Sibrand Houtman

Sympathetic nervous system activity and cardiovascular homeostasis during head-up tilt in patients with spinal cord injuries

The relationship between sympathetic nervous system activity and cardiovascular responses to head-up tilt in patients with spinal cord injuries and in able-bodied subjects was studied. Twenty-seven adults, nine in each of the three groups (tetraplegia, paraplegia, and able-bodied subjects) were tilted 70°, head up, for 12 minutes after 20 minutes supine rest. Differences between steady-state measurements of mean arterial pressure, stroke volume, and sympathetic nervous system activity were estimated in both positions. Sympathetic nervous system activity was reflected by the low-frequency peak of the blood pressure variability spectrum. From supine rest to head-up tilt, low-frequency power increased in able-bodies subjects (median, 0.42 mm Hg2, p=0.003), which was different (p=0.015) from patients with tetraplegia and paraplegia (−0.15 and −0.10 mm Hg2, respectively). Stroke volume and mean arterial pressure decreased in patients with tetraplegia (−40% and −9 mm Hg, respectively; p=0.008, both variables) more than in ablebodied subjects (−33%, 11 mm Hg, respectively) or patients with paraplegia (−24%, 8 mm Hg, respectively). Results indicated increased sympathetic nervous system activity during head-up tilt in able-bodied subjects, but not in patients with paraplegia or tetraplegia, whereas patients with tetraplegia, but not paraplegia, showed poorer cardiovascular homeostasis than able-bodied subjects. This suggests that patients with paraplegia maintained cardiovascular homeostasis during head-up tilt without increased sympathetic nervous system activity.

Physiological responses to asynchronous and synchronous arm-cranking exercise

The purpose of this study was to examine mechanical efficiency (ME) and physiological responses during asynchronous (the pedal arms oriented in opposing directions) arm-cranking exercise (AACE) and compare these responses to those obtained during synchronous (the pedal arms oriented in the same direction) arm-cranking exercise (SACE). Ten male subjects participated in the study and performed two exercise tests, one AACE and the other SACE in counter-balanced order. Each test consisted of submaximal (30, 60 and 90 W) and maximal exercise. At 30 W, gross ME was significantly lower during SACE compared to AACF, whereas at 60 W and 90 W no differences between the two types of exercise could be observed. We found that at lower power output levels the flywheel mass and its moment of inertia may have induced more body movements for compensation, which may have been more pronounced during SACE than during AACE. At higher levels of power output this flywheel mass-effect was less, which explained the lack of differences in ME at these levels. Physiological responses to maximal AACE or SACE exercise were not significantly different. The results indicated that there were no differences in physiological responses to AACE and SACE exercise at higher exercise intensities. However, at lower levels of power output ME seemed to decrease, most likely as a result of the flywheel-mass effect, which was more pronounced during SACE.

Blood Volume and Hemoglobin After Spinal Cord Injury

OBJECTIVE The main purpose of this study was to investigate the difference in total blood volume and hemoglobin mass between spinal cord-injured and able-bodied individuals. DESIGN Total blood volumes of 13 able-bodied and 10 spinal cord-injured individuals (lesion >T4) were determined using the carbon monoxide method. The reproducibility of the total blood volume determination in our setting and the effect of increased physical activity were assessed. RESULTS Comparison of groups showed a significantly higher hemoglobin mass in able-bodied compared with spinal cord-injured individuals. The total blood volume expressed per kilogram of body mass in able-bodied individuals was significantly greater than in spinal cord-injured individuals. CONCLUSIONS These results suggest that total blood volume and hemoglobin mass are decreased in spinal cord-injured individuals with a lesion above T4, which may be related to their inactive lifestyle, because total blood volume increased with increased physical activity in these subjects.

Does peripheral nerve degeneration affect circulatory responses to head-up tilt in spinal cord-injured individuals?

