Heart Rate Recovery, Central Systolic Pressure, and Augmentation Index in Young Healthy Individuals.

PURPOSE: Measuring heart rate recovery (HRR) holds valuable cardiovascular information and requires minimal technical skill and cost. Understanding the associations between HRR and more robust cardiovascular indicators, such as central systolic blood pressure (CSBP), can provide valuable cardiovascular information with less involvement. CSBP is a strong predictor of certain cardiovascular diseases. The study aims to examine the association between measures of HRR and CSBP and the augmentation index (AIx) in a group of young, healthy individuals and based on sex. PARTICIPANTS AND METHODS: One-hundred and seven participants (men - 55, women - 52) were measured for HRR at one minute (HRR1) and two minutes (HRR2) after maximum oxygen consumption (VO2max) testing, CSBP, and the AIx at a heart rate of 75 beats∙min-1 (AIx@75). RESULTS: The Pearson correlation indicated no association between HRR1, HRR2, and CSBP in men and women combined: r = 0.06, P = 0.53; r = 0.05, P = 0.59, respectively, or based on sex: men = r = 0.01, P = 0.95; r = 0.04, P = 0.79, respectively, and women = r = -0.05, P = 0.75; r = -0.09, P = 0.52, respectively. However, there were associations between HRR1 and AIx@75 in men and women combined: r = -0.37, P < 0.001, and based on sex: men = r = -0.31, P = 0.02, and women = r = -0.38, P < 0.01. CONCLUSION: Measures of HRR were not associated with CSBP in a combined group of young men and women or based on sex. Most measures of HRR, especially those established by parasympathetic nervous activity, were associated with lower AIx@75. Though measures of HRR might be good indicators of cardiovascular disease, they might not be good indicators of CSBP in young, healthy individuals.

Impaired autonomic function in normotensive obese children.

PURPOSE: To evaluate baroreflex sensitivity and autonomic control in a cohort of normotensive obese (OB) children. METHOD: A cross-sectional investigation, in which, baroreflex sensitivity and autonomic control were evaluated using transfer function and power spectral density analysis (PSDA) of the electrocardiogram and beat-to-beat blood pressures in normotensive OB children and normal weight normotensive controls. All methods used were noninvasive and data were captured while seated and at rest. RESULTS: The OB and control subjects non-obese (NO) were of similar age (OB 9.1±1.9 years vs. NO 9.2±1.4 years). The OB group was classified as having a BMI greater than the 95th percentile adjusted for age and gender, and a NO group with a BMI at or less than the 95th percentile (BMI percentile OB. 97.5±1.3 vs. NO 58.5±24.1). Both groups had similar normotensive blood pressures. T tests revealed that baroreflex sensitivity and HFRR were significantly reduced in the OB 9.2±2.7 vs. 15.2±4.7 (ms/mmHg) and 6.7±1.1 vs. 8.2±1.1 ms2, respectively, while LFSBP and LF/HF ratio were significantly increased in the OB 11.9±5.6 vs. 8.2±4.7 mmHg2 and 1.2±0.7 vs. 0.54±0.3, respectively. CONCLUSIONS: Normotensive OB children exhibited significantly reduced baroreflex sensitivity, parasympathetic control as well as increased sympathetic control when compared with healthy, NO controls. Findings in this investigation raise the awareness that autonomic dysfunction is present in young OB normotensive children.

Sympathetic drive is modulated by central chemoreceptor activation.

