Carotid baroreceptor reflexes in humans during orthostatic stress

Orthostatic stress, including standing, head-up tilting and lower body suction, results in increases in peripheral vascular resistance but little or no change in mean arterial pressure. This study was undertaken to determine whether the sensitivity of the carotid baroreceptor reflex was enhanced during conditions of decreased venous return. We studied eight healthy subjects and determined responses of pulse interval (ECG) and forearm vascular resistance (mean finger blood pressure divided by Doppler estimate of brachial artery blood velocity) to graded increases and decreases in carotid transmural pressure, effected by a neck suction/pressure device. Responses were determined with and without the application of lower body negative pressure (LBNP) at -40 mmHg. Stimulus-response curves were determined as the responses to graded neck pressure changes and the differential of this provided estimates of reflex sensitivity. Changes in carotid transmural pressure caused graded changes in R-R interval and vascular resistance. The cardiac responses were unaffected by LBNP. Vascular resistance responses, however, were significantly enhanced during LBNP and the peak gain of the reflex was increased from 1.2 +/- 0.3 (mean +/- S.E.M.) to 2.2 +/- 0.3 units (P < 0.05). The increased baroreflex gain may contribute to maintenance of blood pressure during orthostatic stress and limit the pressure decreases during prolonged periods of such stress.     

The influence of small fibre muscle mechanoreceptors on the cardiac vagus in humans

We have previously shown that activation of muscle receptors by passive stretch (PS) increases heart rate (HR) with little change in blood pressure (BP). We proposed that PS selectively inhibits cardiac vagal activity. We attempted to test this by performing PS during experimental alterations in vagal tone. Large decreases in vagal tone were induced using either glycopyrrolate or mild rhythmic exercise. Milder alterations in vagal tone were achieved by altering carotid baroreceptor input: neck pressure (NP) or neck suction (NS). PS of the triceps surae was tested in 14 healthy human volunteers. BP, ECG and respiration were recorded. PS alone caused a significant decrease ( P < 0.05) in R–R interval (962 ± 76 ms at baseline compared to 846 ± 151 ms with PS), and showed a reduction in HR variability, which was not significant. The decrease in R–R interval with PS was significantly less ( P < 0.05, n = 3) following administration of glycopyrrolate (−8.1 ± 4.5 ms) compared to PS alone (−54 ± 11 ms), and also with PS during handgrip (+10 ± 10 ms) compared with PS alone (−74 ± 15 ms) ( P < 0.05, n = 5). Milder reductions in vagal activity (NP) resulted in a small but insignificant further decrease in R–R interval in response to PS (−107 ± 17 ms compared to PS alone −96 ± 13 ms, n = 5). Mild increases in vagal activity (NS) during PS resulted in smaller decreases in R–R interval (−39 ± 5.5 ms) compared to PS alone (−86 ± 17 ms) ( P < 0.05, n = 8). BP was not significantly changed by stretch in any tests. The results indicate that amongst muscle receptors there is a specific group activated by stretch that selectively inhibit cardiac vagal tone to produce tachycardia.     

Lack of interaction between adenosine-induced vasodilatation and carotid baroreflex-induced changes in sympathetic activity in dog hindlimb artery

In anaesthetized dogs, a hindlimb was vascularly isolated and perfused at a constant flow rate of 7.7 +/- 1.9 ml min-1 100 g-1 (mean +/- S.E.M.; n = 5) through the femoral artery. The carotid sinuses were isolated and perfused at high (greater than 145 mmHg) or low (less than 75 mmHg) pressure to enable reflex sympathetic tone on the hindlimb vessels to be controlled. Both vagi were sectioned in the neck and mean aortic blood pressure was held constant by connection of the aorta to a reservoir. The responses to infusion of three doses of adenosine at high and low carotid sinus pressures were not significantly different: infusion of 0.60 +/- 0.16 microM-adenosine reduced femoral arterial perfusion pressure (FAPP) by 11.6 +/- 3.2% (n = 6) at high carotid sinus pressure and by 12.6 +/- 5.1% (n = 4) at low carotid sinus pressure, while 4.71 +/- 0.49 microM-adenosine reduced FAPP by 20.8 +/- 4.8% (n = 6) at high carotid sinus pressure and by 20.7 +/- 4.8% (n = 6) at low carotid sinus pressure; 50.1 +/- 7.3 microM-adenosine reduced FAPP by 36.7 +/- 5.5% (n = 6) at high carotid sinus pressure and by 27.7 +/- 7.8% (n = 5) at low carotid sinus pressure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of neuronal nitric oxide reduces heart rate variability in the anaesthetised dog

