Reflex inputs to the cardiovascular and respiratory centers from dynamically working canine muscles. Some evidence for involvement of group III or IV nerve fibers

Changes of pulse rate and skin temperature evoked by electroacupuncture stimulation with different frequency on both Zusanli acupoints in humans

The purpose of this study was to investigate the effect of electroacupuncture stimulation (EA) of different frequency on pulse rate and skin temperature. Sixteen healthy male medical student volunteers received EA of 2 Hz, and 100 Hz, respectively on the both Zusanli acupoints (St-36) while resting. Their pulse rates were measured on the middle finger, and skin temperature was taken between the thumb and index finger before, during, and after EA stimulation. Each test took 35 minutes. The initial 10 min were defined as baseline period (no EA), the following 15 min as the EA period and the last 10 min as the post-EA period. Three assessments were performed on each subject as follows: A) control assessment: no EA was done throughout the test; B) 2 Hz EA assessment: 2 Hz EA was applied to both Zusanli acupoints during the EA period; and C) 100 Hz EA assessment: 100 Hz EA was applied to both Zusanli acupoints during the EA period. Our results indicate that both 2 Hz EA and 100 Hz EA decreased pulse rates during the EA period, and these changes remained throughout the post-EA period in 2 Hz EA assessment, but not in 100 Hz EA assessment. Both 2 Hz and 100 Hz EA resulted in decreases of skin temperature during the EA period. Our conclusions are that 2 Hz EA and 100 Hz EA applied to both Zusanli acupoints resulted in the decrease of pulse rate, which possibly evoked greater parasympathetic nerve activity on heart beats. 2 Hz EA had a more sustained effect on heart beats than 100 Hz EA. Decreased skin temperatures in the EA period may have resulted from cutaneous vasoconstriction caused by EA induced sympathetic stress response, suggesting EA at least remains for 15 min in clinical application.

Captopril reduces the VE/VCO2 ratio in myocardial infarction patients with low ejection fraction

STUDY OBJECTIVES

To determine whether captopril (CAP) therapy had an effect on the minute ventilation/carbon dioxide output (VE/VCO2) ratio at submaximal levels of exercise in asymptomatic patients with reduced left ventricular function after myocardial infarction.

DESIGN

Double blinded, randomized, prospective, repeated measures.

PATIENTS AND INTERVENTIONS

One hundred thirty-five patients with left ventricular ejection fractions of < 40% were randomly assigned to a treatment group (CAP; n = 62) or a placebo group (PLC; n = 73). Subjects had cycle ergometer exercise tests at 2 to 6 months (T1), 10 to 14 months (T2), and > 22 months (T3) postmyocardial infarction.

MEASUREMENTS

Oxygen uptake (VO2), VCO2, and VE were measured throughout each exercise test. Dependent variables were peak VO2 (VO2peak), the ventilatory anaerobic threshold (VAT), and the VE/VCO2 ratio measured at 30 W and at 75% VO2peak.

RESULTS

VO2peak and VAT did not differ as a result of treatment (CAP vs PLC; p = 0.92 and 0.80) or over time (T1 vs T2 vs T3; p = 0.51 and 0.07). VE/VCO2 was significantly lower for CAP at 30 W (p = 0.05) and, although lower at 75% VO2peak, did not obtain statistical significance (p = 0.22). The between group differences were larger at T2 and T3 when compared with T1.

CONCLUSIONS

CAP resulted in a reduced VE/VCO2 ratio during submaximal exercise. The reduced ventilation may permit patients to perform their normal activities of daily living at a lower perception of difficulty, reduce symptoms, and provide an improved quality of life.

Mechanoreceptors and autonomic responses to movement in humans

Mechanoreceptor contribution to efferent autonomic outflow is incompletely understood. To determine the effects of mechanoreceptor stimulation on autonomic reflexes, we compared autonomic responses in 34 subjects using a cross-over, counter-balanced design, in which hemodynamic, electromyographic, metabolic, and autonomic data were gathered during rest, passive, and active movement protocols. Because metaboreceptors and ventilatory responses influence autonomic outflow we verified and controlled for these influences during all protocols through comparisons of breath-by-breath gas exchange measurements. Verification of active and passive movements was made via electromyographic recordings of the moving legs. Spectral analysis of R-R variability was used to assess autonomic activity, and low to high frequency ratios were considered representative of sympathovagal balance. A repeated measures analysis of variance revealed significant modulating effects of mechanoreceptor stimulation on sympathovagal balance during passive movement upon efferent autonomic outflow (p < 0.01) independent of central command, chemoreceptor, and metaboreceptor stimulation. Furthermore, breathing frequency and volume were identical for both movement protocols. Therefore, findings in this investigation suggest that modulating influences are being exerted by mechanoreceptor stimulation on autonomic outflow to the heart.

Skeletal muscle ECF pH error signal for exercise ventilatory control

An autonomic reflex linking exercising skeletal muscle metabolism to central ventilatory control is thought to be mediated by neural afferents having free endings that terminate in the interstitial fluid of muscle. To determine whether changes in muscle extracellular fluid pH (pHe) can provide an error signal for exercise ventilatory control, pHe was measured during electrically induced contraction by 31P-magnetic resonance spectroscopy and the chemical shift of a phosphorylated, pH-sensitive marker that distributes to the extracellular fluid (phenylphosphonic acid). Seven lightly anesthetized rats underwent unilateral continuous 5-Hz sciatic nerve stimulation in an 8.45-T nuclear magnetic resonance magnet, which resulted in a mixed lactic acidosis and respiratory alkalosis, with no net change in arterial pH. Skeletal muscle intracellular pH fell from 7.30 +/- 0.03 units at rest to 6.72 +/- 0.05 units at 2.4 min of stimulation and then rose to 7.05 +/- 0.01 units (P < 0.05), despite ongoing stimulation and muscle contraction. Despite arterial hypocapnia, pHe showed an immediate drop from its resting baseline of 7.40 +/- 0.01 to 7.16 +/- 0.04 units (P < 0.05) and remained acidic throughout the stimulation protocol. During the on- and off-transients for 5-Hz stimulation, changes in the pH gradient between intracellular and extracellular compartments suggested time-dependent recruitment of sarcolemmal ion-transport mechanisms. pHe of exercising skeletal muscle meets temporal and qualitative criteria necessary for a ventilatory metaboreflex mediator in a setting where arterial pH does not.

Ventilatory and gas exchange response during walking in severe peripheral vascular disease

It has long been recognized that at the onset of a dynamic muscular exercise the ventilatory and the circulatory (blood flow) responses appear to be matched, thereby maintaining arterial blood gas homeostasis. Such a coupling has recently been suggested to rely upon ventilatory reflex triggered by mechanoreceptors encoding changes in muscle blood flow or, more likely, blood volume. The aim of this study was to investigate whether patients with severe peripheral blood flow limitation to the lower extremities have a normal ventilatory response during a light intensity exercise. The ventilatory and gas exchange temporal response characteristics were studied during a 6 min walking test in seven patients with severe ischemic peripheral vascular disease and in six normal age-matched subjects. The magnitude of the overall ventilatory and Vo2 increment at the end of the tests was similar in both groups. However, in contrast to the control subjects, who presented an almost rectangular response, the patients had a considerably slowed response dynamics (t50 = 33 +/- 4 vs. 9 +/- 3 sec for Vo2 and 37 +/- 5 vs. 10 +/- 8 sec for VE) with a dramatic reduction in the magnitude of the initial 20 sec of the responses. Although the slow Vo2 dynamics in patients presumably reflected the impeded perfusion of the working muscles. the accompanying sluggishness of the V1 course implies that either muscular ischemia actually inhibits ventilatory response to exercise or, more likely, that this response is strongly linked to the magnitude of the hyperemia in the exercising muscles.

