Ventilation and cardiac output during the onset of exercise, and during voluntary hyperventilation, in humans

Is breathing frequency a potential means for monitoring exercise intensity in people with atrial fibrillation and coronary heart disease when heart rate is mitigated?

PURPOSE

Moderate-intensity aerobic exercise is safe and beneficial in atrial fibrillation (AF) and coronary heart disease (CHD). Irregular or rapid heart rates (HR) in AF and other heart conditions create a challenge to using HR to monitor exercise intensity. The purpose of this study was to assess the potential of breathing frequency (BF) to monitor exercise intensity in people with AF and CHD without AF.

METHODS

This observational study included 30 AF participants (19 Male, 70.7 ± 8.7 yrs) and 67 non-AF CHD participants (38 Male, 56.9 ± 11.4 yrs). All performed an incremental maximal exercise test with pulmonary gas exchange.

RESULTS

Peak aerobic power in AF ( V ˙ O2peak; 17.8 ± 5.0 ml.kg-1.min-1) was lower than in CHD (26.7 ml.kg-1.min-1) (p < .001). BF responses in AF and CHD were similar (BF peak: AF 34.6 ± 5.4 and CHD 36.5 ± 5.0 breaths.min-1; p = .106); at the 1st ventilatory threshold (BF@VT-1: AF 23.2 ± 4.6; CHD 22.4 ± 4.6 breaths.min-1; p = .240). % V ˙ O2peak at VT-1 were similar in AF and CHD (AF: 59%; CHD: 57%; p = .656).

CONCLUSION

With the use of wearable technologies on the rise, that now include BF, this first study provides an encouraging potential for BF to be used in AF and CHD. As the supporting data are based on incremental ramp protocol results, further research is required to assess BF validity to manage exercise intensity during longer bouts of exercise.

Effects of hyperventilation on oxygenation, apnea breaking points, diving response, and spleen contraction during serial static apneas

PURPOSE

Hyperventilation is considered a major risk factor for hypoxic blackout during breath-hold diving, as it delays the apnea breaking point. However, little is known about how it affects oxygenation, the diving response, and spleen contraction during serial breath-holding.

METHODS

18 volunteers with little or no experience in freediving performed two series of 5 apneas with cold facial immersion to maximal duration at 2-min intervals. In one series, apnea was preceded by normal breathing and in the other by 15 s of hyperventilation. End-tidal oxygen and end-tidal carbon dioxide were measured before and after every apnea, and peripheral oxygen saturation, heart rate, breathing movements, and skin blood flow were measured continuously. Spleen dimensions were measured every 15 s.

RESULTS

Apnea duration was longer after hyperventilation (133 vs 111 s). Hyperventilation reduced pre-apnea end-tidal CO2 (17.4 vs 29.0 mmHg) and post-apnea end-tidal CO2 (38.5 vs 40.3 mmHg), and delayed onset of involuntary breathing movements (112 vs 89 s). End-tidal O2 after apnea was lower in the hyperventilation trial (83.4 vs 89.4 mmHg) and so was the peripheral oxygen saturation nadir after apnea (90.6 vs 93.6%). During hyperventilation, the nadir peripheral oxygen saturation was lower in the last apnea than in the first (94.0% vs 86.7%). There were no differences in diving response or spleen volume reduction between conditions or across series.

CONCLUSIONS

Serial apneas  revealed a previously undescribed aspect of hyperventilation; a progressively increased desaturation across the series, not observed after normal breathing and could heighten the risk of a blackout.

A century of exercise physiology: key concepts on coupling respiratory oxygen flow to muscle energy demand during exercise

After a short historical account, and a discussion of Hill and Meyerhof's theory of the energetics of muscular exercise, we analyse steady-state rest and exercise as the condition wherein coupling of respiration to metabolism is most perfect. The quantitative relationships show that the homeostatic equilibrium, centred around arterial pH of 7.4 and arterial carbon dioxide partial pressure of 40 mmHg, is attained when the ratio of alveolar ventilation to carbon dioxide flow ([Formula: see text]) is - 21.6. Several combinations, exploited during exercise, of pertinent respiratory variables are compatible with this equilibrium, allowing adjustment of oxygen flow to oxygen demand without its alteration. During exercise transients, the balance is broken, but the coupling of respiration to metabolism is preserved when, as during moderate exercise, the respiratory system responds faster than the metabolic pathways. At higher exercise intensities, early blood lactate accumulation suggests that the coupling of respiration to metabolism is transiently broken, to be re-established when, at steady state, blood lactate stabilizes at higher levels than resting. In the severe exercise domain, coupling cannot be re-established, so that anaerobic lactic metabolism also contributes to sustain energy demand, lactate concentration goes up and arterial pH falls continuously. The [Formula: see text] decreases below - 21.6, because of ensuing hyperventilation, while lactate keeps being accumulated, so that exercise is rapidly interrupted. The most extreme rupture of the homeostatic equilibrium occurs during breath-holding, because oxygen flow from ambient air to mitochondria is interrupted. No coupling at all is possible between respiration and metabolism in this case.

Priming Cardiac Function with Voluntary Respiratory Maneuvers and Effect on Early Exercise Oxygen Uptake

Oxygen uptake (V'O₂) at exercise onset is determined in part by acceleration of pulmonary blood flow (Q'p). Impairments in the Q'p response can decrease exercise tolerance. Prior research has shown that voluntary respiratory maneuvers can augment venous return, but the corollary impacts on cardiac function, Q'p and early-exercise V'O₂ remain uncertain. We examined a) the cardiovascular effects of 3 distinct respiratory maneuvers (abdominal, AB; rib cage, RC and deep breathing, DB) under resting conditions in healthy subjects (Protocol 1, n=13) and b) the impact of pre-exercise DB on pulmonary O₂ transfer during initiation of moderate intensity exercise (Protocol 2, n=8). In Protocol 1, echocardiographic analysis showed increased RV and LV cardiac output (RVCO and LVCO, respectively) following AB (by +23±13 and +18±15%, respectively, P<0.05), RC (+23±16; +14±15%, P<0.05) and DB (+27±21; +23±14%, P<0.05). In Protocol 2, DB performed for 12 breaths produced a pre-exercise increase in V'O₂ (+801±254 ml·min^-1 over ~ 6 s), presumably from increased Q'p followed by a reduction in pulmonary O₂ transfer during early phase exercise (first 20 s) compared to the control condition (149±51 vs 233±65 ml, P<0.05). We conclude that (1) respiratory maneuvers enhance RVCO and LVCO in healthy subjects under resting conditions, (2) AB, RC and DB have similar effects on RVCO and LVCO, and (3) DB can increase Q'p prior to exercise onset. These findings suggest that pre-exercise respiratory maneuvers may represent a promising strategy to prime V'O₂ kinetics and thereby to potentially improve exercise tolerance in patients with impaired cardiac function.

