Evaluation of forearm vascular resistance during orthostatic stress: Velocity is proportional to flow and size doesn't matter

Background

The upright posture imposes a significant challenge to blood pressure regulation that is compensated through baroreflex-mediated increases in heart rate and vascular resistance. Orthostatic cardiac responses are easily inferred from heart rate, but vascular resistance responses are harder to elucidate. One approach is to determine vascular resistance as arterial pressure/blood flow, where blood flow is inferred from ultrasound-based measurements of brachial blood velocity. This relies on the as yet unvalidated assumption that brachial artery diameter does not change during orthostatic stress, and so velocity is proportional to flow. It is also unknown whether the orthostatic vascular resistance response is related to initial blood vessel diameter.

Methods

We determined beat-to-beat heart rate (ECG), blood pressure (Portapres) and vascular resistance (Doppler ultrasound) during a combined orthostatic stress test (head-upright tilting and lower body negative pressure) continued until presyncope. Participants were 16 men (aged 38.4±2.3 years) who lived permanently at high altitude (4450m).

Results

The supine brachial diameter ranged from 2.9–5.6mm. Brachial diameter did not change during orthostatic stress (supine: 4.19±0.2mm; tilt: 4.20±0.2mm; -20mmHg lower body negative pressure: 4.19±0.2mm, p = 0.811). There was no significant correlation between supine brachial artery diameter and the maximum vascular resistance response (r = 0.323; p = 0.29). Forearm vascular resistance responses evaluated using brachial arterial flow and velocity were strongly correlated (r = 0.989, p<0.00001) and demonstrated high equivalency with minimal bias (-6.34±24.4%).

Discussion

During severe orthostatic stress the diameter of the brachial artery remains constant, supporting use of brachial velocity for accurate continuous non-invasive orthostatic vascular resistance responses. The magnitude of the orthostatic forearm vascular resistance response was unrelated to the baseline brachial arterial diameter, suggesting that upstream vessel size does not matter in the ability to mount a vasoconstrictor response to orthostasis.

Vasodilator effects of leptin on canine isolated mesenteric arteries and veins

1. Although leptin increases sympathetic nerve activity and blood pressure, its direct action on large arterial rings is to cause relaxation. However, it is the small resistance arteries and veins that are important in blood pressure control. The effects of leptin on these small vessels has not been reported previously in the canine and the effect of leptin on the capacitance vessels is not known. 2. In the present study, third- or fourth-order canine mesenteric arteries and veins were isolated and placed in a perfusion myograph and preconstricted with noradrenaline. The responses to graded concentrations of leptin were determined and the role of nitric oxide was assessed by administration of N(G)-nitro-l-arginine methyl ester (l-NAME), a blocker of nitric oxide synthase. 3. Leptin induced dose-related dilatations in both arterial and venous segments. The mean (+/-SEM) maximum increases in the diameter of the arteries and veins were 25.0 +/- 4.8 and 29.9 +/- 2.0% of the initial preconstriction, respectively. Relaxations of both arteries and veins were abolished by l-NAME or by endothelium denudation, although dilatations were still obtained to sodium nitroprusside, a nitric oxide donor. 4. These results indicate that leptin dilates canine small mesenteric arteries and veins by a mechanism involving endothelial release of nitric oxide. This observation may result in a decrease of peripheral resistance and venous return and, hence, counteract the leptin-induced neurally mediated vasoconstriction that has been reported previously.

Daytime variability of baroreflex function in patients with obstructive sleep apnoea: implications for hypertension