To determine the effects of central chemoreceptor stimulation upon sympathetic modulation while minimizing baroreceptor influences, we performed a single-blind, counter-balanced, placebo-controlled trial of a modified hypercapnic/hyperoxic rebreathe protocol stimulus to activate the central chemoreflex. Muscle sympathetic nerve activity (MSNA), heart rate, blood pressure, and ventilation were recorded dynamically as subjects transitioned from a hypocapnia to hypercapnia state. The stages of data recording were defined as hyperventilation (HyV), pre-threshold (PreT) and post-threshold (PostT), with threshold being defined as the point of non-linear deviation in ventilation. The changes in MSNA (-4.2+/-52.4 arbitrary units (AU) vs. 245.0+/-84.0AU) and burst count (-0.1+/-0.7 bursts/segment vs. 2.5+/-1.7 bursts/segment) were significantly different between control and rebreathe for the HyV to PreT step. There was also a significant difference for PreT to PostT for total MSNA (3.9+/-65.4AU vs. 183.7+/-104.2AU). In a hypercapnic/hyperoxic state, the central chemoreceptors modulate sympathetic activity below the chemoreflex threshold independently of the baroreceptors, possibly contributing to basal autonomic/sympathetic tone. Central chemoreceptors also appear to play a significant role in sympathetic modulation after the threshold.

Autonomic recovery from peak arm exercise in fit and unfit individuals with paraplegia.

INTRODUCTION: Altered autonomic cardiovascular control in persons with paraplegia may reflect peripheral sympathetic denervation caused by the injury or deconditioning due to skeletal muscle paralysis. Parameters of autonomic cardiovascular control may be improved in fit persons with paraplegia similar to effects reported in the noninjured population. PURPOSE: To determine differences in resting and recovery HR and cardiac autonomic control in fit and unfit individuals with paraplegia. METHODS: Eighteen healthy males with paraplegia below T6 were studied; nine participated in aerobic exercise conditioning (fit: >or=30 min.d, >or=3 d.wk, >or=6 months), and nine were sedentary (unfit). Analysis of heart rate variability (HRV) was used to determine spectral power (ln transformed) in the high- (lnHF) and low-frequency (lnLF) bandwidths, and the LF/HF ratio was calculated. Data were collected at baseline (BL) and at 2, 10, 30, 60, and 90 min of recovery from peak arm cycle ergometry. RESULTS: The relative intensity achieved on the peak exercise test was comparable between the groups (i.e., 88% peak predicted HR). However, peak watts (P<0.001) and oxygen consumption (P<0.01) were higher in the fit compared with the unfit group (56 and 51%, respectively). Recovery lnHF was increased (P<0.05), and recovery lnLF (P<0.01) and LF/HF (P<0.05) were reduced in the fit compared with the unfit group. Mean recovery autonomic activity was not different from BL in the fit group. In the unfit group, mean recovery lnHF was reduced, and mean recovery lnLF and LF/HF remained elevated above BL. CONCLUSION: These data suggest that fit individuals with paraplegia have improved cardiac autonomic control during the postexercise recovery period compared with their unfit counterparts.

Evaluation of blood pressure and baroreflex sensitivity by radial artery tonometry versus finger arteriolar photoplethysmography.

BACKGROUND: Published normative data of noninvasive blood pressures (BPs) and autonomic modulations have been primarily derived from the finger arteriole using the Finapres (Ohmeda Co., Englewood, CO), a device that is no longer manufactured. Currently, beat-to-beat BP are obtained from the radial artery using the Colin tonometer. METHODS: We compared BP and autonomic parameters in a crossover design between the two devices in 29 subjects during seated rest and a 0.1-Hz breathing protocol. In addition, we tested whether finger arteriolar BP differences were due to pressure changes exerted by the radial tonometer. RESULTS: Uniformly, BP measured at the radial artery were significantly higher than those from the finger arteriole. Radial BP (106 +/- 19.5 mm Hg) were higher than finger arteriolar BP (95.8 +/- 13.7 mm Hg) (P <.005). Tonometric baroreflex sensitivity (BRS) (24.0 +/- 18 msec/mm Hg) was higher compared to photoplethysmographic BRS (12.0 +/- 7.7 msec/mm Hg; P <.0003). Systolic BP (radial artery) (115 +/- 25 mm Hg) were higher compared to finger arteriolar BP (97.7 +/- 19 mm Hg; P <.0025) during breathing, as was BRS (25.9 +/- 11.6 msec/mm Hg v 21.5 +/- 11.6 msec/mm Hg; P <.05). Differences in the low frequency systolic BP (LF(SBP)), representative of sympathetic vasomotor modulation, between the two methods, whether absolute, normalized, or log-transformed were not observed. CONCLUSIONS: There were no differences in arteriolar BP values in the presence or absence of radial artery tonometric pressure. These findings indicate that differences exist in systolic BP and BRS using the tonometer (radial artery) versus the Finapres (Ohmeda Co.) (finger arteriole). Furthermore, these differences are not due to pressure exerted by the radial artery tonometer that supplies blood to the finger arteriole.