In the vagally intact anaesthetised dog, we have investigated the role of nitric oxide (NO) on a normal sinus arrhythmia using an inhibitor of neuronally released NO, 1-(2-trifluoromethylphenyl) imidazole (TRIM). The mean and S.D. of the R-R interval was used to describe mean heart rate and heart rate variability, respectively. TRIM (0.8 mg I.C.) injected into the sinus node artery increased the mean heart rate slightly but reduced heart rate variability 3-fold from a control of 790 +/- 124 ms (mean +/- S.D.; n = 5) to 666 +/- 36 ms (P < 0.01 Student's paired t test, n = 5). These results suggest that neuronally released NO may have a vagal facilitatory role in the maintenance of sinus arrhythmia in the normal heart.     

A wide range of baroreflex stimulation does not alter forearm blood flow

The contribution to the regulation of forearm blood flow (FBF) by different baroreceptor populations has previously only been studied over a limited range of stimuli. Therefore, FBF and R-R interval were recorded during neck suctions and neck pressures ranging from –60 to +40 mmHg. The change in R-R interval (R-R) during neck suction was significantly increased at each stage when compared to the control (P<0.05). R-R did not show any significant change during any of the neck pressure stages (P>0.05). Suction or pressure applied to the neck did not elicit any significant changes in FBF when compared to the control (P>0.05). These data show that widening the range of applied stimuli to carotid sinus baroreceptors does not induce a change in FBF. However, the small transient changes reported previously cannot be discounted.     

Pitfalls in the interpretation of spectral analysis of the heart rate variability during exercise in humans

The recent use of spectral analysis of the R-R interval variability to assess the autonomic drive during exercise has produced inconsistent results. The purpose of this study was to assess whether the spectral components of the R-R interval variability reflect different mechanisms at rest and during exercise. Autoregressive spectral analysis of the electrocardiographic and breathing signals was performed in 11 healthy young men at rest and during incremental cycle ergometry. The amplitude of respiratory sinus arrhythmia, i.e. the absolute power of the high frequency spectral component, fell at the onset of exercise, consistent with a reduction in cardiac vagal activity. Conversely, the normalized power of the high frequency component, (i.e. the percentage of R-R interval variance due to the respiratory sinus arrhythmia) increased with increasing work rates. The low frequency spectral component of the R-R interval variability, which when expressed in normalized units is believed to reflect cardiac sympathetic activity, was no longer detectable in severe exercise when the adrenergic drive is known to be elevated. In conclusion, autoregressive spectral analysis of the R-R interval variability does not adequately reflect the autonomic changes that occur during incremental exercise. In particular, the evidence indicates that as the cardiac vagal tone falls with increasing levels of exercise, a greater percentage of the residual power of the high frequency component may be due to non-neural mechanisms.     

Carotid-cardiac baroreflex function does not influence blood pressure regulation during head-up tilt in humans

