Measurement of Parasympathetic Activity from 24-Hour Ambulatory

Electrocardiograms and Its Reproducibility and Sensitivity in Normal Subjects,

Patients With Symptomatic Myocardial Ischemia, and Patients With

Diabetes Mellitus James Nolan, MRCP, Andrew D. Flapan, MD, Nicholas E. Goodfield, MRCP,

Robin J. Prescott, PhD, Peter Bloomfield, MD, James M.M. Neilson, PhD, and David J. Ewing, FRCP

The rasympathetic nervous system plays a major role in tr e pathophysiology of many cardiovascular dis- eases, particularly in modulating myocardial electrical stability. Measurements of heart mte variability have been widely used to assess parasym The reproducibility of measurements o r

thetic activity. tained from 24-

hour ambulatory electrocardiograms has not been well documented. We h,ave developed a techni ue for mea- suring parasympathetic activity from clinica 7 quali hour ambulatory electrocardiograms by counting t

24- eat-

to-beat increases in RR interval that are GO ms. To determine the reproducibility and sensitivity of our tech- nique, we analyzed repeated 24-hour electrocardio- grams of 173 subjects (19 normal subiects, 67 patients with ischemic heart disease, and 87 diabetics) followed

A utonomic dysfunction plays a major role in the pathophysiology of many medical conditions. Tests

of cardiovascular reflexes or short-term heart rate (HR) variability have traditionally been used to investigate changes in autonomic activity.’ More recently, measure- ment of HR variability from 24-hour ambulatory elec- trocardiograms has been applied to the investigation of autonomic dysfunction. We developed a time domain method of measuring HR variability from 24-hour ambu- latory electrocardiograms that appears to be a specific index of parasympathetic activity,2 and applied it to the study of normal subjects, 2,3 diabetics,3 and patients with cardiovascular disease.b9 The occurrence of a high degree of variability in repeated measurements obtained with our technique would reduce its value as a means of measuring parasympathetic activity, particularly when assessing patients undergoing interventions or serial fol- low-up. The aim of this study was to investigate the short- and long-term reproducibility and sensitivity of our technique in different subject groups.

From the Department of Cardiology. and University Departments of Medical Physics and Medical Statistics, Royal Infirmaly, Edinburgh, Scotland, United Kingdom. Manuscript received April 4, 1995; revised manuscript received and accepted October 3, 1995.

Address for reprints: James Nolan, MRCP, General Infirmary, Great George St., Leeds, United Kingdom, LSl 3EX.

154 THE AMERICAN JOURNAL OF CARDIOLOGYg? VOL. 77

up over periods of 2 to 16 weeks. In all subject groups, mean values for repeated measurements were virhral-

1 identical. Measurements were stable in all 3 groups

roughout the course of the study, as assessed by intr- aclass correlation coefficients. This technique is sensi- tive enough to detect relatively small changes in para- sympathetic activity in sub+ as demonstrated by the calculated Bland and Altman coefficients of repeatabil- ity. Reproducibili particularly g oaf

and sensitivi in normal su ?i

of our technique are jects and in patients

with ischemic heart disease. The results obtained with this technique imply that other related measurements of parasym thetic activity will show similar excellent short- and ong-term reproducibility and sensitivity. r

(Am J Cardiol 1996;77: 154-l 58)

METHODS Study subjech: We assessed the reproducibility of our

technique in 3 groups of subjects: (1) 19 normal sub- jects, mean age & SD 44 + 13 years. All normal subjects were free of cardiovascular disease, autonomic neu- ropathy, diabetes mellitus, or renal failure, and received no drug therapy during the study; (2) 67 patients with ischemic heart disease; aged 58 f 7 years, who had symptoms of angina, positive Bruce protocol exercise test results, and angiographic evidence of coronary artery disease with varying degrees of left ventricular dys- function (all patients with ischemic heart disease re- mained symptomatically stable with no change in drug therapy during the course of the study); and (3) 87 pa- tients with longstanding diabetes mellitus aged 52 + 12 years. All diabetic patients remained clinically stable with no changes in drug therapy during the course of the study.

Data collection: Twenty-four-hour ambulatory elec- trocardiograms were obtained in all subjects using a min- iature tape recorder (Tracker, Reynolds Medical Ltd., Hertford, United Kingdom) with a crystal-generated tim- ing track that allows correction for recording and replay speed errors (a feature essential for accurate measure- ment of HR variability) and has some practical advan- tages over other recording systems.‘” Recordings were

JANUARY 15, 1996

TABLE I Results of Analysis of Repeated Ambulatory Electrocardiograms

Group Tape 1 Tape 2

lntraclass Coefficient Correlation of

p Value Coefficient Repeatability

Normal subjects

Patients with ischemic heart disease

Patients with diabetes Group 1

Group 2

Group 3

Group 4

6,280 6,208 0.85 0.97 0.21 (503-l 8,620) (328-l 8,030)

2,133 2,275 0.30 0.94 0.41 (103:14,223) (18-l 7,48 1)

258 (18-7,014)

787

(18-12,246) 433

(7-l 2,246) 376

276 0.62 0.91 0.61 (1 O-7,870)