Noxious stimuli do not determine reflex cardiorespiratory effects in anesthetized rabbits

The main purpose of this study is to examine whether the stimulation of an exclusively pain-sensing receptive field (dental pulp) could determine cardiorespiratory effects in animals in which the cortical integration of the peripheral information is abolished by deep anesthesia. In 15 anesthetized (alpha-chloralose and urethan) rabbits, low (3-Hz)- and high-frequency (100-Hz) electrical dental pulp stimulation was performed. Because this stimulation caused dynamic and static reflex contractions of the digastric muscles leading to jaw opening jaw-opening reflex (JOR); an indirect sign of algoceptive fiber activation], experimentally induced direct dynamic and static contractions of the digastric muscle were also performed. The low- and high-frequency stimulation of the dental pulp determined cardiovascular [systolic arterial pressure (SAP): -21.7 +/- 4.6 and 10.8 +/- 4.7 mmHg, respectively] and respiratory [pulmonary ventilation (VE): 145.1 +/- 44.9 and 109.3 +/- 28.4 ml/min, respectively] reflex responses similar to those observed during experimentally induced dynamic (SAP: -17.5 +/- 4.2 mmHg; VE: 228.0 +/- 58.5 ml/min) and static (SAP: 5.8 +/- 1.5 mmHg; VE: 148.0 +/- 75.3 ml/min) muscular contractions. The elimination of digastric muscular contraction (JOR) obtained by muscular paralysis did away with the cardiovascular changes induced by dental pulp stimulation, the effectiveness of which in stimulating dental pulp receptors has been shown by recording trigeminal-evoked potentials in six additional rabbits. The main conclusion was that, in deeply anesthetized animals, an algesic stimulus is unable to determine cardiorespiratory effects, which appear to be exclusively linked to the stimulation of ergoreceptors induced by muscular contraction.

Papaverine injection into the hindlimb circulation stimulates ventilation in sheep

To test the hypothesis, previously suggested by Huszczuk et al. (1993), that distention of the peripheral microvascular network could, per se, stimulate ventilation, the ventilatory effects of papaverine-induced muscular vasodilation were studied in ten anaesthetized sheep. Because systemic action of papaverine may involve the arterial baro- and chemoreceptors, the animals were surgically prepared for a reversible isolation of the hindlimb circulation. Papaverine injection (1-2 mg/kg) into the arterial inflow of the isolated limbs provoked a 13 +/- 6 sec-delayed increase in VE by 1.8 +/- 0.2 L min-1 (p < 0.01) with a concomitant decrease in peripheral vascular resistance and no decrease in the systemic arterial blood pressure. Identical control injection into a jugular vein prior to the hindlimb circulatory separation yielded an increase of VE by 4.95 +/- 0.58 L min-1 with a latency of 21 +/- 2 sec and a coinciding moderate decrease of the systemic arterial pressure. The present data suggest that papaverine injection into the hindlimb circulation can stimulate ventilation independently of its possible effects on the arterial baro- or chemoreceptors, supporting the hypothesis that muscular vasodilation could contribute to the control of breathing through a neural link.

Changes in ventilation related to changes in electromyograph activity during repetitive bouts of isometric exercise in simulated sailing

This study examined the control of ventilation during repetitive bouts of isometric exercise in simulated sailing. Eight male sailors completed four successive 3-min bouts of similar isometric effort on a dinghy simulator; bouts were separated by 15-s rest intervals. Quadriceps muscle integrated electromyograph activity (iEMG) was recorded during each bout and expressed as a percentage of activity during maximal voluntary contraction (%iEMGmax). From the first to the fourth bout, the 3-min mean averages for ventilation and for %iEMGmax increased from 19.8 (SEM 1.1) to 37.5 (SEM 3.0) l.min-1 and from 31 (SEM 4) to 39 (SEM 4)% respectively; also, ventilation and %iEMGmax over each minute throughout the four bouts were significantly correlated (r = 0.85; P < 0.05). Progressive hyperventilation reduced the mean end-tidal partial pressure of carbon dioxide from 5.0 (SEM 0.3) kPa during bout 1 to 4.3 (SEM 0.4) kPa during bout 4 [37.7 (SEM 2.0) to 32.4 (SEM 3.0) mmHg]. From the first to the fourth bout the end-of-bout blood lactate concentration did not increase significantly although the concentration from the third bout onwards was significantly greater than at rest. The results suggested that the development of muscle fatigue, which was enhanced by the insufficiency of recovery during the 15-s intervals and mirrored in the progressive increase in iEMG, was linked with stimuli causing progressive hyperventilation. Though these changes in ventilation and iEMG could not be associated with changes in blood lactate concentration, they could both have been related to accumulating metabolites within the muscles themselves.

Skeletal muscle and the control of ventilation on exercise: evidence for metabolic receptors

Patients with chronic heart failure have an increased ventilatory response to exercise, and have metabolically abnormal skeletal muscle. It has been proposed that a neural signal to ventilation arising from exercising muscle may be heightened in chronic heart failure. Our objective was to detect evidence for such a signal in normal subjects by studying ventilatory behaviour during exercise with muscles in different metabolic states. Fifteen normal subjects undertook treadmill exercise both with and without cuffs inflated around each thigh to suprasystolic pressure. In a second experiment, a group of 11 normal subjects undertook cycle exercise using arms or legs at the same absolute work load. Metabolic gas exchange was measured using mass spectrometry with indicator gas dilution. The ventilatory response was greater at a given workload when subjects exercised with inflated cuffs. Oxygen consumption was reduced in keeping with the isolation of the exercising muscle bulk from the circulation. The ventilation/carbon dioxide output relationship was described by a linear regression function, but the slope of the relationship was increased by 25% from 20.9 (0.46) to 25.43 (0.73) (P < 0.001). Arm exercise at the same load as leg exercise resulted in unchanged oxygen consumption indicating that the same external work was being performed. There was an increase in ventilation at a given workload. The ventilation/carbon dioxide output slope was increased by 25% (from 21.9 (0.9) to 26.3 (0.8)) (P < 0.001). There is a signal to ventilation arising from exercising skeletal muscle which is enhanced by the ischaemia induced by cuff inflation during exercise. This signal appears to be neural.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular distension in muscles contributes to respiratory control in sheep

It has recently been proposed that afferent fibers from skeletal muscle could sense the state of the microvascular circulation, linking ventilation to the degree of peripheral perfusion or vascular distension (Huszczuk et al., Respir. Physiol., 91:207-226, 1993). Ventilatory and circulatory responses to manipulation of peripheral vascular pressures in the hind limbs of anaesthetized (sodium thiopental) sheep were examined. Inflatable balloons were placed at the caudal ends of the abdominal aorta and the vena cava (Vc). Aortic (Ao) occlusion induced a consistent normocapnic decrease in minute ventilation (VE). In contrast, VE increased significantly during vena cava obstruction, leading to hypocapnia. Small changes in systemic blood pressure were observed (+7 mmHg for Ao occlusion and -12 mmHg during Vc obstruction). Moreover, inflation of the caval balloon superimposed on a previously established Ao occlusion, preventing venous drainage of anastomotic inflow, resulted in a significant rise in distal vascular pressures with trivial changes in systolic blood pressure. This led to a gradual rise of VE, despite further reduction of the CO2 flux to the lungs. The subsequent deflation of the aortic balloon, exposing the hindlimb vasculature to aortic pressure, resulted in an even more profound hypocapnic hyperpnea. The concurrent arterial blood pressure changes were too small to possibly involve the ventilatory component of the arterial baroreflex. We therefore hypothesize, that perfusion-related afferent signals within the muscles could contribute to respiratory homeostasis by maintaining ventilation of the lungs commensurate with the circulatory state of the muscular apparatus.

Peripheral and central chemoreceptor control of ventilation during exercise in humans

The stability of arterial blood gas tensions and pH during steady-state moderate exercise has suggested an important humoral element of ventilatory control in humans. However, the involvement of central and peripheral chemoreflexes in this humoral control remains controversial. This reflects, in large part, technical and interpretational limitations inherent in currently used estimators of chemoreflex "sensitivity." Evidence suggests that the central chemoreceptors (a) contribute little during moderate exercise, given the relative stability of cerebrospinal pH, (b) constrain the hyperpnea of high-intensity exercise, consequent to the respiratory compensation for the metabolic acidemia, and (c) may play a role in the respiratory compensation during chronic metabolic acidemia. In contrast, the peripheral chemoreceptors appear to (a) exert considerable influence on ventilatory kinetics in moderate exercise, but are less important in the steady state, and (b) induce much of the respiratory compensation of high-intensity exercise.