The influence of the carotid-cardiac baroreflex on blood pressure regulation was evaluated during supine rest and 40 degrees head-up tilt (HUT) in 9 healthy young subjects with and without full cardiac vagal blockade. The carotid baroreflex responsiveness, or maximal gain (G(MAX)), was assessed from the beat-to-beat changes in heart rate (HR) and mean arterial pressure (MAP) by the variable neck pressure and suction technique ranging in pressure from +40 to -80 Torr, with and without glycopyrrolate (12.0 +/- 1.0 microg/kg body weight; mean +/- SE). In the supine position, glycopyrrolate increased the HR to 91 +/- 3 bpm, from 54 +/- 3; MAP to 89 +/- 2 mmHg, from 76 +/- 2; and cardiac output to 6.8 +/- 0.3 l.min(-1), from 4.9 +/- 0.3 (P < 0.05). The G(MAX) of the carotid baroreflex control of HR was reduced to -0.06 +/- 0.01 bpm.mmHg(-1), from -0.30 +/- 0.02 (P < 0.05) with no significant effect on the G(MAX) of the carotid baroreflex control of MAP. During HUT the carotid baroreflex control of MAP was unchanged, though the G(MAX) of the carotid baroreflex control of HR was increased (P < 0.05). During HUT, central blood volume, assessed by electrical thoracic admittance, and total vascular conductance were decreased with and without glycopyrrolate. Furthermore, glycopyrrolate reduced G(MAX) of the carotid baroreflex control of HR during HUT (P < 0.05) with no significant effect on G(MAX) of the carotid baroreflex control of MAP. These data suggest that during supine rest and HUT-induced decreases in central blood volume, the carotid baroreflex control of HR is mediated primarily via parasympathetic activity. Furthermore, the maintenance of arterial blood pressure during postural stress is primarily mediated by arterial and cardiopulmonary reflex regulation of sympathetic activity and its effects on the systemic vasculature.     

Differences in autonomic modulation of heart rate during arm and leg exercise.

Heart rate (HR) is higher during dynamic arm exercise than during leg exercise at equal oxygen consumption levels, but the physiological background for this difference is not completely understood. The vagally mediated beat-to-beat R-R interval fluctuation decreases until the level of approximately 50% of maximal oxygen consumption during an incremental bicycle exercise, but the vagal responses to arm exercise are not well known. Changes in autonomic modulation of HR were compared during arm and leg exercise by measuring beat-to-beat R-R interval variability from a Poincaré plot normalized for the average R-R interval (SD1n), a measure of vagal activity, in 14 healthy male subjects (age 20 +/- 4 years) who performed graded bicycle and arm cranking tests until exhaustion. Seven of the subjects also performed the dynamic arm and leg tests after beta-adrenergic blockade (propranolol 0.2 mg kg-1 i.v.). More rapid reduction occurred in SD1n during the low-intensity level of dynamic arm exercise than during dynamic leg exercise without beta-blockade (e.g. 11 +/- 6 vs. 20 +/- 10 at the oxygen consumption level of 1.2 l min-1; P < 0.001) and with beta-blockade (e.g. 13 +/- 4 vs. 25 +/- 10 at the level of 1.0 l min-1; P < 0.05), and the mean HR was significantly higher during submaximal arm work than during leg work in both cases (e.g. during beta-blockade 81 +/- 12 vs. 74 +/- 6 beats min-1 at the level of 1.0 l min-1; P < 0.05). These data show that dynamic arm exercise results in more rapid withdrawal of vagal outflow than dynamic leg exercise.     

Cardiovascular responses to carotid sinus baroreceptor stimulation during moderate to severe exercise in man

Our objective was to assess the importance of arterial baroreflexes in maintaining vasoconstriction in active muscle during moderate to severe exercise. Eight subjects exercised for 8-15 min on a cycle ergometer at three levels (averages 94, 194, 261 W) requiring 40-88% of VO2 max. Four times during each exercise level pulsatile negative pressure (-50 mmHg) was applied over the carotid sinuses for 30 s; suction was applied at each ECG R-wave for 250-400 ms. Before and during each neck suction, femoral venous blood flow (FVBF) was measured by constant infusion thermal dilution. At 94 W neck suction significantly reduced blood pressure (BP) (15 mmHg) and heart rate (HR) (7 beats min-1), and raised leg vascular conductance (LVC) (11.4%) without changing FVBF. At 194 W, neck suction reduced BP (9 mmHg), HR (4 beats min-1) and FVBF (5.1%, 240 ml min-1), and raised LVC (5.2%). At 261 W, LVC was unchanged by neck suction, but BP and FVBF both fell (9 mmHg and 650 ml min-1 or 7.4%). We conclude that competing local vasodilation and sympathetic vasoconstriction control muscle blood flow during moderate exercise, and vasoconstrictor tone can be withdrawn by baroreceptor stimulation. High levels of vasoconstrictor outflow to muscle in severe exercise may not originate from baroreflexes.     