875 0.60 0.93 0.68

(30-10,641) 370 0.41 0.80 0.98

(1 O-8,166) 383 0.91 0.86 0.87

(6-l 1,766) (17-9,397)

Values are expressed as group mean (range) sNN50 counts and measurements of introsubject reproducibility. sNN50 = individual number of increases in successive NN intervals x50 ms over o 24.hour period.

obtained during normal ambulant out-of-hospital activi- ties. Ambulatory electrocardiograms were repeated in normal subjects after 13 f 9 da>Js. Ambulatory clectro- cardiograms were repeated in patients with ischemic heart disease after 14 + 13 days. Baseline recordings ob- tained in 87 diabetic patients were repeated at intervals of 29 + I1 days in 28 (group 1): 63 +- 7 days in 16 (group 2). 92 + 9 days in 36 (group 3), and 115 f 1.5 days in 36 (group 4). Diabetics were chosen for the long-term fol- low-up study because they remained clinically stable over several months, and underwent regular hospital fol- low-up and thus were suitable for repeated monitoring.

Measurement of heart rate variability: Twenty-four- hour ambulatory electrocardiograms were repla}Ted through a Pathfinder arrhythmia analyzer (Reynolds Medical Ltd, Hertford: United Kingdom) at I20 limes the original recording speed. Segments of tape in which changes in RR interval duration arise due to the occur- rencc of supraventricular or ventricular cctopics were cx- eluded from analysis by the analyzer, which can detect such cctopics by their difierencc in timing and mor- phology.” Because the analyzer takes no account of P- wave morphology, and is thus unable to determine whether complexes showing minor degrees of prematu- rity are of sinus origin: the signal was also closely mon- itored by an experienced operator. who discarded seg- ments of tape in which frequent late-coupled supraven- tricular complexes ol’ possible ectopic origin occurred. The overall accuracy of QRS detection with this system is high,12 and the speed surveyor in the replay unit min- imizes inaccuracy due to speed variation: the system is therefore ideal for HR \/ariability measurements.“’ The remaining normal-to-normal (NN) RR intervals were measured, and increases in successive NN intervals >50 ms were counted as previously described in detai1.2.s.” To facilitate comparison between subjects. counts ob- taincd from suitable recordings were normalized to the exact 24-hour value and this standardized value was pre- scnted as the sNNS0 count.

Statistical analysis: Since the sNN.50 co~mls obtained with our technique are not normally distributcd,2 we log- transl’ormcd the values before statistical analysis. All sta- tistical inferences were based on analysis of the trans-

formed data. Group sNNS0 results are expressed as geo- metric mean (range). A paired t test was used to com- pare group mean counts obtained from repeated record- ings.

Group mean values for repeated studies may be sim- ilar despite the presence of considerable variation in indi- vidual measurements. To quantify the degree of agree- ment lor repeated measurements within sub.jects, data from repeated studies were analyzed by means of 1 -way random ell’ects analysis of variance. Study subjects defined the random factor, with 2 times measurement for each. Components of variance due to differences within patients were d&cd and the intraclass correlation coef- ticient was calculated as a mcasurc ol’intrasubject repro- ducibilitvI”.IA; . . h f., >t’ it is t e lclc Ion of person-lo-person vari- ancc of& observed measurements that is due to the variance of steady-state values (the remaining fraction represents lability or mcasurcment error). An intraclass correlation coefficient of >0.79 indicates good repro- ducibility in sub,jects.ls

To define the magnitude of change in log-transformed sNN50 counts necessary for an individual difference in repeated measurements to be coniidently ascribed to a true change in HR varjability rather than inherent mea- surement variation: we calculated the coeflicient of repeatability of Bland and Altman.‘” The values defined by the coefficient of repealability can be considered as limits for a biologically important change in HR vari- ability. 1-f the coetlicient of repeatability is large, the sen- sitivity of the technique to small changes in cardiac parasympathetic activity in patients will be poor.

RESULTS Able 1 details the group mean sNNS0 counts for

repeated ambulatory electrocardiograms in each study group. As cxpectcd, count rates are highest in normal subjects and lowest in diabetics. Total 24-hour sNNSO counts from the repeated ambulatory electrocardiograms in each study group arc plotted against each other on a logarithmic scale in Figure I. The solid line on the graphs is the lint of identity, along which all the points would lie if’ there was perfect agreement f’or repeated measure- mcnts. There is limited scatter of points around the line

sNNS0 COUNTS sNN50 COUNTS (first recording)

sNN50 COUNTS (first recording)

E

0 10 101 102 103 104

sNN50 COUNTS (first recording)

0

D

104-

103-

23 E$ 102-

uu Ott

z” 2 2 lol- WY

%

10 -

10 101 10 2 103 104

sNN50 COUNTS (first recording)

. 0 10 101 10 2 10.3 104

F sNNS0 COUNTS (first recording)

FIGURE 1. lndiidual (sNN50) counts for repeated ambulatory electrocardiograms in normal subjects (., patients with cardiovas- cular disease (61, and diabetic group 1 [Cl, 2 (D), 3 (El, and 4 IFI. Wd I I mes re number of increases in successive NN intervals ~50 ms over a 24-hour period sNN50 counts) on the first occasion are plotted on the x-axis, and those for the second occasion on the y-axis.

