An improved isolated, left ventricular ejecting, murine heart model

 An improved, isolated, left ventricular-ejecting, murine heart model is described and evaluated. Special attention was paid to the design and impedance characteristics of the artificial aortic outflow tract and perfusate composition, which contained glucose (10 mM plus insulin) and pyruvate (1.5 mM) as substrates. Temperature of the isolated perfused hearts was maintained at 38.5 °C. During antegrade perfusion (preload 10 mm Hg, afterload 50 mm Hg, 2.5 mM Ca²⁺) proper design of the aortic outflow tract provided baseline values for cardiac output (CO), left ventricular developed pressure (LVDP) and the maximum first derivative of left ventricular pressure (LV dP/dt _max) of 11.1±1.7 ml min^–1, 83±5 mm Hg and 6283±552 mm Hg s^–1, respectively, resembling findings in the intact mouse. During 100 min normoxic antegrade perfusion CO declined non-significantly by less than 10%. Varying pre- and afterloads resulted in typical Frank-Starling relationships with maximal CO values of 18.6±1.8 ml min^–1 at pre- and afterload pressures of 25 and 50 mm Hg, respectively. Left ventricular function curves were constructed at free [Ca²⁺] of 1.5 and 2.5 mM in the perfusion medium. Significantly higher values for CO, LVDP and LV dP/dt _max and LV dP/dt _min were obtained at 2.5 mM Ca²⁺ at all loading conditions investigated. Phosphocreatine and creatine levels remained stable throughout the perfusion period. Despite a small but significant decline in tissue ATP content, the sum of adenine nucleotides did not change during the normoxic perfusion period. The tissue content of glycogen increased significantly. Received: 28 April 1998 / Received after revision and accepted: 10 September 1998     

Relationship of exercise test variables to cycling performance in an Ironman triathlon

The purpose of this study was, firstly, to investigate the intensity of exercise performanceof highly trained ultra-endurance triathletes during the cycling portion of an Ironman triathlon, and, secondly, to examine the anaerobic threshold and its relationship to this performance. Following a peak oxygen consumption (VO_2peak) test on a cycle ergometer to determine the heart rate (HR_Th,vent) and power output (PO_Th,vent) at the ventilatory threshold (Th_vent), 11 highly trained male triathletes [mean (SEM) age 35.8 (1.6) years, body fat 11.7 (1.2)%. VO_2peak 67.5 (1.0) ml·kg^–1·min^–1] who were participating in an Ironman triathlon, in random order: (1) cycled at their PO_Th,vent (Bi_Th,vent) until they were exhausted, and (2) cycled for 5 h at a self-selected intensity (Bi_SSI). Cycling power output (PO), oxygen uptake (VO₂), heart rate (HR) and blood lactate concentration ([La^–]_b) were recorded at regular intervals during these trials, while performance HR was recorded during the cycling phase of the Ironman triathlon. Significantly greater (P<0.05) values were attained during Bi_Th,vent than during Bi_SSI for PO [274 (9) compared to 188 (9) W], VO₂ [3.61 (0.15) compared to 2.64 (0.09) l·min^–1], and [La^–]_b [6.7 (0.8) compared to 2.8 (0.4) mmol·l^–1]. Moreover, mean HR during the Ironman triathlon cycle phase [146.3 (2.4) beats·min^–1; n=7] was significantly greater than mean HR during Bi_SSI [130 (4) beats·min^–1], and significantly less than mean HR during Bi_Th,vent [159 (3) beats·min^–1; all P<0.05]. However, HR during the cycle portion of the Ironman triathlon was highly related to (r=0.873; P<0.05) and not significantly different to HR_Th,vent [150 (4) beats·min^–1]. These data suggest that ultra-endurance triathletes cycle during the Ironman triathlon at a HR intensity that approximates to HR_Th,vent, but at a PO that is significantly below PO_Th,vent. Electronic Publication     

Vastus medialis muscle oxygenation trends during a simulated 20-km cycle time trial