Acute Effects of the Wim Hof Breathing Method on Repeated Sprint Ability: A Pilot Study

The Wim Hof breathing method (WHBM) combines periods of hyperventilation (HV) followed by voluntary breath-holds (BH) at low lung volume. It has been increasingly adopted by coaches and their athletes to improve performance, but there was no published research on its effects. We determined the feasibility of implementing a single WHBM session before repeated sprinting performance and evaluated any acute ergogenic effects. Fifteen amateur runners performed a single WHBM session prior to a Repeated Ability Sprint Test (RAST) in comparison to voluntary HV or spontaneous breathing (SB) (control) in a randomized cross-over design. Gas exchange, heart rate, and finger pulse oxygen saturation (SpO2) were monitored. Despite large physiological effects in the SpO2 and expired carbon dioxide (VCO2) levels of both HV and WHBM, no significant positive or negative condition effects were found on RAST peak power, average power, or fatigue index. Finger SpO2 dropped to 60 ± 12% at the end of the BHs. Upon the last HV in the WHBM and HV conditions, end-tidal CO2 partial pressure (PETCO2) values were 19 ± 3 and 17 ± 3 mmHg, indicative of respiratory alkalosis with estimated arterial pH increases of +0.171 and of +0.181, respectively. Upon completion of RAST, 8 min cumulated expired carbon dioxide volumes in the WHBM and HV were greater than in SB, suggesting lingering carbon dioxide stores depletion. These findings indicate that despite large physiological effects, a single WHBM session does not improve anaerobic performance in repeated sprinting exercise.

Vagal blockade suppresses the phase I heart rate response but not the phase I cardiac output response at exercise onset in humans

Purpose

We tested the vagal withdrawal concept for heart rate (HR) and cardiac output (CO) kinetics upon moderate exercise onset, by analysing the effects of vagal blockade on cardiovascular kinetics in humans. We hypothesized that, under atropine, the φ₁ amplitude (A₁) for HR would reduce to nil, whereas the A₁ for CO would still be positive, due to the sudden increase in stroke volume (SV) at exercise onset.

Methods

On nine young non-smoking men, during 0–80 W exercise transients of 5-min duration on the cycle ergometer, preceded by 5-min rest, we continuously recorded HR, CO, SV and oxygen uptake (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{V} $$\end{document}V˙O₂) upright and supine, in control condition and after full vagal blockade with atropine. Kinetics were analysed with the double exponential model, wherein we computed the amplitudes (A) and time constants (τ) of phase 1 (φ₁) and phase 2 (φ₂).

Results

In atropine versus control, A₁ for HR was strongly reduced and fell to 0 bpm in seven out of nine subjects for HR was practically suppressed by atropine in them. The A₁ for CO was lower in atropine, but not reduced to nil. Thus, SV only determined A₁ for CO in atropine. A₂ did not differ between control and atropine. No effect on τ₁ and τ₂ was found. These patterns were independent of posture.

Conclusion

The results are fully compatible with the tested hypothesis. They provide the first direct demonstration that vagal blockade, while suppressing HR φ₁, did not affect φ₁ of CO.

Influence of breathing on variation in cardiac stroke volume at the onset of cycling

PURPOSE

During cycling, the variation in cardiac stroke volume (SVV) is similar to that at rest. However, SVV may be influenced by ventilation at the start of cycling, e.g., by a Valsalva-like maneuver used to stabilize the body. This study evaluated the influence of ventilation on SV during initiation of cycling.

METHODS

Ten healthy recreationally physical active males (mean ± SD: age 26 ± 3 years, height 184 ± 9 cm, weight 85 ± 9 kg) cycled on an ergometer for four 30 s intervals at submaximal workloads while synchronizing ventilatory and cardiovascular variables derived from gas exchange and arterial pulse contour analysis, respectively.

RESULTS

At exercise onset, cardiac output increased by an instantaneous rise in heart rate and SV (P < 0.05). In contrast, blood pressure increased only after 15 s (P < 0.05), reflected in a decline in total peripheral resistance from exercise onset (P < 0.05). SVV was similar at rest (20 ± 6%) and during exercise (21 ± 5%) except for the first 5 s of exercise when a ~ 2.5-fold elevation (47 ± 6%; P < 0.05) was correlated to variation in respiratory frequency (= 0.71, P = 0.02) and tidal volume (R = 0.66, P = 0.04) but not to variation in heart rate or blood pressure. Stepwise multiple regression analysis indicated a respiratory frequency influence on SVV at the onset of ergometer cycling.

CONCLUSION

The data provide evidence for a ventilatory influence on SVV at the onset of cycling exercise.

Priming the cardiodynamic phase of pulmonary oxygen uptake through voluntary modulations of the respiratory pump at the onset of exercise

NEW FINDINGS

What is the central question of this study? The initial increase in oxygen uptake ( V ̇ O 2 ) at exercise onset results from pulmonary perfusion changes secondary to an increased venous return. Breathing mechanics contribute to venous return through abdominal and intrathoracic pressures variation. Can voluntary breathing techniques (abdominal or rib cage breathing) increase venous return and improve V ̇ O 2 at exercise onset? What is the main finding and its importance? Abdominal and rib cage breathing increase venous return and V ̇ O 2 at exercise onset. This mechanism could be clinically relevant in patients with impaired cardiac function limiting oxygen transport.

ABSTRACT

We examined how different breathing patterns can modulate venous return and alveolar gas transfer during exercise transients in humans. Ten healthy men transitioned from rest to moderate cycling while breathing spontaneously (SP) or with voluntary increases in abdominal (AB) or intrathoracic (RC) pressure swings. We used double body plethysmography to determine blood displacements between the trunk and the extremities (V_bs ). From continuous signals of airflow and O₂ fraction, we calculated breath-by-breath oxygen uptake at the mouth and used optoelectronic plethysmography to correct for lung O₂ store changes and calculate alveolar O₂ transfer ( V ̇ O 2 A ). Oesophageal (P_oes ) and gastric (P_ga ) pressures were monitored using balloon-tipped catheters. Cardiac stroke volume was measured using impedance cardiography. During the cardiodynamic phase (Φ1) of V ̇ O 2 A -on kinetics (20 s following exercise onset), AB and RC increased total alveolar oxygen transfer compared to SP (227 ± 32, P = 0.019 vs. 235 ± 27, P = 0.001 vs. 206 ± 20 ml, mean ± SD). P_ga and P_oes swings increased with AB (by 24.4 ± 9.6 cmH₂ O, P < 0.001) and RC (by 14.5 ± 5.7 cmH₂ O, P < 0.001), respectively. AB yielded a greater increase in intra-breath V_bs swings compared with RC and SP (+0.30 ± 0.14 vs. +0.16 ± 0.11, P < 0.001 vs. +0.10 ± 0.05 ml, P = 0.006) and increased the sum of stroke volumes compared to SP (4.47 ± 1.28 vs. 3.89 ± 0.96 litres, P = 0.053), while RC produced significant central blood translocation from the extremities compared with SP (by 493 ± 311 ml, P < 0.001). Our findings indicate that combining exercise onset with AB or RC increases venous return, thus increasing mass oxygen transport above metabolic consumption during Φ1 and limiting the oxygen deficit incurred.

