Andrew P. Blaber

Effect of Endurance Exercise on Autonomic Control of Heart Rate

Long-term endurance training significantly influences how the autonomic nervous system controls heart function. Endurance training increases parasympathetic activity and decreases sympathetic activity in the human heart at rest. These two training-induced autonomic effects, coupled with a possible reduction in intrinsic heart rate, decrease resting heart rate. Long-term endurance training also decreases submaximal exercise heart rate by reducing sympathetic activity to the heart. Physiological ageing is associated with a reduction in parasympathetic control of the heart; this decline in parasympathetic activity can be reduced by regular endurance exercise. Some research has indicated that females have increased parasympathetic and decreased sympathetic control of heart rate. These gender-specific autonomic differences probably contribute to a decreased cardiovascular risk and increased longevity observed in females.

Transfer Function Analysis of Cerebral Autoregulation Dynamics in Autonomic Failure Patients

BACKGROUND AND PURPOSE Autonomic nervous system diseases affect systemic blood pressure regulation. Patients with autonomic nervous system diseases have consistently larger drops in blood pressure associated with standing than the normal population. Autonomic dysfunction and/or these changes in blood pressure may affect dynamic cerebral autoregulation. METHODS Heart rate, mean blood flow velocity (MBFV) of the middle cerebral artery via transcranial Doppler ultrasound, mean arterial blood pressure adjusted to brain level (MABPbrain) via Finapres, and end tidal CO2 were measured continuously during graded tilt (after 5 minutes in supine position as baseline, -10 degrees, +10 degrees, +30 degrees, +60 degrees, -10 degrees, and supine recovery) in autonomic failure patients and their age- and sex-matched control subjects. The dynamic response of MBFV to spontaneous variations in MABPbrain was investigated by cross-spectral analysis. The transfer gain and phase relationships between MBFV and MABPbrain were determined from the final 256 beats of each 5-minute-tilt segment. The transfer gain was normalized to mean MABPbrain and MBFV and then converted to decibels (dB). RESULTS MBFV variation (0.03 to 0.14 Hz) preceded MABPbrain by similar phase angles in patients and control subjects and in all tilt conditions (patients: 31 +/- 5 degrees; control subjects: 30 +/- 5 degrees; mean +/- SEM). Patients had a higher supine gain than control subjects (P < .05). Both patients and control subjects showed a significant decrease in gain with tilt and by 60 degrees the patients were not different from the control subjects (supine to 60 degrees: patients = 5.23 +/- 0.77 to -1.65 +/- 0.89 dB; control subjects = 1.74 +/- 0.82 to -1.80 +/- 0.62 dB). CONCLUSIONS These data indicate an altered, yet present, autoregulatory response with autonomic failure.

The effect of age and gender on heart rate variability after endurance training.

PURPOSE This research investigated the age and gender differences in cardiovascular adaptation to a standardized/quantified endurance-training program that included two taper periods. METHODS The latter was analyzed from spectral analysis of electrocardiogram records of heart rate variability (HRV) at rest in groups of young (19-21 yr) and middle aged (40-45 yr), mixed gender groups (6 males and 6 females), pre- and poststandardized training. All subjects were recreational runners who completed the same 12-wk running program. Before, and subsequent to training, HRV was measured during supine rest and submaximal cycling. RESULTS There was a significant decrease in heart rate both at rest (2.7 +/- 0.45 beats x min-1) and during submaximal exercise (8.1 +/- 0.67 beats x min-1) in both age groups after training. After training, total spectral power increased (560.7 +/- 308.9 ms2), as well as high-frequency power (362.3 +/- 405.5 ms2), in both age groups at rest. The young group showed a greater increase in total power (849.0 +/- 308.7 ms2) after the training program. CONCLUSION It is concluded that a well-designed 12-wk endurance-training program will decrease resting and submaximal heart rate in both younger and older adults. The significant increase in HRV, total power, and high-frequency power in all groups after endurance training indicates that HRV measurement appears to provide an effective, noninvasive assessment of cardiovascular adaptation to aerobic training.