Differential effect of head-up tilt on cardiovagal and sympathetic baroreflex sensitivity in humans

The purpose of this study was to determine the effect of baroreceptor unloading on the sensitivity of the cardiovagal and sympathetic arms of the baroreflex during upright posture. Beat-by-beat R-R interval, arterial blood pressure and cardiac output (Doppler ultrasound), as well as muscle sympathetic nerve activity (MSNA) were recorded during periods in supine (Supine) and 60 deg head-up tilt (HUT) positions (n = 8 volunteers). Cardiovagal baroreflex sensitivity (BRS) was measured by the spontaneous sequence analysis method using systolic blood pressure and R-R interval, while sympathetic BRS was determined using the slope of the linear relationship between decreasing segments of diastolic blood pressure (DBP) and corresponding increases in MSNA. On changing to HUT, mean R-R interval and cardiac output decreased, while mean measures of MSNA, DBP and total peripheral resistance increased (P < 0.05). Cardiovagal BRS decreased from Supine to 60 deg HUT (19 +/- 2 ms mmHg(-1) versus 7.6 +/- 1.2 ms mmHg(-1); P < 0.01). In contrast, sympathetic BRS increased from -6.1 +/- 1.4 a.u. mmHg(-1) in Supine to -14 +/- 2 a.u. mmHg(-1) in HUT (P < 0.01). Thus, HUT produced differential effects on cardiac versus sympathetic BRS. The data suggest that dynamic baroreflex-mediated cardiovascular control is dominated by sympathetic control during baroreceptor unloading.     

Arterial blood pressure and carotid baroreflex function during arm and combined arm and leg exercise in humans

AIM: During arm cranking (A) blood pressure is higher than during combined arm and leg exercise (A + L), while the carotid baroreflex (CBR) is suggested to reset to control a higher blood pressure in direct relation to work intensity and the engaged muscle mass. METHOD: This study evaluated the function of the CBR by using neck pressure and neck suction during upright A, L and A + L in 12 subjects and, in order to evaluate a potential influence of the central blood volume on the CBR, also during supine A in five subjects. Exercise intensities for A and L were planned to elicit a heart rate response of c. 100 and 120 beats min(-1), respectively, in the upright position and both workloads were maintained during A + L and supine A. RESULTS: The CBR operating point, corresponding to the pre-stimulus blood pressure, was 88 +/- 6 mmHg (mean +/- SE) at rest. During upright A, L and A + L and supine A it increased to 109 +/- 9, 95 +/- 7, 103 +/- 7 and 104 +/- 4 mmHg, respectively, and it was thus higher during upright A than during A + L and supine A (P < 0.05). In addition, the CBR threshold and saturation pressures, corresponding to the minimum and maximum carotid sinus pressure, respectively, were higher during upright A than during supine A, A + L, L and at rest (P < 0.05) with no significant change in the maximal reflex gain. CONCLUSION: These findings demonstrate that during combined arm and leg and exercise in the upright position the CBR resets to a lower blood pressure than during arm cranking likely because the central blood volume is enhanced by the muscle pump of the legs.     

Baroreflex inhibition of the human sinus node: importance of stimulus intensity, duration, and rate of pressure change.

1. Carotid baroreceptors were stimulated with electronically controlled neck suction in five healthy young men and pulse interval prolongation was measured. Timing of the onset of stimuli in relation to cardiac activity was held constant, and stimulus intensity, duration, and dP/dt were varied independently. 2. In the subjects studied, sinus node responses to neck suction were proportional to dP/dt. However, variations of stimulus dP/dt within or above the normal range for arterial dP/dt did not influence the magnitude of integrated baroreflex responses, or the earliest portion of baroreflex sinus node inhibition. 3. Carotid baroreflex responses were linear over a wide range which extended beyond the normal range for human systolic arterial pressures. 4. Saturation of the carotid baroreceptor-cardiac reflex occurred at distending pressures of about 160 mmHg. 5. The average baroreflex responses of the group studied were highly reproducible over time. 6. Baroreflex gain correlated very strongly with base line pulse interval. 7. The magnitude of baroreflex responses increased linearly with the duration of carotid sinus distension and reached a maximum level with stimuli lasting 0-5 sec or more. 8. The results demonstrate that carotid sinus transfer characteristics can be measured in normal man, and that human response patterns are strikingly similar to those observed earlier in experimental animals.     

