Mechanisms responsible for changes in abdominal vascular volume during sympathetic nerve stimulation in anaesthetized dogs

This study was designed to determine the extent to which the decrease in volume of blood in the abdominal circulation in response to sympathetic stimulation was due to a passive effect of decreasing flow rather than active constriction of the capacitance vessels. In dogs anaesthetized with alpha-chloralose (100 mg kg-1 i.v.) the abdominal circulation was vascularly isolated and perfused either at constant flow or at constant pressure, and drained at constant pressure from the inferior vena cava. Changes in volume were determined by integration of the differences between inflow and outflow. Supramaximal stimulation of both splanchnic (sympathetic) nerves at 1 Hz decreased abdominal volume during constant pressure perfusion (active and passive components) by 3.04 +/- 0.58 ml kg-1 and at constant flow (active responses only) by 2.30 +/- 0.49 ml kg-1 (means +/- S.E.M.). The responses at 8 Hz were respectively 9.52 +/- 0.91 and 5.09 +/- 0.49 ml kg-1. The proportion of the responses calculated to be passive at 1 and 8 Hz was 23 +/- 6.3 and 45 +/- 5.1%, respectively. These responses were almost identical to those induced by changing inflow by increasing the pump speed. Following ligation of the splenic pedicle, the responses during both constant pressure and constant flow were reduced by similar amounts, indicating that only the active response was affected. After ligation of the splenic pedicle, the proportion of the response calculated to be passive at 1 and 8 Hz increased to 44 +/- 8.0 and 62 +/- 3.7% respectively. These results indicate the importance of passive volume change in affecting abdominal volume, particularly following ligation of the splenic circulation.

Delayed sympathetic efferent responses to coronary baroreceptor unloading in anaesthetized dogs

1. We previously reported that, although stimulation of coronary arterial baroreceptors results in reflex vasodilatation of a magnitude and a time course similar to that seen in response to carotid baroreceptor stimulation, the vasoconstriction that occurs when the stimulus to coronary baroreceptors is removed develops more slowly. We now report the results of experiments designed to investigate the site on the reflex are that is responsible for the delayed vasoconstriction. 2. In alpha-chloralose anaesthetized, artificially ventilated dogs, a perfusion circuit allowed independent control of pressures to the aortic root, including the coronary arteries, the aortic arch and the carotid sinuses. Electrophysiological recordings were made of afferent discharge in nerve fibres dissected from the vagus nerve, which responded to changes in coronary pressure, and from renal and lumbar efferent sympathetic nerves. Reflex vascular responses were assessed from changes in perfusion pressure to the systemic circulation, which was perfused at constant flow. 3. The afferent discharge from the coronary baroreceptors responded rapidly to both increases and decreases in coronary perfusion pressure. This indicates that prolonged activation of the coronary receptors cannot be the cause of the delayed vasoconstriction. 4. An increase in pressure to the coronary baroreceptors resulted in an immediate decrease in activity in either renal or lumbar sympathetic nerves. A decrease in coronary pressure, however, was followed by a slow gradual increase in sympathetic discharge. This contrasts with the responses to decreases in carotid or aortic arch pressures, which were followed by rapid increases in efferent discharge, often with an overshoot. 5. We conclude that the slow recovery of efferent sympathetic activity following a reduction in coronary pressure is likely to explain the previously reported slow recovery of vascular resistance.

Vascular responses to stimulation of carotid, aortic and coronary artery baroreceptors with pulsatile and non-pulsatile pressures in anaesthetized dogs