Deriving heart period variability from blood pressure waveforms.

International standards for calculating heart period variability (HPV) from a series of R-wave intervals (R-R) in an electrocardiographic (ECG) recording have been widely accepted. It is possible, and potentially useful in various settings, to use systolic blood pressure waveform intervals to estimate HPV, but the validity of HPV derived from blood pressure (BP) waveforms has not been established. To test the reliability between BP- and ECG-derived HPV indexes, we evaluated data from 234 healthy adults in four studies of HPV reactivity to stress. Study conditions included resting baseline, arithmetic, Stroop test, speech presentation, and orthostatic tilt. Continuous ECG and BP recordings were sampled at a rate of 500 Hz, scored by the same methods, and used to calculate heart rate and time- and frequency-domain measures of HPV. Overall, reliability between the two methods was very high for computing heart rate and HPV indexes. High-frequency HPV indexes were somewhat less reliably computed. In conclusion, in healthy adults, with the use of appropriate methods, BP waveforms can produce reliable indexes of HPV.

The effect of ventilation on spectral analysis of heart rate and blood pressure variability during exercise.

Heart rate variability (HRV) and systolic blood pressure variability (BPV) during incremental exercise at 50, 75, and 100% of previously determined ventilatory threshold (VT) were compared to that of resting controlled breathing (CB) in 12 healthy subjects. CB was matched with exercise-associated respiratory rate, tidal volume, and end-tidal CO(2) for all stages of exercise. Power in the low frequency (LF, 0.04-0.15 Hz) and high frequency (HF, >0.15-0.4 Hz) for HRV and BPV were calculated, using time-frequency domain analysis, from beat-to-beat ECG and non-invasive radial artery blood pressure, respectively. During CB absolute and normalized power in the LF and HF of HRV and BPV were not significantly changed from baseline to maximal breathing. Conversely, during exercise HRV, LF and HF power significantly decreased from baseline to 100% VT while BPV, LF and HF power significantly increased for the same period. These findings suggest that the increases in ventilation associated with incremental exercise do not significantly affect spectral analysis of cardiovascular autonomic modulation in healthy subjects.

Supplemental oxygen increases arterial stiffness in chronic obstructive pulmonary disease.

STUDY OBJECTIVES: Recently, we demonstrated significantly improved baroreflex sensitivity (BRS) and autonomic balance after 31% supplemental oxygen (SuppO2) in resting patients with chronic obstructive pulmonary disease (COPD). In order to investigate whether peripheral arterial stiffness changes may play a role, we evaluated changes in peripheral arterial stiffness and BRS after SuppO2. DESIGN: Single blinded crossover design. SETTING: Pulmonary exercise testing laboratory. PARTICIPANTS: Seventy subjects with moderate to severe COPD. INTERVENTIONS: We measured arterial vascular stiffness using the augmentation index via contour analysis of the radial pulse obtained from applanation tonometry. BRS was derived using the sequence method before and after treatments with compressed air (CA) and 30% SuppO2 in 70 individuals with COPD via a counterbalanced crossover design. RESULTS: Paired t-tests indicated significant differences in oxygen saturation (SaO2) following SuppO2 when compared to CA (mean 96.0+/-2.0% SuppO2 versus mean 92.6+/-3.6% CA, P<0.001). BRS was significantly greater following SuppO2 compared to CA (mean 3.5+/-2.3 ms/mmHg SuppO2 versus mean 3.1+/-2.1 CA ms/mmHg, P<0.03). Vascular stiffness was significantly increased with SuppO2 when compared with CA (mean 13.3+/-6.1% SuppO2 versus mean 10.8+/-4.9% CA, P<0.001). CONCLUSIONS: Our findings indicate that oxygen supplementation ameliorates BRS by changes in vasomotor activity. The amelioration of the BRS into a more normal range is a move towards the restoration of more normal physiology.

