Determinants of diastolic cardiac filling during exercise

AIM: To examine relative contributions of factors responsible for cardiac diastolic filling during exercise by investigating changes in systemic venous return (as indicated by alterations in cardiac output) after abrupt cessation of skeletal muscle pump function. METHODS: Two cycle exercise studies differing in subject population (men, boys), intensity (submaximal, maximal), and technique for assessing cardiac output (thoracic bioimpedance, Doppler echocardiography). Study 1 involved 12 healthy boys (mean age 12.0+/-1.3 years) and study 2 was composed of 9 young adult men (mean age 27.0+/-3.7 years). RESULTS: Decline in cardiac output averaged 15.8% within the first 15 sec of passive recovery in study 1, while a decrease of 10.9% was observed at 20 sec of passive recovery in study 2. Active pedaling recovery in study 2 slowed the decrease in cardiac output. CONCLUSION: Factors other than the skeletal muscle pump, particularly forward cardiac flow, are capable of maintaining high levels of systemic venous return during early passive recovery. However, this model is unlikely to reflect hemodynamics during exercise, since skeletal muscle contractions impede arterial inflow but augment systemic venous return by increasing the arterial-venous pressure gradient.     

Echocardiography and circulatory response to progressive endurance exercise

Cardiac ultrasound techniques have provided an abundance of empirical information regarding normal circulatory responses to dynamic exercise. These data are consistent with a schema by which alterations in peripheral resistance, effected by arteriolar dilatation, facilitate blood flow to exercising muscle and match these responses to increasing metabolic demand. In this model, cardiac responses are governed by quantity of systemic venous return, matched by increases in heart rate to maintain a constant ventricular filling volume. In a progressive test in the upright position, stroke volume rises early, then plateaus as work intensity rises. The initial increase in stroke volume reflects refilling of the heart from the sequestration of blood in the dependent extremities by gravity that occurs when assuming the upright position. Well documented improvements in both inotropic and lusitropic function during progressive exercise therefore serve to maintain constancy of stroke volume and ventricular filling, respectively, with progressive shortening of systolic and diastolic time periods as work intensity rises. During exercise, then, the circulatory system appears to act like an arterial venous fistula, with peripheral resistance serving as the principal factor facilitating and controlling blood flow. Observations in subjects with altered circulatory dynamics during exercise (patients with cardiac disease, highly trained endurance athletes) can be understood within the context of this physiological model.     

Several hemodynamic indices during human breathing of oxygen under excess pressure]

The paper gives the experimental findings on the pattern and value of circulation changes in ten test subjects exposed to oxygen breathing at positive pressure of 20 mm Hg. The major parameters of central circulation were registered by the method of radioactive label (I131) dilution. During the exposure the test subjects showed an average 27% decrease in the cardiac output as compared with the control level. The test subjects who were pretrained to the exposure displayed a smaller reduction of cardiac output. It is suggested that the circulation changes found under the experimental conditions were mainly induced by peripheral changes rather than by the cardiac function. It is concluded that the reduction of cardiac output is associated with a decrease of the venous return to the heart caused by the diminished circulating blood volume and by the increased total peripheral resistance.     

Potential benefits of maximal exercise just prior to return from weightlessness

The purpose of this study was to determine whether performance of a single maximal bout of exercise during weightlessness within hours of return to earth would enhance recovery of aerobic fitness and physical work capacities under a 1G environment. Ten healthy men (36-51 yr) underwent maximal supine exercise followed by upright maximal exercise before and after a 10-d bedrest period in the 6 degrees headdown position. A graded maximal supine cycle ergometer test was performed before and at the end of bedrest to simulate exercise during weightlessness. Following 3 h of resumption of the upright posture from the supine exercise test, a second maximal exercise test was performed on a treadmill to measure work capacity under conditions of 1G. Compared to before bedrest, peak VO2 decreased (p less than 0.05) by 8.7% and peak HR increased (p less than 0.05) by 5.6% in the supine cycle test at the end of bedrest. However, there were no significant changes in peak VO2 and peak HR in the upright treadmill test following bedrest. These data, based on a simulation, suggest that one bout of maximal leg exercise prior to return from 10 d of weightlessness may be adequate to restore preflight aerobic fitness and physical work capacity.     

