Differences in pulse rate and heart rate and effects on the calculation ...

Journal of Behavioral Medicine, Vol. 17, No. 1, 1994

Differences in Pulse Rate and Heart Rate and Effects on the Calculation of Heart Rate Reactivity During Periods of Mental Stress Christopher F. Sharpley 1 Accepted for publication: March 24, 1993

Heart rate and reactivity from pulse and ECG were compared over rest and mental arithmetic periods of 2-min duration each for 32 males and 50 females. Data from the two sources of heart rate were not significantly different during the rest period but did differ significantly during periods of heart rate acceleration and deceleration. Sex effects were also noted, with females having consistently higher heart rates from both sources of measurement. Calculation of heart rate reactivity via five procedures based upon the wider literature revealed significant differences between data from different sources of heart rate. Implications for assessment of heart rate reactivity to laboratory stressors are discussed, with suggestions for future research. KEY WORDS: heart rate reactivity; gender differences; methodology.

INTRODUCTION The reliable measurement of heart rate is necessary for much research in behavioral medicine which focuses on psychophysiological responsiveness to stimuli. One method of assessing heart rate which is recommended in basic methodological texts as "sometimes preferred for . . . safety and convenience" (Hassett, 1978, p. 173), and providing a "signal (which) is relatively stable and could be used for the measurement of heart rate" (Stem et al., 1980, p. 190), is plethysmography. In the experimental literature devoted to heart rate reactivity, the use of plethysmography is not infrequent. 1Centre for Stress Management & Research, Monash University, Clayton, Victoria, 3168, Australia. 99

0160-7715/94/020o-0099507.00/09 1994 PlenumPublishingCorporation

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For example, Abbot, et al. (1987) measured heart rate "continuously as digital pulsation using a photocell plethysmograph attached to the subject's ear" (p. 3) in their study of subjects' heart rate responses to a memory reaction task. Other similar studies of heart rate reactivity assessed via plethysmography are not difficult to find in the literature (e.g., Cinciripini et al., 1989; Emmons and Weidner, 1988; Houston et al., 1989; Jorgensen and Houston, 1989; Linden, 1987). In support of this practice, one study presented data from resting heart rate measured by ECG and an automated sphygmomanometer on the nondominant arm as being correlated between .94 and .99 in a sample of 45 female adults (Matthews et al., 1987). However, the sites at which pulse is assessed by plethysmography (e.g., the nondominant arm, earlobe) are some distance from the heart itself. Thus, measurement of "heart rate" at these sites relies on pulse rather than the actual electrical activity of the heart. It may be that pulse transit time ( F I T ) varies as actual heart rate increases or decreases, leading in turn to differences between "heart rate" as measured by ECG (i.e., the "real" heart rate) and pulse. Although this variation may be confined to the period immediately following an increase/decrease of heart rate, it may contribute to variations in heart rate reactivity (HRR) if heart rate data used to calculate H R R are based upon pulse versus ECG. That is, plethysmography "heart rate" and H R R data may vary from ECG "heart rate" and H R R data. This may not necessarily be a source of invalidity in those studies mentioned above, largely because the assessment of heart rate reactivity in those s t u d i e s was based u p o n d a t a c o l l e c t e d a f t e r the hypothesized change in pulse rate due to hypothesized variations in PTI'. However, even if only because plethysmography represents a relatively noninvasive procedure compared to ECG, there is a need to clarify (a) the existence and degree of disagreement between heart rate data collected via ECG and plethysmography; (b) the conditions (if any) under which such disagreement exists; and (c) the potential effect which this disagreement may have upon the calculation of heart rate reactivity. Because it was largely exploratory in nature, the present study also included both sexes so as to enable comparison of males' versus females' data on this issue.