This research was designed to compare coronary, carotid and aortic arch baroreceptors in terms of the ranges of pressures required to elicit reflex vascular responses and the possible differences between the responses to pulsatile and non-pulsatile stimuli. Dogs were anaesthetized with alpha-chloralose, artificially ventilated and the chests opened wide. A perfusion circuit allowed independent control of pressures distending the three baroreceptor regions. A cardiopulmonary bypass and ventricular fibrillation prevented cardiac pulsations from influencing coronary baroreceptor pressure. The caudal region of the animal was perfused at constant flow and vascular resistance responses were assessed from changes in perfusion pressure. Only tests in which the overall response exceeded 3 kPa (22.5 mmHg) were analyzed. Reflex responses were obtained to significantly lower coronary pressures than were required to induce responses from other regions. The inflexion points of the stimulus-response curves for pulsatile coronary, carotid and aortic pressures were 10.5 +/- 0.6, 15.5 +/- 1.8 and 16.4 +/- 1.7 kPa (79 +/- 5, 116 +/- 14 and 123 +/- 13 mmHg, respectively; values are means +/- S.E.M.). When the responses to pulsatile stimuli were compared with those to non-pulsatile stimuli, it was noted that for the carotid receptors, lower pressures were required to induce responses (inflexion pressure less) and the slope of the stimulus-response curve was less. Pulsatile aortic pressures induced a parallel (downward) displacement of the curve but no change in inflexion point or slope. The coronary baroreceptor stimulus-response relationship was unaffected by pulsatility. These results show differences between the characteristics of the three baroreceptors with coronary receptors being unaffected by pressure pulsatility but likely to be of importance in hypotensive situations.

Reflex vascular responses to abdominal venous distension in the anesthetized dog

This was undertaken to determine whether distension of the subdiaphragmatic veins results in reflex vasoconstriction and interacts with the carotid baroreflex. In alpha-chloralose-anesthetized open-chest dogs, a perfusion circuit controlled carotid and thoracic aortic pressures, splanchnic and limb blood flows, and cardiopulmonary blood flows. At carotid sinus pressures below approximately 90 mmHg, increases in splanchnic pressure of 7 mmHg or more resulted in increases in vascular resistance in both the splanchnic and limb circulations; there was no response at higher carotid pressures. At high venous pressures, the average maximum gains of the carotid baroreflex for splanchnic and limb resistance responses were increased by 106 and 67%, respectively. The responses were not abolished by cutting the vagal or phrenic nerves but were prevented by cutting the splanchnic nerves and, for the limb, the sciatic and femoral nerves. These results suggest that splanchnic congestion, by causing vasoconstriction and augmentation of the carotid baroreflex, may be important in the maintenance of blood pressure during gravitational stress.

Differences between syncope resulting from rapid onset acceleration and orthostatic stress

BACKGROUND AND HYPOTHESIS

Orthostatically-induced syncope is accompanied by venous pooling and vasodilatation. Loss of consciousness during head-to-foot acceleration (G-LOC) in aviators may be caused by a different mechanism, as venous pooling should be prevented through the use of an anti-G suit. This research was conducted to test the hypothesis that in individuals wearing a well-fitted anti-G garment, no important changes occur in the volume of dependent regions during loss of consciousness resulting from rapid onset acceleration stress. Further, this work compares venous pooling patterns in G-LOC subjects to patterns seen during syncope in volunteers and patients subjected to orthostatic stress. We conducted the tilt/LBNP tests to establish what level of venous pooling was required to induce syncope in the absence of a hydrostatic component (other than 1 G) and to confirm that our equipment was sensitive enough to detect volume changes large enough to cause syncope.

METHODS

Shifts in blood volume to the calf, thigh and abdominal segments were compared in subjects with G-LOC to those in subjects taken to presyncope with orthostatic stress created by upright tilt and lower body negative pressure (LBNP). Centrifuge subjects were exposed to a 15 s rapid onset (6 G.s-1) + 5 Gz exposure on the centrifuge while remaining relaxed and wearing a well-fitting anti-G suit, but with the anti-G suit pressure inactivated.

RESULTS

Blood volume decreased an average of 14.9 +/- 22.1 ml in the calf segment; increased an average of 64.1 +/- 7.9 ml in the thigh segment, and decreased an average of 80.1 +/- 29.7 ml in the abdominal segment. The mean net change in volume of the three combined regions was not significantly different from zero. Presyncope was induced in subjects by a progressive exposure to upright tilt, and then addition of LBNP at -20 mm Hg and -40 mm Hg. In the tilt/LBNP group, there was a net increase of 1022 +/- 269.8 ml for the combined segments. Changes in all three segments were significantly different than the mean segmental volume changes seen in centrifuge subjects at G-LOC endpoints. Significant changes from baseline mean arterial pressure, but not heart rate were also seen within, but not between the 2 groups, with mean eye level blood pressures (ELBP) falling an average of 45.6 +/- 7.7 mm Hg in the tilt/LBNP group at syncope and 105.1 +/- 15.5 mm Hg in the centrifuge subjects at G-LOC.