Ventilatory responses at the onset of passive movement and voluntary exercise with arms and legs

This study was undertaken to elucidate whether phase I appeared at the onset of voluntary and passive arm movements and to compare these results with those of similar leg movements. Instead of the conventional cranking exercise, seven male subjects performed alternately flexion-relaxation of both arms, extension-relaxation of both legs, and combined arm and leg exercise at the rate of about 60 min-1 for four breaths in a sitting position. Similar movements were accomplished passively by the experimenters. In all experiments, minute ventilation increased rapidly within the first breath after the onset of exercise. The difference of ventilation (delta value) between the mean of the first two breaths at the onset of voluntary exercise and that of five breaths during rest was significantly (P < 0.05) greater in arm (7.75 l min-1) than in leg (5.19 l min-1). Passive movement showed a similar tendency. Arm delta ventilation correlated highly (r = 0.74-0.91) with leg delta ventilation and the slope of the regression lines was about 1.2. Heart rate increased abruptly while cardiac output did not always increase rapidly at the onset of locomotion. Oxygen uptake in the voluntary leg exercise continued for 3 min was slightly but nonsignificantly higher than in the arm exercise, indicating the equality of the exercise intensity. In conclusion, ventilatory responses at the onset of the arm exercise are larger than those of the leg in both voluntary and passive conditions regardless of the muscle mass, suggesting the different neurogenic mechanism between arm and leg.

Independence of ventilation and blood lactate responses during graded exercise

The effect of power output increment, based on an increase in pedal rate, on blood lactate accumulation during graded exercise is unknown. Therefore, in the present study, we examined the effect of two different rates of power output increments employing two pedal rates on pulmonary ventilation and blood lactate responses during graded cycle ergometry in young men. Males (n = 8) with an mean (SD) peak oxygen uptake of 4.2 (0.1) l.min-1 served as subjects. Each subject performed two graded cycle ergometer tests. The first test, conducted at 60 rev.min-1, employed 4 min of unloaded pedaling followed by a standard power output step increment (SI) of 60 W every 3rd min. The second test, conducted at 90 rev.min-1, employed 4 min of unloaded pedaling followed by a high power output step increment (HI) of 90 W every 3rd min. Venous blood was sampled from a forearm vein after 5 min of seated rest, 4 min of unloaded pedaling, and every 3rd min of graded exercise. Peak exercise values for heart rate, oxygen uptake (VO2), and ventilation (VE) were similar (P > 0.05) for SI and HI exercise, as was the relationship between VE and VO2, and between VE and carbon dioxide production (VCO2). However, the relationship between blood lactate concentration and VO2 was dissimilar between SI and HI exercise with blood lactate accumulation beyond the lowest ventilatory equivalent of oxygen, and peak exercise blood lactate concentration values significantly higher (P < 0.05) for SI [12.8 (2.6) mmol.l-1] compared to HI [8.0 (1.9) mmol.l-1] exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute ventilatory responses to hypoxia during voluntary and electrically induced leg exercise in man

1. The acute ventilatory response to a brief period of hypoxia (AHVR) was measured in six subjects (a) at rest, (b) during electrically induced leg exercise (EEL), (c) during voluntary leg exercise at an external work rate matched to electrical exercise (EV1) and (d) during voluntary leg exercise at an internal work rate (i.e. metabolic rate) matched to electrical exercise (EV2). The end-tidal PO2 during hypoxia was 50 mmHg and the end-tidal PCO2 was held constant at 1-2 mmHg above resting values throughout each of these four protocols. 2. EEL was produced by surface electrode stimulation of the quadriceps muscles so as to cause the legs to extend at the knee and lift a set of weights via a pulley system. During EV1, each subject lifted the same weight through the same height and at the same frequency as during his EEL protocol. During EV2, the weight, the height through which it was lifted and the frequency of voluntary contractions were altered to produce a similar O2 consumption and CO2 production as during EEL. 3. In each subject, end-tidal PCO2 values showed no change between the four protocols, and in three subjects in whom they were measured, arterial PCO2 values were also similar between the protocols. Venous lactate levels did not increase after EEL or EV2. 4. The AHVR during EEL (14.1 +/- 1.42 l min-1; mean +/- S.E.M) was significantly increased (Student's paired t test) compared with rest (7.55 +/- 1.10 l min-1; P < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)

Femoral vascular occlusion and ventilation during recovery from heavy exercise

Ventilation and cardiac output subside gradually following cessation of exercise, which is commonly linked to the slow wash-out of materials from the recovering muscles. The effect of hindering the removal of the metabolic products of heavy cycle exercise on the kinetics of ventilation and gas exchange was studied in 5 subjects by occluding the femoral circulation with cuffs during the first 2 min of recovery (15 tests). Fifteen undisturbed recoveries served as controls. Compared to spontaneous recovery, circulatory obstruction induced an immediate (from the first breath) decrease in minute ventilation (VE), while end-tidal CO2 (PETCO2) as well as lactate and K+ in venous blood at forearm did not change significantly. A ventilatory deficit of 27 +/- 9 L was observed from the 2 min of occlusion. Following cuff deflation, VE rose 2-3 breaths after PETCO2 began to increase in every subject. The mechanisms of the normocapnic reduction of VE during occlusion, as well as the rise of ventilation following cuff release, are still unclear. However, these results argue against any significant role for hyperpnea-inducing intramuscular chemoreception, or point to muscular perfusion as a prerequisite of such a mechanism to operate.

Blood gas dynamics at the onset of exercise in heart transplant recipients

One hypothesis to explain the rapid neural component of exercise hyperpnea contends that afferent stimuli originating in the ventricles of the heart act reflexly on the respiratory center at the onset of exercise, ie, "cardiodynamic hyperpnea." Orthotopic cardiac transplantation (Tx) results in the loss of afferent information from the ventricles. Thus, Tx possibly results in transient hypercapnia and hypoxemia in deafferented heart transplant recipients (HTR) at the onset of exercise due to hypoventilation. To examine the cardiodynamic hypothesis, we collected serial arterial blood gas (ABG) samples during both the transient and the steady-state responses to moderate cycle exercise in 5 HTRs (55 +/- 7 years) 14 +/- 7 months post-Tx and 5 control subjects matched with respect to gender, age, and body composition. Forced vital capacity, forced expiratory volume in 1 s, total lung capacity, and diffusion capacity did not differ (p > or = 0.05) between groups. Resting arterial PO2, PCO2, and pH did not differ between groups (p > or = 0.05). The ABGs were drawn every 30 s during the first 5 min and at 6, 8, and 10 min of constant load square wave cycle exercise at 40 percent of the peak power output (watts). Absolute and relative changes in arterial PO2, PCO2, and pH were similar (p > or = 0.05) between HTR and the control group at all measurement periods during exercise. Heart rate (%HRmax reserve), rating of perceived exertion, and reductions in plasma volume (% delta from baseline) did not differ between HTR and control during exercise at 40 percent of peak power output (p > or = 0.05). Our results demonstrate that there is no discernible abnormality in ABG dynamics during the transient response to exercise at 40 percent of peak power output in patients with known cardiac denervation. These data do not support the cardiodynamic hyperpnea hypothesis of ventilatory control in humans. The absence of hypercapnia in HTRs is further evidence for the existence of redundant mechanisms capable of stimulating exercise hyperpnea.

Role of muscle perfusion and baroreception in the hyperpnea following muscle contraction in dog

The influence of impeding muscle perfusion on the time course of ventilatory decline during recovery from electrically induced hindlimb contractions has been studied in 14 anesthetized dogs. When intravascular balloons, placed in abdominal aorta and inferior vena cava just rostral to the iliac bifurcation, were inflated at the cessation of contraction bout, minute ventilation (VE) was significantly reduced during recovery compared with control. The subsequent restoration of iliac circulation rapidly augmented VE, which peaked at the fifth breath after release, by an average of +4.97 L.min-1; VE then returned exponentially to resting (pre-contraction) level. Breathing 100% O2 did not affect the VE recovery pattern neither during iliac occlusion nor immediately after its release (the peak average delta VE = +4.42 L.min-1). When a local anesthetic (5% Lidocaine) was applied bilaterally to the regions of carotid bifurcation, systemic blood pressure was significantly increased and the VE response to both iliac occlusion and release were nearly abolished. The VE response to inhalation of 5% CO2 in air was not affected by this procedure, whereas the stimulation of VE with 2 mg i.v. bolus of NaCN was attenuated. When the local anesthetic was thoroughly washed out (and systemic blood pressure had returned to control level) the previously observed VE responses to iliac occlusion and release were restored. These results and analysis of the VE response timing (transits and latencies) suggest that the vascular rather than humoral effects or tissue 'metaboreception' modulate ventilatory recovery from muscular contractions; baroreception appears to be important in this process.