METHOD Subjects Thirty males and 52 females (M age = 28.2 years; range, 18 to 61 years) who were either staff or students at the author's institution volunteered for this study. The preponderance of females was an unexpected

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experimental artifact. Data analyses were conducted with sex as a factor, thus enabling identification of any effects due to this imbalance in the number of males and females sampled. All subjects were in good health, with blood pressure less than 140/90, and none had ever suffered from any cardiac disease. All subjects were tested in a laboratory on campus, and all procedures were standardized and applied by a research assistant who had used the protocol with over 200 other subjects.

Apparatus Because the major arguments supporting the use of plethysmography are based upon noninvasiveness and convenience, two comparably noninvasive measuring devices were used in the present study. That is, if a significant difference was found to exist between the heart rate (HR) and the heart rate reactivity data obtained from plethysmography versus ECG, then a convenient and relatively noninvasive E C G measuring device might be favored by the arguments put by Stern and colleagues (1980). Pulse was continuously measured by a photoelectric pulse transducer attached to the earlobe on the subject's nondominant side, and data were fed into an Apple 11e personal computer and stored on floppy disk for later analysis. These data were collapsed into 5-sec means to match the process used in the PE 4000 described below. E C G was measured by the Sports Tester PE 4000 device, which consists of a rubber strap containing sensors designed to detect the electrical activity of the heart. This strap is worn around the subject's chest, and the data collected are transmitted by radio waves to a wristwatch recorder which is used to store the data. The time period between the subject's R-waves is sampled, and the calculation of heart rate is based on a pulse-to-pulse time-averaging algorithm which uses the first four pulse values to calculate heart rate via a moving-average process described by Karvonen et al. (1984). The PE 4000 has high agreement with standardized E C G in the laboratory [r = .97 to .99 (Treiber et al., 1989)]. Both instruments were mechanically coupled together so as to commence sampling heart rate and pulse together and to gather data on these variables every 5-sec. The stressor task used was mental arithmetic (MA) as described elsewhere (Sharpley, 1989). Briefly, this task consists of a series of mental arithmetic questions of the type "89 - 34 + 14 = ?" which are presented via audiotape for standardization purposes. Each question takes 4 sec for presentation and 2 sec are allowed for the subject to write the answer to the question onto a sheet of paper provided for this purpose before the next question is presented. In addition, a $10 note is attached to the wall

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in front of the subjects, and this is "for the person who gets most mental arithmetic questions correct." Thus, the three required aspects of any psychological stressor (cognitive effort, time-pressure, and c o m p e t i t i o n ) (Cinciripini, 1986) were present in this task.

Procedure After having the purpose of the two heart rate measurement devices explained to them and being fitted with both, subjects underwent a 15-min adaptation period, 2 rain rest, and 2 min of the MA task.

Statistical Analyses To test the degree of similarity/difference between the two sets of heart rate data, Pearson correlations and M A N O V A s with repeated measures were planned for Rest and MA data separately. Heart rate reactivity is usually calculated by subtracting some value obtained during Rest from a value observed during the stressor period. To use the procedure most easily generalizable to the wider literature, a survey of research published in the major journals during the last 4 years was performed. From the studies found, 40% used the means of 1-min periods; 22% used the means of 2-, 3-, 4-, or 5-min periods; 15% used means of periods between 30 and 45 sec; 15% used means of periods of between 10 and 30 sec; 4% used the mean heart rate per second; and the remainder used the maximum and minimum heart rates during 10-sec periods. Thus, there appears to be some lack of consistency in the choice of what length of time heart rate is measured for and what reductions of these data may be performed when determining heart rate reactivity to selected stressors. As mentioned above, heart rate was sampled every 5 sec in the present study and used as the unit of analysis to test for differences between heart rate and pulse. Following this, five separate methods were used for calculating heart rate reactivity in an attempt to cover as many as possible of those procedures in the literature noted above. With each of these five chosen methods, heart rate reactivity was calculated for heart rate from plethysmography and E C G separately, then A N O V A was used to test for the presence of significant differences between the reactivity values obtained from each source of heart rate. Each of these five methods has a high degree of face validity and is at least as justifiable as the wide range of procedures found in the literature. First, the mean of the entire Rest period (i.e., 2 rain) was subtracted from the mean of the entire MA period (2 min). Second, the mean of the last 30-sec Rest was subtracted from the mean of the first 30-sec