Characteristics of the carotid baroreflex in man during normal and flow-restricted exercise.

Eight subjects were studied in the supine position at rest, during normal dynamic leg exercise (control exercise) and with blood-flow restriction in the working legs (flow-restricted exercise). Graded muscle blood-flow restriction was accomplished by applying a supra-atmospheric pressure of 50 mmHg to the working legs. During incremental-load exercise, flow restriction reduced exercise performance and peak heart rate by 36% and 13%, respectively. The function of the cardiac branch of the carotid baroreflex was studied over its full operational range, at rest and during constant-load control and flow-restricted exercise, by measuring R-R intervals during application of pulse-synchronous graded pressures (40 to -65 mmHg) in a neck-chamber device. Heart rate and arterial pressure were higher during flow-restricted than control exercise, indicating that the flow restriction activated the muscle chemoreflex. Raising the carotid transmural pressure (systolic arterial pressure minus neck-chamber pressure) was accompanied by increasing R-R intervals in all conditions. The set point (point of baseline carotid transmural pressure and R-R interval) coincided with the midportion of the pressure-response curve at rest and with the threshold point of the curve during exercise. The maximal rate of change in relative R-R intervals and the corresponding carotid transmural pressure range were higher during control exercise than at rest and highest during flow-restricted exercise, indicating that exercise and especially flow-restricted exercise increased carotid baroflex sensitivity, and shifted the carotid baroreflex optimal buffering range to higher pressures. The results suggest that the carotid baroflex attenuates exercise heart rate increases mediated by the muscle chemoreflex and/or by central command.     

Effect of Carotid Sinus Stimulation on Cardiac Output and Peripheral Vascular Resistance during Changes in Blood Volume Distribution in Man

In ten healthy subjects (mean age 29.6 years) the hemodynamic response to carotid sinus stimulation (neck suction - 40 mmHg) was studied under control conditions and during peripheral pooling of blood (lower body negative pressure). Heart rate, arterial and central venous pressure, cardiac output and forearm blood flow were measured. The time sequence of the heart rate response was studied separately in six healthy subjects. During control conditions, carotid sinus stimulation induced a significant decrease in arterial pressure and heart rate. The blood pressure decrease mainly reflected a reduction in cardiac output, total peripheral vascular resistance being essentially unchanged. However, in the skeletal muscle, represented by a forearm segment, vascular resistance decreased significantly. During lower body negative pressure (LBNP) the same stimulation of the carotid sinus induced a significantly greater fall in mean arterial pressure even though the reduction in cardiac output was slightly smaller on the average than in the control condition. The heart rate increased, probably secondary to a time dependent increase in heart rate elicited by the continuous LBNP stimulus. Total peripheral vascular resistance decreased significantly during LBNP, the reaction likewise differing significantly from that in the control condition. Thus the augmented blood pressure response was due to a more pronounced vasodilatation when the carotid sinus was stimulated during lower body negative pressure. The results indicate that the hemodynamic changes elicited by carotid sinus stimulation are modified by changes in the distribution of blood volume and in the tone of resistance vessels.     

The effects of lower body negative pressure on baroreceptor responses in humans

In healthy human subjects the immediate responses of pulse interval and the steady-state responses of arterial blood pressure and cardiac output to changes in carotid sinus transmural pressure were determined before and during the application of a subatmospheric pressure to the lower part of the body. Increases in carotid sinus transmural pressure, effected by applications of subatmospheric pressure to the neck (neck suction) resulted in prolongation of pulse interval and decrease in blood pressure; opposite responses were obtained to application of a positive pressure (neck pressure). Application of lower body negative pressure resulted in a decrease in pulse interval (heart rate increase) but little change in blood pressure. During lower body negative pressure, the responses of pulse interval to neck pressure were reduced but those to neck suction were unaffected; the responses of blood pressure to neck suction were enhanced but those to neck pressure were unaffected. From experiments in which cardiac output was also determined, it was seen that lower body negative pressure reduced cardiac output, increased calculated total body vascular resistance and augmented the resistance response to neck suction although not to neck pressure. These results are compatible with the view that application of lower body negative pressure does not change the sensitivity of the baroreceptor reflex and that the changes in the responses are due to non-linearities of the stimulus-response curves.     