P

resent the line of identity. Values for individual

156 THE AMERICAN JOURNAL OF CARDIOlOGY”’ VOl 77 ]A.NUARY 15. 1996

of identity for all study groups. suggesting that intra- subject reproducibility will bc good.

Table I also details the reproducibility mcasuremcnts for each study group. The intraclass correlation coetii- cient is >0.8 in all patients, confirming the good intra- subject reproducibility (Figures 1A to IF). The mean of the differences between paired recordings was close to zero in all study groups. allowing Bland and Altman coefficients of repeatability to bc reliably calculated. The coefficients of repeatability detailed in Table I indicate that a relatively small chang.e in log-translormed counts is significant, particularly m normal subjects and in patients with ischemic heart disease.

DISCUSSION This study investigates the reproducibility and scnsi-

tivity of our technique for measuring HR variability from ambulatory electrocardiograms. Previous studies of reproducibility in mcasurcments of HR variability have been restricted to small numbers ol’ normal subjccts17 I’) or short-term hollow-up.- ‘() 22 We studied I73 subjects (including normal persons, patients with ischemic heart disease, and diabetics) followed up over 2 to 16 weeks.

There is very little variation in group mean sNNS0 counts for mcasuremcnts repeated over short time intcr- vals in normal sub.jects or in patients with ischemic heart disease (p >O.OS for all differences) (Table I). Similarly. there is very little variation in group mean sNT’J50 counts for measurements repeated in diabetic subjects at all of the time intervals investigated (p >0.0.5 Ii-jr all dilfer- ences). Thus, group mean values arc stable and highly reproducible in all subject groups over periods of 2 to 16 weeks.

The results presented in Figure 1 and Table I demon- strate that short- and long-term intrasubject reproducibil- ity for measurements obtained with our technique is good in normal subjects: patients with ischcmic heart disease, and diabetics. We have pre\Gously demonstrated that our technique is more sensitive than conventional cardio- vascular reflex tests3 The present study enables us to define the sensitivity of our technique in more detail. The coeilicients of repeatability detailed in Table I d&e the limits lor a biologically important change in log-trans- formed counts in a subject. These coeficients indicate that our technique is sensitive enough to dctcct relative- ly small temporal changes in parasympathetic activity associated with alterations in disease state or intervcn- tions, such as drug therapy. particularly in normal sub- jccts or in patients with ischemic heart disease. The supe- rior reproducibility demonstrated for measurements obtained in normal subjects and in patients with ischc- mic heart disease compared with that in diabetic subjects may be due to the greatct statistical reliability associal- ed with high count rates in the lirst 2 groups. The find- ing of the highest intraclass correlation coeflicicnt and lowest coefficient ol’ repeatability in the group with the highest sNNSO count rate (normal subjects, Table I) would support this contention. Because diabetics with low count rates showed ,g”od reproducibility over the 16 week followup period, It IS likely that subjects with high- cr count rates will have superior long-term reliability.

Parasympathetic activity can be assessed using a va- riety of dilrerent techniyucs. Short-term changes in HR that occur in response to stimuli, such as deep breathing, change in posture, or the Valsalva maneuver. are well- validated simple tests that have been widely applied to the investigation of parasympathetic integrity.’ More re- cently a number of djll%rent time and lrequency domain techniques have been developed to ineasure para- sympathetic activity li-om 24-hour ambulatory electro- cardiograms.*‘~*” The amount of high-frequency vari- ability present in the power spectrum: the root-mean- square successive difference index, and our sNN5O index and its derivatives are all highly correlated quantitative indexes of parasympathetic activity.‘“*” The results ob- tained in this study with our sNNS0 index imply that mca- surements of parasympathetic activity obtained with these other relalcd indexes are also likely to show good short- and long-term reproducibility and scnsitivitp al- though this has not been assessed in the present study. Broad-based time-domain measures ol‘ HR variability and other frequency domain indexes. such as mid- and low-frequency components of the power spectrum, are modulated by a wide variety 01‘ factors, such as changes in sympathetic activity, thermorcgulatory mechanisms, and the renin-angjotcnsin system.23.2J The results ob- tained with our specific parasympathetic index therefore cannot be taken to indicate that these broad-based measurements are also likely to demonstrate similar re- liability.

In summary. we have demonstrated that measurement of parasympathetic activity from 24-hour ambulatory electrocardiograms is rcproduciblc and sensitive in both the short and long term in different subject groups, even when severe parasympathetic impairment is present. Bc- cause our sNN5CJ technique is valid, even in the pres- cnce of frequent arrhythmias,” artifact. I0 and heart rhythm changes that can adversely affect measurements obtained with other techniques, ‘2.26-30 it may be prcfcrable for assessing parasympathetic activity from routine clinical quality 24-hour ambulatory electrocardiograms.

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158 THE ,AMERiCAN JOURNAI. OF CA.RDIOLOGF VCL. 77 jP.NUA,RY 15, 1996