Obstructive events during sleep in patients with obstructive sleep apnoea (OSA) cause large alterations in blood pressure, and this may lead to changes in baroreflex function with implications for long-term blood pressure control. This study examined the daytime variations in the responses to carotid baroreceptor stimulation in OSA patients. We determined the cardiac and vascular responses every 3 h between 09.00 and 21.00 h in 20 patients with OSA, using graded suctions and pressures applied to a neck collar. These responses were plotted against estimated carotid sinus pressures and, from these plots, baroreflex sensitivities and operating points were taken as the maximal slopes and the corresponding carotid sinus pressures, respectively. We found that at 09.00 h, sensitivity for the control of vascular resistance was at its lowest (--1.2 +/- 0.2% mmHg(-1), compared with --1.9 +/- 0.3% mmHg(-1) at 12.00 h, P < 0.02) and operating point for control of mean arterial pressure was at its highest (101.1 +/- 5.8 mmHg, compared with 94.1 +/- 5.8 mmHg at 12.00 h, P < 0.05). This is in contrast to previous data from normal subjects, in whom sensitivity was highest and operating point lowest at 09.00 h. We suggest that the higher baroreflex sensitivity and lower operating point seen in the mornings in normal subjects may provide a protective mechanism against hypertension and that this protection is absent in patients with OSA. It is possible that the reduced reflex sensitivity and increased operating point in the mornings may actually promote hypertension.

The coronary baroreflex in humans

Previous studies have identified the presence of coronary baroreceptors in animal models. We set up a study to explore the presence of coronary baroreceptors in humans, which was performed with isolated, graded aortic root perfusion in patients during cardiopulmonary bypass. With ethical approval 12 patients with normal coronary arteries, aged 58-75 (mean 69) years undergoing mitral valve surgery were recruited to the study with informed consent. Those with aortic valve incompetence, coronary, or peripheral artery disease and diabetes mellitus were excluded. They were randomized to have their coronary perfusion pressure set low at 50 mmHg for 90 seconds and then adjusted high to 80 mmHg for 90 seconds (group L-H) or the reverse sequence (group H-L). Average arterial pressure and approximately constant systemic flow over 30-second periods were used to calculate vascular resistance (SVR). The first six experiments followed initiation of cardiopulmonary bypass and aortic clamping but before the delivery of cold blood cardioplegia; the blood temperature for these experiments was kept at 32 degrees C. The remaining six were conducted prior to removal of the aortic cross clamp at 37 degrees C. Coronary sinus blood samples were analyzed to exclude myocardial ischemia. Coronary sinus blood samples showed insignificant variation in oxygen saturation, lactate, and troponin T. Three patients were excluded because of unstable blood pressure. In the (L-H) group SVR reduced in 4 of 4 remaining patients (mean -9.4%, range -3.9 to -19.6%). In the (H-L) group SVR increased in three patients (mean +2.0%, range 1.1 to 3.7%) but decreased in two (-8.9% and -15.8%). These preliminary results, although not statistically different, suggest the presence of coronary baroreceptors in humans. The reflex vascular responses are similar to those previously reported in animal models.

Interaction of chemoreceptor and baroreceptor reflexes by hypoxia and hypercapnia - a mechanism for promoting hypertension in obstructive sleep apnoea