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MA. Third, heart rate 30 sec prior to the end of the Rest period was subtracted from heart rate 30 sec after the commencement of the MA period (i.e., at a single point in time during each period). Fourth, heart rate 60 sec prior to the end of the Rest period was subtracted from heart rate at the end of the first minute of the MA period (again at a single point in time). Finally, the mean of the three observations (i.e., over a 15-see period) which gave the lowest value for heart rate during the Rest period was subtracted from the mean of the three observations which gave the highest value for heart rate during the MA period (these data were taken from the group means as shown in Fig. 1).

RESULTS Data from the two measures of heart rate are presented in Fig. 1 and clearly indicate that resting heart rate was relatively steady during the final minute of the Rest period, following a brief increase about 15 sec after the commencement of the Rest period, perhaps due to the instructions to relax which followed the Adaptation period. Although the actual

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mean values for heart rate during this period were slightly higher than the commonly considered "normal" level of about 72 bpm, they were stable (particularly ECG), and not dissimilar to those in at least one other source (Guyton, 1977). They may thus be considered to represent the resting level for these subjects. Following the onset of the MA task there was a dramatic increase in mean heart rate as measured by both instruments, with the rate of increase apparently greater for pulse than for the ECG, lending initial support for the suggestion referred to above that decreases in pulse transit time during acceleration may lead to an additional increase in pulse. In fact, if the E C G is taken as the yardstick of heart rate here, then it may be seen that pulse was consistently faster than actual heart rate (i.e., via E C G ) during the first 40 sec of the MA period. Statistical analysis of these data was performed in a series of steps. First, Pearson correlations were calculated for the relationship between pulse and E C G at each 5-sec observation of Rest and MA. All these 48 correlations were significant at the p < .001 level, and ranged from .436 (at the first observation of the MA period) to .983 (at several points during the final 70 sec of the MA period). In contrast to this finding (based upon similarity), M A N O V A with repeated measures with two within-groups factors (sex, measure) was performed to detect the presence of significant differences between the two sets of heart rate data. [Violations in the assumption of sphericity which sometimes arise when repeated measurements are taken on the same subjects at closely spaced points in time are resolved by M A N O V A (Jennings, 1987).] Data from the Rest period indicated that there was no significant difference in E C G heart rate versus pulse over all 24 Rest observations. There was a significant sex effect [Gender: F(1,80) = 9.11, p < .01], with females having higher mean heart rates than males during the Rest period, regardless of how heart rate was measured. This relationship did not differ significantly over the 24 observations of the Rest period. As might be expected from Fig. 1, there was a significant change in heart rate over all 24 observations of the Rest period [Time: F(23,58) = 5.146, p < .001], regardless of measure or sex. From Fig. 1, it is apparent that H R during the MA period should not be treated as a homogeneous set of data. Therefore, analysis was performed in sections, following the trends evident in Fig. 1. First, observations 1 to 4 of this period were examined. Again, females had significantly higher heart rates regardless of how this was measured [Gender: F(1,80) = 5.780, p < .05]. Unlike the Rest period, there was a significant difference in heart rate as measured by the two instruments during these 20 sec [Method: F(1,80) = 64.34, p < .001], with pulse being clearly higher than E C G .