CONCLUSIONS

These differences suggest that G-LOC may be due entirely to hydrostatic effects, with venous pooling being prevented by the wearing of an ant-G garment, even when it remains uninflated.

Reflex vascular responses from aortic arch, carotid sinus and coronary baroreceptors in the anaesthetized dog

In chloralose-anaesthetized dogs, pressure applied to coronary, carotid and aortic baroreceptors were changed independently and the resulting reflex vascular responses were determined. Increases in pressure to each group of baroreceptors resulted in reflex vasodilatation; the maximal responses to distension of carotid and coronary baroreceptors were significantly larger than those to aortic receptors, but not different from each other. Increases in pressure in all three regions induced maximal responses at similar times from the onset of the pressure stimulus. However, the time for recovery of vascular resistance following a decrease in baroreceptor pressure differed. Vasoconstriction following a period of coronary hypertension occurred slowly, requiring 70 s for 90% of the response to develop. This was significantly longer than the corresponding times for carotid and aortic receptors (about 28 s). The rate of vasoconstriction in response to coronary baroreceptor unloading was influenced by the period for which the pressure stimulus was applied and vasoconstriction was even slower when the pressure stimulus had been maintained for 8 min. The mechanism responsible for delaying the vasoconstriction following a period of coronary hypertension is not known, but this effect may have important implications for the control of arterial blood pressure.

Salt supplement increases plasma volume and orthostatic tolerance in patients with unexplained syncope

OBJECTIVE

To determine whether in patients presenting with posturally related syncope administration of salt increases plasma volume and improves orthostatic tolerance. Patients with poor tolerance of orthostatic stress tend to have lower than average plasma and blood volumes.

DESIGN

A double blind placebo controlled study in 20 patients and an open study in 11 of the effects of giving 120 mmol/day of sodium chloride.

PATIENTS

31 patients presenting with episodes of syncope who had no apparent cardiac or neurological disease. Plasma volume was determined by Evans blue dye dilution, orthostatic tolerance by time to presyncope in a test of combined head-up tilt and lower body suction, and baroreceptor sensitivity by the effect of neck suction on pulse interval.

RESULTS

8 weeks after treatment, 15 (70%) of the 21 patients given salt and three (30%) of the placebo group showed increases in plasma and blood volumes and in orthostatic tolerance, and decreases in baroreceptor sensitivity. Improvement was related to initial salt excretion in that patients who responded to salt had a daily excretion below 170 mmol. The patients in the placebo group who improved also showed increases in salt excretion.

CONCLUSIONS

In patients with unexplained syncope who had a relatively low salt intake administration of salt increased plasma volume and orthostatic tolerance, and in the absence of contraindications, salt is suggested as a first line of treatment.

Re-evaluation of Evans blue dye dilution method of plasma volume measurement

To simplify the method of plasma volume measurement by Evans blue dye dilution we used, for the first time, the same venous site for injection of dye and collection of samples. In a series of 49 studies the dye decay between 10 and 35 min after injection was highly linear (r = 0.991 +/- 0.01), indicating that contamination of samples is very unlikely. We repeated the measurements after eight weeks in nine patients; the mean difference was 16.4 +/- 19.6 ml, indicating a high degree of reproducibility. We found that extrapolation of the dye decay curve to time zero is required for accurate estimates of plasma volume. There was good agreement between the estimates of plasma volume obtained by extrapolation from only three samples taken at 10, 20 and 30 min after dye injection with the results obtained using all six samples. We also found good agreement between the estimates of plasma volume obtained by using standard curves constructed from four standard dilutions of 1.25, 2.5, 5 and 10 mg/l and those obtained by the use of standard curves constructed from the blank and only one standard dilution of 10 mg/l. We therefore conclude that the Evans blue technique can be simplified with minimal loss of accuracy, by using only one venous site for injection and withdrawal, withdrawing only three samples between 10 and 30 min after injection and using a two point calibration line.