In this study we examined the oxygenation trend of the vastus medialis muscle during sustained high-intensity exercise. Ten cyclists performed an incremental cycle ergometer test to voluntary exhaustion [mean (SD) maximum oxygen uptake 4.29 (0.63) l·min^–1; relative to body mass 60.8 (2.4) ml·kg^–1·min^–1] and a simulated 20-km time trial (20TT) on a wind-loaded roller system using their own bicycle (group time = 23–31 min) in two separate sessions. Cardiorespiratory responses were monitored using an automated metabolic cart and a wireless heart rate monitor. Tissue absorbency, which was used as an index of muscle oxygenation, was recorded simultaneously from the vastus medialis using near-infrared spectroscopy. Group mean values for oxygen uptake, ventilation, heart rate, respiratory exchange ratio, power output, and rating of perceived exhaustion were significantly (P≤0.05) higher during the incremental test compared to the 20TT [4.29 (0.63) l·min^–1 vs 4.01 (0.55) l·min^–1, 120.4 (26) l·min^–1 vs 97.6 (16.1) l·min^–1, 195 (8) beats·min^–1 vs 177 (9) beats·min^–1, 1.15 (0.06) vs 0.93 (0.06), 330.1 (31) W vs 307.2 (24.5) W, and 19 (1.5) vs 16 (1.7), respectively]. Oxygen uptake and heart rate during the 20TT corresponded to 93.5% and 90.7%, respectively, of the maximal values observed during the incremental test. Comparison of the muscle oxygenation trends between the two tests indicated a significantly greater degree of deoxygenation during the 20TT [–699 (250) mV vs –439 (273) mV; P≤0.05] and a significant delay in the recovery oxygenation from the 20TT. The mismatching of whole-body oxygen uptake and localised tissue oxygenation between the two tests could be due to differences in muscle temperature, pH, localised blood flow and motor unit recruitment patterns between the two tests. Electronic Publication     

Heart rate as an indicator of the intensity of physical activity in human adolescents

The aims of this study were, in a group of adolescents, firstly to identify the absolute heart rates (HR) and the percentages of maximal heart rates (HR_max) corresponding to 40%, 60% and 80% of peak oxygen uptake ( ), secondly to identify absolute and relative ( ) oxygen uptakes ( ) corresponding to HR of 120, 140 and 160 beats·min^–1, and thirdly to examine a possible effect of fatness and fitness on the relationship between HR and . The subjects were 127 (60 boys, 67 girls) adolescents with a mean age of 14.8 (SD 0.3) years. The HR and were measured by means of an incremental exercise test to exhaustion. Linear regressions were performed for the and relationships using absolute and relative (%HR_max, ) data for each individual. From these regressions, target HR and were computed. Average target HR corresponding to 40%, 60% and 80% of were: 119 (SD 9), 145 (SD 9), 171 (SD 8), and 120 (SD 10), 146 (SD 8), 172 (SD 8) beats·min^–1 for boys and girls, respectively. Average corresponding to HR of 120, 140 and 160 beats·min^–1 were: 22 (SD 5), 30 (SD 5), 38 (SD 6) and 18 (SD 4), 24 (SD 4), 31 (SD 4) mlO₂·kg^–1·min^–1for boys and girls, respectively. An analysis of covariance showed a significant fitness effect (P<0.001) for predicted at all HR studied. The results suggest that the use of absolute HR to define exercise intensity levels when assessing young people's physical activity using HR monitoring detracts from the validity of the interpretation of the data. Electronic Publication     