Cardiorespiratory response at the onset of passive leg movements during sleep in humans

To examine the ventilatory response at the onset of passive leg movements during sleep in man and the concomitant changes in cardiac output (Qc), five healthy male subjects had their knee joints extended and flexed alternately at a frequency of about 60.min-1 for about 8 s. Minute ventilation (VI), respiratory frequency, tidal volume, end-tidal partial pressure of carbon dioxide and of oxygen, stroke volume (SV), heart rate (fc) and Qc were measured before, during and after passive leg movement during sleep stage III or IV (SLEEP). These values were compared with those of the awake condition (AWAKE). The VI increased significantly (P < 0.05) compared with the mean of five breaths preceding the movement (pre-movement) within one or two breaths at the onset of passive leg movements in both conditions. The difference between the mean of the first and second breaths after the onset of leg movement and pre-movement was 5.2 (SEM 1.9) l.min-1 for SLEEP and 2.7 (SEM 1.1) l.min-1 for AWAKE, respectively. Four of the five subjects showed a larger increase in ventilation during SLEEP compared with AWAKE. The fc increased significantly (P < 0.05) at the beginning of the passive movement in all cases, while SV showed an increase or decrease so that Qc showed no significant change in either condition. These results would suggest that afferent drive from moving limbs could produce an increase in ventilation without any change in Qc.

Changes in muscle sympathetic nerve activity and calf blood flow during combined leg and forearm exercise

In order to examine efferent sympathetic nerve control of the peripheral circulation during exercise, muscle sympathetic nerve activity (MSNA), calf blood flow (CBF), heart rate (HR), blood pressure (BP) and oxygen uptake were measured during combined foot and forearm exercise. An initial period of rhythmic foot exercise (RFE) (60 min-1 at 10% of maximal voluntary contraction (MVC) was followed by the addition of rhythmic handgrip exercise (RFE + OCCL) (60 min at 30% of MVC) and by forearm ischaemia after handgrip exercise while continuing RFE (RFE + OCCL). During RFE, CBF in the working leg, HR and oxygen increased respectively by 560%, 121% and 144% when compared with the control rest period, but MSNA (burst rate) was reduced by 13% (P > 0.05) and BP was unchanged. During RFE + RHG, HR, BP and oxygen uptake were greater than during RFE alone. There was no change in CBF, but a significant increase occurred in calf vascular resistance (CVR) and MSNA increased to 121% of the control level. During RFE + OCCL, MSNA, CVR and BP were all higher than during RFE alone, whereas HR and oxygen uptake decreased slightly, although they remained higher than the control values. The increase in CVR in the working leg and the rise in BP during RFE + RHG or RFE + OCCL might be linked to enhancement of MSNA, which may have been reflexly evoked by input from muscle metabolic receptors in the working forearm.

Relation between ventilation and carbon dioxide production in patients with chronic heart failure

OBJECTIVES

The aim of this study was to analyze the relation between ventilation and carbon dioxide production and the control of ventilation in patients with chronic heart failure.

BACKGROUND

Patients with chronic heart failure exhibit an increased ventilatory response to exercise. Ventilation is closely linked to carbon dioxide production, producing a high correlation between the two variables. This relation is nonlinear at high levels of exercise.

METHODS

The ventilation/carbon dioxide production ratio during exercise was examined in 29 patients with chronic heart failure and 9 normal volunteers.

RESULTS

In the patients with heart failure, there were three patterns: in the least severely affected patients, the pattern was similar to that of the normal subjects, with an initial decrease in the ventilation/carbon dioxide production ratio to a plateau maintained during exercise; in more severely affected patients, there was an increase in the ratio at the end of exercise, and in the most severely affected patients, the ratio increased from the outset of exercise. The ventilation/carbon dioxide relation is not adequately described by a straight line relation.

CONCLUSIONS

The ventilation/carbon dioxide ratio is not fixed, and the changes that occur in this ratio reflect either a noncarbon dioxide-driven ventilatory stimulus or an increase in ventilation-perfusion mismatch due to increased dead space ventilation. The different patterns of this ratio may provide clues to the pathophysiologic mechanisms of the excessive ventilation and breathlessness seen during exercise in chronic heart failure.

Muscle chemoreflexes and exercise in humans

This review focuses on the role afferent nerves from the contracting muscles play in linking muscle metabolism to the cardiovascular adjustments during exercise by means of a 'muscle chemoreflex'. In the 1930s Alam and Smirk provided the first clear evidence that human (and animal) skeletal muscles are innervated by chemosensitive afferents that can evoke increases in arterial blood pressure. They proposed that the purpose of the increase in pressure was to improve blood flow to the active muscles. Subsequent studies have identified the slowly conducting group IV afferents as the major class of fibres participating in the sensory arm of this reflex. Most of these fibres travel via the dorsal roots to the ipsilateral spinal cord where they synapse in the substantia gelatinosa and release substance P or other peptide transmitters. The second order (or higher) neurons cross to the contralateral side of the spinal cord and travel rostrally to stimulate brainstem cardiovascular centres and increase arterial pressure. Current evidence favours the concept that substances associated with muscle acidosis provide the stimulus to the afferents. In humans, chemosensitive afferent activation causes a marked increase in vasoconstrictor efferent muscle sympathetic nerve activity. It is unclear if the muscle chemoreflex improves blood flow to 'underperfused' active muscles by augmenting arterial pressure, or if the increase in sympathetic outflow restrains metabolic vasodilatation to regulate arterial blood pressure during activities like running or cycling.

Spinal cholinergic modulation of cardiovascular tone and a somatosympathetic reflex response

The purpose of this study was to examine the relationship between spinal cholinergic pressor neurons and a somatosympathetic reflex response in rats. Intrathecal (IT) injection of the cholinesterase inhibitor, neostigmine (NEO), produced marked pressor and tachycardic responses without any changes in respiratory parameters. On the other hand, stimulation of the sciatic nerve produced increases in both cardiovascular and respiratory (tidal volume, minute volume, respiratory rate) responses. These cardiorespiratory responses to nerve stimulation were inhibited by IT NEO. A pressor response could also be induced by topical application of NEO to the surface of lower spinal cord, which was not altered by prior dorsal rhizotomy. These results indicate that two independent cholinergic systems exist in the spinal cord, one of which participates in the inhibitory modulation of the somatosympathetic reflex, and the other which mediates a sympathoexcitatory response. It is unlikely that the pressor response to spinal administration of NEO is mediated through this somatosympathetic response.

The sympathoexcitatory response following selective activation of a spinal cholinergic system in anesthetized rats

This study was performed to help elucidate the role of spinal cholinergic neurons in cardiorespiratory function by selective activation of spinal or medullary cholinergic systems in anesthetized rats. A selective site of action of cholinergic drugs on the spinal cord was obtained by refining the method of intrathecal (i.t.) drug injection to localize drug distribution to specific spinal segments. I.t. injection of the cholinesterase (ChE) inhibitor, neostigmine (NEO), produced a significant reduction in spinal, but not medullary tissue levels of ChE, and evoked marked pressor and tachycardic responses without any changes in respiratory parameters. In contrast to i.t. injection, intracisternal (i.c.) injection of NEO which inhibited both spinal and medullary ChE, produced characteristic respiratory changes--increased tidal volume and decreased respiratory rate and minute volume, as well as pressor and tachycardic responses. I.t. injection of the muscarinic antagonist, methylatropine, inhibited the cardiovascular responses to i.t. NEO, but not the cardiorespiratory responses to i.c. NEO. These cardiovascular responses to i.t. NEO were blocked by spinal transection, but not by midcollicular transection. Finally, the pressor and tachycardic responses to i.t. NEO were inhibited following peripheral alpha-adrenergic and beta-adrenergic blockade, respectively. These results indicate that activation of the spinal cholinergic system selectively produces a sympathoexcitatory response through spinal muscarinic receptor activation independent of respiratory changes. This finding is consistent with the possibility that such responses are elicited by activation of a non-cholinergic bulbo-spinal sympathoexcitatory pathway at the spinal level, or at higher centers through an ascending pathway. In either case, the spinal cholinergic system appears to be anatomically and pharmacologically distinct from the medullary pathway and may subserve a different function.