Breath isoprene excretion during rest and low-intensity cycling exercise is associated with skeletal muscle mass in healthy human subjects

The physiological roles of isoprene, which is one of the many endogenous volatile organic compounds contained in exhaled breath, are not well understood. In recent years, exhaled isoprene has been associated with the skeletal muscle. Some studies have suggested that the skeletal muscle produces and/or stores some of the isoprene. However, the evidence supporting this association remains sparse and inconclusive. Furthermore, aging may affect breath isoprene response because of changes in the skeletal muscle quantity and quality. Therefore, we investigated the association between the breath isoprene excretion ([Formula: see text]) and skeletal muscle mass in young (n = 7) and old (n = 7) adults. The participants performed an 18 min cycling exercise after a 3 min rest. The workload corresponded to an intensity of 30% of the heart rate reserve, as calculated by the Karvonen formula. The exhaled breath of each participant was collected during the exercise test. We calculated [Formula: see text] from the product minute ventilation and isoprene concentration and, then, investigated the relationships between [Formula: see text] and muscle mass, which was measured by multi-frequency bioelectrical impedance analysis. Importantly, muscle mass persisted as a significant determinant that explained the variance in [Formula: see text] at rest even after adjusting for age. Furthermore, the muscle mass was a significant determinative factor for [Formula: see text] response during exercise, regardless of age. These data indicated that skeletal muscle mass could be one of the determinative factors for [Formula: see text] during rest and response to exercise. Thus, we suggest that the skeletal muscle may play an important role in generating and/or storing some of the endogenous isoprene. This new knowledge will help to better understand the physiological functions of isoprene in humans (Approval No. 20190079).

Effect of Lower Body Negative Pressure on Phase I Cardiovascular Responses at Exercise Onset

We hypothesised that vagal withdrawal and increased venous return interact in determining the rapid cardiac output (CO) response (phase I) at exercise onset. We used lower body negative pressure (LBNP) to increase blood distribution to the heart by muscle pump action and reduce resting vagal activity. We expected a larger increase in stroke volume (SV) and smaller for heart rate (HR) at progressively stronger LBNP levels, therefore CO response would remain unchanged. To this aim ten young, healthy males performed a 50 W exercise in supine position at 0 (Control), −15, −30 and −45 mmHg LBNP exposure. On single beat basis, we measured HR, SV, and CO. Oxygen uptake was measured breath-by-breath. Phase I response amplitudes were obtained applying an exponential model. LBNP increased SV response amplitude threefold from Control to −45 mmHg. HR response amplitude tended to decrease and prevented changes in CO response. The rapid response of CO explained that of oxygen uptake. The rapid SV kinetics at exercise onset is compatible with an increased venous return, whereas the vagal withdrawal conjecture cannot be dismissed for HR. The rapid CO response may indeed be the result of two independent yet parallel mechanisms, one acting on SV, the other on HR.

Submaximal exercise cardiac output is increased by 4 weeks of sprint interval training in young healthy males with low initial Q̇-V̇O2: Importance of cardiac response phenotype

Cardiovascular adaptations to exercise, particularly at the individual level, remain poorly understood. Previous group level research suggests the relationship between cardiac output and oxygen consumption (Q˙-V˙O2) is unaffected by training as submaximal Q˙ is unchanged. We recently identified substantial inter-individual variation in the exercise Q˙-V˙O2 relationship that was correlated to stroke volume (SV) as opposed to arterial oxygen content. Therefore we explored the effects of sprint interval training (SIT) on modulating Q˙-V˙O2 given an individual’s specific Q˙-V˙O2 relationship. 22 (21±2 yrs) healthy, recreationally active males participated in a 4-week SIT (8, 20 second sprints; 4x/week, 170% of the work rate at V˙O2 peak) study with progressive exercise tests (PET) until exhaustion. Cardiac output (Q˙ L/min; inert gas rebreathe, Finometer Modelflow™), oxygen consumption (V˙O2 L/min; breath-by-breath pulmonary gas exchange), quadriceps oxygenation (near infrared spectroscopy) and exercise tolerance (6–20; Borg Scale RPE) were measured throughout PET both before and after training. Data are mean Δ from bsl±SD. Higher Q˙ (HQ˙) and lower Q˙ (LQ˙) responders were identified post hoc (n = 8/group). SIT increased the Q˙-V˙O2 post-training in LQ˙ (3.8±0.2 vs. 4.7±0.2; P = 0.02) while HQ˙ was unaffected (5.8±0.1 vs. 5.3±0.6; P = 0.5). ΔQ˙ was elevated beyond 80 watts in LQ˙ due to a greater increase in SV (all P<0.04). Peak V˙O2 (ml/kg/min) was increased in LQ˙ (39.7±6.7 vs. 44.5±7.3; P = 0.015) and HQ˙ (47.2±4.4 vs. 52.4±6.0; P = 0.009) following SIT, with HQ˙ having a greater peak V˙O2 both pre (P = 0.02) and post (P = 0.03) training. Quadriceps muscle oxygenation and RPE were not different between groups (all P>0.1). In contrast to HQ˙, LQ˙ responders are capable of improving submaximal Q˙-V˙O2 in response to SIT via increased SV. However, the increased submaximal exercise Q˙ does not benefit exercising muscle oxygenation.

Do interindividual differences in cardiac output during submaximal exercise explain differences in exercising muscle oxygenation and ratings of perceived exertion?

Considerable interindividual differences in the Q˙-V˙O2 relationship during exercise have been documented but implications for submaximal exercise tolerance have not been considered. We tested the hypothesis that these interindividual differences were associated with differences in exercising muscle deoxygenation and ratings of perceived exertion (RPE) across a range of submaximal exercise intensities. A total of 31 (21 ± 3 years) healthy recreationally active males performed an incremental exercise test to exhaustion 24 h following a resting muscle biopsy. Cardiac output (Q˙ L/min; inert gas rebreathe), oxygen uptake (V˙O2 L/min; breath-by-breath pulmonary gas exchange), quadriceps saturation (near infrared spectroscopy) and exercise tolerance (6-20; Borg Scale RPE) were measured. The Q˙-V˙O2 relationship from 40 to 160 W was used to partition individuals post hoc into higher (n = 10; 6.3 ± 0.4) versus lower (n = 10; 3.7 ± 0.4, P < 0.001) responders. The Q˙-V˙O2 difference between responder types was not explained by arterial oxygen content differences (P = 0.5) or peripheral skeletal muscle characteristics (P from 0.1 to 0.8) but was strongly associated with stroke volume (P < 0.05). Despite considerable Q˙-V˙O2 difference between groups, no difference in quadriceps deoxygenation was observed during exercise (all P > 0.4). Lower cardiac responders had greater leg (P = 0.027) and whole body (P = 0.03) RPE only at 185 W, but this represented a higher %peak V˙O2 in lower cardiac responders (87 ± 15% vs. 66 ± 12%, P = 0.005). Substantially lower Q˙-V˙O2 in the lower responder group did not result in altered RPE or exercising muscle deoxygenation. This suggests substantial recruitment of blood flow redistribution in the lower responder group as part of protecting matching of exercising muscle oxygen delivery to demand.