Impairment of Cerebral Blood Flow Regulation in Astronauts With Orthostatic Intolerance After Flight

Background and Purpose— We investigated cerebral blood flow regulation in astronauts before and after flights. We hypothesized that autoregulation would be different before flight and after flight between nonfinishers and the finishers of a stand test. Methods— Twenty-seven astronauts from shuttle missions lasting 8 to 16 days underwent a 10-minute stand test: 10 days before flight, 1 to 2 hours and 3 days after landing. Mean blood flow velocity of the middle cerebral artery (MCA) was measured using transcranial Doppler; Mean arterial pressure was measured using a Finapres (Ohmeda, Englewood, CO) and was adjusted to the level of the MCA (BPMCA). Cross-spectral power, gain, phase, and coherence were determined for the relation between BPMCA and the cerebrovascular resistance index mean blood flow velocity/BPMCA. Results— BPMCA was reduced with stand (P<0.001). Differences between finishers and nonfinishers (P=0.011) and over test days (P=0.004) were observed. Cerebrovascular conductance was affected by stand (P<0.001), by group (P<0.001) with a group by stand, and test day interaction (P<0.01). Preflight data suggest that the nonfinishers were operating at a higher cerebral vasodilation than finishers for a given BPMCA, and on landing day the nonfinishers had a greater decrease in mean blood flow velocity as a function of BPMCA with standing compared to finishers and preflight. There was a significant interaction effect of gender over the test days and from supine to stand (P=0.035). Conclusions— Our results indicate that the cause of presyncope in astronauts may be related to a mismatch of cerebral blood flow with blood pressure. Astronaut gender may also play a role in susceptibility to orthostatic intolerance after flight.

Cardiovascular regulation during long-duration spaceflights to the International Space Station.

Early evidence from long-duration flights indicates general cardiovascular deconditioning, including reduced arterial baroreflex gain. The current study investigated the spontaneous baroreflex and markers of cardiovascular control in six male astronauts living for 2-6 mo on the International Space Station. Measurements were made from the finger arterial pressure waves during spontaneous breathing (SB) in the supine posture pre- and postflight and during SB and paced breathing (PB, 0.1 Hz) in a seated posture pre- and postflight, as well as early and late in the missions. There were no changes in preflight measurements of heart rate (HR), blood pressure (BP), or spontaneous baroreflex compared with in-flight measurements. There were, however, increases in the estimate of left ventricular ejection time index and a late in-flight increase in cardiac output (CO). The high-frequency component of RR interval spectral power, arterial pulse pressure, and stroke volume were reduced in-flight. Postflight there was a small increase compared with preflight in HR (60.0 ± 9.4 vs. 54.9 ± 9.6 beats/min in the seated posture, P < 0.05) and CO (5.6 ± 0.8 vs. 5.0 ± 1.0 l/min, P < 0.01). Arterial baroreflex response slope was not changed during spaceflight, while a 34% reduction from preflight in baroreflex slope during postflight PB was significant (7.1 ± 2.4 vs. 13.4 ± 6.8 ms/mmHg), but a smaller average reduction (25%) during SB (8.0 ± 2.1 vs. 13.6 ± 7.4 ms/mmHg) was not significant. Overall, these data show no change in markers of cardiovascular stability during long-duration spaceflight and only relatively small changes postflight at rest in the seated position. The current program routine of countermeasures on the International Space Station provided sufficient stimulus to maintain cardiovascular stability under resting conditions during long-duration spaceflight.

Precordial vibrations provide noninvasive detection of early-stage hemorrhage.

ABSTRACT Graded lower-body negative pressure was used to create a hemodynamic response similar to hemorrhage. Echocardiogram measurements showed a maximal reduction of 32.4% in stroke volume. Analysis of systolic time intervals, such as pre-ejection period and left ventricular ejection time (LVET), derived from a seismocardiogram (SCG), were demonstrated to be more sensitive in detection of early-stage hemorrhage compared with pulse pressure, heart rate, and the amplitude features extracted from SCG. In particular, the LVET and pre-ejection period/LVET features, extracted from SCG, were significantly different between, and correlated with, the different stages of lower-body negative pressure (r = 0.9 and 0.88, P < 0.05), for 32 subjects. These results suggest a portable, cost-effective solution for identification of mild or moderate hemorrhage using accelerometers.