Central and peripheral cardiovascular adaptations during a maximal cycle exercise in boys and men

PURPOSE: Stroke volume response to exercise depends on changes in cardiac filling, intrinsic myocardial contractility, and left ventricular afterload. The purpose of this study was to compare these responses during an upright cycle test performed until exhaustion in children and adults. METHODS: Stroke volume, cardiac output (Doppler echocardiography), left ventricular dimensions (two-dimensional and time-movement echocardiography), as well as arterial pressure and systemic vascular resistance (SVR) were assessed in 17 boys (mean age, 11.7 +/- 0.6 yr) and 23 young adult men (mean age, 21.2 +/- 2.7 yr) having a similar aerobic potential. All variables were measured at the end of the resting period, during the final minute of each workload, and during the last minute of the test. RESULTS: No significant differences were obtained for stroke volume, cardiac output, and left ventricular dimensions when they were scaled to body surface area at rest and whatever the exercise intensity. However, arteriovenous oxygen uptake was higher and the SVR lower in the adults than in the children. CONCLUSION: The patterns of stroke volume, as well as its underlying mechanisms, were not age-related during an upright maximal exercise test. However, other studies are required to understand further the effect of pubertal status on the peripheral cardiovascular system.     

Central and general hemodynamics in healthy persons during simulated weightlessness

The pumping and contractile functions of the left ventricle were investigated during combined exposure to water immersion and head-down tilt (at -6 degrees) that simulated microgravity. This 7-day exposure caused noticeable changes in central and systemic circulation which developed as a function of time. It is assumed that the deficiency of venous blood return seen on test days 1-3 was a compensatory response to initial blood displacement which was induced by a reduction of the circulating blood volume. The reduction was in turn a result of responses from volumoreceptors of the heart and intrathoracic vessels to "excessive" intrathoracic blood volume.     

Reliability and validity of measures of cardiac output during incremental to maximal aerobic exercise. Part I: Conventional techniques

The assessment of cardiac function, particularly cardiac output (Q) during heavy exercise is essential for the evaluation of cardiovascular factors that might limit oxygen transport. A series of invasive and noninvasive techniques has been developed for the assessment and monitoring of Q during resting and submaximal exercise conditions. However, very few techniques have been found to give accurate and reliable determinations of Q during vigorous to maximum exercise. For exercise physiologists and sport cardiologists, maximal exercise data are of primary importance. The 'gold standard' measures of cardiac function are considered to be the direct Fick and dye-dilution methods. These have been widely shown to give accurate and reliable determinations of Q during resting and submaximal exercise conditions; however, their use during maximal exercise conditions is debatable due to the inherent risks involved with each and their increasing inaccuracy during the later stages of vigorous exercise. Thermodilution has also been considered to be a relatively good method for the determination of Q during rest and exercise conditions, but recent authors have questioned its use due to the nature of the measure and its inaccuracy during strenuous exercise. Various noninvasive measures of cardiac function have been developed to overcome the problems associated with the 'gold standard' measures. The first part of this article discusses conventional techniques used in exercise physiology settings. The majority of these provide accurate and reliable determinations of Q during rest and submaximal exercise. However, very few techniques are suitable for maximal exercise conditions. Perhaps only the foreign gas rebreathe using acetylene (C2H2) meets all the criteria of being noninvasive, simple to use, reliable over repeated measurements, accurate and useful during maximal exercise.     

Variant of quantitative evaluation of the mechanisms of orthostatic response of central hemodynamics

This paper describes an approach that can help clarify mechanisms of central circulation of orthostatic men using a mathematical model and noninvasive methods of examinations. Circulation parameters such as peripheral resistance (PR), arterial compliance (Ca), and ratio of vein compliance (Cv) to the pump coefficient of the heart (beta) were determined by the "partial identification" method of the two-component circulation model with the aid of cardiac output and arterial blood pressure measured by tetrapolar thoracic rheography and tachooscillography. The paper also contains physiological interpretation of the above parameters as related to the upright posture of man. Peripheral resistance in the head-up position characterizes both the degree of arterial vasoconstriction and the state of the so-called muscle pump. Blood displacement to the lower body results in an increase of the ratio Cv . beta. The orthostatic reaction of circulation of 28 healthy male subjects was investigated. Blood pooling in the lower body, with venous and cardiac reactions being manifest, led to the fall of arterial pressure and cardiac output to 67% when compared with the pretest level. The change in the properties of resistance vessels (Ca and PR) accelerated the recovery of the arterial pressure value.     