A new interpolation-free procedure for breath-by-breath analysis of oxygen uptake in exercise transients

INTRODUCTION

Interpolation methods circumvent poor time resolution of breath-by-breath oxygen uptake (VO₂) kinetics at exercise onset. We report an interpolation-free approach to the improvement of poor time resolution in the analysis of VO₂ kinetics.

METHODS

Noiseless and noisy (10% Gaussian noise) synthetic data were generated by Monte Carlo method from pre-selected parameters (Exact Parameters). Each data set comprised 10 (VO₂)-on transitions with noisy breath distribution within a physiological range. Transitions were superposed (no interpolation, None), then analysed by bi-exponential model. Fitted model parameters were compared with those from interpolation methods (average transition after Linear or Step 1-s interpolations), applied on the same data. Experimental data during cycling were also analysed. The 95% confidence interval around a line of parameters' equality was computed to analyse agreement between exact parameters and corresponding parameters of fitted functions.

RESULTS

The line of parameters' equality stayed within confidence intervals for noiseless synthetic parameters with None, unlike Step and Linear, indicating that None reproduced Exact Parameters. Noise addition reduced differences among pre-treatment procedures. Experimental data provided lower phase I time constants with None than with Step.

CONCLUSION

In conclusion, None revealed better precision and accuracy than Step and Linear, especially when phenomena characterized by time constants of <30 s are to be analysed. Therefore, we endorse the utilization of None to improve the quality of breath-by-breath [Formula: see text] data during exercise transients, especially when a double exponential model is applied and phase I is accounted for.

Body height and blood pressure regulation in humans during anti-orthostatic tilting

The hypothesis was tested that the cardiovascular changes during an upper body anti-orthostatic maneuver in humans are more pronounced in tall than in short individuals, because of the larger intravascular hydrostatic pressure gradients. In 34 males and 41 females [20–30 yr, body height (BH) = 147–206 cm], inter-individual multiple linear regression analyses adjusted for gender and body weight were conducted between changes in cardiovascular variables versus BH during tilting of the upper body from vertical to horizontal while keeping the legs horizontal. In all the subjects, tilting induced increases in stroke volume and arterial pulse pressure and a decrease in heart rate, which each correlated significantly with BH. In males ( n = 51, BH = 163–206 cm), 24-h ambulatory mean arterial pressure increased significantly with BH ( P = 0.004, r = 0.40, α = 0.15 mmHg/cm) so that systolic/diastolic blood pressure increased by 2/2 mmHg per 15 cm increase in BH. There was no significant correlation between mean arterial pressure and BH in females ( n = 53, BH = 147–193 cm). In conclusion, a larger BH induces larger cardiovascular changes during anti-orthostatic tilting, and in males 24-h ambulatory mean arterial pressure increases with BH. The lack of a mean arterial pressure to BH correlation in females is probably because of their lower BH and greater variability in blood pressure.

Mechanisms of increase in cardiac output during acute weightlessness in humans

Based on previous water immersion results, we tested the hypothesis that the acute 0-G-induced increase in cardiac output (CO) is primarily caused by redistribution of blood from the vasculature above the legs to the cardiopulmonary circulation. In seated subjects (n = 8), 20 s of 0 G induced by parabolic flight increased CO by 1.7 ± 0.4 l/min (P < 0.001). This increase was diminished to 0.8 ± 0.4 l/min (P = 0.028), when venous return from the legs was prevented by bilateral venous thigh-cuff inflation (CI) of 60 mmHg. Because the increase in stroke volume during 0 G was unaffected by CI, the lesser increase in CO during 0 G + CI was entirely caused by a lower heart rate (HR). Thus blood from vascular beds above the legs in seated subjects can alone account for some 50% of the increase in CO during acute 0 G. The remaining increase in CO is caused by a higher HR, of which the origin of blood is unresolved. In supine subjects, CO increased from 7.1 ± 0.7 to 7.9 ± 0.8 l/min (P = 0.037) when entering 0 G, which was solely caused by an increase in HR, because stroke volume was unaffected. In conclusion, blood originating from vascular beds above the legs can alone account for one-half of the increase in CO during acute 0 G in seated humans. A Bainbridge-like reflex could be the mechanism for the HR-induced increase in CO during 0 G in particular in supine subjects.

Desensitization of the cough reflex by exercise and voluntary isocapnic hyperpnea

Little is known about the effects of exercise on the sensory and cognitive aspects of coughing evoked by inhalation of tussigenic agents. The threshold for the cough reflex induced by inhalation of increasing nebulizer outputs of ultrasonically nebulized distilled water (fog), an index of cough reflex sensitivity, was assessed in twelve healthy humans in control conditions, during exercise and during voluntary isocapnic hyperpnea (VIH) at the same ventilatory level as the exercise. The intensity of the urge to cough (UTC), a cognitive component of coughing, was recorded throughout the trials on a linear scale. The relationships between inhaled fog nebulizer outputs and the correspondingly evoked UTC values, an index of the perceptual magnitude of the UTC sensitivity, were also calculated. Cough appearance was always assessed audiovisually. At an exercise level of 80% of anaerobic threshold, the median cough threshold was increased from a control value of 0.73 to 2.22 ml/min ( P < 0.01), i.e., cough sensitivity was downregulated. With VIH, the threshold increased from 0.73 to 2.22 ml/min ( P < 0.01), a similar downregulation. With exercise and VIH compared with control, mean UTC values at cough threshold were unchanged, i.e., control, 3.83 cm; exercise, 3.12 cm; VIH, 4.08 cm. The relationship of the fog nebulizer output/UTC value was linear in control conditions and logarithmic during both exercise and VIH. The perception of the magnitude of the UTC seems to be influenced by signals or sensations arising from exercising limb and thoracic muscles and/or by higher nervous (cortical) mechanisms. The results indicate that the adjustments brought into action by exercise-induced or voluntary hyperpnea exert inhibitory influences on the sensory and cognitive components of fog-induced cough.