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T h e r e was a significant change in heart rate (regardless of measure or sex) during this initial section of the MA period [Time: F(3,78) = 33.629, p < .001], indicating that the MA task continued to significantly elevate subjects' heart rates during this first 20 sec of the MA period. During the next six observations (30 sec), heart rate continued to increase, but at a slower rate (see Fig. 1). Analysis of these data showed that females had significantly higher H R than males over all six observations, independent of measure [Gender: F(1,80) = 5.610, p < .05]. Pulse was again significantly higher than heart rate from ECG [Method: F(1,80) = 7.710, p < .01], and there was a significant increase In H R during these six observations [Time: F(5,70) = 5.236, p < .001], regardless of sex or measure. Finally, H R data from the final 14 observations of the MA period were consistent in their gradual decrease. Again females' H R were significantly higher than males' [Gender: F(1,78) = 6.930, p < .05], and there was a significant difference in heart rate as measured by the two instruments [Method: F(1,78) = 9.340, p < .005]. However, unlike the periods of H R acceleration, pulse H R was lower than E C G H R during these 14 observations. There was a significant change in heart rate over this part of the MA period [Time: F(13,66) = 6.126, p < .001], but this did not vary according to sex or measure. The next step in data analysis was to examine the effects of using pulse versus E C G for the calculation of heart rate reactivity ( H R R ) acc o r d i n g to the five p r o c e d u r e s d e s c r i b e d a b o v e . R e s u l t s of t h e s e comparisons for the total sample, males, and females are presented in Table I and indicate that 6 of the 15 comparisons made were significant. That is, using pulse to calculate H R R would result in a significant overor underestimate in a considerable subset of the possible procedures used in this study (which are generalizable to those used in the wider literature).

DISCUSSION The first conclusion which may be drawn from these data is that there is some evidence that there are differences in heart rate as assessed by pulse versus E C G during periods of mental stress and that these differences extend to the calculation of H R R based upon these two indices. However, the issue of validity of H R and H R R via pulse versus E C G is not a question of instrumentation exclusively, but more an interaction between instrument a t i o n and the two f a c t o r s of (a) e x p e r i m e n t a l c o n d i t i o n s and (b) methodology used for calculating H R R .

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Concerning experimental conditions, it is apparent from these data that plethysmography and E C G produce H R data which do not differ significantly during rest, thus allowing use of either instrumentation. However, during either HR-acceleration or HR-deceleration periods induced by the onset and continuance of the MA stressor used here, there were significant differences between pulse and E C G HR. During H R acceleration, pulse was significantly higher than E C G HR. During H R deceleration, pulse was significantly lower than ECG, supporting the suggestion that variations in P T T do effect the values for H R from pulse. When H R data are used to calculate H R R to the stressor condition, there is some basis for suggesting that the selection of a single H R value taken 1 min before and after the onset of the stressor period will produce the most reliable estimate from plethysmography. Although it is true that either pulse or E C G may be used with a satisfactory degree of validity in m o s t cases, there was a substantial number of significant differences between the H R R values obtained from pulse and E C G when the methods of calculation were similar to those used quite often in the literature. If, for example, the differences noted here were generalizable to some of the other studies which were mentioned in the Introduction to this paper, then it might be that the conclusions drawn from those data would not be the same. However, this paper is not intended to be a critique of previous literature, merely an exploratory investigation. Several issues arise for f u r t h e r c o n s i d e r a t i o n and investigation. Quite clearly there is a need for some standardization in terms of the use of plethysmography versus E C G in studies of H R under nonrest conditions. Related to this is the requirement for agreement on how H R R is calculated from HR. The five procedures used here were devised to cover most of those noted in the recent literature. It may be that further development of alternative procedures could help resolve this issue and provide a standard method for determining H R R , regardless of, or in interaction with, the measuring device used to collect heart rate data. A further issue which requires attention is the interaction of sex and m e t h o d of calculating H R R , as suggested by the inconsistencies in the occurrence of significant differences between pulse and E C G for males and females over the five methods. Additionally, different stressors may produce different "patterns" of H R responses during the period of the stressor. Alt h o u g h t h e r e is some e v i d e n c e that generalizability of H R R across various stressors is legitimate (Turner, 1988), those data did not refer to the interaction of stressor, method of calculating H R R , and device used for collecting H R data. This issue may be of particular relevance when considering the effects which variations in blood pressure have upon PTI'. Isolation of each of these factors could elucidate this relationship and