Responses of upper airway muscles to gastrocnemius muscle contraction in dogs

We studied electromyographic (EMG) responses of the alae nasi (AN) and the posterior cricoarytenoid (PCA) muscles, which act as upper airway dilators, during contraction of gastrocnemius muscle in six chest-intact anesthetized dogs with spontaneous breathing and in four thoracotomized, phrenicotomized and mechanically ventilated dogs with right thoracic and left cervical vagotomy. Muscle contraction was phasically induced by electrical stimulation of the intact gastrocnemius nerve or the distal cut end of this nerve for 20-30 sec. Stimulation intensity was determined as twice the motor threshold in each dog. In chest-intact animals, phasic contraction induced by intact nerve stimulation produced initial rapid increases in upper airway muscle activity, but stimulation of the distal cut end of the nerve did not show the rapid increase in upper airway muscle activity. Furthermore, stimulation of the proximal cut end did not produce any transient response with the stimulation intensity used in this study. In chest-open and vagotomized animals with artificial ventilation, responses of the upper airway muscles to contraction during the intact nerve stimulation were observed. These results suggest that the contraction of the gastrocnemius muscle activates upper airway dilating muscles via reflex mechanisms.

Kinetics of cardiorespiratory response to dynamic and rhythmic-static exercise in men

Kinetics of cardiorespiratory response to dynamic (DE) and then to rhythmic-static exercise (RSE) was compared in nine male subjects exercising in an upright position on a cycle ergometer at an intensity of about 50% VO2max and a mean pedalling frequency of 60 rpm over 5 min. Respiratory frequency (fR), tidal volume (VT), minute ventilation (VE), heart rate (fc), stroke volume (SV), and cardiac output (Qt) were measured continuously. The RSE caused a greater increase in fR than DE, whereas VT increased more during DE. The effect of reciprocal changes in fR and VT was that VE and its kinetics, expressed as a time constant (tau), did not differ between experimental situations. The ventilatory equivalent for O2 (VE: VO2) was greater for RSE (31.3) than for DE (23.0, P less than 0.01). Elevation of fc was similar for both types of exercise. The SV increased suddenly at the beginning of DE from 54 ml to 74 ml and then decreased to the end of exercise. At the onset of RSE only a moderate increase in SV was observed, from 56 ml to 62 ml, and then SV remained stable. The DE caused a greater and faster increase in Qt (4.20 l.min-1, for tau equal to 16.1 s) than RSE (3.25 l.min-1, for tau equal to 57.0 s, P less than 0.05 and P less than 0.002, respectively). Total peripheral resistance was almost 40% greater for RSE than for DE. No relationship was found between Qt and VE at the first 15 s of both types of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between the ventilation and lactate thresholds following normal, low and high carbohydrate diets

Five men performed an incremental exercise test following a normal, low and high carbohydrate dietary regimen over a 7-day period, to examine the influence of an altered carbohydrate energy intake on the relationship between the ventilation (VET) and lactate (LaT) thresholds. VET and LaT were determined from the ventilatory equivalents for O2 (VE.VO2(-1) and CO2 (VE.VCO2(-1) and the log-log transformation of the lactate (La) to power output relationship, respectively. The total duration of the incremental exercise test, carbon dioxide output (VCO2), respiratory exchange ratio, blood La values and arterialized venous partial pressure of CO2 (PCO2) were reduced, and VE.VCO2(-1), the slope of the VE-VCO2 relationship, blood beta-hydroxybutyrate and pH were increased during the low carbohydrate trial compared with the other conditions. Total plasma protein and Na+, K+, and Cl- were similar across conditions. LaT and VET were unaffected by the altered proportions of carbohydrate in the diets and occurred at a similar oxygen consumption (mean VO2 across trials was 1.98 L.min-1 for VET and 2.01 L.min-1 for LaT). A significant relationship (r = 0.86) was observed for the VO2 that represented individual VET and LaT values. The increased VE.VCO2(-1) and slope of the VE-VCO2 relationship could be accounted for by the lower PCO2. It is concluded that alterations in carbohydrate energy intake do not produce an uncoupling of VET and LaT as has been reported previously.

The nucleus reticularis gigantocellularis modulates the cardiopulmonary responses to central and peripheral drives related to exercise

It is known that muscle afferents and the hypothalamic locomotor region (HLR) both project to the nucleus reticularis gigantocellularis (NGC) and that the NGC is capable of influencing cardiovascular and respiratory variables. Therefore, the role of NGC in the cardiovascular and respiratory response to exercise-related signals was investigated in anesthetized cats. These signals were generated by stimulation of: (1) spinal ventral roots to induce hindlimb muscle contraction (MC) and (2) the HLR. Bilateral electrolytic lesion of the NGC at the pontomedullary border caused tidal volume, respiratory frequency and heart rate responses to HLR stimulation to be greater than the responses recorded prior to lesioning. Lesioning had no effect on the ventilatory or cardiovascular responses to MC but did decrease phrenic responsiveness; lesion had no effect on any resting values. In this preparation, the pontomedullary NGC acts as an inhibitory influence on tidal volume, breathing frequency and heart rate responses to the central command for exercise. In addition, NGC modulation of ventilation would appear to be selective for certain respiratory muscle groups.

The relationship between lactic acid and work load: a measure for endurance capacity or an indicator of carbohydrate deficiency?

The influence of low and high carbohydrate diets on the relationship between blood lactate concentration ([Lac]) and work load (WL) in incremental exercise tests (cycle ergometer) and endurance tests was evaluated in trained subjects. The relationship between relative work load (WLrel) and [Lac] in arterialized blood was compared in untrained subjects (UT) and trained male athletes (TR) after 2 days without training while consuming a high carbohydrate diet (HCD). In both groups [Lac] of 2 mmol.l-1 was reached at about 60% [(mean +/- SD) UT 57.7% +/- 6%, TR 62.7% +/- 3.8%] and 4 mmol.l-1 at about 75% (UT 75.2% +/- 3.6%, TR 77.8 +/- 2.2) of the maximal work load (WLmax). In eight cyclists the relationship between [Lac] and WL was not influenced by a 13-day training camp; however, heart rate was lower after the training camp. During their normal training programme, trained subjects had high relative work loads at their [Lac] thresholds, but after an HCD combined with an interruption of the training of 3 days, the relationship between [Lac] and WLrel was the same as in UT. In six TR a low carbohydrate diet (LCD) combined with training led to high absolute (WLabs) and WLrel at [Lac] at 2 and 4 mmol.l-1; an HCD combined with 3 days without training led to low WLabs and WLrel at the same [Lac] and to higher WLmax. In spite of the apparently lower endurance capacities TR were able to work significantly longer after HCD than after LCD (23 +/- 10.5 min and 49 +/- 16.2 min, respectively) at 65% of their WLmax. The variability of the relationship between [Lac] and WL following the dietary regimes leads to the conclusion that the "typical" [Lac] versus WL curve of endurance TR may result from a permanent glycogen deficiency.

Regional left ventricular mechanical function during isometric exercise in patients with coronary artery disease: correlation with regional coronary blood flow changes

The effects of isometric exercise on regional left ventricular mechanical function and regional coronary blood flow were evaluated in 17 patients with significant proximal stenosis of the left anterior descending coronary artery and 10 patients with normal coronary arteriograms. All patients had normal myocardial contractility in the basal condition. All performed isometric handgrip exercise at 50% of the maximal voluntary contraction for 3 min during two-dimensional echocardiographic monitoring and hemodynamic evaluation of great cardiac vein flow by thermodilution technique. During isometric exercise, 7 of the 17 patients with left anterior descending coronary stenosis developed asynergy in the anterior territory (anterior or septal segment, or both) (group I); the remaining 10 showed normal myocardial contraction during the test (group II). The 10 normal subjects manifested no regional asynergy during the test (control group). The increase in great cardiac vein flow at peak isometric exercise was significantly smaller (p less than 0.01) in group I (+15 +/- 8%) than that in group II (+98 +/- 48%) and the control group (+64 +/- 22%). Anterior coronary vascular resistance decreased in group II (-32 +/- 13%) and in the control group (-25 +/- 8%) but increased in group I (+6 +/- 8%, p less than 0.01 versus group II and control group). These data demonstrate that handgrip-induced myocardial asynergy is associated, in our study patients, with an abnormal response of the regional coronary circulation. The increase in coronary vascular resistance in group I patients with asynergy demonstrates that functional mechanisms play a dominant role in left ventricular mechanical dysfunction induced by isometric exercise.