Despite the loss of centrally mediated sympathetic vasoconstriction, spinal cord-injured (SCI) individuals cope surprisingly well with orthostatic challenges. In the pathophysiology of this intriguing observation spinal sympathetic—, veno-arteriolar—(VAR), and myogenic reflexes seem to play a role. The purpose of this study was to assess whether central (stroke volume, heart rate, blood pressure and total peripheral resistance) and peripheral (leg blood flow, leg vascular resistance and femoral arterial diameter) hemodynamic responses to head-up tilt are different in two groups of SCI patients, i. e., SCI individuals with upper motor neuron lesions (who have spinal reflexes, VAR and myogenic reflexes) (U; n=6) and those with lower motor neuron lesion (who have no spinal reflexes, perhaps no VAR due to nerve degeneration, but intact myogenic reflexes) (L; n=5). Ten healthy male individuals served as controls (C) (normal supraspinal sympathetic control and presence of all reflexes). After 10 min supine rest all individuals were tilted to 30° head-up tilt. Red blood cell velocity (measured by echo Doppler ultrasound) in the femoral artery decreased and vascular resistance increased significantly in all three groups in the upright position compared with supine. Mean arterial pressure (MAP) remained unchanged in U and L and increased significantly in C in the upright versus supine position. The present study shows that all SCI individuals were able to maintain MAP by increasing leg vascular resistance during head-up tilt, despite nerve degeneration in L and lack of centrally mediated sympathetic control in all SCI individuals. Results of the present study suggest that not spinal reflexes but local (myogenic) reflex activity plays a pivotal role in peripheral vascular responses upon head-up tilt when central control mechanisms fail.

Non-invasive assessment of autonomic nervous system integrity in able-bodied and spinal cord-injured individuals

The purpose of this study is to identify features of the Valsalva maneuver (VM) that differentiate between able-bodied (AB,n=14) and spinal cord-injured individuals with high (T,n=11) and low (P,n=10) lesions. The systolic blood pressure (BP) recovery during the VM was used as a measure of sympathetic and vagal activity, whereas from the BP rise in phase IV of the VM baroreflex sensitivity was derived. After a similar initial BP decrease in all groups, BP recovered faster VM. After release of the VM, the BP overshoot was very similar in AB and P, suggesting that the BP overshoot reflects sympathetic activity onto the heart. The baroreflex sensitivity was smaller in P, but not in T, compared to AB. However, 36% of the observations of T had such a low correlation between BP and RR interval that a slope could not be determined, which caused loss of observations, but suggested a low baroreflex sensitivity as well.The purpose of this study is to identify features of the Valsalva maneuver (VM) that differentiate between able-bodied (AB,n=14) and spinal cord-injured individuals with high (T,n=11) and low (P,n=10) lesions. The systolic blood pressure (BP) recovery during the VM was used as a measure of sympathetic and vagal activity, whereas from the BP rise in phase IV of the VM baroreflex sensitivity was derived. After a similar initial BP decrease in all groups, BP recovered faster VM. After release of the VM, the BP overshoot was very similar in AB and P, suggesting that the BP overshoot reflects sympathetic activity onto the heart. The baroreflex sensitivity was smaller in P, but not in T, compared to AB. However, 36% of the observations of T had such a low correlation between BP and RR interval that a slope could not be determined, which caused loss of observations, but suggested a low baroreflex sensitivity as well.

Effect of a pulsating anti-gravity suit on peak exercise performance in individual with spinal cord injuries

Abstract The aim of this study was to examine effects of a pulsating pressure anti-gravity suit on the peak values of oxygen uptake (V˙O2) and power during maximal arm exercise in spinal-cord-injured (SCI) individuals. Five well-trained SCI men (with lesions at levels between T6 and L1) and seven well-trained able-bodied men (ABC) performed two incremental (10 W · min−1) arm-cranking tests. During one test the pressure in the anti-G suit pulsated between 4.7 kPa (35 mmHg) and 9.3 kPa (70 mmHg) every 2 s (PPG+), during the other test (PPG−) all the subjects wore the anti-G suit in a deflated state. Tests were performed in a counter-balanced order. Peak V˙O2 in SCI was 1 ml · kg−1 · min−1 lower during PPG+ compared to PPG− (P = 0.05). Peak power and peak heart rate were not significantly different during PPG+ compared to PPG−. These results would suggest that no increase in work capacity can be obtained with a pulsating pressure anti-gravity suit in either SCI or ABC.

The effect of inspired oxygen fraction on peak oxygen uptake during arm exercise

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Non-invasive cardiac output assessment during moderate exercise: pulse contour compared with CO2 rebreathing

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