Asphyxia, which occurs during obstructive sleep apnoeic events, alters the baroreceptor reflex and this may lead to hypertension. We have recently reported that breathing an asphyxic gas resets the baroreceptor-vascular resistance reflex towards higher pressures. The present study was designed to determine whether this effect was caused by the reduced oxygen tension, which affects mainly peripheral chemoreceptors, or by the increased carbon dioxide, which acts mainly on central chemoreceptors. We studied 11 healthy volunteer subjects aged between 20 and 55 years old (6 male). The stimulus to the carotid baroreceptors was changed using graded pressures of -40 to +60 mmHg applied to a neck chamber. Responses of vascular resistance were assessed in the forearm from changes in blood pressure (Finapres) divided by brachial blood flow velocity (Doppler) and cardiac responses from the changes in RR interval and heart rate. Stimulus-response curves were defined during (i) air breathing, (ii) hypoxia (12% O(2) in N(2)), and (iii) hypercapnia (5% CO(2) in 95% O(2)). Responses during air breathing were assessed both prior to and after either hypoxia or hypercapnia. We applied a sigmoid function or third order polynomial to the curves and determined the maximal differential (equivalent to peak sensitivity) and the corresponding carotid sinus pressure (equivalent to 'set point'). Hypoxia resulted in an increase in heart rate but no significant change in mean blood pressure or vascular resistance. However, there was an increase in vascular resistance in the post-stimulus period. Hypoxia had no significant effect on baroreflex sensitivity or 'set point' for the control of RR interval, heart rate or mean arterial pressure. Peak sensitivity of the vascular resistance response to baroreceptor stimulation was significantly reduced from -2.5 +/- 0.4 units to -1.4 +/- 0.1 units (P < 0.05) and this was restored in the post-stimulus period to -2.6 +/- 0.5 units. There was no effect on 'set point'. Hypercapnia, on the other hand, resulted in a decrease in heart rate, which remained reduced in the post-stimulus period and significantly increased mean blood pressure. Baseline vascular resistance was significantly increased and then further increased in the post-control period. Like hypoxia, hypercapnia had no effect on baroreflex control of RR interval, heart rate or mean arterial pressure. There was, also no significant change in the sensitivity of the vascular resistance responses, however, 'set point' was significantly increased from 74.7 +/- 4 to 87.0 +/- 2 mmHg (P < 0.02). This was not completely restored to pre-stimulus control levels in the post-stimulus control period (82.2 +/- 3 mmHg). These results suggest that the hypoxic component of asphyxia reduces baroreceptor-vascular resistance reflex sensitivity, whilst the hypercapnic component is responsible for increasing blood pressure and reflex 'set point'. Hypercapnia appears to have a lasting effect after the removal of the stimulus. Thus the effect of both peripheral and central chemoreceptors on baroreflex function may contribute to promoting hypertension in patients with obstructive sleep apnoea.

Cerebrovascular responses to hypoxia and hypocapnia in high-altitude dwellers

Cerebral blood flow is known to increase in response to hypoxia and to decrease with hypocapnia. It is not known, however, whether these responses are altered in high-altitude dwellers who are not only chronically hypoxic and hypocapnic, but also polycythaemic. Here we examined cerebral blood flow responses to hypoxia and hypocapnia, separately and together, in Andean high-altitude dwellers, including some with chronic mountain sickness (CMS), which is characterized by excessive polycythaemia. Studies were carried out at high altitude (Cerro de Pasco (CP), Peru; barometric pressure (P(B)) 450 mmHg) and repeated, following relief of the hypoxia, on the day following arrival at sea level (Lima, Peru; P(B) 755 mmHg). We compared these results with those from eight sea-level residents studied at sea level. In nine high-altitude normal subjects (HA) and nine CMS patients, we recorded middle cerebral artery mean blood flow velocity (MCAVm) using transcranial Doppler ultrasonography, and expressed responses as changes from baseline. MCAVm responses to hypoxia were determined by changing end-tidal partial pressure of oxygen (P(ET,O2)) from 100 to 50 mmHg, with end-tidal partial pressure of carbon dioxide clamped. MCAVm responses to hypocapnia were studied by voluntary hyperventilation with (P(ET,O2)) clamped at 100 and 50 mmHg. There were no significant differences between the cerebrovascular responses of the two groups to any of the interventions at either location. In both groups, the MCAVm responses to hypoxia were significantly greater at Lima than at CP (HA, 12.1 +/- 1.3 and 6.1 +/- 1.0%; CMS, 12.5 +/- 0.8 and 5.6 +/- 1.2%; P < 0.01 both groups). The responses at Lima were similar to those in the sea-level subjects (13.6 +/- 2.3%). The responses to normoxic hypocapnia in the altitude subjects were also similar at both locations and greater than those in sea-level residents. During hypoxia, both high-altitude groups showed responses to hypocapnia that were significantly smaller at Lima than at CP (HA, 2.17 +/- 0.23 and 3.29 +/- 0.34% mmHg(-1), P < 0.05; CMS, 1.87 +/- 0.16 and 3.23 +/- 0.24% mmHg(-1); P < 0.01). The similarity of the results from the two groups of altitude dwellers suggests that haematocrit is unlikely to greatly affect cerebrovascular reactivity to hypoxia and hypocapnia. The smaller vasodilatation to hypoxia and larger vasoconstriction to hypoxic hypocapnia at high altitude suggest that cerebrovascular responses may be impaired at the high altitude, i.e. a maladaptation. The changes in the responses within less than 24 h at sea level indicate that this impairment is rapidly reversible.