Increased QT interval variability in 3 recently concussed athletes: an exploratory observation.

CONTEXT: The QT interval variability index (QTVI) is a noninvasive measure of beat-to-beat fluctuations of the QT interval as seen from a single electrocardiographic lead. It represents the relationship between the respective variabilities of the QT and R-R intervals. Recently, the QTVI was demonstrated to be an index of vagal cardiac autonomic modulation in resting conditions. OBJECTIVE: To determine whether QTVI varied in athletes at 48 hours, 1 week, and 2 weeks after a concussive head injury. DESIGN: Case series. SETTING: Testing facility. PATIENTS OR OTHER PARTICIPANTS: Three athletes with recent concussions and 3 uninjured athletes with similar demographic factors. MAIN OUTCOME MEASURE(S): Continuous 3-lead electrocardiograms were obtained in a seated, resting position over 2 successive weeks. Separate, unpaired t tests were performed to determine whether group-visit differences were present in the QTVI at 48 hours, 1 week, or 2 weeks. RESULTS: No demographic differences were present between groups. At 48 hours, the QTVI was greater in the concussion group than in the matched controls. At weeks 1 and 2, the QTVI in the concussion group was lower than at 48 hours and not different from that of the control group. CONCLUSIONS: Vagal cardiac autonomic modulation, as quantified by the QTVI, appeared to be negatively affected in concussed athletes within 48 hours of injury, resolved within 1 week, and remained at control group levels 2 weeks later. Serial assessments of QTVI may be of clinical utility in identifying suspected cases of acute concussion and may provide helpful information for determining when an athlete can return to play safely.

The acute effects of acupuncture upon autonomic balance in healthy subjects.

Restoration of the sympathovagal (S/V) balance, involving a lowering of sympathetic and/or an augmentation of vagal modulation or a combination of both is associated with improvements in cardiovascular morbidity and mortality. To determine whether acupuncture exerts a favorable influence upon resting blood pressure and sympathovagal balance, a single-blind cross-over investigation was used to study the acute effects of acupuncture on S/V balance in normal healthy subjects. The ANOVA revealed a significant lowering of the sympathovagal balance (LF:HF) during rest for the acupuncture treatment from pre (4 +/- 2 nu) to post (2.2 +/- 1.8 nu)(p < 0.05). No such change was seen during sham treatment. The ANOVA revealed significant differences in systolic blood pressures during rest (114 +/- 4 vs. 108 +/- 3 mmHg) for the acupuncture treatment (p < 0.05). No significance was found during the sham treatment. The ANOVA failed to reveal any significant improvements in sympathovagal balance during the sustained isometric contraction. The clinical significance of these findings appears to suggest that acupuncture treatment might be beneficial in lowering blood pressure at rest. Furthermore, the lowering of the blood pressure might be in part due to a lowering of the sympathovagal balance. These findings are of importance since acupuncture treatments are non-pharmacological and have no known detrimental side-effects. This investigation employed healthy volunteers, yet acupuncture has been found to have more potent effects in animal models of hypertension and or in the presence of an autonomic imbalance.

The effect of chemoreceptor stimulation upon muscle sympathetic nerve activity.

