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Original Studies |
MRC Metabolic Programming Group (D.E.F., D.I.P.), University of Southampton, Southampton General Hospital, Southampton SO16 6YD, United Kingdom; Department of Cardiovascular Medicine (J.C.V.), Queen Elizabeth Medical Centre, University of Birmingham B15 2TH, United Kingdom; Department of Medical Physics (G.W.P.), University of Southampton, Southampton General Hospital, Southampton S01 66YD, United Kingdom; Department of Obstetrics and Gynaecology (V.M.M., J.S.R.), University of Adelaide, South Australia 5005; Wynn Department of Metabolic Medicine (I.F.G.), Imperial College School of Medicine, London NW8 95Q, United Kingdom; and Child Development Unit (R.A.C.), Womens and Childrens Hospital, Adelaide, South Australia 5006
Address all correspondence and requests for reprints to: Dr. David Phillips, Medical Research Council Unit, Southampton General Hospital, Southampton SO16 6YD, United Kingdom. E-mail: diwp{at}mrc.soton.ac.uk
| Abstract |
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| Introduction |
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Most studies relating HR to the insulin resistance syndrome have depended on the resting pulse rate, which is an imprecise and highly variable physiological measurement, influenced by many factors. Analysis of the small beat-to-beat variations of the HR has been shown to provide a more accurate assessment of the neural regulation of the heart than the HR itself. The recent development of computer techniques to quantify HR variability (HRV) provides quantitative indices of sympathetic and vagal activity by means of power spectral analysis (10). We have applied this technique to examine the relationship between autonomic activity, as indicated by HRV, and Si in a group of young men and women. Si was measured using the iv glucose tolerance test (IVGTT) with minimal model analysis.
| Subjects and Methods |
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An ECG was performed on each subject using a portable analogue tape recorder (Oxford Instruments Co. Ltd., Osney Mead, Oxford) via bipolar skin electrodes applied to the chest wall. Subjects lay supine on a couch in a quiet room; and before the start of each recording, a 30-min rest period was allowed, to enable HR, blood pressure, and ventilation to stabilize. A 15-min undisturbed ECG recording was then obtained.
Subjects then underwent a fifteen-point frequently-sampled IVGTT. Each subject received a glucose dose of 0.5 g/kg BW, as 50% wt/vol dextrose via an antecubital vein, over a 3-min period. Blood was sampled from the opposite arm at the following time points: -30, -5, 3, 5, 7, 10, 15, 20, 30, 45, 60, 75, 90, 120, and 180 min. Plasma samples were analyzed for glucose using the hexokinase method and insulin using two-site immunometric assays with alkaline phosphatase as the label (12). The within-assay coefficient of variation of the insulin measurements was less than 10%.
Data analysis
Si was determined, from the IVGTT glucose and insulin profiles, using the minimal model of glucose disappearance (13), with programs written in Fortran 77 run on a PDP-11/83 microcomputer. The IVGTT protocol employed in the present study differs, in two respects, from that traditionally used with mathematical modeling analysis. First, after glucose injection, a reduced sample schedule of 15 (rather than 26) samples is followed, the reduced schedule being more useful for relatively large studies. Second, a glucose load of 0.5 g/kg (rather than 0.3 g/kg) is employed, which provides for a sufficient endogenous insulin response in nondiabetic volunteers, without recourse to additional augmentation of pancreatic insulin secretion. The validity and effectiveness of the IVGTT protocol employed in the present study, with regard to the measurement of Si, is apparent in the high rate of model identification and good correlation with measures of Si derived from the euglycemic clamp (r = 0.92) that it provides (14, 15).
The ECG tape cassettes were replayed on a separate playback deck (Oxford Instruments Co. Ltd.). The ECG signal was sampled at 125 Hz, digitized into 12 bits, and analyzed by an Apple Macintosh IIci microcomputer, running Lab-View software (National Instruments Corp., Austin, TX). R waves were detected by individually adjusted thresholds for each recording. Of the initial 15-min recordings, 256-beat segments were selected for analysis, based on the absence of ectopic beats and stationarity of the time series, by an observer who was blinded to the insulin resistance results. HRV was analyzed off-line using the Lab-View 3.1 software.
Frequency domain analysis was performed to determine the power of the underlying component oscillations. Power spectral analysis was performed using the Burg Algorithm (16) with a model order between 8 and 12 (17). The power of each underlying frequency was quantified by decomposing the total variability signal with the method of Zetterberg (18). This enabled the determination of power at the two major peaks in the HRV spectrum: low-frequency (LF) power (arising between 0.050.15 Hz) and high-frequency (HF) power (0.150.40 Hz). Because total power varies greatly between individual subjects, power was expressed as normalized units, calculated by dividing the [absolute power of a given component (area under the component curve)] by the [total variance minus the DC component] (10). Evidence suggests that the power of the LF peak is determined by sympathetic activity with vagal modulation (19), whereas the HF peak corresponds to respiratory sinus arrhythmia and provides an index of cardiac vagal activity (10). Thus, the high-to-low ratio (HLratio) is taken as a measure of sympathovagal balance (10, 20).
Statistical methods
Where necessary, the data was transformed to normality using
logarithms or square-root transformation (Si), and the means are
therefore presented as back-transformed values. Independent samples
(t tests) were used to compare means in Table 1
. We analyzed the data using multiple linear
regression; all analyses were undertaken using continuous variables.
P values in Tables 24![]()
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are derived from the relevant
correlation or regression equations.