Phase I dynamics of cardiac output, systemic O2 delivery, and lung O2 uptake at exercise onset in men in acute normobaric hypoxia

We tested the hypothesis that vagal withdrawal plays a role in the rapid (phase I) cardiopulmonary response to exercise. To this aim, in five men (24.6 ± 3.4 yr, 82.1 ± 13.7 kg, maximal aerobic power 330 ± 67 W), we determined beat-by-beat cardiac output (Q̇), oxygen delivery (Q̇aO2), and breath-by-breath lung oxygen uptake (V̇o2) at light exercise (50 and 100 W) in normoxia and acute hypoxia (fraction of inspired O2 = 0.11), because the latter reduces resting vagal activity. We computed Q̇ from stroke volume (Qst, by model flow) and heart rate ( fH, electrocardiography), and Q̇aO2 from Q̇ and arterial O2 concentration. Double exponentials were fitted to the data. In hypoxia compared with normoxia, steady-state fH and Q̇ were higher, and Qst and V̇o2 were unchanged. Q̇aO2 was unchanged at rest and lower at exercise. During transients, amplitude of phase I (A1) for V̇o2 was unchanged. For fH, Q̇ and Q̇aO2, A1 was lower. Phase I time constant (τ1) for Q̇aO2 and V̇o2 was unchanged. The same was the case for Q̇ at 100 W and for fH at 50 W. Qst kinetics were unaffected. In conclusion, the results do not fully support the hypothesis that vagal withdrawal determines phase I, because it was not completely suppressed. Although we can attribute the decrease in A1 of fH to a diminished degree of vagal withdrawal in hypoxia, this is not so for Qst. Thus the dual origin of the phase I of Q̇ and Q̇aO2, neural (vagal) and mechanical (venous return increase by muscle pump action), would rather be confirmed.

Central circulatory and peripheral O2 extraction changes as interactive facilitators of pulmonary O2 uptake during a repeated high-intensity exercise protocol in humans

It has frequently been demonstrated that prior high-intensity exercise facilitates pulmonary oxygen uptake [Formula: see text] response at the onset of subsequent identical exercise. To clarify the roles of central O(2) delivery and/or peripheral O(2) extraction in determining this phenomenon, we investigated the relative contributions of cardiac output (CO) and arteriovenous O(2) content difference [Formula: see text] to the [Formula: see text] transient during repeated bouts of high-intensity knee extension (KE) exercise. Nine healthy subjects volunteered to participate in this study. The protocol consisted of two consecutive 6-min KE exercise bouts in a supine position (work rate 70-75% of peak power) separated by 6 min of rest. Throughout the protocol, continuous-wave Doppler ultrasound was used to measure beat-by-beat CO (i.e., via simultaneous measurement of stroke volume and the diameter of the arterial aorta). The phase II [Formula: see text] response was significantly faster and the slow component (phase III) was significantly attenuated during the second KE bout compared to the first. This was a result of increased CO during the first 30 s of exercise: CO contributing to 100 and 56% of the [Formula: see text] speeding at 10 and 30 s, respectively. After this, the contribution of [Formula: see text] became increasingly more predominant: being responsible to an estimated 64% of the [Formula: see text] speeding at 90 s, which rose to 100% by 180 s. This suggests that, while both CO and [Formula: see text] clearly interact to determine the [Formula: see text] response, the speeding of [Formula: see text] kinetics by prior high-intensity KE exercise is predominantly attributable to increases in [Formula: see text].

Simultaneous determination of the kinetics of cardiac output, systemic O2 delivery, and lung O2 uptake at exercise onset in men

We tested whether the kinetics of systemic O2 delivery (Q̇aO2) at exercise start was faster than that of lung O2 uptake (V̇o2), being dictated by that of cardiac output (Q̇), and whether changes in Q̇ would explain the postulated rapid phase of the V̇o2 increase. Simultaneous determinations of beat-by-beat (BBB) Q̇ and Q̇aO2, and breath-by-breath V̇o2 at the onset of constant load exercises at 50 and 100 W were obtained on six men (age 24.2 ± 3.2 years, maximal aerobic power 333 ± 61 W). V̇o2 was determined using Grønlund's algorithm. Q̇ was computed from BBB stroke volume (Qst, from arterial pulse pressure profiles) and heart rate ( fh, electrocardiograpy) and calibrated against a steady-state method. This, along with the time course of hemoglobin concentration and arterial O2 saturation (infrared oximetry) allowed computation of BBB Q̇aO2. The Q̇, Q̇aO2 and V̇o2 kinetics were analyzed with single and double exponential models. fh, Qst, Q̇, and V̇o2 increased upon exercise onset to reach a new steady state. The kinetics of Q̇aO2 had the same time constants as that of Q̇. The latter was twofold faster than that of V̇o2. The V̇o2 kinetics were faster than previously reported for muscle phosphocreatine decrease. Within a two-phase model, because of the Fick equation, the amplitude of phase I Q̇ changes fully explained the phase I of V̇o2 increase. We suggest that in unsteady states, lung V̇o2 is dissociated from muscle O2 consumption. The two components of Q̇ and Q̇aO2 kinetics may reflect vagal withdrawal and sympathetic activation.

Pulmonary O2 Uptake during Exercise: Conflating Muscular and Cardiovascular Responses

For moderate-intensity exercise (below lactate threshold, thetaL), muscle O(2) consumption (VO(2)) kinetics are expressed in a first-order phase 2 (or fundamental) pulmonary O(2) uptake (VO(2)) response: dVO(2)/dt . tau + DeltaVO(2)((t)) = DeltaVO(2)((ss)); where DeltaVO(2)(ss) is the steady-state VO(2) increment, and tau the VO(2) time constant (which is within approximately 10% of tauQVO(2)). A likely source of VO(2) control in this intensity domain is ADP-mediated, for which intramuscular phosphocreatine (PCr) may serve as a proxy variable. Whether, in reality, this behavior reflects the operation of a single homogeneous compartment is unclear, however; a multicompartment structure comprised of units having a similar DeltaVO(2)((ss)) but with widely varying tau can also yield a "well-fit" exponential response with an apparent single tau. In support of this is the inverse (although poorly predictive) correlation between tau and both theta(L) and VO(2max). Above theta(L), the fundamental VO(2) kinetics are supplemented with a delayed, slowly developing component that can set VO(2) on a trajectory towards VO(2max), and that has complex temporal- and intensity-related kinetics. This VO(2) slow component is also demonstrable in [PCr], suggesting that the decreased efficiency above theta(L) predominantly reflects a high phosphate cost of force production rather than a high O(2) cost of phosphate production. In addition, the oxygen deficit for the slow component is more likely to reflect a progressive shifting of DeltaVO(2)((ss)) rather than a single DeltaVO(2)((ss)) having a single tau.