Characteristics of the carotid baroreflex in man during normal and flow-restricted exercise

EIKEN, O., CONVERTINO, V. A., DOERR, D. F., DUDLEY, G. A., MORARIU, G. & Mekjavic; I. B. 1992. Characteristics of the carotid baroreflex in man during normal and flow-restricted exercise. Acta Physiol Scand 144 , 325–331. Received 27 May 1 991 , accepted 29 October 1991. ISSN 0001–6772. School of Kinesiology, Simon Fraser University, Burnaby, B.C., Canada. Eight subjects were studied in the supine position at rest, during normal dynamic leg exercise (control exercise) and with blood-flow restriction in the working legs (flow-restricted exercise). Graded muscle blood-flow restriction was accomplished by applying a supra-atmospheric pressure of 50 mmHg to the working legs. During incremental-load exercise, flow restriction reduced exercise performance and peak heart rate by 36% and 13%, respectively. The function of the cardiac branch of the carotid baroreflex was studied over its full operational range, at rest and during constant-load control and flow-restricted exercise, by measuring R-R intervals during application of pulse-synchronous graded pressures (40 to -65 mmHg) in a neck-chamber device. Heart rate and arterial pressure were higher during flow-restricted than control exercise, indicating that the flow restriction activated the muscle chemoreflex. Raising the carotid transmural pressure (systolic arterial pressure minus neck-chamber pressure) was accompanied by increasing R-R intervals in all conditions. The set point (point of baseline carotid transmural pressure and R-R interval) coincided with the midportion of the pressure-response curve at rest and with the threshold point of the curve during exercise. The maximal rate of change in relative R-R intervals and the corresponding carotid transmural pressure range were higher during control exercise than at rest and highest during flow-restricted exercise, indicating that exercise and especially flow-restricted exercise increased carotid baroflex sensitivity, and shifted the carotid baroreflex optimal buffering range to higher pressures. The results suggest that the carotid baroflex attenuates exercise heart rate increases mediated by the muscle chemoreflex and/or by central command.     

Interplay among carotid sinus, cardiopulmonary, and carotid body reflexes in dogs.

Interactions among vascular reflexes evoked from carotid sinuses, carotid bodies, and cardiopulmonary region were examined in anesthetized, atropinized, and respired dogs with aortic nerves cut. The carotid sinuses were perfused at 220, 150, and 40-50 mmHg; the chemoreceptors were stimulated by perfusion with hypoxic hypercapnic blood. Cardiopulmonary vasomotor inhibition was interrupted by vagal cold block. Measurements were made of arterial blood pressure and of kidney and hindlimb vascular resistance. At sinus pressures less than 170-160 mmHg, cardiopulmonary vasomotor inhibition increased with increase in blood volume. At high sinus pressure, interruption of this augmented cardiopulmonary inhibition was as ineffective in changing vascular resistance as interruption of the lesser inhibition present during normovolemia. Chemoreceptor stimulation increased the response to vagal block at intermediate but not at high or low sinus pressure. The studies demonstrate the dominant role of the carotid sinus reflex when the three systems interact and the ineffectiveness of chemoreceptor stimulation when carotid or cardiopulmonary inhibition is maximal.     

Effect of cervical sympathetic nerve stimulation on canine carotid sinus reflex.