The role of the right ventricle during hypobaric hypoxic exercise: insights from patients after the Fontan operation

OBJECTIVES: The principal objective of this study was to examine the importance of the right ventricle for maximal systemic oxygen transport during exercise at high altitude by studying patients after the Fontan operation. BACKGROUND: High-altitude-induced hypoxia causes a reduction in maximal oxygen uptake. Normal right ventricular pump function may be critical to sustain cardiac output in the face of hypoxic pulmonary vasoconstriction. We hypothesized that patients after the Fontan operation, who lack a functional subpulmonary ventricle, would have a limited exercise capacity at altitude, with an inability to increase cardiac output. METHODS: We measured oxygen uptake (VO2, Douglas bag), cardiac output (Qc, C2H2 rebreathing), heart rate (HR) (ECG), blood pressure (BP) (cuff), and O2 Sat (pulse oximetry) in 11 patients aged 14.5+/-5.2 yr (mean +/- SD) at 4.7+/-1.6 yr after surgery. Data were obtained at rest, at three submaximal steady state workrates, and at peak exercise on a cycle ergometer. All tests were performed at sea level (SL) and at simulated altitude (ALT) of 3048 m (10,000 ft, 522 torr) in a hypobaric chamber. RESULTS: At SL, resting O2 sat was 92.6+/-4%. At ALT, O2 sat decreased to 88.2+/-4.6% (P < 0.05) at rest and decreased further to 80+/-6.3% (P < 0.05) with peak exercise. At SL, VO2 increased from 5.1+/-0.9 mL x kg(-1) x min(-1) at rest to 23.5+/-5.3 mL x kg(-1) x min(-1) at peak exercise and CI (Qc x m(-2)) increased from 3.3+/-0.7 L x m(-2) to 6.2+/-1.2 L x m(-2). VO2 peak, 17.8+/-4 mL x kg(-1) x min(-1) (P < 0.05), and CI peak, 5.0+/-1.5 L x m(-2) (P < 0.05), were both decreased at ALT. Remarkably, the relationship between Qc and VO2 was normal during submaximal exercise at both SL and ALT. However at ALT, stroke volume index (SVI, SV x m(-2)) decreased from 37.7+/-8.6 mL x min(-1) x m2 at rest, to 31.3+/-8.6 mL x min(-1) x m2 at peak exercise (P < 0.05), whereas it did not fall during sea level exercise. CONCLUSIONS: During submaximal exercise at altitude, right ventricular contractile function is not necessary to increase cardiac output appropriately for oxygen uptake. However, normal right ventricular pump function may be necessary to achieve maximal cardiac output during exercise with acute high altitude exposure.     

Cardiorespiratory responses to arm cranking and electrical stimulation leg cycling in people with paraplegia.

PURPOSE: The purpose of this study was to assess the cardiorespiratory responses during arm exercise with and without concurrent electrical stimulation-induced leg cycling in people with paraplegia. METHODS: On separate days, 10 subjects with spinal cord injuries (T5-T12) performed either arm cranking (ACE), or simultaneous arm cranking + electrical stimulation-induced leg cycling (ACE+ES-LCE) graded exercise tests. RESULTS: During submaximal, steady-state exercise, ACE+ES-LCE elicited significantly higher VO2, (by 0.25-0.28 L x min(-1)) stroke volume (by 13 mL), and VE(BTPS) (by 9.4 L x min(-1)) compared with ACE alone. In contrast, there were no significant differences of submaximal HR, cardiac output, or power output between the exercise modes. At maximal exercise, ACE+ES-LCE elicited significantly higher VO2 (by 0.23 L x min(-1)) compared with ACE alone, but there were no differences in power output, HR, or VE(BTPS). CONCLUSIONS: These results demonstrate that during submaximal or maximal exercise there was a greater metabolic stress elicited during ACE+ES-LCE compared with during ACE alone. The higher stroke volume observed during submaximal ACE+ES-LCE, in the absence of any difference in HR, implied a reduced venous pooling and higher cardiac volume loading during ACE+ES-LCE. These results suggest that training incorporating ACE+ES-LCE may be more effective in improving aerobic fitness in people with paraplegia than ACE alone.     