Associative conditioning with leg cycling and inspiratory resistance enhances the early exercise ventilatory response in humans

Repeated trials of hypercapnic exercise [deltaPET CO2 = 7 (1) mmHg] augment the increase in inspired minute ventilation and tidal volume (V(T)) in the early phase of subsequent trials of unencumbered exercise alone. The increase in V(T) in the first 20 s of exercise was correlated to the increase in V(T) evoked during hypercapnic exercise trials, suggesting that the evoked increase in V(T) during conditioning may be a factor in mediating associative conditioning. To test this hypothesis, inspiratory resistive loading (IRL) was employed to evoke an increase in V(T) [deltaV(T) = 0.4 (0.1) I(BTPS)] during conditioning exercise trials [IRL + EX; deltaP(ET)CO2 = 2 (l) mmHg]. IRL + EX associative conditioning elicited a significant augmentation of the early minute ventilation (+46%) and V(T) (+100%) responses to subsequent unencumbered exercise. The latter was correlated to the evoked increase in V(T) during associative conditioning with IRL + EX. The results support the hypothesis that an evoked increase in V(T) during associative conditioning could be a factor in eliciting long-term modulation of minute ventilation in subsequent unencumbered exercise. The results further indicated that the modulation of ventilation early in exercise is not due to sensitisation to repeated trials of either IRL or exercise alone. Associative conditioning may shape the ventilatory response to exercise through a process of motor learning. Data are presented as mean (SEM) unless otherwise stated.

Effects of ventilation on cardiac output determined by inert gas rebreathing

One of the most important methodological problems of the foreign gas rebreathing technique is that outcome of the measurements depends on procedural variables such as rebreathing frequency (RF), rebreathing bag volume (V(reb)), lung volume at start of rebreathing and intervals between measurements. Therefore, in 10 healthy males we investigated the effects of changes in ventilation pattern on cardiac output (CO) estimated by an N(2)O-rebreathing technique. Reducing the rebreathing volume (V(reb)) from 1.5 to 1.0 l diminished CO by 0.5 +/- 0.2 l min(-1), whereas an increase in V(reb) from 1.5 to 2.5 l had no effects. CO was 1.0 +/- 0.2 l min(-1) higher when, rebreathing was performed after a forced expiration than following a normal tidal expiration. Serial determinations of CO required a 3-min interval between the measurements to avoid effects of recirculation of N(2)O. Changing RF from 15 to 30 breaths min(-1) or adding serial dead space by up to 600 ml did not affect the determination of CO. In conclusion, the rebreathing procedure for determination of CO at rest should be performed following a normal tidal expiration with a rebreathing bag volume of between 1.5 and 2.5 l and with manoeuvres separated by at least 3-5 min. Variations in RF within the physiological range from 15 to 30 breaths min(-1) do not affect outcome of the measurements.

Oxygen uptake kinetic response to exercise in children

The oxygen uptake (.VO2) kinetic response to exercise assesses the integrated response of the cardiovascular system and the metabolic requirements of the exercising muscle. The response differs both qualitatively and quantitatively according to the exercise intensity domain (moderate, heavy, very heavy and severe) in which it lies. In each domain, a rapid cardiodynamic phase 1 response is followed by an exponential rise in .VO2 toward a projected steady state (for which the inverse of the rate constant is represented as the time constant [tau]). The achievement of the new steady state may be delayed and elevated due to a slow component of .VO2 in the heavy intensity domain, or above this exercise intensity, the achievement of peak .VO2 truncates the exercise period. For each of these domains, specific mathematical models have been identified and may be applied to appropriate breath-by-breath response data in order to allow quantification of the response. Much of our understanding of the .VO2 kinetic response and the methodologies required to obtain meaningful assessment are derived from adult studies. Although pioneering, early studies with young people were lacking in suitable equipment and the methodologies used may consequently have clouded the true interpretation of the kinetic response. More recently, with the advent of online breath-by-breath analysis systems, studies using mathematical modelling procedures have been hindered by the low signal-to-noise ratio which is inherent to children's response profiles. This has the effect of widening the confidence intervals for estimated parameters, and therefore questions the validity in making inter- and intra-study comparisons. In addition, the difficulty in accurately assessing domain demarcators, especially critical power, often confounds the interpretation of age and sex effects on the exercise response.This review therefore analyses the literature to date on the .VO2 kinetic response during childhood and adolescence, and specifically highlights concerns with technical rigour in its determination. Rigorously determined data indicate that the exponential rise in .VO2 is more rapid in children than adults and that at exercise intensities above the anaerobic threshold, the slow component of .VO2 may be attenuated in the young. Sex differences have not been found in the response to moderate intensity exercise, and there does not appear to be a consistent correlation between peak .VO2 and tau in children. However, sex differences in the response to exercise intensities above the anaerobic threshold are identified and discussed.

Associative conditioning of the exercise ventilatory response in humans

Repeated hypercapnic exercise augmented the ventilatory response to subsequent trials of exercise alone in running goats and in humans performing arm exercise, suggesting a form of associative conditioning or 'long-term modulation' had taken place. These studies did not include 'control' single stimulus conditioning paradigms. This study demonstrated that ten repeated trials of familiar leg bicycling exercise with dead-space induced hypercapnia also elicited similar significant increases in inspired ventilation (+ 22%; P < 0.009) and tidal volume (VT; + 255 +/- 73 ml(BTPS); mean +/- S.E.M.; P = 0.004) within the first 20 sec of subsequent exercise only trials. Long-term modulation of the early ventilatory response to cycling was not fully replicated by ten trials of 'control' paradigms involving either repeated exercise alone or resting dead space alone. This study thus demonstrated that long term modulation of the early ventilatory response exercise was due to an explicit effect of associative conditioning and not simply sensitisation to repeated trials of a single stimulus.

Control of the exercise hyperpnoea in humans: a modeling perspective

Models of the exercise hyperpnoea have classically incorporated elements of proportional feedback (carotid and medullary chemosensory) and feedforward (central and/or peripheral neurogenic) control. However, the precise details of the control process remain unresolved, reflecting in part both technical and interpretational limitations inherent in isolating putative control mechanisms in the intact human, and also the challenges to linear control theory presented by multiple-input integration, especially with regard to the ventilatory and gas-exchange complexities encountered at work rates which engender a metabolic acidosis. While some combination of neurogenic, chemoreflex and circulatory-coupled processes are likely to contribute to the control, the system appears to evidence considerable redundancy. This, coupled with the lack of appreciable error signals in the mean levels of arterial blood gas tensions and pH over a wide range of work rates, has motivated the formulation of innovative control models that reflect not only spatial interactions but also temporal interactions (i.e. memory). The challenge is to discriminate between robust competing control models that: (a) integrate such processes within plausible physiological equivalents; and (b) account for both the dynamic and steady-state system response over a range of exercise intensities. Such models are not yet available.