The physiological demands of sail pumping in Olympic level windsurfers

This study investigated the physiological effects of sail pumping (PB) – a manoeuvre often adopted to provide additional propulsion to the board – in Olympic Class Windsurfing, following relaxation of the "no-pumping" rules by the International Federation. Fifteen Olympic-level windsurfers (10 men) from nine different countries volunteered for the study, which was performed during two international Olympic regattas. The measurements were carried out during actual sailing when both PB and not-pumping (NPB) using a portable metabolimeter. Windsurfing, when PB, elicited a dramatic increase in cardiorespiratory responses compared to NPB. Mean (SD) values for oxygen uptake and heart rate during NPB for the men and women were: 19.2 (4.4) and 15.7 (3.3) ml·kg^–1·min^–1, and 110 (10) and 122 (12) beats·min^–1, respectively, whereas the values in PB were: 48.4 (5.7) and 40.2 (4.2) ml·kg^–1·min^–1, and 165 (12) and 172 (13) beats·min^–1, respectively. All the PB parameters, with the exception of heart rate (HR), were significantly higher in the men than in the women but no differences were observed between the sexes in NPB with the exception of HR, which was higher in the women. Our results suggest sail pumping is as physically demanding as most aerobic sporting activities. In the context of the need to deal with a highly demanding athletic branch of sailing as part of an Olympic regatta, recommendations are made on how best to make physical and dietary preparations. Electronic Publication     

The inotropic effect of atrial natriuretic factor in the anesthetized rabbit

This study was designed to investigate whether atrial natriuretic factor (ANF) administered over the physiological, pathological and pharmacological range has a negative inotropic action on the heart. Anesthetized rabbits were infused with increasing doses of ANF (0.05, 0.25 and 0.5g kg^–1min^–1), while measuring hemodynamic variables including the maximum rate of change of left ventricular pressure (dP/dt _max) as an index of inotropic state. Plasma levels of immunoreactive ANF (iANF) were measured to relate the hemodynamic changes to actual plasma levels of the peptide. Administration of ANF was associated with decreases in blood pressure, left ventricular pressure and dP/dt _max so that after 0.5 g kg^–1 min^–1 infusion, these variables had decreased by 21±2 mmHg, 21±5.3 mmHg and 925±175 mmHg/s, respectively (P<0.01). There were no significant changes in right atrial pressure, left ventricular end-diastolic pressure or heart rate. Since dP/dt _max can be influenced by changing hemodynamic variables and baroreflex changes, a second group of rabbits was studied in which afterload and heart rate were held artificially constant. Again, in this group of rabbits, infusions of ANF led to decreasing inotropic state, so that at the highest infusion rate, a 14% decrease in dP/dt _max was observed (P<0.05). By comparison, hydralazine, a drug which causes active vasodilatation but no direct inotropic action, significantly (P<0.01) decreased blood pressure, left ventricular pressure and dP/dt _max when infused at a rate of 10 g kg^–1 min^–1. However, in animals in which afterload was controlled, hydralazine did not affect any of the variables measured. The results indicate that ANF does have a negative inotropic action in the anesthetized rabbit.     

Effects of aerobatics flight on oxygen consumption and heart rate control: influence on autonomic cardiovascular regulation during recovery

Oxygen consumption ( ) and blood pressure regulation were measured on five pilots during and after normal training aerobatics flights of a mean duration of 35 min. The acceleration vector along the longitudinal axis of the body (Gz) ranged from +6.5 Gz to –3.5 Gz. was continuously monitored by a miniature telemetric system (K2). Heart rate (f _c), the abdominal muscle electromyogram (EMG) and Gz levels were recorded synchronously on a magnetic tape recorder. A tilt test was performed pre- and post-flight to evaluate f _c and blood-pressure variability. The left forearm blood flow was measured by strain-gauge plethysmography. The mean during flight was 1.2 l·min^–1, with a peak of 2.1 l·min^–1. f _c ranged between 55 and 165 beats·min^–1 and showed a progressive increase under the effect of +Gz, with a sudden fall during –Gz. The abdominal muscle EMG indicated the occurrence of muscle contraction under Gz load. Maximal responses were observed during the –Gz phase. Comparison between pre- and post-flight data showed lower post-flight systolic blood pressure with higher f _c. Before flight, upright tilt induced a significant increase in low/high frequency f _c, as assessed using spectral analysis. This change was suppressed after flight. In summary, these data show that aerobatics flight leads to enhanced energy expenditure, mainly because of increased skeletal muscle work. The post-flight tilt test showed that aerobatic flight favors parasympathetic drive and, consequently, modifies blood pressure regulation during recovery. This action may decrease +Gz tolerance to a second aerobatics flight performed shortly after the first. Electronic Publication     