心律失常运动员心输出量的研究

近年来由于训练强度不断增大,高水平运动员心律失常的发生率显著增加。本文研究资料表明,男心律失常组安静时心搏量明显大于正常组(P<0.05),而运动后心搏量降低(正常组心搏量增加),说明心律失常的运动员心脏储备能力差,心脏泵血功能降低,应予慎重对待。     

Exercise training improves cardiac function postinfarction: special emphasis on recent controversies on na+/ca2+ exchanger

Exercise training instituted after myocardial infarction improves many steps involved in cardiac excitation-contraction coupling. Focusing on Na/Caexchange, current controversies regarding whether it mediates Cainflux during an action potential, whether it is increased or decreased in disease models, whether protein kinase A alters its activity, and whether exercise training affects its function are reviewed. Finally, a novel target for exercise training in the heart is suggested.     

Cardiovascular adjustments induced by training evaluated during semisupine isotonic exercise and recovery period: an echocardiographic study

Two groups of subjects were examined: trained athletes (group A) and a sedentary control group (group B). The subjects performed submaximal bicycle exercise in the semisupine position to evaluate the differences between the two groups with regard to cardiovascular response during exercise and recovery and to point out all the changes due to training. During the first part of exercise, cardiac output increased contemporary with heart rate and myocardial contractility as shown by the trend of the ejection fraction, higher in group A, under the same level of total vascular peripheral resistances. Later there was an increase of cardiac output for a further increase of heart rate and cardiac inotropism due to the homeometric mechanism. During recovery the heart rate and peripheral vascular resistance reduction led to an increase of venous return which set up the Frank-Starling mechanism via an increase of left ventricular dimensions. These adjustments were more efficient in group A. During exercise and recovery the heart rate-pressure product was constantly lower in group A with a significant difference to group B. Therefore, trained athletes' myocardium is more efficient than that of the sedentary group because it performs an external work load with a lower oxygen consumption.     

The influence of recovery posture on post-exercise hypotension in normotensive men

PURPOSE: Postexercise hypotension may be the result of an impaired vasoconstrictor response. This hypothesis was investigated by examining the central and peripheral hemodynamic responses during supine and seated recovery after maximal upright exercise. METHODS: After supine or seated baseline measurements, seven normotensive male volunteers completed a graded upright cycling protocol to volitional exhaustion. This was immediately followed by either supine or seated recovery. Measurements of pulsatile arterial blood pressure and central and peripheral hemodynamic variables recorded 30 min before exercise were compared with those taken throughout 60 min of recovery. RESULTS: Compared with baseline, mean arterial pressure (MAP) was reduced after exercise (P < 0.05) although the degree of change was not different between the supine (-9 +/- 4 mm Hg) and seated positions (-6 +/- 2 mm Hg). This change in MAP was associated with a reduction in diastolic blood pressure (DBP) (P < 0.05) and arterial pulse pressure (APP) (P < 0.01) for the supine and seated positions, respectively. The reduction in APP during seated recovery was accompanied by a decline in stroke volume (SV) (P < 0.05), not seen in the supine position, that limited the contribution of cardiac output (CO) to the maintenance of MAP. This effect of seated recovery was compensated by greater systemic (SVR) and regional vascular resistances in the forearm (FVR) and the forearm skin (SkVRA). There was also evidence of an augmented return of FVR and SkVRA to resting levels in the seated position after exercise. CONCLUSION: The lower peripheral resistance in the supine compared with seated recovery position suggests there is potential for greater vasoconstriction, although this is not evoked to increase blood pressure. This further suggests that the arterial baroreceptor reflex is reset to a lower operating pressure after exercise.     

Exercise Intolerance in Patients with Chronic Iliocaval Venous Occlusion: Initial Experience with Noninvasive Exercise Testing before and after Intervention

Patients with chronic iliocaval occlusions after thrombosis often present with exercise intolerance, which improves after venous reconstruction. Three male patients with chronic iliocaval occlusions underwent a cardiorespiratory fitness test before and 2.5–11 months after venous reconstruction using stents. After the intervention, average absolute oxygen consumption increased by 29.5%, maximal oxygen consumption relative to body weight increased by 38.7%, total work at maximum exercise increased by 74.4%, and exercise time increased by 18.7%. The cardiorespiratory fitness test may be a useful noninvasive tool to objectively evaluate exercise intolerance due to chronic venous occlusions and response to therapy.     