Inferences from pulmonary O2 uptake with respect to intramuscular [phosphocreatine] kinetics during moderate exercise in humans

1. In the non-steady state of moderate intensity exercise, pulmonary O2 uptake (Vp,O2) is temporally dissociated from muscle O2 consumption (Vm,O2) due to the influence of the intervening venous blood volume and the contribution of body O2 stores to ATP synthesis. A monoexponential model of Vp,O2 without a delay term, therefore, implies an obligatory slowing of Vp,O2 kinetics in comparison to Vm, O2. 2. During moderate exercise, an association of Vm,O2 and [phosphocreatine] ([PCr]) kinetics is a necessary consequence of the control of muscular oxidative phosphorylation mediated by some function of [PCr]. It has also been suggested that the kinetics of Vp,O2 will be expressed with a time constant within 10 % of that of Vm,O2. 3. Vp,O2 and intramuscular [PCr] kinetics were investigated simultaneously during moderate exercise of a large muscle mass in a whole-body NMR spectrometer. Six healthy males performed prone constant-load quadriceps exercise. A transmit-receive coil under the right quadriceps allowed determination of intramuscular [PCr]; Vp,O2 was measured breath-by-breath, in concert with [PCr], using a turbine and a mass spectrometer system. 4. Intramuscular [PCr] decreased monoexponentially with no delay in response to exercise. The mean of the time constants (tauPCr) was 35 s (range, 20-64 s) for the six subjects. 5. Two temporal phases were evident in the Vp, O2 response. When the entire Vp,O2 response was modelled to be exponential with no delay, its time constant (tau'Vp,O2) was longer in all subjects (group mean = 62 s; range, 52-92 s) than that of [PCr], reflecting the energy contribution of the O2 stores. 6. Restricting the Vp,O2 model fit to phase II resulted in matching kinetics for Vp,O2 (group mean tauVp,O2 = 36 s; range, 20-68 s) and [PCr], for all subjects. 7. We conclude that during moderate intensity exercise the phase II tauVp,O2 provides a good estimate of tauPCr and by implication that of Vm,O2 (tauVm,O2).

Measurement of minute ventilation with different DDDR pacemaker electrode configurations. Investigators of a Multicenter Study Evaluating the Chorus RM and Opus RM Pacemakers

A rate responsive minute ventilation (VE) pacemaker was implanted in 49 patients (70.8 +/- 40.0 years). A Chorus RM 7034 pacemaker was implanted in 43 patients and an Opus RM 4534 in six patients. Four sensor configurations were compared: atrial configuration (bipolar atrial lead) in 34 patients; ventricular configuration (bipolar ventricular lead) in 6 patients; unipolar configuration (double unipolar leads) in 6 patients; and floating configuration (VDD single-pass lead) in 3 patients. The patients carried out 57 exercise tests in all with cardiopulmonary recording (CPX). Real VE and oxygen consumption (VO2) were recorded by the CPX, the VE measured by the sensor (VEsensor) was recorded in the pacemaker memory. The mean correlation between VE and VEsensor was 0.90 +/- 0.08 (P < 0.001) and between VO2 and VEsensor was 0.86 +/- 0.10 (P < 0.001). The mean correlation between VE and VEsensor by configuration type were as follows: atrial configuration = 0.89 +/- 0.08; ventricular configuration = 0.95 +/- 0.05; unipolar configuration = 0.87 +/- 0.14; and floating configuration = 0.88 +/- 0.05. In conclusion, VE may be reliably measured using different electrode configurations. A study conducted in a larger population should allow one to conclude that unipolar electrodes can be used in VDDR, AAIR, VVIR, or DDDR modes to measure VE.

The role of pulmonary CO2 flow in the control of the phase I ventilatory response to exercise in humans

To gain an insight into the origin of the phase I ventilatory response to exercise (ph I) in humans, pulmonary ventilation (VE) and end-tidal partial pressures of oxygen and carbon dioxide (PETO2 and PETCO2, respectively) were measured breath-by-breath in six male subjects during constant-intensity exercise on the cycle ergometer at 50, 100 and 150 W, with eupnoeic normocapnia (N) or hyperpnoeic hypocapnia (H) established prior to the exercise test. Cardiac output (Qc) was also determined beat-by-beat by impedance cardiography on eight subjects during moderate exercise (50 W), and the CO2 flow to the lungs (Qc.Cv-CO2 where Cv-CO2 is concentration of CO2 in mixed veneous blood) was estimated with a time resolution of one breathing cycle. In N, the initial abrupt increase of VE during ph I (delta VE approximately 18 1.min-1 above rest) was followed by a transient fall. When PETCO2 started to increase (and PETO2 decreased) VE increased again (phase II ventilatory response, ph II). In H, during ph I delta VE was similar to that of N. By contrast, during ph II delta VE kept gradually decreasing and started to increase only when PETCO2 had returned to approximately 40 mmHg (5.3 kPa). Thus, as a result of the prevailing initial conditions (N or H) a temporal shift of the time-course of VE during ph II became apparent. No correlation was found between CO2 flow to the lungs and VE during ph I. These results are interpreted as suggesting that an increased CO2 flow to the lungs does not constitute an important factor for the initial hyperventilatory response to exercise. They are rather compatible with a neural origin of ph I, and would support the "neurohumoral" theory of ventilatory control during exercise.

Technical improvements in sensors for rate adaptive pacemakers

Many types of sensors have been developed and applied clinically during recent years. Technical improvements can be achieved through greater sensitivity and especially through more specificity for various physical or preferably physiologic signals. However, to date no single sensor properly reflects metabolic demands under all circumstances. In a manner analogous to the normal sinus node, the input from different sources will have to be considered. This leads to the development of dual-sensor or eventually multisensor pacemakers in which the rate is a computed result of blended and cross-checked information on the various parameters that are analyzed.

Relationship between cardiac output and oxygen uptake at the onset of exercise

The purpose of the present study was to assess the relationship between the rapidity of increased gas exchange (i.e. oxygen uptake VO2) and increased cardiac output (Qc) during the transient phase following the onset of exercise. Five healthy male subjects performed multiple rest-exercise or light exercise (25 W)-exercise transitions on an electrically braked ergometer at exercise intensities of 50, 75, or 100 W for 6 min, respectively. Each transition was performed at least eight times for each load in random order. The VO2 was obtained by a breath-by-breath method, and Qc was measured by an impedance method during normal breathing, using an ensemble average. On transitions from rest to exercise, VO2 rapidly increased during phase I with time constants of 6.8-7.3 s. The Qc also showed a similar rapid increment with time constants of 6.0-6.8 s with an apparent increase in stroke volume (SV). In this phase I, VO2 increased to about 29.7%-34.1% of the steady-state value and Qc increased to about 58.3%-87.0%. Thereafter, some 20 s after the onset of exercise a mono-exponential increase to steady-state occurred both in VO2 and Qc with time constants of 26.7-32.3 and 23.7-34.4 s, respectively. The insignificant difference between Qc and VO2 time constants in phase I and the abrupt increase in both Qc and SV at the onset of exercise from rest provided further evidence for a "cardiodynamic" contribution to VO2 following the onset of exercise from rest.