Maximal oxygen uptake during field running does not exceed that measured during treadmill exercise

Modern ergometric equipment enables the simulation of laboratory maximal oxygen uptake (V˙O_2max) testing in the field. Therefore, it was investigated whether the improved event specificity on the track might lead to higher V˙O_2max measurements in running. Identical protocols were used on the treadmill and on the track (speed was indicated by a computer-driven flashing light system). Ambulatory measurements of gas exchange were carried out throughout both tests, which were executed in randomized order. There were no significant differences (P=0.71) in V˙O_2max between treadmill [4.65 (0.51) ml·min^–1] and field tests [4.63 (0.55) ml·min^–1]. However, the test duration differed significantly (P<0.001) by approximately 5%: treadmill 691 (39) s; field test 727 (42) s. With the exception of maximum heart rate (HR_max; significantly higher in the field with P=0.02) all criteria for the degree of effort were similar between the two tests. However, the difference in HR_max at less than 2 beats·min^–1, was practically negligible. Submaximal measurements of oxygen uptake and minute ventilation were significantly higher on the treadmill (P<0.001 for both parameters). In summary, field tests with incremental running protocols do not result in higher V˙O_2max measurements compared to laboratory treadmill exercise. A better running economy on the track results in higher maximal velocities and longer exercise durations being sustained. The determination of V˙O_2max is not a reasonable application for ambulatory gas exchange measurements because laboratory values are not surpassed. Electronic Publication     

Heart rate is lower during ergometer rowing than during treadmill running

This study evaluated whether the heart rate (HR) response to exercise depends on body position and on the active muscle mass. The HR response to ergometer rowing (sitting and using both arms and legs) was compared to treadmill running (upright exercise involving mainly the legs) using a progressive exercise intensity protocol in 55 healthy men [mean (SD) height 176 (5) cm, body mass 71 (6) kg, age 21 (3) years]. During rowing HR was lower than during running at a blood lactate concentration of 2 mmol·l^–1 [145 (13) compared to 150 (11) beat·min^–1, P<0.05], 4 mmol·l^–1 [170 (10) compared to 177 (13) beat·min^–1, P<0.05], and 6 mmol·l^–1 [182 (10) compared to 188 (10) beat·min^–1, P<0.05]. Also during maximal intensity rowing, HR was lower than during maximal intensity running [194 (9) compared to 198 (11) beat·min^–1, P<0.05]. These results were accompanied by a higher maximal oxygen uptake during rowing than during running [rowing compared to running, 4.50 (0.5) and 4.35 (0.4) l·min^–1, respectively, P<0.01]. Thus, the oxygen pulse, as an index of the stroke volume of the heart, was higher during rowing than during running at any given intensity. The results suggest that compared to running, the seated position and/or the involvement of more muscles during rowing facilitate venous return and elicit a smaller HR response for the same relative exercise intensity. Electronic Publication     

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

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. Accepted: 1 September 1998     

Role of aspirin in modulating myocardial ischemic reperfusion injury

The role of low-dose aspirin (3 mg/kg, i.v.) in attenuating ischemic reperfusion injury was studied in a canine model. Regional ischemia for 40 min was produced by temporary occlusion of the left anterior descending coronary artery and thereafter reperfusion instituted for 3 h. Mean arterial pressure (MAP), heart rate (HR), left ventricular end diastolic pressure (LVEDP), positive (+) LV dP/dt _max and negative (–) LV dP/dt _max were monitored alongwith myocardial adenosine triphosphate (ATP), creatine phosphate (CP), glycogen and lactate. Following reperfusion, there was a significant fall in (i) MAP, (ii) (+) LV dP/dt _max and (iii) (–) LV dP/dt _max. LVEDP was corrected after about 2h of reperfusion. Replenishment of only myocardial CP occurred, without any change in ATP and glycogen, although lactate accumulation was corrected.Aspirin administered 15 min before reperfusion (posttreatment) caused normalisation of LVEDP within 15 min and prevented any deterioration in (–) LV dP/dt _max, although it had no effect on MAP and (+) LV dP/dt _max. After 3h of reperfusion (post-treatment), myocardial ATP, CP, glycogen and lactate contents became normal. The number of premature ventricular complexes was significantly reduced after aspirin treatment. The present study indicates that low-dose aspirin post-treatment can ameliorate at least some of the deleterious consequences of reperfusion injury of the myocardium.     