Validation of noninvasive maximal cardiac output measurement

Due to the invasiveness of direct techniques and problems that constrain the use of popular indirect techniques during very heavy (non-steady-state) exercise, measurements of maximal cardiac output are seldom included in studies of exercise. The acetylene-rebreathing technique is well-suited for maximal exercise; however, until recent technological advances, difficulties involved in collecting and measuring alveolar acetylene samples have restricted its use. We compared cardiac output values measured via the acetylene-rebreathing technique (QA) (modified for use with a mass spectrometer) and the dye-dilution technique (QD) at rest and during light to maximal exercise in six moderately active males. Although QA consistently underestimated QD, the two techniques showed a significant correlation of 0.87 throughout all levels of exercise. During maximal exercise, QA and QD values were not significantly different (24.7 and 26.7 l X min-1, respectively). Modifications in the QA technique that reduce potential sources of error were also examined. We conclude that the acetylene-rebreathing technique, modified for use with a mass spectrometer, is a simple and valid procedure for measuring maximal cardiac output.     

Myocardial performance during maximal exercise in adolescents with anorexia nervosa

AIM: To examine cardiac responses and indicators of myocardial function during maximal exercise in adolescent girls with anorexia nervosa. METHODS: Eight girls (mean age 16.3+/-2.7 years) who satisfied criteria for the diagnosis of anorexia nervosa underwent maximal cycle testing. Cardiac stroke volume and peak aortic velocity and mean acceleration of flow (markers of myocardial contractility) were assessed using Doppler echocardiography and compared to healthy control subjects. Gas exchange variables were measured using open circuit spirometry techniques. RESULTS: Resting and maximal heart rates were less in the patients, and maximal oxygen uptake was significantly lower than controls. Maximal stroke index was greater in the patients than controls, with a normal pattern of response to progressive exercise. Peak aortic velocity and mean acceleration of flow were similar in the two groups when adjusted for heart rate. CONCLUSION: Findings of low heart rate and aerobic fitness previously described in patients with anorexia nervosa were confirmed. However, there was no evidence of abnormal myocardial performance during maximal exercise testing.     

Effect of prolonged exercise on insulin level in the portal and peripheral venous circulation in rats

The purpose of the present study was to compare the plasma insulin levels in portal and peripheral veins in rats submitted to a prolonged exercise to exhaustion (mean +/- SE: 67 +/- 7 min). Plasma insulin levels were reduced significantly (p less than 0.01) during exercise in both the portal and peripheral venous circulation. The reduction was, however, more pronounced in the portal than in the peripheral vein (mean: -113 vs -64 and -162 vs -88 pmol.l-1 after 30 and 45 min of exercise, respectively), indicating that exercise reduced not only insulin secretion but also its removal. Accordingly, no significant differences were found for the portal/peripheral ratio of insulin at rest and during the first 45 min of exercise (1.86 +/- 0.2, 1.76 +/- 0.1, and 1.57 +/- 0.2 at rest, and after 30 and 45 min of exercise, respectively). This suggests that insulin removal during exercise is regulated in proportion to portal insulin concentrations.     

Experimental preparations to understand the cardiovascular effect of training: in situ preparation

Although exercise training is generally thought to exert a beneficial effect on the cardiovascular system, additional experimental work is required to determine the mechanism(s) by which training influences cardiovascular function. The utilization of an in situ preparation for cardiovascular studies should provide valuable information since both peripheral circulatory and myocardial performances can be evaluated. However, this paper specifically considers functional evaluation of the intact, in situ heart. The underlying physiological basis for ventricular pump performance evaluation in the intact heart is presented. Isolated muscle mechanics (force-velocity) principles are discussed and their application to the intact heart is illustrated. Proposed biochemical correlates of total tension development, the maximal rate of tension development, and maximal velocity of shortening are presented. The methods and procedures currently available for functional evaluation are described. Finally, the results of previous exercise studies where in situ cardiac functional evaluation has been utilized are reviewed.