Relationship between plasma volume, carotid baroreceptor sensitivity and orthostatic tolerance

1. Studies were carried out on 43 otherwise healthy patients referred for investigation for attacks of syncope of unknown cause and on six healthy volunteers. 2. Plasma volume was determined by Evans Blue dye dilution and blood volume was estimated using haematocrit. Carotid baroreceptor sensitivity was determined from the changes in pulse interval in response to subatmospheric pressures applied to the neck overlying the carotid sinuses, and orthostatic tolerance was assessed as the time to presyncope in a test of head-up tilt, followed by the addition of graded lower body suction. 3. Eight patients and one volunteer fainted during head-up tilt alone, 23 patients and two volunteers fainted during tilt with lower body suction at -20 mmHg and 12 patients and three volunteers either fainted during suction at -40 mmHg or tolerated the entire procedure. 4. Although plasma and blood volumes were higher in males than females, the values normalized for either body weight or for calculated lean body mass were not different between male and female patients and asymptomatic volunteers. The subjects showing the greatest resistance to syncope were found to have significantly larger plasma and blood volumes (P < 0.0001) and significantly smaller baroreceptor sensitivities (P < 0.0002) than those who fainted earlier.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular reflexes from ventricular and coronary receptors

Ventricular receptors are distributed throughout the left ventricle and most, if not all, are attached to nonmyelinated nerve fibers. The receptors may be chemosensitive, mechanosensitive or both. Chemosensitive afferents are classically excited by exogenous chemicals such as veratridine, although endogenous chemicals such as bradykinin and prostaglandins, which are released during ischemia, also excite these nerves. The reflex responses can be very powerful, resulting in profound bradycardia and hypotension. A normal physiological role for these receptors seems unlikely although it is probable that they contribute to the changes occuring in some pathological states. Ventricular mechanoreceptors, some of which may also exhibit chemosensitivity, are excited by increases in ventricular systolic pressure, but only when the pressure increases to extreme levels. They also appear to react to increases in inotropic state and increases, and possibly also to decreases, in ventricular filling. It seems that ventricular mechanoreceptors do not show the same intense response as is seen in the chemosensitive afferents following chemical stimulation and probably as a consequence of this their reflex responses are also weak and probably of little importance. Previous assertions that they are involved in the vasovagal reaction can probably now be discounted. The existence of coronary arterial baroreceptors has been suspected for about 30 years. This has now been confirmed and they have been shown to respond to pressure changes in much the same way as the well known carotid and aortic baroreceptors. There are, however, some interesting differences. Coronary baroreceptors, at least in the dog, do not control the heart rate, although they do influence respiratory activity. Another intriguing difference is that when vascular resistance has been inhibited reflexly by perfusing coronary receptors at a high pressure, it takes several minutes for the vascular resistance to increase when coronary pressure is again lowered. The implications of this are uncertain, but it is conceivable that, whereas carotid baroreceptors are involved in the responses to rapid changes in pressure, coronary baroreceptors may be more concerned with the regulation of the long-term level of arterial blood pressure.

The influence of myocardial systolic shortening on action potential duration following changes in left ventricular end-diastolic pressure

INTRODUCTION

Contraction-excitation feedback may be an important factor in arrhythmogenesis in patients with heart failure. We have previously demonstrated the contrasting effects of raising left ventricular end-diastolic pressure on action potential duration in dog and guinea pig hearts. The current study was undertaken to assess whether these differing effects might reflect differences in the effect of varying left ventricular end-diastolic pressure on systolic shortening in the two models.