Cardiorespiratory responses to fatiguing dynamic and isometric hand-grip exercise

In occupational work, continuous repetitive and isometric actions performed with the upper extremity primarily cause local muscle strain and musculoskeletal disorders. They may also have some adverse effects on the cardiorespiratory system, particularly, through the elevation of blood pressure. The aim of the present study was to compare peak cardiorespiratory responses to fatiguing dynamic and isometric hand-grip exercise. The subjects were 21 untrained healthy men aged 24–45 years. The dynamic hand-grip exercise (DHGE) was performed using the left hand-grip muscles at the 57 (SD 4)% level of each individual's maximal voluntary contraction (MVC) with a frequency of 51 (SD 4) grips · min^−l. The isometric hand-grip exercise (IHGE) was done using the right hand at 46 (SD 3)% of the MVC. The endurance time, ventilatory gas exchange, heart rate (HR) and blood pressure were mea- sured during both kinds of exercise. The mean endurance times for DHGE and IHGE were different, 170 (SD 62) and 99 (SD 27) s, respectively (P < 0.001). During DHGE the mean peak values of the breathing frequency [20 (SD 6) breaths · min⁻¹] and tidal volume [0.89 (SD 0.34) l] differed significantly (P < 0.01) from peak values obtained during IHGE [15 (SD 5) breaths · min⁻¹, and 1.14 (SD 0.32) l, respectively]. The corresponding peak oxygen consumptions, pulmonary ventilations, HR and systolic blood pressures did not differ, and were 0.51 (SD 0.06) and 0.46 (SD 0.11) l · min⁻¹, 17.1 (SD 3.0) and 16.7 (SD 4.7) l · min⁻¹, 103 (SD 18) and 102 (SD 17) beats · min⁻¹, and 156 (SD 17) and 161 (SD 17) mmHg, respectively. The endurance times of both DHGE and IHGE were short (<240 s). The results indicate that the peak responses for the ventilatory gas exchange, HR and blood pressure were similar during fatiguing DHGE and IHGE, whereas the breathing patterns differed significantly between the two types of exercise. The present findings emphasize the importance of following ergonomic design principles in occupational settings which aim to reduce the output of force, particularly in tasks requiring isometric and/or one-sided repetitive muscle actions. Accepted: 16 February 2000     

The effects of blood loss on the performance of physical exercise

Summary Seven male subjects were studied before and up to 53 days after the loss of 11 of blood. The resting hematocrit fell from 44.0 to 38.7% and returned to control level after 3 weeks. Maximal oxygen uptake decreased from 4.00 l/min to 3.54 l/min and returned to the initial level within 2 weeks. Submaximal oxygen uptake, pulmonary ventilation, maximal heart rate and blood lactate were not found to change significantly. Submaximal heart rate was increased from 125 beats·min⁻¹ to about 135 beats·min⁻¹ and remained elevated for 3 weeks, whereas blood lactate was increased only in the first week. Maximal work time decreased from 5.1 min to 3.8 min and remained low for the first 2 weeks, but rose thereafter above the starting level. Comparison with a control study suggested that there is some training effect, which, when allowed for, indicates that maximal work time returns to starting values at the same time as does the maximal oxygen uptake. It is concluded that the drop in Hct, maximal oxygen uptake and work capacity, found after the loss of 11 of blood, are related to each other both in magnitude and duration.     