Hemodynamic, ventilatory and metabolic effects of light isometric exercise in patients with chronic heart failure

Light isometric exercise, such as lifting or carrying loads that require 25% of a maximal voluntary contraction, is frequently reported to cause dyspnea in patients with heart failure. The pathophysiologic mechanisms responsible for the appearance of this symptom, however, are unknown. Accordingly, hemodynamic, metabolic and ventilatory responses to 6 min of light isometric forearm exercise were examined and compared in 20 patients with chronic heart failure and abnormal ejection fraction (24 +/- 9%) and 17 normal individuals. In contrast to findings in normal volunteers, exercise cardiac index did not increase whereas exercising forearm and mixed venous lactate concentrations increased (p less than 0.05) above levels at rest in patients with heart failure; at 90 s of recovery, blood lactate concentration remained elevated (p less than 0.05). The venous lactate concentration of the nonexercising arm, unlike that of the exercising forearm, was not altered. Oxygen uptake, carbon dioxide production and minute ventilation increased similarly in patients and normal subjects during exercise, but only in patients did each increase further (p less than 0.05) during recovery. Thus, in patients with heart failure, light isometric forearm exercise represents an anaerobic contraction with lactate production. The subsequent increase in carbon dioxide production leads to a disproportionate increase in minute ventilation and oxygen uptake during recovery that may be perceived as breathlessness.

Cardiorespiratory reflex responses to static contraction of vascularly isolated hindleg muscles of the rat

One hindleg of an anasthetized rat (n = 15) was isolated from systemic blood circulation. The preparation was connected to the body only by nerve and bone. A. and V. femorales were cannulated and perfused with normoxic (PO2 = 530 mm Hg) or hypoxic (PO2 = 60 mm Hg) Tyrode solutions. Static contractions of the muscle were elicited by electrical stimulation on the sciatic nerve (2 x motor threshold, 400-800 mV, 50 s-1). A 1 s stimulus was followed by a 2 s rest period. Total test time amounted to 40 min. It was proceeded and succeeded by 20 min periods of control perfusions without stimulation. Heart rate (HR) and respiratory rate (f) were measured and cross correlated with the following outflow parameters from V. femoralis of the experimental muscle: [K+], [Na+], PO2, PCO2, pH and [lactate]. During the test period HR and f increased significantly within 20 min of the start of stimulation: HR 5.8% (p less than 0.005) and f 24.3% (p less than 0.005) for hypoxic perfusion (n = 6) and HR 3.2% (p less than 0.005) and f (p less than 0.001, ANOVA) for normoxic perfusion (n = 3). The dynamic changes of several outflow parameters were nearly simultaneous with the cardiorespiratory responses. Cross correlation analyses revealed an excellent temporal relationship between HR and PO2 or [lactate] and between f and PO2 or [lactate]. In addition PCO2 and pH correlated well with HR as well as with f. Comparison of the threshold of the cardiorespiratory response revealed an optimal relationship to pH, a good one to PCO2 and lactate concentration but no correlation to PO2.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory and cardiac responses to exercise-simulating peripheral perfusion in endurance trained and untrained rats. II. Temporal relationships between outflow parameters and cardiac and respiratory responses

In endurance trained (TR) and untrained (UTR) rats heart rate (HR) and respiratory rate (RR) were recorded during perfusion of the circulatorily isolated hind leg of the rat with exercise simulating modified tyrode solutions (TR:n = 10, UTR:n = 10; compare part I). During the 20 min test period and the preceding and succeeding periods of control perfusions with an unmodified tyrode solution, [lactate], pH, [K+], [Na+], PO2 and PCO2 were measured in the outflow of the femoral vein. In 3 experimental series: (1) hypoxic tyrode solution enriched with lactic acid (15 mmol.l-1), (2) normoxic solution with lactic acid, (3) hypoxic solution without lactic acid, were applied. The outflow parameters were cross correlated with both HR and RR. The analysis revealed a significant temporal relationship between [lactate], pH, PO2, PCO2 and [K+] and both HR and RR. In the trained rats no temporal correlation between either of the outflow and reflex parameters could be determined. This result was not due to low [lactate], but was also found during perfusion with lactic acid. In all 3 test conditions [lactate] in untrained individuals was best correlated with both HR and RR. Although the correlation peaks of the respiratory response, but not of the HR response were definitely lower in normoxic lactic and perfusion than in the two other experimental conditions, both inter- and intraindividual correlation analyses revealed a high degree of interdependence between respiratory and cardiac responses.

Respiratory and cardiac responses to exercise-simulating peripheral perfusion in endurance trained and untrained rats. I. Reflex responses and changes in perfusion outflow

Ventilatory and circulatory drives elicited by exercise-simulating perfusion of the circulatory isolated hindleg were examined in 10 trained (TR) and untrained (UTR) rats. TR were submitted to endurance training on a motordriven treadmill (30.min-1 at a grade of 10%, 5 days a week for 30 min). Exercise was simulated by perfusion with modified tyrode solutions: I.) hypoxic, enriched with lactic acid (15 mmol.l-1), II.) normoxic, enriched with lactic acid. III.) hypoxic without lactic acid. Perfusion was performed in anaesthetized animals through cannulae in the femoral artery and vein; the hindled was connected to the rest of the body only by nerve and bone. 10 min of control perfusion (normoxic tyrode solution) was followed by a 20 min test period and another 10 min control perfusion. Apart from heart rate (HR), respiratory rate (RR) and several outflow parameters were measured ([K+], [Na+], [lactate], pH, PO2, PCO2). During control period HR was slightly higher in UTR than in TR (375.5 +/- 3.9 (SE) vs. 364.1 +/- 5.5 beats/min-1, p less than 0.6 n.s.), and RR in UTR was significantly higher than those in TR (61.5 +/- 0.4 bpm vs. 55.5 +/- 3.9 breaths.min-1, p less than 0.001). During the test periods both HR and RR in UTR increased significantly while in TR they did not (e.g. in series I mean HR and RR in UTR increased by 8.9 +/- 1.2 beats.min-1 and 1.4 +/- 0.1 breaths.min-1 respectively, whereas in TR the changes were - 2.9 +/- 1.5 beats/min-1 and -0.8 +/- 0.2 breaths.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of angiotensin II in the renal responses to somatic nerve stimulation in the rat

1. Electrical stimulation of the brachial nerves at 3 Hz (15 V, 0.2 ms), in sodium pentobarbitone-anaesthetized rats whose renal arterial pressure was held constant, elicited a 26% increase in systemic blood pressure, a 15% rise in heart rate, an 11% reduction in renal blood flow, did not alter glomerular filtration rate and significantly reduced absolute and fractional sodium excretions and urine flow by 44, 49 and 31%, respectively. 2. In a separate group of rats, brachial nerve stimulation at 3 Hz increased plasma renin activity approximately 2-fold, while in animals in which the brachial nerves were not stimulated plasma renin activity did not change. 3. Following inhibition of the renin-angiotensin system with captopril or sar-1-ile-8-angiotensin II, brachial nerve stimulation resulted in similar increases in systemic blood pressure and heart rate as in the animals with an intact renin-angiotensin system but, in captopril-infused rats, did not change renal haemodynamics or urine flow while absolute and fractional sodium excretions were reduced by 20 and 25%, respectively. In sar-1-ile-8-angiotensin II-infused animals, similar nerve stimulation decreased renal blood flow by 12%, glomerular filtration rate by 7% and absolute and fractional sodium excretions and urine flow by 25, 18 and 18%, respectively. These decreases in sodium and water output were significantly smaller than those observed in animals with an intact renin-angiotensin system. 4. Stimulation of the brachial nerves increased post-ganglionic efferent renal nerve activity by 20% and the magnitude of this response was unaffected following inhibition of the renin-angiotensin system. 5. The results show that low rates of brachial nerve stimulation in the rat can increase efferent renal nerve activity and result in an antinatriuresis and antidiuresis which is dependent on the presence of angiotensin II, and appears to be due to an action of angiotensin II at the level of the kidney.