METHODS AND RESULTS

Two models were studied and compared. In open chest dog hearts and isolated guinea pig hearts, measurements of myocardial segment length were made while left ventricular end-diastolic pressure was raised and lowered at constant left ventricular peak systolic pressure. Action potentials were also recorded while left ventricular end-diastolic pressure was changed. The dog hearts were studied further in a manner aimed at reproducing the contraction pattern of the guinea pig hearts. In the in situ dog heart, elevation of left ventricular end-diastolic pressure, and the consequent increase in end-diastolic segment length, was accompanied by a marked increase in systolic shortening, such that minimum systolic segment length remained unchanged. Elevation of left ventricular end-diastolic pressure was accompanied by a prolongation of action potential duration. In the in vitro guinea pig model, elevation of left ventricular end-diastolic pressure was accompanied by more modest changes in systolic shortening, which were not sufficient to compensate for increased diastolic segment length. Consequently, minimum systolic segment length increased as the hearts dilated. Elevation of left ventricular end-diastolic pressure was accompanied by a shortening of action potential duration. In a further series of experiments, the effects of increased left ventricular end-diastolic pressure were studied in the dog model while allowing aortic pressure to rise, thereby restricting systolic shortening. Under these circumstances, the dog model was similar to the guinea pig model, with an increase in left ventricular end-diastolic pressure causing a shortening of action potential duration.

CONCLUSION

Our results suggest that the effects of preload changes on action potential duration depend on accompanying changes in systolic shortening. This suggests a possible role for contraction-excitation feedback in arrhythmogenesis in patients with regional wall-motion abnormalities.

Cardiac pacing does not improve orthostatic tolerance in patients with vasovagal syncope

This study was undertaken to assess the value of dual chamber pacing in the treatment of vasovagal syncope. In a preliminary study, on two patients the time to presyncope during head-up tilt before and after implanting pacemakers was determined. Both patients fainted with similar decreases in blood pressure at almost exactly the same time after tilting. In the main study, nine patients with pacemakers implanted as treatment for syncope were studied, in random order, with pacemakers on and either off or turned to minimum rate. The pacemakers prevented bradycardia but had no effect on the time to syncope in a progressive test of head-up tilt followed by the addition of graded lower body suction. It is concluded that cardiac pacing does not prevent or even delay the onset of postural syncope and infer that bradycardia is an unimportant component of vasovagal attacks.

Orthostatic tolerance in patients with unexplained syncope

Orthostatic tolerance in 79 patients complaining of attacks of unexplained syncope, was assessed as the time to imminent syncope in a test involving: head-up tilt by 60 degrees for 20 min, followed by tilt and lower body suction at -20 and -40 mmHg for 10 min at each. Blood pressure and heart rate were determined noninvasively. Ninety-five per cent of patients developed signs of presyncope during the test. After 10 min of lower body suction at -20 mmHg, presyncope had occurred in 85% of the patients compared with only 23% in a recently reported group of asymptomatic controls. Both patients and controls were divided into four groups: men and women, under and over 50 years, and the times at which each group of control subjects showed a 20% incidence of syncope were taken as the limits of normality. By those times, overall 85% of patients had developed syncope. It is concluded that the new combined test is able to discriminate patients who have poor orthostatic tolerance and is likely to be of value in assessing the effects of treatment regimes.

Sensory functions of the heart

Stretch receptors are situated in the walls of the atria, left ventricle and coronary arteries. Atrial receptors are stimulated by atrial distension due to increased filling as the result, amongst other things, of an increased blood volume. Their reflex responses are increases in heart rate and in urine flow. Ventricular receptors may be stimulated by chemical agents, mechanical stimuli, or both. They are responsible for the Bezold-Jarisch reflex of bradycardia and hypotension, but their normal physiological role, if any, is uncertain. The proximal part of the left coronary artery contains baroreceptors which are excited by increases in blood pressure. Their role, like other baroreceptors, is to control blood pressure by regulation of the diameter of the peripheral blood vessels.