The aim of this investigation was to quantify the combined peripheral and central chemoreceptor contribution to sympathetic outflow above (post) and below (pre) the chemoreceptor ventilatory threshold (CVT). We measured muscle sympathetic nerve activity (MSNA) in seven subjects during hypoxic/hypercapnic and room air rebreathe protocols. Comparisons were made using a repeated measures analysis of variance with two within subject factors. One factor contained three levels--hyperventilation, pre-CVT, and post-CVT. The other factor contained two levels--rebreathe and control. Total MSNA increased from hyperventilation to pre-CVT to post-CVT in the rebreathe trial (385.7+/-95.9, 592.4+/-155.7, 882.0+/-235.4 au/15s respectively) and remained constant in the control trial (433.0+/-189.3, 409.1+/-183.4, 406.1+/-161.4 au/15s respectively). Ventilation increased in the rebreathe trial only. Heart rate and blood pressure did not change in either trial. These data suggest that the chemoreceptors significantly contribute to the modulation of sympathetic outflow.

Transient suppression of heart rate complexity in concussed athletes.

Heart rate variability (HRV) and complexity (HRC) were calculated at rest and during an isometric hand grip test (IHGT) within 48-hours (48 h) and two weeks (Week Two) of a concussion in athletes (CG) and control subjects. No differences were present at rest or in HRV during IGHT. HRC was significantly lower in the CG compared to controls at 48 h during IHGT. In CG at Week Two during IHGT, HRC was significantly greater than 48 h observations and not significantly different than controls. The findings suggest that HRC may have utility in detecting efferent cardiac autonomic anomalies within two weeks of concussion.

Reproducibility of heart rate and blood pressure variability in patients with chronic obstructive pulmonary disease.

Wide variations in respiratory rate and hypoxic stimulation of chemoreceptors may produce unreliable autonomic results in patients with COPD. We studied the reproducibility of two consecutive measurements of heart rate variability (HRV) and blood pressure variability (BPV) by time frequency analysis in patients with COPD while controlling respiratory rate and oxygen hemoglobin saturation (SaO(2)). Reproducibility was assessed by paired t-tests and correlation analyses between repeated measures. Correlation analyses of the log transformed low (LF) and high frequency (HF) HRV were x 11.5 +/- 1.1 in measurement A and x 11.5 +/- 1.0 in measurement B (r = 0.89, p < 0.0001), and x 10.5 +/- 1.1 in measurement A and x 10.6 +/- 1.1 in measurement B (r = 0.89, p < 0.0001) respectively. The log transformed LF and HF BPV were x 4.9 +/- 1.3 in measurement A and x 5.3 +/- 0.9 in measurement B (r = 0.70, p < 0.0002), and x 6.4 +/- 1.3 in measurement A and 6.6 +/- 1.2 in measurement B (R = 0.71 p < 0.0001) respectively. In conclusion, time frequency analysis of HRV and BPV is reproducible and reliable in patients with COPD while controlling their respiratory rate and oxygen hemoglobin saturation. Reproducibility of these measurements may allow for a non-invasive evaluation of autonomic tone in response to treatments in COPD patients.

Cardiac autonomic responses to progressive head-up tilt in individuals with paraplegia.

Beat-to-beat fluctuations in heart rate are partially controlled by the autonomic nervous system and may be altered by a spinal cord injury. The purpose of the present investigation was to examine the role of the autonomic nervous system in modulating the heart rate response to head-up tilt (HUT) in subjects with low lesion paraplegia. Nineteen subjects with paraplegia and nine age-, height-, and weight-matched control subjects consented to participate. A three lead ECG was used to acquire heart rate (HR), cardiac sympathetic [low frequency component of R-R interval variability (LFRRI)], vagal [high frequency component of R-R interval variability (HFRRI)] and sympatho-vagal balance (LF/HF). A finger photoplethysmograph was used to assess beat-to-beat blood pressure for the estimation of sympathetic vasomotor tone [low frequency component of peak systolic blood pressure variability (LFSBP)]. The results showed a significant main effect for tilt angle for the HR response to HUT, which was comparable between the groups. LFRRI was significantly increased (P < 0.001) and HFRRI was significantly reduced (P < 0.001) across tilt angle in the control group, whereas subjects with paraplegia demonstrated no significant change in LFRRI, but significantly reduced HFRRI (P < 0.001) across tilt angle. There was a significant interaction effect for LF/HF (P < 0.05). LFSBP was significantly reduced in the group with paraplegia compared to controls (P < 0.05). These findings suggest that although cardiac autonomic control is intact, there is a blunted sympathetic response to HUT in subjects with low lesion paraplegia, which may implicate an altered baroreceptor response to acute orthostatic provocation.