Complexity of middle cerebral artery blood flow velocity: effects of tilt and autonomic failure

We examined spectral fractal characteristics of middle cerebral artery (MCA) mean blood flow velocity (MFV) and mean arterial blood pressure adjusted to the level of the brain (MAPbrain) during graded tilt (5 min supine, -10°, 10°, 30°, 60°, -10°, supine) in eight autonomic failure patients and age- and sex-matched controls. From supine to 60°, patients had a larger drop in MAPbrain (62 ± 4.7 vs. 23 ± 4.5 mmHg, P < 0.001; means ± SE) and MFV (16.4 ± 3.8 vs. 7.0 ± 2.5 cm/s, P < 0.001) than in controls. From supine to 60°, there was a trend toward a decrease in the slope of the fractal component (β) of MFV (MFV-β) in both the patients and the controls, but only the patients had a significant decrease in MFV-β (supine: patient = 2.21 ± 0.18, control = 1.99 ± 0.60; 60°: patient = 1.46 ± 0.24, control = 1.62 ± 0.19). The β value of MAPbrain(MAPbrain-β; 2.19 ± 0.05) was not significantly different between patients and controls and did not change with tilt. High and low degrees of regulatory complexity are indicated by values of β close to 1.0 and 2.0, respectively. The increase in fractal complexity of cerebral MFV in the patients with tilt suggests an increase in the degree of autoregulation in the patients. This may be related to the drop in MAPbrain. The different response of MFV-β compared with that of MAPbrain-β also indicates that MFV-β is related to the regulation of cerebral vascular resistance and not systemic blood pressure.We examined spectral fractal characteristics of middle cerebral artery (MCA) mean blood flow velocity (MFV) and mean arterial blood pressure adjusted to the level of the brain (MAPbrain) during graded tilt (5 min supine, -10 degrees, 10 degrees, 30 degrees, 60 degrees, -10 degrees, supine) in eight autonomic failure patients and age- and sex-matched controls. From supine to 60 degrees, patients had a larger drop in MAPbrain (62 +/- 4.7 vs. 23 +/- 4.5 mmHg, P < 0.001; means +/- SE) and MFV (16.4 +/- 3.8 vs. 7.0 +/- 2.5 cm/s, P < 0.001) than in controls. From supine to 60 degrees, there was a trend toward a decrease in the slope of the fractal component (beta) of MFV (MFV-beta) in both the patients and the controls, but only the patients had a significant decrease in MFV-beta (supine: patient = 2.21 +/- 0.18, control = 1.99 +/- 0.60; 60 degrees: patient = 1.46 +/- 0.24, control = 1.62 +/- 0.19). The beta value of MAPbrain (MAPbrain-beta; 2.19 +/- 0.05) was not significantly different between patients and controls and did not change with tilt. High and low degrees of regulatory complexity are indicated by values of beta close to 1.0 and 2.0, respectively. The increase in fractal complexity of cerebral MFV in the patients with tilt suggests an increase in the degree of autoregulation in the patients. This may be related to the drop in MAPbrain. The different response of MFV-beta compared with that of MAPbrain-beta also indicates that MFV-beta is related to the regulation of cerebral vascular resistance and not systemic blood pressure.