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its effects u p o n H R R . A n o t h e r issue for f u t u r e r e s e a r c h is the site used for m e a s u r e m e n t of pulse. If it is t r u e t h a t v a r i a t i o n s in P T T c a n d i s t o r t the p u l s e interval, t h e n the total d i s t a n c e o f the p e r i p h e r a l p u l s e f r o m the h e a r t m a y c h a n g e the d e g r e e o f d i s t o r t i o n . F o r e x a m p l e , p u l s e collected from the e a r l o b e m a y p r o d u c e m o r e d i s t o r t i o n t h a n a c a r o t i d p u l s e b u t less t h a n a f i n g e r pulse. T h e e a r l o b e site was c h o s e n in this r e s e a r c h b e c a u s e o f logistic ease at this stage o f i n v e s t i g a t i o n of the issue of a g r e e m e n t b e t w e e n p u l s e a n d h e a r t rate. W h a t a p p e a r s clear from this initial study into the issue of a g r e e m e n t b e t w e e n these two m e t h o d s of m e a s u r i n g " h e a r t rate" a n d H R R is the n e e d for r e s e a r c h e r s to take the distorting effects of P T T u p o n pulse (wherever it is m e a s u r e d ) into c o n s i d e r a t i o n w h e n investigating psychophysiological reactivity to stressful events.

REFERENCES Abbot, J., Sutherland, C., and Watt, D. (1987). Cooperative dyadic interactions, perceived control, and task difficulty in Type A and Type B individuals: A cardiovascular study. Psychophysiology, 24: 1-13. Cinciripini, P. M. (1986). Cognitive stress and cardiovascular reactivity: Relationship to hypertension. Am. Heart J. 112: 1044-1050. Cinciripini, P. M., Benedict, C. E., Vunakis, H. V., Mace, R., Lapitsky, L., Kitchens, K., Nezami, E., and Gjika, H. B. (1989). The effects of smoking on the mood, cardiovascular and adrenergic reactivity of heavy and light smokers in a non-stressful environment. BioL Psychol. 29: 273-289. Emmons, K. M., and Weidner, G. (1988). The effects of cognitive and physical stress on cardiovascular reactivity among smokers and oral contraceptive Psychophysiology, 25: 166-171. Guyton, A. C. (1977). Basic Human Physiology, Saunders, London. Hassett, J. (1978). A Primer of Psychophysiology, Freeman, New York. Houston, B. K., Smith, M. A., and Cates, D. S. (1989). Hostility patterns and cardiovascular reactivity to stress. Psychophysiology, 26: 337-342. Jennings, J. R. (1987). Editorial policy on analyses of variance with repeated measures. Psychophysiology, 24: 474-475. Jorgensen, R. S., and Houston, B. K. (1989). Reporting life events and family history of hypertension, and cardiovascular activity at rest and during psychological stress. Biol. Psyehol. 28: 135-148. Karvonen, J. J., Cwalbinska-Moneta, J., and Saynajakangas, S. (1984). Comparison of heart rates measured by ECG and microcomputer. Physiol. Sports Med. 12: 65-69. Linden, W. (1987). Effect of noise distraction during mental arithmetic on phasic cardiovascular reactivity. Psychophysiology 24: 328-333. Matthews, K. A., Rakaczky, C. J., Stoney, C. M., and Manuck, S. B. (1987). Are cardiovascular responses to behavioral stressors stable individual difference variable in childhood'? Psychophysiology, 24: 464-473. Sharpley, C. F. (1989). Biofeedback training versus simple instructions to reduce heart rate reactivity to a psychological stressor. J. Behav. Med. 12: 435-447. Stern, R. M., Ray, W. J., and Davis, C. M. (1980). Psychophysiological Recording, Oxford University Press, New York.

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Treiber, F., Musante, L., Hartdagan, S., Davis, H., Levy, M., and Strong, W. (1989). Validation of a heart rate monitor with children in laboratory and field settings. Med. Sci. Sports Exer. 21: 338-342. Turner, R. (1988). Inter-task consistency: An integrative re-evaluation. Psychophysiology, 25: 235-238.