Cardiac homeostasis is independent of calf venous compliance in subjects with paraplegia.

The purpose of this study was to examine cardiac hemodynamics during acute head-up tilt (HUT) and calf venous function during acute head-down tilt (HDT) in subjects with paraplegia compared with sedentary nondisabled controls. Nineteen paraplegic males (below T6) and nine age-, height-, and weight-matched control subjects participated. Heart rate, stroke volume, and cardiac output were assessed using the noninvasive acetylene uptake method. Venous vascular function of the calf was assessed using venous occlusion plethysmography. After supine measurements were collected, the table was moved to 10 degrees HDT followed by the three levels of HUT (10, 35, and 75 degrees ) in random order. Cardiac hemodynamics were similar between the groups at all positions. Calf circumference was significantly reduced in the paraplegic group compared with the control group (P < 0.001). Venous capacitance and compliance were significantly reduced in the paraplegic compared with control group at supine and HDT. Neither venous capacitance (P = 0.37) nor compliance (P = 0.19) increased from supine with 10 degrees HDT in the paraplegic group. A significant linear relationship was established between supine venous compliance and supine cardiac output in the control group (r = 0.80, P < 0.02) but not in the paraplegic group. The findings of reduced calf circumference and similar venous capacitance at supine rest and 10 degrees HDT in the paraplegic group imply that structural changes may have limited venous dispensability in individuals with chronic paraplegia. Furthermore, the lack of a relationship between supine venous compliance and supine cardiac output suggests that cardiac homeostasis does not rely on venous compliance in subjects with paraplegia.

Low arterial compliance in young African-American males.

Hypertension remains a common public health challenge because of its prevalence and increase in co-morbid cardiovascular diseases. Black males have disproportionate pathophysiological consequences of hypertension compared with any other group in the United States. Alterations in arterial wall compliance and autonomic function often precede the onset of disease. Accordingly, our purpose was to investigate whether differences exist in arterial compliance and autonomic function between young, healthy African-American males without evidence of hypertension and age- and gender-matched non-African-American males. All procedures were carried out noninvasively following rest. Arterial compliance was calculated as the integrated area starting at the well-defined nadir of the incisura of the dicrotic notch to the end of diastole of the radial artery pulse wave. Power spectral analysis of heart rate and blood pressure variability provided distributions representative of parasympathetic and sympathetic modulations and sympathovagal balance. Baroreflex sensitivity (BRS) was calculated using the sequence method. Thirty-two African-American and twenty-nine non-African-American males were comparable in anthropometrics and negative family history of hypertension. t-Tests revealed lower arterial compliance (5.8 +/- 2.4 vs. 8.6 +/- 4.0 mmHg. s; P = 0.0017), parasympathetic modulation (8.9 +/- 1.1 vs. 9.7 +/- 1.1 ln ms2; P = 0.0063), and BRS (13.7 +/- 7.3 vs. 21.1 +/- 8.5 ms/mmHg; P = 0.0007) and higher sympathovagal balance (2.9 +/- 3.2 vs. 1.5 +/- 1.1; P = 0.03) in the African-American group. In summary, differences exist in arterial compliance and autonomic balance in African-American males. These alterations may be antecedent markers of disease and valuable in the detection of degenerative cardiovascular processes in individuals at risk.