An acute animal model that simulates the hemodynamic situations present during +Gz acceleration

Air combat maneuver acceleration (G) profiles with onset/offset patterns that occur faster than the response characteristics of the human cardiovascular system may lead to regulatory instability and, ultimately, acceleration-induced loss of consciousness (G-LOC) incidents. We have developed an acute animal model that simulates the hemodynamic situations seen under acceleration to study the effects of complex G environments on individual reflexogenic areas. This preparation allowed us to individually isolate the effects of high gravity on venous return and cardiac preload, arterial baroreflexes and splanchnic capacity. This report describes the preparation and presents examples of the types of +Gz simulations possible and recordings of the responses of the animals. Further, we tested the hypothesis that the volume of blood displaced from the cephalic regions of the circulation and the rate of displacement into the splanchnic capacitance with G onset is affected by distending pressure at the carotid/aortic baroreceptor sites. Early results from 7 dogs show that resistance to flow into the splanchnic beds is affected by changes in distending pressure occurring at arterial baroreceptor sites. When pressure distending the carotid/aortic baroreceptors was increased, resistance to flow into the abdominal vascular beds was decreased. This result suggests that sudden increases in +Gz loads occurring during the overshoot phase from a previous G-peak may result in reduced tolerance.

Combined head-up tilt and lower body suction: a test of orthostatic tolerance

A combined tilt-table and lower body suction chamber to provide a progressive test of orthostatic tolerance which avoided the use of drugs and had a defined end point even in most asymptomatic subjects has been constructed and evaluated. An air-tight cover, sealed to a tilt-table and to the subject at the level of the iliac crest, was used to study the responses to: head-up tilting for 20 min, then tilting plus lower body suction at -20 and -40 mmHg for 10 min at each. Blood pressure, heart rate and cardiac output were measured noninvasively and orthostatic tolerance was assessed as the time to imminent onset of syncope. All subjects tolerated tilt alone but 84% developed signs and symptoms of presyncope during the suction. Younger women had a lower orthostatic tolerance than other groups. Values of the variables measured during tilting alone did not correlate with the measured orthostatic tolerance, but during the suction subjects who developed early syncope showed larger decreases in cardiac output and smaller maximal heart rates than the more resistant subjects. The test is repeatable and is likely to be of value in the assessment of orthostatic tolerance in patients and for evaluating the effects of various interventions.

Reflex responses to stimulation of mechanoreceptors in the left ventricle and coronary arteries in anaesthetized dogs

1. Previous work has shown that physiological increases in mean aortic root pressure, which change the pressure in both the coronary circulation and the left ventricle, result in reflex vasodilatation. This study was undertaken to attempt to localize the reflexogenic area mainly responsible for the reflex. 2. In anaesthetized, artificially ventilated dogs, cannulae connected to perfusion systems were inserted in the ascending aorta, left ventricular apex and left atrium. This allowed us to change the pressures in: (a) the aortic root including both the coronary arteries and the left ventricle; (b) aortic root and coronary arteries, at constant ventricular pressure; and (c) in the ventricle, with mean (although not pulse) aortic pressure constant. Aortic and carotid baroreceptors were perfused at constant pressure and reflex responses were determined from changes in perfusion pressures (flows constant) to a vascularly isolated hindlimb and to the remainder of the systemic circulation. 3. Combined changes in mean aortic root (coronary arterial) and ventricular systolic pressures consistently resulted in decreases in perfusion pressures. A change in only mean aortic root (coronary arterial) pressure, with ventricular pressure constant, also resulted in decreases in perfusion pressures and these were only a little smaller than those to the combined stimulus. Changes in ventricular systolic pressure resulted in responses averaging only about 30% of those to the combined stimulus. 4. Setting mean aortic root or ventricular systolic pressures at different levels did not affect the responses to changes in pressures in the other region. 5. These results show that physiological increases in pressure in the aortic root and coronary arteries, in the absence of changes in pressure in the left ventricle, cause reflex vasodilatation. The relatively small response occurring when ventricular pressure was changed could be due either to a contribution from ventricular receptors or to a change in the stimulus to coronary receptors resulting from changes in the ventricular or aortic pulse. 6. We conclude that the reflex effects of increases in mean aortic root pressure are due mainly to stimulation of coronary arterial baroreceptors.