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| Results |
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Figure 1
shows the relationship between
Si and HR for men and women. Men, with a lower HR, were more insulin
sensitive (r = -0.410, P = 0.004). However, in
women, there was no association between Si and the resting HR
(P = 0.67). Although Si correlated strongly with all
indices of obesity in both genders (P < 0.001), HR was
not associated with indices of obesity in either men or women. HR was
higher in subjects reporting a lower level of physical activity (none,
69; moderate exercise or more, 64; P = 0.04) but was
not associated with either smoking habit or alcohol consumption. In a
multiple regression analysis in men (with Si as the dependent variable
and HR, BMI, and the usual level of physical activity as independent
variables), the effects of HR (P < 0.001) and BMI
(P = 0.001), but not physical activity, were
statistically significant. In a similar regression analysis in women,
Si was associated with BMI (P < 0.001) but not HR or
physical activity. The interaction between gender and HR was tested in
a least-squares regression model with Si as the dependent variable and
BMI, HR, gender, and the interaction term (gender x HR). The
interaction term was significant at P = 0.002.
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Table 3
shows the relationship between
resting HR and HF and LF power obtained for men and women. In both
genders, a higher HR was associated with a lower HF power, an increase
in the LF power, and a lower ratio of HF to LF power (HLratio),
suggesting a shift in the balance (away from vagal, towards sympathetic
influence) as HR increases. Table 4
shows the
relationship between Si and the HLratio for men and women. As was
observed in Fig. 1
, the trends differ in men and women. In men, greater
Si was associated with a fall in the LF component of HR
(P = 0.008), an increase in HF component
(P = 0.001), and (as shown in Table 4
) a rise in
HLratio (r = 0.291, P = 0.002). These
relationships were not seen in women. Neither the HF nor the LF peaks
were associated with indices of obesity or exercise in either gender.
In multiple regression analyses, the relationship between Si and the
HLratio in men was independent of BMI, fat distribution, or the current
level of physical exercise.
Because Si did not relate to either the HR or HRV in women, we
investigated the hypothesis that exposure to hormonal contraception or
endogenous steroids, as indicated by the menstrual cycle, might
confound the association between Si and HR. A total of 32 women were
taking oral contraceptives; but neither their Si, HR, nor their HRV
differed significantly from that of the other women. Furthermore, in
the subset of women not on hormonal contraception, there was no
evidence of a relationship between Si and HR or HRV. In Table 5
, we have analyzed the relation-ship
between the phase of the menstrual cycle and the measurements of
HR, HRV, and Si in women. During the follicular phase (days 014), the
HR was significantly lower and Si significantly higher than during the
luteal phase(days 1528). However, the higher luteal HR does not
seem to be caused by altered sympathovagal influence, because neither
the measurements of HF and LF power nor their ratio were related to the
phase of the menstrual cycle.
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| Discussion |
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The resting HR is a highly variable physiological measurement and is affected by many influences, including stress, activity, and stimulant drugs (including caffeine or nicotine). Although we carried out this study under standardized conditions, and we controlled for these factors as much as possible (by dietary restriction before the study and by performing a relatively long resting ECG), the measurements we obtained are likely to be imprecise. Despite this imprecision, which will tend to lead to underestimation of the strength of the associations, we have found strong and statistically significant relationships between Si and HR in male subjects.
The power spectral analysis of HRV provides further evidence that the relationships between HR and Si are attributable to alterations in autonomic function. Power spectral analysis of HR fluctuations enables quantitative evaluation of beat-to-beat cardiovascular control. Although cardiac automaticity is intrinsic, the resting HR is largely under the control of the autonomic nervous system. The parasympathetic influence on HR is mediated via cholinergic vagal nerve fibers. In contrast, the sympathetic influence is mediated by catecholamines. Under resting conditions, beat-to-beat variations in HR are dependent on the interaction of vagal and sympathetic activity. HF power is recognized as a marker of the influence of vagal tone on HR, whereas LF power is thought to represent a combination of sympathetic and vagal mechanisms (10, 20). Hence, the HLratio is considered to represent the balance between vagal and sympathetic contributions to HRV (10, 20). Our data showing that insulin-resistant individuals have a lower HF component (together with an increased LF component and a lower HLratio) suggest that reduced Si in men is associated with a reduced vagal contribution and an increased sympathetic contribution to cardiac autonomic control.
A striking feature of our study is the finding of gender differences in
the association between Si and HR. Although the women in our study had
a similar BMI to that of the men, they had a greater degree of upper or
truncal adiposity, as shown by their increased bicep and tricep
skinfold thicknesses, and were less insulin sensitive (Table 1
).
Compared with the men, women also had a higher resting HR, but
evidence of reduced sympathetic cardiac autonomic activity, as
indicated by an increased HLratio. These findings suggest that the
interrelationships between Si, HR, and HRV differ in women. Together
with our finding of marked gender differences in the correlation
between Si and HR (Fig. 1
), they suggest that Si is less dependent on
autonomic tone in women. It is possible that this reflects an
underlying gender difference in the response to stressful stimuli,
which has been reported in both human and animal studies (21). Another
possible explanation, which is supported by our data, is that the
influence of gonadal steroids on HR obscures the influence of autonomic
effects. We showed that women had a higher HR and lower Si during the
luteal (than during the follicular) phase of the menstrual cycle (Table 5
). Similar relationships have been reported in some, but not all,
previous studies (22, 23). Because HRV was not related to stage of the
menstrual cycle, it seems that these menstrual variations in HR are not
mediated by alterations in autonomic tone.
In conclusion, we have shown, in young men but not young women, that a raised resting HR and increased HRV are associated with greater insulin resistance. Though our data provide support for the hypothesis that SNS activity and insulin resistance are linked, they also show that the interrelationships are complex and differ substantially between the sexes.
| Acknowledgments |
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| Footnotes |
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Received August 12, 1998.
Revised December 28, 1998.
Accepted January 5, 1999.
| References |
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