New and combined sensors for adaptive-rate pacing

A new potential indication for cardiac pacing is chronotropic incompetence, that is, an inadequate cardiac rate response to exercise and other metabolic demands. Many patients who have been paced for indications such as complete heart block or sick sinus syndrome also have chronotropic incompetence. Such patients are not adequately treated when fitted with a constant rate pacemaker. Adaptive-rate pacemakers increase the pacing rate in proportion to signals derived from a biosensor which is sensitive to exertion and possibly to other metabolic requirements. These pacemakers have proven valuable for patients with overt chronotropic incompetence. However, no single sensor/algorithm is ideal and improvement has been sought by introducing new sensors, adjusting the algorithms by which biosensor signals are converted to the most appropriate pacing rate, or by combining sensors in such a way that a composite biosensor signal is derived which bears a close linear relationship with the appropriate heart rate. An example of a new sensor is the accelerometer, which is sensitive to a fuller range of movements than the piezo crystal. A successful new algorithm is the rate augmentation algorithm for use with minute ventilation, which provides a better initial pacing rate response. A combination of minute ventilation sensed by impedance changes and movement sensed with piezo crystals maintains the rapid response from the piezo crystal and overcomes its lack of proportionality. Another successful new combination of sensors is QT sensing from the evoked ventricular potential and motion sensing with a piezo crystal. As yet, these innovations have not been exhaustively tested and shown to confer clinical benefit but the improvements are such that an advantage can be expected.

The range of sensors and algorithms used in rate adaptive cardiac pacing

Implantable sensors play an important role in physiological cardiac pacing. Sensors can be classified according to the technical methods in which sensing is achieved: the sensing of the evoked ventricular response, intrathoracic impedance and body acceleration forces, and the incorporation of special sensors on pacing electrodes. These sensors differ in their relative merits in terms of speed, proportionality, sensitivity, and specificity of rate response. The efficacy of a sensor can be significantly modified by the algorithm used in relating sensor signal to a pacing rate change. The currently available types of sensors and algorithms are summarized and compared in this review article. The relative merits of these sensors and algorithms form the basis for designing a multisensor pacing system.

The effect of a meal on cardiac output in man at rest and during moderate exercise

Cardiac output at rest increased by 11-63% in a group of healthy individuals after the consumption of a medium-sized, mixed meal. The maximum post-prandial levels of cardiac output were reached from 10 to 30 min after termination of the meal. Cardiac output values at rest fluctuate around a mean level, and this fluctuation was considerably more marked after a meal, when changes in cardiac output from one 15-s period to another could be of the order of 1-1.5 l min-1. Recording of flow in the superior mesenteric artery before and also after a meal was successful in two subjects in whom anatomical conditions were favourable. Flow in the artery was approximately doubled from the fasting to the post-prandial situation, an augmentation that accounted for about 50% of the concomitant increase in cardiac output. The increases in cardiac output caused by 2-min bouts of standardized, moderate and rhythmic exercise were consistently larger in the post-prandial than in the fasting situation. It thus appears that any tendency for redistribution of blood flow, for example from the gastrointestinal tract to the working muscles, during moderately intense exercise is less marked after a meal than before.

Dynamics and dimensions of cardiac output changes in humans at the onset and at the end of moderate rhythmic exercise

1. An improved Doppler ultrasound technique was used to measure stroke volume (SV) and cardiac output (CO) on a beat-to-beat basis in a group of supine humans before, during and after periods of standardized, rhythmic exercise, involving the quadriceps muscle groups on both sides. The development of CO on such bouts of exercise was compared to Doppler ultrasound records of the simultaneous femoral arterial flow (FF) response. 2. Records of CO at rest revealed spontaneous fluctuations around a mean level, with differences between the minimal and maximal values of the order of 1 l min-1. The mean CO level at rest again varied considerably from one day to another and from test run to test run. 3. Upon start of exercise an immediate and rapid increase in heart rate (HR) and CO took place. The entire increase, the size of which varied appreciably from test run to test run, was completed within 10-15 s. No or only minor changes were seen in the mean SV level during the exercise periods. 4. The time course of the increase in FF was indistinguishable from that of the increase in CO, which occurred without any detectable delay relative to the changes in FF. These closely parallel developments indicate a tight regulatory coupling between the two types of flow changes. 5. In the majority of tests the total and two-sided increase in FF seen in the steady-state situation in the last part of an exercise period was significantly larger than the recorded increase in CO. This discrepancy implies that some redistribution of flow from tissues other than the working muscles might take place, even at this moderate level of work. 6. Upon the end of exercise a striking but transient increase in CO occurred, resulting from an increase in SV concomitant with a maintained HR. In the course of five to eight post-exercise cardiac cycles about 100 extra milliliters of blood were expelled from the heart. This cardiac outflow overshoot was found to occur during a post-exercise fall in mean arterial blood pressure (MAP).

Relationship between heart rate and minute ventilation, tidal volume and respiratory rate during brief and low level exercise

The correlation between heart rate (HR) and three respiratory parameters, minute ventilation (VE), tidal volume (Vt), and respiratory rate (RR), were studied. Four healthy subjects performed four exercise tests (duration 30 seconds at 50, 100, 150, or 200 W), in random order. Cardio-respiratory parameters were recorded respiratory cycle by respiratory cycle. The results of these low level exercise tests showed that oxygen consumption (VO2) was strongly correlated with VE (r = 0.91 +/- 0.10; P less than 0.01) (except in one test) and Vt (r = 0.91 +/- 0.07; P less than 0.001) (except in one test). There was no significant correlation between VO2 and RR. At exercise onset HR, VE, and Vt were modified in a matter of a few heart beats while RR varied depending on the subject and the level of exercise. During exercise average HR, VE, and Vt were significantly higher than at rest in most cases; but RR was not significantly changed by exercise. The correlations between HR and VE, Vt and RR varied from one individual to another. Nevertheless, the correlation coefficients were positive for VE and Vt, while they were negative for RR. Sensing respiratory rate thus appears to be insufficient for responsive pacing at exercise onset, but sensing respiratory volumes (Vt, VE) should give satisfactory results.

Immediate ventilatory response to sudden changes in venous return in humans

We changed venous return transiently by postural manoeuvres, and by lower body positive pressure, to see what happened simultaneously to ventilation. Cardiac output was measured by a Doppler technique. In seven subjects, after inflation of a pressure suit to 80 and 40 mmHg at 30 deg head-up tilt, both cardiac output and ventilation increased. Ventilation increased rapidly to a peak in the first 5 s, cardiac output more slowly to a steady state in about 20 s, at 80 mmHg inflation. After inflation to 80 mmHg in six subjects at 12.5 deg head-up and 30 deg head-down tilt, cardiac output did not change in the first, and fell in the second case. There were no significant changes in ventilation. On release of pressure there were transient increases in both cardiac output and ventilation, with ventilation lagging behind cardiac output, in contrast to (2) above. In five subjects, elevation of the legs at 30 deg head-up tilt caused a rise in both cardiac output and ventilation, but in two subjects neither occurred. In all seven subjects there was a transient increase in cardiac output and ventilation when the legs were lowered. Ventilation and cardiac output changes were approximately in phase. We were therefore unable to dissociate entirely increasing cardiac output from increasing ventilation. The relation between them was certainly not a simple proportional one.