The effect of treadmill speed on ventilation at the start of exercise in man

1. The change in ventilation at the start of exercise was determined during both hyperoxic rebreathing and air breathing in four volunteers. 2. In order to differentiate between the effects of limb-movement frequency and exercise load in terms of oxygen uptake, three treadmill exercises were tested: E1, at an oxygen uptake of 1 l/min on a level treadmill; E2, at 2 l/min on an inclined treadmill at the same speed as E1; E3, at 2 l/min on a level treadmill at a higher speed. All of the exercises were performed at a walking pace. 3. Prior to rebreathing, hyperventilation for 5 min to 20 mmHg was used to reduce carbon dioxide to below the central chemoreceptor threshold. From eleven to fourteen rebreathing experiments were done on each volunteer for each of the three exercises, with the treadmill started at carbon dioxide levels which ranged from 36 (below threshold) to 58 mmHg (above threshold). 4. Ten experiments were performed on each volunteer for each of the three exercises during air breathing, with the treadmill started after 5 min of rest. 5. In both the rebreathing experiments and the air breathing experiments it was found that the change in ventilation at the start of exercise was the same for exercises E1 and E2, and significantly greater for exercise E3. 6. It was concluded that the frequency of limb movement, rather than exercise load (oxygen consumption) is a determinant of the change in ventilation at the start of exercise.

Response of chemosensitive nerve fibers of group III and IV to metabolic changes in rat muscles

Spike recordings were obtained with preparations of group III and IV fibers from the nervus peroneus of the rat. During the recordings the muscle was stimulated by chemical substances simulating metabolic effects of static exercise: increase of [K+], enhancement of osmolality and increase of concentrations of lactic acid and inorganic phosphates. Two experimental setups were used: in series I application was performed by a perfusion of the circulatorily isolated hindleg, and in series II a single muscle of the hindleg (musculus extensor digitorum longus) was superfused by control or test solutions. Only those fiber preparations were further investigated which did not respond to pressure, tension or squeezing of the muscle. Only few fibers that were exposed to all of our stimuli responded to none of them; from the rest, about the half were selective or only preferential for one stimulus. The majority of the fibers adapted their response after 8 min while the applications still endured. A comparison of all fibers (in series II) proved that all the four stimuli elicited significant increases of activity. The greatest significant effects were found for lactic acid and potassium (in series I and II). Since the concentrations used in the test applications were characteristic for medium and heavy exercise these results support the hypothesis that metabolic muscle receptors participate in the peripheral control of circulatory and respiratory drives during static exercise.

Gas exchange parameters, muscle blood flow and electromechanical properties of the plantar flexors

Sixteen men were tested to determine VO2max (ml X kg-1 X min-1), anaerobic threshold VO2 (ATVO2) and oxygen kinetics (time constant, T.C.) during running on a treadmill. For measuring maximal calf blood flow (maxBF, ml X 100 ml-1 X min-1), venous occlusion plethysmography was employed immediately following a combination of arterial occlusion and toe raising exercise to exhaustion. In addition, supramaximal electrical stimulations were given to determine maximal calf twitch force (Fmax, N), maximal rate of twitch force development (dF/dt) and relaxation (R X dF/dt, N X ms-1) and electro-mechanical delay time (EMD, ms). Results demonstrated that VO2max, ATVO2 and maxBF were all inversely related to T.C. (p less than 0.05). MaxBF and ATVO2 showed the highest correlation (r = 0.89, p less than 0.01). Stepwise multiple linear regression analyses revealed that variance in VO2max (60%) and ATVO2 (84%) could be accounted for by the combined effects of the following peripheral factors: VO2max = 51,25-3.24(dF/dt) + 0.14(maxBF), and ATVO2 = 11.68 + 0.42(maxBF) - 0.2(Fmax). These findings, together with the results of cluster analysis, suggest a tight link between ATVO2 and peripheral blood flow capacity. On the other hand, a moderate correlation (r = 0.64, p less than 0.01) between VO2max and maxBF might be due in part to individual differences in oxygen extraction-utilization capacity during heavy exercise above anaerobic threshold.

Spinal opiate modulation of cardiovascular reflexes in the exercising dog

The possible role of spinal opiate receptors on hind limb muscle afferents, participating in arterial blood pressure and heart rate responses to ischemic exercise, were investigated following lumbosacral intrathecal injection of morphine and naloxone in chronically instrumented dogs. Morphine markedly attenuated these responses. Naloxone had no effect alone but blocked the morphine responses. Thus, spinal opiate receptors may modulate ascending afferent information mediating cardiovascular reflexes from ischemic muscle.

The relationship between lactate and ventilatory thresholds: coincidental or cause and effect?

To determine if blood lactate (LA) is the stimulus responsible for 'breakaway' ventilation (VE), the lactate (LT) and ventilation (VT) thresholds were monitored during one-legged cycling exercise. Ten healthy volunteer male subjects (Mean 2-legged VO2max = 4.27 l X min-1) performed prior exercise (PE) to reduce muscle glycogen stores by cycling at 75-85% of maximal heart rate (HR max) for 60-75 min, followed by a 30 h low carbohydrate diet. Pre- and post- LT and VT tests were performed on a cycle ergometer employing a continuous protocol with increments of 16 W every 3 min. Muscle biopsies were taken from the vastus lateralis muscle before the PE ride, prior to the threshold test 24 h later, and before testing the non-exercised (NE) leg. An I.V. catheter placed in the antecubital vein was used for serial blood samples taken at rest, and during the final 30 s of each progressive load. Gas analysis was calculated every 30 s (Beckman Metabolic Measurement Cart). Biopsies (N = 3) showed that the exercise and diet regimen elicited glycogen reduction which significantly (p less than 0.05) reduced R and the blood LA concentration in both the PE (2.62 to 1.99 mmol X l-1) and NE (2.87 to 2.26 mmol X l-1) legs at LT. At VT, LA concentrations were also significantly reduced in the PE (3.35 to 2.56 mmol X l-1) and NE (3.59 to 2.74 mmol X l-1) legs. VO2 and VE, however, were similar between pre- and post- tests.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of exercise training on the ventilatory threshold of patients with coronary heart disease

Out of 156 patients with stable coronary heart disease randomized to either an exercise intervention group or a control group, 41 had complete gas analysis data. Continuous gas exchange data, including the ventilatory threshold, and selected heart rates were determined initially and at 1 year. The mean attendance for the exercise group was 2.2 +/- 0.7 days a week at an intensity of 60 +/- 9% of estimated peak oxygen uptake for 1 year of the study. Statistically significant differences (p less than 0.05) were observed between the exercise group (n = 19) and the control group (n = 22) for peak oxygen uptake (L/min), total treadmill time, and supine rest and submaximal heart rates after 1 year. The most remarkable change was a 16% increase in treadmill time. There was no difference between groups for the ventilatory threshold expressed either as an absolute oxygen uptake or as a percentage of peak oxygen uptake at 1 year. However, there was a significant correlation (r = 0.45; p less than 0.05) between the absolute change in peak oxygen uptake and the absolute change in the ventilatory threshold. These results indicate that a moderate exercise program is inadequate to alter the ventilatory threshold in patients with coronary heart disease and that changes in ventilatory threshold do not explain the increase in treadmill time that usually occurs.

Control of ventilation during submaximal exercise: a brief review

This review discusses the leading hypotheses concerning ventilatory control during submaximal exercise. The ventilatory response at the onset of submaximal exercise has been studied extensively. It is generally agreed that expired ventilation (VE) increases rapidly at the initiation of exercise followed by a slower increase in VE until a steady state is reached. In general, there are four schools of thought concerning the mechanisms that are responsible for the exercise hyperpnoea. Two of the hypotheses relate the increase in VE to neural regulation. One group argues that the increase in VE during work is primarily due to afferent neural feedback to the ventilatory control centre while the other group proposes that efferent neural activity can explain the hyperpnoea. A third group of hypotheses submit that humoral mechanisms must be actively involved in the increase in VE during exercise. The leading hypothesis in this area is based on experiments that suggest that CO2 return to the lung provides a stimulus for ventilatory control. Finally, the fourth supposition is that the exercise hyperpnoea may be due to both neural and humoral mechanisms. In summary, although there is persuasive evidence that both humoral and neural factors may play a role in mediating the exercise hyperpnoea, the basic question of whether the response is due solely to humoral or neural mechanisms remains unresolved.

