Biomedical subjects

D Essfeld

Publications and source records attributed to D Essfeld.

44 records · Page 3Linked to original sources

PubMed1988

The influence of muscle interstitial volume on K+-induced heart rate drive in rats.

During exercise heart rate is influenced by reflexogenic drives which are elicited by receptors situated in the interstitial space. Since the structure of interstitial tissue is complex (e.g. fixed negative charges of glycosaminoglycans), the situation in the immediate surrounding of the receptors might differ from the free fluid phases of blood or lymph in which the concentrations of stimulating substances are usually determined. Physiological variations of the interstitial structure may be due to changes in interstitial volume induced by exercise or the hydrostatic effects on body fluids. The objective of the present study was to investigate the effect of the interstitial volume on the relationship between heart rate and K+ stimuli applied through the muscle blood vessels. The calves of 12 male Wistar rats were artificially perfused and separated from the rest of the body with the sciatic nerve remaining intact. In these preparations the heart rate (HR) responses to low (4 mM) and high (8 mM) potassium concentrations were determined at different interstitial volumes. Expansion of the interstitial volume was obtained by reducing the colloid-osmotic pressure of the perfusate. The combination of intracellular oedema and mechanical limitation of total volume expansion (tapeing) was utilized to decrease the interstitial volume. When switching between the low and high potassium concentrations, significant heart rate responses could be observed only with reduced interstitial volume. It is suggested that the interstitial structure surrounding the muscular receptors modifies the relationship between heart rate response and the K+ stimuli determined in blood or lymph.

Animals↗

PubMed1988

Local fluid losses enhance heart rate drives in light to moderate exercise.

UNLABELLED: Dynamic calf muscle exercise of different intensities was performed at 90 degrees knee-joint flexion. To obtain different hydrostatic pressures and calf volumes, all exercise tests were conducted both in prone and supine position after 10 min of rest in the respective positions. Severe prone exercise was associated with increased anaerobic metabolism, reduced exercise tolerance time, and pain sensation. Despite these differences, maximal heart rates were nearly identical in both positions. During light to moderate exercise, heart rate was significantly increased in the prone position. In a steady-state exercise test this difference developed gradually during the first 10 min of exercise. Neither changes in muscle metabolism nor baroreceptor influence or pain sensation could account for this effect. DISCUSSION: 1) At high exercise intensities in the prone position, the decreased hydrostatic pressure reduces maximal exercise duration by virtue of reduced maximal muscle perfusion. 2) In steady-state prone exercise, the local fluid loss appears to increase peripheral heart rate drives.

Adult↗

PubMed1987

VO2 kinetics in subjects differing in aerobic capacity: investigation by spectral analysis.

The power-VO2 transfer functions of 38 subjects differing in aerobic capacity have been determined on the basis of breath-by-breath total oxygen uptake (VO2,t) measurements during light cycle ergometer exercise (lactic acid concentrations below 2 mmol.l-1). At constant pedalling frequency (1 Hz) pseudorandom binary sequences (PRBS) of workload were used as the testing signal. The VO2,t response was analysed by autocorrelating the ergometer power input and crosscorrelating the power input and VO2,t output. From the spectra of these functions the amplitude ratios and phase relationships were computed for the first six harmonics of the PRBS fundamental (14 mrad X s-1). We found that differences in aerobic capacity are associated with significant differences in the amplitude plots of the VO2 transfer function.

Adult↗

PubMed1986

The influence of furosemide on heart rate and oxygen uptake in exercising man.

We recently observed that heart rate (HR) related methods for assessing physical fitness lead to an overestimation of endurance capacity in subjects treated with furosemide. To gain a more detailed description of this effect, the relationships between work load (WL), oxygen uptake (VO2), and HR were determined in the present study. To this end, nine healthy male subjects performed two incremental exercise tests (10 W increase per 30 s) on a bicycle ergometer. In one test 40 mg furosemide (Lasix) was applied orally 90 min before exercise started. Compared with control conditions, furosemide led to a change in mean blood volume of -4.5% (range: +7.8% to -11.5%). Neither the maximal VO2 (VO2max) nor the maximal work load (WLmax) were significantly altered after furosemide application. Though the WL-VO2 relationship was not significantly affected, the HR-VO2 relationship showed significant alterations which depended on both the loss of blood volume (BV) and work intensity: When the reduction in BV was less than approximately 5%, HR was found to be lowered at all workloads. When the BV reduction was greater than about 5% HR was significantly reduced only in the lower ranges of work load but significantly increased at the higher work intensities. Since BV reductions are known to increase HR during exercise, our findings suggest that, in addition to the blood volume induced changes in HR, furosemide exerts further direct or indirect effects on heart rate adjustment.

Adult↗

PubMed1985

Effects of a 7-day head-down tilt (-6 degrees) on the dynamics of oxygen uptake and heart rate adjustment in upright exercise.

