardial infarction is correlated with many factors, but only heart rate was independently predictive and sharply so over 90 bpm ." Furthermore, clinicians have come to realize that physically fit individuals, particularly trained athletes, have slower heart rates, often well below 60 or even 50 bpm, and that such rates are compatible with perfect health and even with exceptional cardiovascular performance. A review of the literature for normal control subjects recruited for various studies and free of invasive instrumentation (since the mere presence of invasive instrumentation would exact a physiologic cost'? yields the results tabulated in Table 1, which appear to support at least the mean values in this study. Indeed, Levine's perceptive analysis of heart rate issues assumes the average "rate of the normal heart" to be 75 bpm.!' Even in the Cardiac Arrhythmia Pilot Study, 12 mean heart rates in 40 patients who were receiving placebos averaged approximately 73 bpm . OPERATIONAL DEFINITION OF NORMAL SINUS HEART

RATE

It is easy to acknowledge that there is not an Table I-Mean Daytime Sinm Heart &tu in Nonnal Control Subjecu &ee oflnooaive lnatrumentat10n Study

Heart Rate, bpm

Kostis et alI3 Kitzman et all< Mltlgaard et ailS Vybiral et al'· Harrison et alI 1

66.9 84.0 77.9 66.9 70.0 69.0

Remes et

all.

RedefinItion 01Normal SInusHeart Rate (David H. Spodlck)

overwhelming amount of data on the normal distribution of human measurements.' For practical purposes, thresholds or cutoffs are necessary, always to be recognized as not rigidly precise. Moreover, although mean values can be determined, it is not clear at what point off-mean values become abnormal.' as implied earlier, statistics based on Gaussian distributions are mainly conveniences. Numerous a priori assumptions are involved, including the generally agreed-upon value of 2 SD from the mean to define the extremes of "normal." Indeed, if there were some method of ascertaining a clinical outcome associated with a particular rate range, one might have a superior definition of that range (although, as noted, postinfarction patients with resting rates in the 90s have an especially impaired prognosis." This does not appear to be practical for either routine bedside diagnosis or ascertainment of abnormality related to heart rate from a single resting value (ie, the data from any individual examination). Whatever the hopes for the future state-of-thescience, the current state-of-the-art strongly suggests an operational interpretation of our results' and those already cited in agreement. 2 •3 .6-8.11 For practical purposes, the range of normal resting sinus heart rate should be shifted down to new limits (rounded for convenience)-SO and 90 bpm, rather than 60 and 100 bpm - to improve the sensitivity of the tachycardia threshold and the specificity of the bradycardia threshold. REFERENCES 1 Spodiek DH, Raju P, Bishop RL, RiOdn RD . Operational definition of normal sinus heart rate . Am J Cardioll992; 69:124546 2 Fleg JL . Alterations in cardiovascular structure and function with advancing age. Am J Cardioll986; 57:33C-44C 3 Kannel WB, Kannel C, Paffenbarger KS Jr, Cupples LA. Heart rate and cardiovascular mortality: the Framingham study. Am

Heart J 1987; 113:1489-94 4 Marshall E . Immune system theories on trial . Science 1986; 1490-92 5 Murphy EA. The logic of medicine. Baltimore: Johns Hopkins University Press, 1970; 124-27 6 Sapoznikov 0, Luria MH. Mahler Y,Gotsman MS. Day vs night ECG and heart rate variability patterns in patients without obvious heart disease . J Electroeardioll992; 25:175-84 7 Knaus WA, Sun X, Nystrom PO, Wagner DP. Evaluation of definitions for sepsis. Chest 1992; 101:1656-62 8 Bone RC. Gram-negative sepsis: background , clinical features and interaction. Chest 1991; l00 :802-oB 9 Hjalmarson A, Gilpin EA, Kjekshus J, Schieman G , Nicod P, Henniey H, et aI. Influence of heart rate on mortality after myocardial infarction . Am J Cardioll990; 65:547-53 10 Spodick DH. Physiologic and prognostic implications oflnvasive monitoring. Am J Cardioll980; 46:173-75 11 Levine HJ. Optimum heart rate in large failing hearts. Am J Cardioll988; 61:633-36 12 Bigger JT [r, Fleiss JL , Rolnitzki LM, Steinman RC. Stability over time of heart rate period variability in patients with previous myocardial infarction and ventricular arrhythmias. Am J Cardiol 1992; 66:718-23 13 Kostis JB, Wilson AC, Moreyra AE, Cosgrove NM, Hosler M, Jaganathan N, et aI. Effect of aging on heart rate and rhythm in subjects free from heart disease: a 7-year prospective study. Am J Noninvasive Cardioll991; 5:285-90 14 Kitzman OW Sheikh Kil, Beere PA, Philips JL, Higginbothain MB. Age-related alterations of Doppler len ventricular filling indexes in normal subjects are independent of len ventricular mass, heart rate , contractility and loading conditions. J Am Coli Cardioll991; 18:1243-50 15 MlSlgaard H, SlSrensen KE, Bjerregaard P. Circadian variation and influence of risk factors on heart rate variability in healthy subjects. Am J Cardioll991; 68:777-84 16 Vybiral T, Bryg RJ, Maddens ME, Boden WE. Effect of passive tilt on sympathetic and parasympathetic components of heart rate variability in normal subjects. Am J Cardiol 1989; 63:11720 17 Harrison MR, Clil\on GO, Pennell AT. DeMaria AN. Effect of heart rate on len ventricular diastolic transmitral flow velocity patterns assessed by Doppler echocardiography in normal subjects. Am J Cardioll991; 67:622-27 18 Remes J, TIkkanen I, Fyrhquist F, Pyoriilli. K. Neuroendocrine activity in untreated heart failure. Br Heart J 1991; 65:249-55

CHEST I 104 I 3 I SEPTEMBER. 1993

941