Afferent discharges from coronary arterial and ventricular receptors in anaesthetized dogs

1. Previous work has shown that increases in aortic root pressure result in reflex vasodilation, and that this response is likely to result mainly from stimulation of receptors in the coronary arteries, although contribution from left ventricular receptors was not excluded. This investigation was undertaken to resolve this question and to determine the afferent nerve fibres likely to be involved in this reflex. 2. In chloralose-anaesthetized dogs a perfusion circuit was used which allowed us to change the pressures in: (a) the aortic root, coronary arteries and the left ventricle; (b) aortic root and coronary arteries at constant ventricular pressure; and (c) the left ventricle with mean (although not pulse) aortic pressure constant. Electrophysiological recordings were made from slips dissected from the vagus nerve which responded with an increase in discharge to either combined increases in the pressures, or to aortic root injections of veratridine. 3. Recordings were made from twenty-one vagal afferents. On the basis of their conduction velocities, eleven were classified as non-myelinated and ten as myelinated. 4. Three non-myelinated afferents responded to veratridine injections only, three to both veratridine and combined aortic root and ventricular pressure changes, and five to pressure changes only. Responses to pressure occurred only when ventricular systolic pressure exceeded 30 kPa. 5. None of the myelinated afferents responded to veratridine. All showed increases in discharge to combined increases in mean aortic root, coronary arterial and left ventricular systolic pressures, which would be graded over a range similar to that which caused reflex changes. All were more sensitive to changes in mean coronary pressure than to changes in ventricular systolic pressure. 6. We conclude that myelinated vagal afferent nerve fibres, which respond predominantly to changes in mean coronary arterial pressure, are likely to be responsible for the vasodilation to the changes in mean aortic root pressure previously reported. These fibres are probably attached to coronary arterial mechanoreceptors.

The use of a microcomputer to automate measurement of action potential duration for both transmembrane and monophasic action potentials

Measurement of action potential duration is made more valuable if it can be made simultaneously with other variables, to which it may be related. We have developed a microcomputer-based system which allows measurement of action potential duration, both for transmembrane action potentials and for monophasic action potentials. The system allows simultaneous recording and analysis of action potentials and intraventricular pressures. Both end-diastolic and maximum systolic pressures have been analysed. Action potential duration was assessed at four different levels of the repolarization curve. We have analysed the consistency of measurements made by the computer, and compared them to measurements made manually, using results from six dog experiments. For action potential duration, there was no systematic difference between the manual and the computer methods, but the computer was significantly more consistent. In the case of the pressure measurements, the two methods were approximately the same in their consistency, and again there was no systematic difference. We have demonstrated that potential errors in determination of the average diastolic potential did not significantly affect the results obtained by our method. The variances of action potential duration measurements made at different levels of repolarization were equal. We demonstrated that there was no effect of amplitude on the action potential duration of potentials recorded under steady-state conditions.

The effects of ventricular end-diastolic and systolic pressures on action potential and duration in anaesthetized dogs

1. Although it is known that mechanical events in the heart influence the duration of the cardiac action potential, there is no quantitative information on the effects of independent changes in ventricular end-diastolic and systolic pressures. 2. Experiments were carried out on open-chest anaesthetized dogs in which the autonomic nervous influences on the heart were prevented and monophasic action potentials were recorded form the epicardial surface of the left ventricle. The duration of these action potentials was taken as the interval from the upstroke to the point of 90% repolarization. 3. Elevation of left ventricular peak systolic pressure, at constant end-diastolic pressure, significantly shortened the monophasic action potential. 4. Elevation of end-diastolic pressure at constant peak systolic pressure significantly lengthened the monophasic action potential. 5. Responses were not dependent on release of noradrenaline from sympathetic nerve terminals because they persisted after administration of bretylium tosylate. They were also not due to myocardial ischaemia because they persisted when coronary perfusion pressure was maintained at a constant high level. 6. Simultaneous recordings of changes in myocardial segment length showed the expected responses to changes in ventricular pressures: increases in shortening in response to increases in diastolic pressure and no consistent effect from changes in systolic pressure. 7. These investigations demonstrate the independent effects of changes in systolic and end-diastolic pressures on cardiac action potential duration. This effect is likely to be an effect of the mechanical events, i.e. contraction-excitation feedback. This response may be mediated through changes in myocardial fibre tension, the consequent changes in fibre shortening, or both.