Oxygen uptake kinetics during treadmill running across exercise intensity domains

The purpose of the present study was to examine comprehensively the kinetics of oxygen uptake ( ) during treadmill running across the moderate, heavy and severe exercise intensity domains. Nine subjects [mean (SD age, 27 (7) years; mass, 69.8 (9.0) kg; maximum , , 4,137 (697) ml·min^–1] performed a series of "square-wave" rest-to-exercise transitions of 6 min duration at running speeds equivalent to 80% and 100% of the at lactate threshold (LT; moderate exercise); and at 20%, 40%, 60%, 80% and 100% of the difference between the at LT and (Δ, heavy and severe exercise). Critical velocity (CV) was also determined using four maximal treadmill runs designed to result in exhaustion in 2–15 min. The response was modelled using non-linear regression techniques. As expected, the amplitude of the primary component increased with exercise intensity [from 1,868 (136) ml·min^–1 at 80% LT to 3,296 (218) ml·min^–1 at 100% Δ, P<0.05]. However, there was a non-significant trend for the "gain" of the primary component to decrease as exercise intensity increased [181 (7) ml·kg^–1·km^–1 at 80% LT to 160 (6) ml·kg^–1·km^–1 at 100% Δ]. The time constant of the primary component was not different between supra-LT running speeds (mean value range = 17.9–19.1 s), but was significantly shorter during the 80% LT trial [12.7 (1.4) s, P<0.05]. The slow component increased with exercise intensity from 139 (39) ml·min^–1 at 20% Δ to 487 (57) ml·min^–1 at 80% Δ (P<0.05), but decreased to 317 (84) ml·min^–1 during the 100% Δ trial (P<0.05). During both the 80% Δ and 100% Δ trials, the at the end of exercise reached [4,152 (242) ml·min^–1 and 4,154 (114) ml·min^–1, respectively]. Our results suggest that the "gain" of the primary component is not constant as exercise intensity increases across the moderate, heavy and severe domains of treadmill running. These intensity-dependent changes in the amplitudes and kinetics of the response profiles may be associated with the changing patterns of muscle fibre recruitment that occur as exercise intensity increases. Electronic Publication     

The effect of prolonged submaximal exercise on gas exchange kinetics and ventilation during heavy exercise in humans

This study compared ventilation, gas exchange (oxygen uptake, V̇O₂) and the surface electromyogram (EMG) activity of four major lower limb muscles during heavy exercise before (Pre-Ex) and after (Post-Ex) a sustained 90-min cycling exercise at 60% V̇O_2peak. The 90-min exercise was incorporated under the hypothesis that sustained exercise would alter substrate availability in the second exercise bout causing differences in fibre recruitment patterns, gas exchange and ventilation. Nine trained male subjects [V̇O_2peak=60.2 (1.7) ml·kg⁻¹·min⁻¹] completed two identical 6-min bouts of cycling performed at high intensity [~90% V̇O_2peak; 307 (6) W, mean (SE)]. Ventilation and gas exchange were measured breath-by-breath and the EMG was recorded during the last 12 s of each minute of the two 6-min bouts. EMG signals were analysed to determine integrated EMG (iEMG) and mean power frequency (MPF). V̇O₂ at min 3 and min 6 in Post-Ex were significantly higher (i.e., +201 and 141 ml·min⁻¹, respectively, P<0.05) than in Pre-Ex but there was a ~25% decrease of the slow component, taken as the difference between min 6 and min 3 [187 (27) vs 249 (35) ml·min⁻¹, respectively, P<0.05]. The greater whole-body V̇O₂ after 3 min of exercise in Post-Ex was not accompanied by clear alterations in the iEMG and MPF of the examined leg muscles. Ventilation and heart rate were elevated (~12–16 l·min⁻¹ and ~10 beats·min⁻¹, respectively, P<0.05) as were the ratios V̇ _E/O₂ and V̇ _E/V̇CO₂ in the Post-Ex tests. It was concluded that the V̇O₂ and ventilation responses to high-intensity exercise can be altered following prolonged moderate intensity exercise in terms of increased amplitude without associated major changes in either iEMG or MPF values among conditions.     