Automatic Annotation of Seismocardiogram With High-Frequency Precordial Accelerations

Seismocardiogram (SCG) is the low-frequency vibrations signal recorded from the chest using accelerometers. Peaks on dorsoventral and sternal SCG correspond to specific cardiac events. Prior research work has shown the potential of extracting such peaks for various types of monitoring and diagnosis applications. However, annotation of these peaks is not a trivial task and complicated in some subjects. In this paper, an automated method is proposed to annotate these peaks. The high-frequency accelerations obtained from the same accelerometer, used to record SCG with, were used to facilitate the annotation of the SCG. Algorithms were developed for detection of isovolumic moment (IM) and aortic valve closure (AC) points of SCG. Four different envelope calculation methods were used: cardiac sound characteristic waveform (CSCW), Shannon, absolute, and Hilbert. The algorithms were evaluated based on a dataset including 18 subjects undergoing lower body negative pressure and were further tested with another dataset, which included 67 subjects. These datasets had been previously manually annotated. The algorithm based on CSCW envelope calculation produced the highest detection accuracy for both IM and AC. The overall CSCW algorithm detection accuracy for the test dataset was 98.7% and 99.1% for the IM and AC points, respectively.

Cerebrovascular autoregulation: lessons learned from spaceflight research.

This review summarizes our current understanding of cerebral blood flow regulation with exposure to microgravity, outlines potential mechanisms associated with post-flight orthostatic intolerance, and proposes future directions for research and linkages with cerebrovascular disorders found in the general population. It encompasses research from cellular mechanisms (e.g. hind limb suspension: tissue, animal studies) to whole body analysis with respect to understanding human responses using space analogue studies (bed rest, parabolic flight) as well as data collected before, during, and after spaceflight. Recent evidence indicates that cerebrovascular autoregulation may be impaired in some astronauts leading to increased susceptibility to syncope upon return to a gravitational environment. The proposed review not only provides insights into the mechanisms of post-flight orthostatic intolerance, but also increases our understanding of the mechanisms associated with pathophysiological conditions (e.g. unexplained syncope) with clinical applications in relation to postural hypotension or intradialytic hypotension.

Heart rate variability and short duration spaceflight: relationship to post-flight orthostatic intolerance

BackgroundUpon return from space many astronauts experience symptoms of orthostatic intolerance. Research has implicated altered autonomic cardiovascular regulation due to spaceflight with further evidence to suggest that there might be pre-flight autonomic indicators of post-flight orthostatic intolerance. We used heart rate variability (HRV) to determine whether autonomic regulation of the heart in astronauts who did or did not experience post-flight orthostatic intolerance was different pre-flight and/or was differentially affected by short duration (8 – 16 days) spaceflight. HRV data from ten-minute stand tests collected from the 29 astronauts 10 days pre-flight, on landing day and three days post-flight were analysed using coarse graining spectral analysis. From the total power (PTOT), the harmonic component was extracted and divided into high (PHI: >0.15 Hz) and low (PLO: = 0.15 Hz) frequency power regions. Given the distribution of autonomic nervous system activity with frequency at the sinus node, PHI/PTOT was used as an indicator of parasympathetic activity; PLO/PTOT as an indicator of sympathetic activity; and, PLO/PHI as an estimate of sympathovagal balance.ResultsTwenty-one astronauts were classified as finishers, and eight as non-finishers, based on their ability to remain standing for 10 minutes on landing day. Pre-flight, non-finishers had a higher supine PHI/PTOT than finishers. Supine PHI/PTOT was the same pre-flight and on landing day in the finishers; whereas, in the non-finishers it was reduced. The ratio PLO/PHI was lower in non-finishers compared to finishers and was unaffected by spaceflight. Pre-flight, both finishers and non-finishers had similar supine values of PLO/PTOT, which increased from supine to stand. Following spaceflight, only the finishers had an increase in PLO/PTOT from supine to stand.ConclusionsBoth finishers and non-finishers had an increase in sympathetic activity with stand on pre-flight, yet only finishers retained this response on landing day. Non-finishers also had lower sympathovagal balance and higher pre-flight supine parasympathetic activity than finishers. These results suggest pre-flight autonomic status and post-flight impairment in autonomic control of the heart may contribute to orthostatic intolerance. The mechanism by which higher pre-flight parasympathetic activity might contribute to post-flight orthostatic intolerance is not understood and requires further investigation.