Oxygen uptake (VO2) kinetics and heart rate (HR) kinetics were studied in six healthy male students before and on days 1,3 and 5 after a continuous 7-d antiorthostatic bedrest (-6 degrees). The exercise test protocol consisted of pseudorandom binary sequences (PRBS) of workload (W) performed on a bicycle ergometer in the upright position (20 W - 80 W, 15 bits, 30 s per bit; the sequence was repeated three times). Amplitude ratio and phase of the W-VO2 and W-HR relations were computed at six harmonic frequencies in the range 0.014 - 0.084 rad X s-1. After bedrest the VO2 kinetics was found to be impaired at the harmonic frequencies greater than 0.056 rad X s-1. Additionally, the mean heart rate during the PRBS cycles was increased (108 +/- 15 as compared to 92 +/- 10 min-1). There were no significant effects on HR kinetics and on the static W-VO2 relation. During an endurance training program both VO2 and HR changes were restored to the pre-bedrest levels. It is concluded that the impairment of VO2 kinetics can be attributed mainly to muscular factors.

Adult↗

PubMed1983

CO2-H+ stimuli and neural muscular drive to ventilation during dynamic exercise: comparison of stimuli at constant levels of ventilation.

In exercising man, the ventilatory responses to CO2-H+ stimuli and neural muscular drives were compared at constant ventilation (VE). For that purpose, a small increase of the CO2-H+ stimulus in exercise was to be counterbalanced by work load reductions in such a way that the magnitude of ventilation remained unchanged. Control of end-tidal PO2 and PCO2 (PETO2, PETCO2) was established to minimize the influence of changed mixed venous gas tensions on the arterial levels. Only in metabolic acidosis could the additional CO2 stimulus be compensated by work load reduction. This compensation was due to the concomitant decrease of acidosis. Below the 2 mmol X l-1 [La]a threshold, decrements of work load, VO2, and VCO2 showed no effect on VE, when PETCO2 and PETO2 were regulated at constant levels. After the termination of end-tidal clamps, the proportional relation of VE to VO2, VCO2, and work load was largely reestablished. The results show that neural muscular drives cannot decrease ventilation against a background of constant arterial feedback stimuli. Transient decreases of the CO2-H+ stimulus seem to be necessary to readjust the ventilation to a decreased CO2 flow to the lungs. It is suggested that the overall effect of decreasing CO2 is to inhibit the respiratory centers and that positive ventilatory effects of CO2 are the result of a disinhibitory influence.

Acidosis↗

PubMedDate not supplied

Glycolytic ATP production estimated from 31P magnetic resonance spectroscopy measurements during ischemic exercise in vivo.

In an oxygen-depleted muscle, glycolytically produced ATP is inversely related to the ([ATP]+ creatine phosphate [PCr]) decrease because ATP, PCr, and glycolysis are virtually the only energy sources under these conditions. In particular, the onset of glycolysis or any appreciable increase in the rate of glycolytic ATP production will lead to a slower rate of ([ATP]+ [PCr]) breakdown at a given energy consumption. To quantify this relationship, endurance athletes performed isometric foot plantar flexion (20% of a test force [TF], n = 10; 50% TF, n = 5) during local arterial occlusion. Parameters of energy metabolism were measured with 31P magnetic resonance spectroscopy (31P-MRS). During exercise, [PCr] decreased to 80 +/- 10 (20% TF) and 11 +/- 4% (50% TF) of its resting concentration, and pH dropped from 7.04 +/- 0.01 to 6.98 +/- 0.10 (20% TF) and from 7.03 +/- 0.02 to 6.70 +/- 0.10 (50% TF). In both experiments, two phases of ([ATP]+ [PCr]) decrease were observed: an initial faster decrease was followed by a slower decline. The latter phase started at about the time when the pH began to drop. The difference between a line extrapolated from the slope of the initial phase and the measured ([ATP]+[PCr]) decrease was used as an estimate for glycolytically produced ATP. This estimate and pH were significantly correlated with r = -0.97 (20% TF) and r = -0.99 (50% TF). These results indicate that glycolytically produced ATP can be estimated from the ([ATP]+ [PCr]) decrease during exercise.

Adenosine Triphosphate↗

PubMedDate not supplied

Cardiovascular responses to apnea during dynamic exercise.

Breath holding maneuvers induce hypoxia, hypercapnia, and various cardiovascular responses typically including increases in total peripheral resistance, mean arterial pressure (MAP) and decreases in heart rate (HR). During dynamic exercise these responses may have a generally negative impact on performance. Moreover, they deserve particular attention in cardiovascular risk subjects. In 26 healthy sport students we studied the HR and MAP effects induced by the combination of dynamic exercise (cycle ergometry, 30 W and 250 W) with 20 s of either respiratory arrest (mouth piece pressure held constant at 20 mm Hg), free breathing, or rebreathing, i. e. periods of unimpeded breathing leading to similar levels of hypercapnia and hypoxia as the respiratory arrest. The measurements yielded no major differences between the conditions of rebreathing and free breathing. In contrast, 20 s of apnea led to a marked increase in MAP and a HR depression at both levels of exercise intensity. Additionally, there was a delayed MAP recovery after this stimulus. The present findings show that breath holding has marked effects on MAP and HR during dynamic exercise, which are essentially independent of the resulting hypoxia and of increases in intrathoracic pressure. The key factor seems to be an increase in total peripheral resistance, probably including a vasoconstriction in the exercising muscles.

Adult↗