Stimulation by central command of locomotion, respiration and circulation during exercise

We studied the relationships between exercise (locomotion) and respiratory and circulatory responses in 19 cats that walked or ran normally on a treadmill, and in 16 paralyzed animals during fictive locomotion, i.e., locomotory activity in motor nerves to the legs. Preparations included anesthetized cats with intact brains and unanesthetized decorticate (hypothalamic) and decerebrate (mesencephalic) animals. Spontaneous actual locomotion and fictive locomotion occurred in all preparations except the mesencephalic cats. Electrical stimulation or injection of a GABA antagonist (picrotoxin) into the hypothalamic locomotor region caused locomotion to develop. In all cases when locomotion occurred, respiration and arterial pressure increased in proportion to the level of locomotor activity despite control or ablation of respiratory feedback mechanisms. Respiration and arterial pressure increased similarly during fictive locomotion despite the absence of muscular contraction or limb movement and the lack of change of metabolic rate. We conclude that the study provides experimental support for the feed-forward, or command signal, hypothesis for the genesis of proportional changes of respiration and circulation that occur during exercise. Feedback mechanisms are not required for its operation. We suggest that command signals emanating from the hypothalamus provide the primary drive for changes of respiration and circulation during exercise.

The role of spinal cord transmission in the ventilatory response to exercise in man

The ventilatory response to electrically induced exercise was studied in thirteen patients with traumatic spinal cord transection at or about the level of T6. The steady-state and on-transient responses to this exercise were compared with those obtained in eighteen normal subjects (Adams, Garlick, Guz, Murphy & Semple, 1984). Exercise was produced by surface electrode stimulation of the quadriceps and hamstring muscles so as to produce a pushing movement at 1 HZ against a spring load. At rest there was no significant difference between normals and patients, except that the patients had a lower CO2 elimination (VCO2) and end-tidal PCO2 (PET,CO2) and a higher heart rate. On exercise the mean rise in VCO2 for the patients was 172 ml min-1 (S.D. 72), and for the normals was 287 ml min-1 (S.D. 143). The corresponding mean changes in ventilation (VI) were 4.4 l min-1 (S.D. 2.2) and 7.6 l min-1 (S.D. 3.2). However, the ventilatory equivalent for CO2 (delta VI/delta VCO2) in the steady state was not significantly different between patients (26.0, S.D. 5.9) and normals (28.5, S.D. 7.4). In the steady state there was a mean rise in PET,CO2 of 0.9 mmHg (S.D. 1.4) in the normals, and 3.2 mmHg (S.D. 2.7) in the patients, but there was overlap between the two groups. In many experimental runs in both groups, PET,CO2 did not rise, and sometimes fell. Where PCO2 did rise, the ventilatory response to exercise could not be accounted for on the basis of the ventilatory sensitivity to CO2 inhalation. From arterial sampling in three of the patients it was found that when PET,CO2 rose, the corresponding change in Pa,CO2 was less. During the on transient, there was a significant rise in both VCO2 and VI by the second breath in both groups. At the end of the on transient the normal subjects had achieved 84% (S.D. 40) of the steady-state increase in VCO2 and 88% (S.D. 24) of the increase in VI. The corresponding values for the patients were 67% (S.D. 17) and 77% (S.D. 16) respectively; these differences between normals and patients are significant. The increase of VI during the on transient in the patients was achieved almost entirely by an increase in tidal volume whereas in normals, an increase in respiratory rate was a more important component. We conclude therefore that in man, spinal cord transection with a presumed loss of muscle afferents allows a ventilatory response to electrically induced exercise that cannot be explained by classical chemoreception.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory responses to shivering produced by external and central cooling in the pigeon

Respiratory responses of pigeons to spinal cord cooling (5-6 degrees C) in neutral environment (Ta = 28 degrees C), to ambient cooling (Ta = 5 degrees C), and to simultaneous spinal cord and ambient cooling were measured. Spinal cord cooling produced shivering and a 242% increased in heat production (M); expiratory flow rate (VE) increased 216%, a result of increases in both respiratory frequency (160%) and tidal volume (140%). Increases produced by ambient cooling compared to thermoneutral controls were slightly, but not significantly, less than those during spinal cord cooling: M = 203%, VE = 199%, respiratory frequency (fR) = 146%, tidal volume (VT) = 138%. Spinal cord cooling at low ambient temperature produced greater increases in shivering, heat production and respiration compared to thermoneutral controls than either type of cooling alone: M = 337%, VE = 326%, fR = 198%, VT = 178%. The oxygen extraction from the ventilatory gas remained relatively constant among the different groups. fR, VT and VE were all significantly linearly related to M over the wide range studied. These relationships were independent of whether cooling was central or external. Respiratory changes induced by the onset and end of spinal cord cooling were immediate and closely correlated with the magnitude of shivering. It is unlikely that changes in arterial and venous blood gases during shivering effected the major portion of the respiratory response. Thus, it is suggested that a control mechanism of the respiratory center via afferents from the shivering muscles is important in increasing respiration during central or external cooling.

Reflex increases in heart-rate induced by perfusing the hind leg of the rat with solutions containing lactic acid

The hypothesis that metabolic receptors in skeletal muscle influence heart-rate during exercise was tested by means of a perfused preparation of the rat's hind legs. The isolated leg was connected to the body only by nerve and bone and was perfused with tyrode solution. The humoral changes of exercise were simulated by perfusing with modified tyrode solutions in which concentration of K+, osmolality, concentrations of lactic acid, and inorganic phosphate were changed to reflect to those occurring during heavy exercise. Only perfusion with a solution enriched with lactic acid elicited a significant increase in heart-rate. The response disappeared when the nerve supply to the leg was cooled or sectioned. 20-60 s after the start of perfusion with solution of high [lactic acid] heart-rate began to increase reaching a maximum (delta HR +/- SE = 20.2 +/- 8.2, n = 7) after about 2 min. The effect on heart-rate increased when the venous concentration of lactic acid was increased the range from 3 to 10 mmol/l. In further experiments, we tried to separate the effects of pH and lactate. Heart-rate responses were induced only at low pH and at low pH the extent to which heart-rate changed increased with increases in lactate concentration.

Responses in muscle afferent fibres of slow conduction velocity to contractions and ischaemia in the cat

The aim of the study was to find out to what extent muscle receptors with slowly conducting afferent fibres (group III and IV) are activated by muscular contractions of moderate force, and what kind of muscle afferents could mediate the pain of ischaemic exercise. In chloralose-anaesthetized cats, the impulse activity of single afferent units from the triceps surae muscle was recorded from dorsal root filaments during muscular contractions with intact blood supply and after occlusion of the muscle artery. Two types of responses were observed to contractions without muscular ischaemia. One was characterized by sudden onset and a graded response amplitude to contractions of increasing force. In most cases stretching the muscle was also an effective stimulus. Units showing this response behaviour were labelled c.s.m (contraction-sensitive with mechanical mechanism of activation). The other response type had a more delayed onset and often outlasted the exercise period; because of the unknown mechanism of activation, units of this kind were labelled c.s.x. The proportion of c.s.m receptors was significantly higher amongst group III than amongst group IV units. During ischaemic contractions of comparable force the c.s.m and c.s.x receptors exhibited an unchanged or a decreased response amplitude. Under these conditions another receptor type (N, for nociceptive) was activated which did not respond to contractions with intact blood supply. Vigorous activations during ischaemic work were only observed in group IV receptors. The majority of the 131 group III and IV units tested did not respond to contractions at all. These contraction-insensitive (c.i.) endings probably comprised different receptor populations (nociceptors, thermoreceptors, low-threshold mechanoreceptors). It is concluded that the various central nervous effects of muscular exercise without ischaemia which are known to be due to raised activity in thin muscle afferents (e.g. cardiopulmonary adjustments, spinal locomotor reflexes) are probably produced by the c.s.m and c.s.x types. The pain of ischaemic contractions is most likely mediated by the N receptors most of which possess non-myelinated afferent fibres.