Noninvasive assessment of cardiac contractility by using (dP/dt)/P of carotid artery pulses during exercise

In earlier studies we have shown that both the pressure (P) of the carotid artery pulse (CAP) and its first derivative (CAP dP/dt) could be recorded during moderate exercise. To establish that the CAP (dP/dt)/P is a noninvasive substitute for the left ventricular (LV) value, LV (dP/dt)/P, an index of cardiac contractility, we studied CAP (dP/dt)/P under various states of activity in the autonomic nervous system in 12 healthy male subjects. Increased sympathetic nerve activities yielded by passive tilting, emotional load, or cold stress increased CAP (dP/dt)/P significantly (P< 0.05). Increased parasympathetic nerve activity by ocular compression, however, did not significantly affect the value. Moderate exercise at a heart rate of approximately 150 beats·min^–1 increased it significantly from 16.7 to 25.2·s^–1 in a supine position (P<0.001) and from 16.6 to 24.8·s^–1 in an upright position (P<0.001). It increased monotonically as heart rate increased, but the slope was steeper when the heart rate was greater than approximately 100 beats·min^–1 than it was when the rate was less than 100 beats·min^–1. In conclusion, the present study indicated that CAP (dP/dt)/P can be used as a noninvasive index of cardiac contractility even in moderate exercise.     

Reproducibility of the parameters of the on-transient cardiopulmonary responses during moderate exercise in patients with chronic obstructive pulmonary disease

To be clinically useful as indices reflective of altered physiological function consequent to interventions in patients with chronic obstructive pulmonary disease (COPD), the time constant (τ) and steady-state amplitude of the kinetic responses for oxygen uptake ( ) carbon dioxide output ( ) ventilation ( ) and heart rate (HR) have to be appropriately differentiable and reproducible. We therefore assessed the reproducibility of τ and steady state amplitude values in 41 patients with severe COPD [mean (SD)] [forced expiratory volume in 1 s=41 (7)% predicted], aged 64 (5) years. Of the total, 6 of the patients (15%) did not produce breath-by-breath data of sufficient quality to warrant kinetic analysis. The remaining 35 patients completed two moderate-intensity 10 min square-wave exercise tests separated by 2 h, both before and after an endurance training programme. Tests were conducted on an electromagnetically-braked cycle ergometer at an exercise intensity corresponding to 80% of the estimated lactate threshold (θ_La) or 50% of peak oxygen uptake if θ_La was insufficiently differentiable. Breath-by-breath measurements of , , and HR were averaged into 10 s bins and the on-transient response kinetics were estimated using a mono-exponential model. Analysing the pre-training and the post-training test 1 and test 2 comparisons together, the test 1 –test 2 differences were not significantly different from 0 for either τ or A. The standard deviation of the test 1 –test 2 differences allowed us to define the magnitude of change that would reach statistical significance. For τ, this averaged some 8, 10, 11 and 8 s, for , , and HR, respectively, for a one-tailed paired-comparisons test (i.e. appropriate for assessing hypothesised improvements resulting from an intervention); for a two-tailed comparison, the differences were approximately 2 s greater. The corresponding one-tailed values for A were 100 ml·min^–1, 95 ml·min^–1, 2.5 1·min^–1 and 4 beats·min^–1, respectively; the two-tailed values were 10%–15% greater. We therefore conclude that both τ and A for moderate-intensity exercise can be reproducibly estimated in patients with COPD when the data set provides a sufficiently large amplitude of response and sufficiently low sample variability to allow appropriate parameter estimation. Electronic Publication     

Blood flow to the brown adipose tissue of conscious young rabbits during hypoxia in cold and warm conditions