In the chloralose-anesthetized dog the carotid sinus on one side of the neck was isolated vascularly. Pressure in the isolated sinus [carotid sinus pressure (CSP)], electrocardiogram, and systemic arterial pressure were recorded. Both vagosympathetic trunks were cut and the contralateral common carotid artery was occluded or the contralateral sinus nerve was cut to reduce reflex buffering of arterial pressure changes. By varying CSP from 50 to 250 mmHg the full range of the reflex response was examined. Electrical stimulation of the peripheral end of the cut ipsilateral cervical sympathetic nerve brought about a rapid decrease in mean arterial pressure (MAP) and heart rate (HR) at lower CSPs, no change in these variables at midrange CSPs, and a gradual increase at higher CSPs, such that the gain of the reflex was reduced (1.89 +/- 0.19 to 1.33 +/- 0.15 mmHg/mmHg). The decrease in MAP and HR at lower CSPs implies an increase in baroreceptor activity whereas the converse would appear to occur at higher CSPs. These responses attained a maximum value at low stimulus frequencies (less than 10 Hz).     

Discharge patterns of preganglionic neurones with axons in a cardiac vagal branch in the rat

The fibre types that run in a vagal branch projecting to the rat heart are described in this study. In order to obtain spontaneous discharge in this vagal branch and optimal recording conditions, we compared the decerebrate state to urethane, urethane-chloralose and pentobarbital-chloralose anaesthesia with regard to level of chronotropic cardiac vagal tone. Administration of atropine (2 mg kg(-1), I.V.) significantly decreased baseline cardiac interval only in the decerebrate and urethane-anaesthetised rat (by 0.018 +/- 0.001 s and 0.019 +/- 0.002 s, respectively). As a result of these experiments, urethane was chosen as the anaesthetic for all subsequent studies. Using a heart rate signal-averaging method we demonstrated that rat cardiac vagal preganglionic neurones innervating the sinoatrial node should have an expiratory discharge pattern, as reported in other species. However, only 5 % of chronotropic vagal tone was found to be subject to respiratory sinus arrhythmia. A suction microelectrode method, combined with spike-triggered averaging, was employed to record activity from a total of 58 vagal afferents that had axons in this branch. Approximately 75 % of these latter sensory fibres displayed cardiac rhythm. In a separate study we also recorded 318 preganglionic neurones with axons in the right cardiac vagal branch of the rat. Respiratory-modulated preganglionic units were statistically less common than tonically firing units. Six preganglionic subtypes were categorised according to conduction velocity and respiratory discharge pattern. Myelinated B-fibre and unmyelinated C-fibre types were found to be equally prevalent and equally likely to be reflexly excited during the pulmonary chemoreflex and the peripheral arterial chemoreflexes. The electrophysiological analysis has shown how diverse the discharge patterns of the preganglionic neurones or interneurones are whose axons course in the right cardiac vagal branch of the rat. The results of these experiments demonstrate the usefulness of combining spike discrimination with multiple spike-triggered averaging to simultaneously record B and C centrifugal vagal efferents.     

Left ventricular responses to experimental aortic coarctation in growing puppies.

The haemodynamic status of 8 coarctated and 7 sham-operated beagle puppies was studied by a catheterization technique at rest and during isoproterenol and volume loading at the ages of 7 (I) and 9 (II) months (5 and 7 months after the experimental coarctation). Proximal aortic systolic and pulse pressures were constantly higher in the coarctation group than in the control group (P less than 0.05), and the systolic pressure gradient across the coarctation was always significantly higher in the coarctation group [I at rest mean 45 +/- 5 (SD) vs 5 +/- 4 mmHg, P less than 0.001, and after I isoproterenol infusion 56 +/- 9 vs 10 +/- 6 mmHg, P less than 0.001, and after I dextran infusion 58 +/- 10 vs 8 +/- 7 mmHg, P less than 0.001]. The time constant of exponential isovolumic left ventricular pressure fall after the isoproterenol tests was longer in the coarctation group (I 28 +/- 8 ms and II, 30 +/- 4 ms) than in the control group (I, 21 +/- 2, P less than 0.05 and II, 19 +/- 3 ms, P less than 0.005), indicating impaired relaxation. The tension time index during the volume loading tests increased in the coarctation dogs (I, 4150 +/- 660 and II, 4080 +/- 810 mmHg s min-1) to higher levels than in the control group (I, 3550 +/- 220, II, 2540 +/- 1140 mmHg s min-1, P less than 0.05 both). Cardiac output, left ventricular end diastolic pressure, inotropic parameters and heart rate were similar in both groups during the infusions.(ABSTRACT TRUNCATED AT 250 WORDS)