An investigation of cardiovascular reflexes during a trimix saturation dive to 450 msw (GUSI 17)

The study examines the hypothesis that the carotid sinus heart rate baroreflex responses are changed in human subjects on exposure to 450 msw. Baroreceptor reflex changes in heart rate (expressed as ms/mmHg applied pressure) were evoked by application of negative or positive pressure to a cuff surrounding the neck. At 450 msw using trimix, the mean resting heart rate of divers slowed significantly from 64 +/- 1.3 beats/min at surface to 55 +/- 1.4 beats/min at 450 msw, respiratory rate decreased from 15 +/- 1.4 at surface to 11 +/- 2 at 450 msw, and sinus arrhythmia increased. There was no change in arterial blood pressure. Baroreceptor reflex sensitivity to an increased carotid sinus transmural pressure was reduced from 5.6 +/- 2.9 (mean +/- SEM) at surface to 2.4 +/- 0.8 ms.mmHg-1 at 450 msw; sensitivity to decreased carotid sinus transmural pressure increased from 2.2 +/- 0.4 ms.mmHg-1 at surface to 5.1 +/- 0.2 ms.mmHg-1 at 450 msw. A progressive shortening of cardiac interval during breath hold in expiration was also noted. When this shortening of interval was incorporated into the analysis of baroreceptor reflex sensitivity, no significant change in sensitivity was observed but the overall baroreflex stimulus-response relationship shifted downward.

Reflex vascular responses to changes in left ventricular pressures, heart rate and inotropic state in dogs

Dogs were anaesthetized with chloralose, artificially ventilated and the chests widely opened. Left ventricular mechanoreceptors, including those in or near the coronary arteries, were stimulated by changing the pressure in the aortic root. The pressures distending the left atrium and the aortic and carotid baroreceptors were controlled. Reflex vascular responses were assessed from changes in perfusion pressures to a hind limb and to the rest of the systemic circulation, which were perfused independently at constant flows. Physiological increases in peak left ventricular and coronary arterial pressures resulted in vasodilatation in both regions. These responses were not influenced by changes in the heart rate. Stimulation of the left cardiac sympathetic nerves resulted in increases in peak ventricular pressure and in the maximal rate of change of pressure (dP/dtmax). This also resulted in increases in perfusion pressures (vasoconstriction) at all levels of peak ventricular pressure although there was little effect on the responses to changes in ventricular pressure. Sympathetic stimulation had little effect on the relationship between perfusion pressures and aortic root pressure. Increases in ventricular filling also resulted in vasoconstriction at all levels of peak ventricular pressure. Increases in filling, however, did not affect the relationship between either perfusion pressure and aortic root pressure. Conversely, decreases in left ventricular filling, by bypassing some of the left atrial blood, resulted in vasodilatation at all levels of peak ventricular pressures but had no effect on the perfusion pressures at any aortic root pressure. The combination of sympathetic stimulation with decreased ventricular filling resulted in little effect on perfusion pressures or on their responses to changes in either aortic root or ventricular systolic pressures. We conclude that the vascular responses to stimulation of left ventricular mechanoreceptors are not enhanced by sympathetic stimulation, decreases in ventricular filling or the combination of the two. The apparent effects of each of these interventions alone on the relationships between perfusion pressures and ventricular, but not aortic root, pressure, could be explained if the receptors responsible were sensitive more to changes in aortic root and coronary arterial pressures than to pressure changes in the ventricle itself.

The Importance of Vascular Capacitance in Cardiovascular Control

Vascular capacitance refers to degree of active constriction of vessels (mainly veins) which affects return of blood to the heart and thus cardiac output. Capacitance changes participate in cardiovascular reflexes but passive volume changes resulting from changes in transmural pressure are likely to be at least as important.