 Brown adipose tissue (BAT) non-shivering thermogenesis is stimulated by cold temperature and depressed by hypoxia. We investigated the extent to which changes in metabolic rate during cold and hypoxia, singly or combined, were accompanied by changes in BAT perfusion. One-month-old rabbits were instrumented for measurements of regional blood flow by the coloured microsphere technique. One group of rabbits was tested in warm (24 °C, n=17), and the other in cold (13 °C, n=9) conditions, first in normoxia (inspired oxygen concentration FIO₂ about 21%, arterial oxygen saturation S _aO₂ approximately 88%) followed by hypoxia (FIO₂ approximately 10%, S _aO₂ approximately 54%). In warm conditions, oxygen consumption (V·O₂, measured by an open-flow method) averaged 22 ml·kg^–1·min^–1 (STPD), and BAT blood flow 98 ml·100g^–1·min^–1. In hypoxia, V·O₂ dropped on average to 87%, whereas BAT flow dropped to 43% of the normoxic values. In the cold during normoxia, V·O₂ averaged 31 ml·kg^–1·min^–1 (STPD), and BAT blood flow was 155 ml·100g^–1·min^–1. In cold and hypoxia V·O₂ dropped to 19 ml·kg^–1·min^–1 (STPD) (i.e. 60% of the normoxic value), whereas BAT blood flow was not altered significantly (148 ml·100g^–1·min^–1). Hence, BAT blood flow decreased in hypoxia in absence of cold stimuli, whereas it remained high when hypoxia occurred during cold, despite the major drop in V·O₂. We conclude that cold is more important than hypoxia in determining BAT perfusion, and that changes in BAT blood flow are not a mechanism for the hypoxic control of V·O₂. Received: 24 June 1998 / Received after revision: 21 September 1998 / Accepted: 29 September 1998     

Short-term cardiovascular responses to rapid whole-body tilting during exercise

Our objective was to characterize the responses of heart rate (HR) and arterial blood pressure (BP) to changes in posture during concomitant dynamic leg exercise. Ten men performed dynamic leg exercise at 50, 100, and 150 W and were rapidly and repeatedly tilted between supine (0°) and upright (80°) positions at 2-min intervals. Continuous recordings of BP and HR were made, and changes in central blood volume were estimated from transthoracic impedance. Short-lasting increases in BP were observed immediately upon tilting from the upright to the supine position (down-tilt), averaging +18 mmHg (50 W) to +31 mmHg (150 W), and there were equally short-lasting decreases in BP, ranging from −26 to −38 mmHg upon tilting from supine to upright (up-tilt). These components occurred for all pressure parameters (systolic, mean, diastolic, and pulse pressures). We propose that these transients reflect mainly tilt-induced changes in total peripheral resistance resulting from decreases and increases of the efficiency of the venous muscle pump. After 3–4 s (down-tilt) and 7–11 s (up-tilt) there were large HR transients in a direction opposite to the pressure transients. These HR transients were larger during the down-tilt (−15 to −26 beats · min⁻¹) than during the up-tilt (+13 to +17 beats · min⁻¹), and increased in amplitude with work intensity during the down-tilt. The tilt-induced HR fluctuations could be modelled as a basically linear function of an arterial baroreflex input from a site half-way between the heart and the carotid sinus, and with varying contributions of fast vagal and slow sympathetic HR responses resulting in attenuated tachycardic responses to hypotensive stimuli during exercise. Accepted: 24 August 1999     

Non-invasive cardiac output evaluation during a maximal progressive exercise test, using a new impedance cardiograph device

One of the greatest challenges in exercise physiology is to develop a valid, reliable, non-invasive and affordable measurement of cardiac output (CO). The purpose of this study was to evaluate the reproducibility and accuracy of a new impedance cardiograph device, the Physio Flow, during a 1-min step incremental exercise test from rest to maximal peak effort. A group of 12 subjects was evaluated to determine the reproducibility of the method as follows: (1) each subject performed two comparable tests while their CO was measured by impedance cardiography using the new device (CO_Imp1, CO_Imp2), and (2) in a subgroup of 7 subjects CO was also determined by the direct Fick method (CO_Fick) during the second test. The mean difference between the values obtained by impedance (i.e. CO_Imp1–CO_Imp2) was –0.009 l·min^–1 (95% confidence interval: –4.2 l·min^–1, 4.2 l·min^–1), and CO ranged from 3.55 l·min^–1 to 26.75 l·min^–1 (n=146). When expressed as a percentage, the difference (CO_Imp1–CO_Imp2) did not vary with increasing CO. The correlation coefficient between the values of CO_Imp and CO_Fick obtained during the second exercise test was r=0.94 (P<0.01, n=50). The mean difference expressed as percentage was –2.78% (95% confidence interval: –27.44%, 21.78%). We conclude that CO_Imp provides a clinically acceptable evaluation of CO in healthy subjects during an incremental exercise. Electronic Publication