Cardiovascular responses to orthostatic stress in healthy altitude dwellers, and altitude residents with chronic mountain sickness

High altitude (HA) dwellers have an exceptionally high tolerance to orthostatic stress, and this may partly be related to their high packed cell and blood volumes. However, it is not known whether their orthostatic tolerance would be changed after relief of the altitude-related hypoxia. Furthermore, orthostatic tolerance is known also to be influenced by the efficiency of the control of peripheral vascular resistance and by the effectiveness of cerebral autoregulation and these have not been reported in HA dwellers. In this study we examined plasma volume, orthostatic tolerance and peripheral vascular and cerebrovascular responses to orthostatic stress in HA dwellers, including some with chronic mountain sickness (CMS) in whom packed cell and blood volumes are particularly large. Eleven HA control subjects and 11 CMS patients underwent orthostatic stress testing, comprising head-up tilting with lower body suction, at their resident altitude (4338 m) and at sea level. Blood pressure (Portapres), heart rate (ECG), brachial and middle cerebral artery blood velocities (Doppler) were recorded during the test. Plasma volumes were found to be similar in both groups and at both locations. Packed cell and blood volumes were higher in CMS patients than controls. All subjects had very good orthostatic tolerances at both locations, compared to previously published data in lowland dwellers. In CMS patients responses of forearm vascular resistance to the orthostatic stress, at sea level, were smaller than controls (P < 0.05). Cerebral blood velocity was less in CMS than in controls (P < 0.01) and, at sea level, it decreased more than the controls in response to head-up tilting (P < 0.02). Cerebral autoregulation, assessed from the relationship between cerebral pressure and velocity, was also impaired in CMS patients compared to HA controls, when examined at sea level (P < 0.02). These results have shown that the good orthostatic tolerance seen in high altitude dwellers at altitude is also seen at sea level. There was no difference in orthostatic tolerance between CMS patients, with their exceptionally large blood volumes, and the HA controls. This may be because peripheral vascular and cerebrovascular responses (at least at sea level) are impaired in the CMS patients relative to HA controls. Thus, the advantage of the large blood volume may be offset by the smaller vascular responses.

High-salt diet and responses of the pressurized mesenteric artery of the dog to noradrenaline and acetylcholine

A high-salt diet in rats has been shown to result in enhanced vasoconstrictor and/or reduced vasodilator responses of isolated arteries to agonists. The present experiments were designed to investigate the effects of dietary salt on the responses of the pressurized mesenteric resistance artery of the dog to constrictor and dilator agents. Dogs were fed diets containing three different levels of salt with sodium concentrations (in mmol/kg per day) of 0.4 (low salt; LS), 3.0 (intermediate salt; IS) and 6.0 (high salt; HS) for a period of 4 weeks. At the end of the feeding period, animals were killed and lengths of third-order mesenteric artery were obtained and mounted in a perfusion myograph and changes in internal diameter were measured using a microscope and video-tracking device. The responses to noradrenaline (NA), acetylcholine (ACh) and sodium nitroprusside (SNP) were then determined. The vasoconstrictor responses to NA were identical in the three groups. However, the relaxation response of the vessels to ACh was attenuated in HS dogs compared with LS dogs (P < 0.05), but not with IS dogs. The application of N(G)-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthase, reduced the relaxation responses to ACh comparably in all three groups. The relaxation responses of the vessels to SNP were similar in all groups. These results indicate that, in the dog mesenteric resistance artery, a high-salt diet does not affect vasoconstrictor responses to NA, but does attenuate the vasorelaxant action of ACh, largely by inhibiting the production of endothelium-derived relaxing factor.

Orthostatic tolerance and blood volumes in Andean high altitude dwellers

Orthostatic tolerance is a measure of the ability to prevent hypotension during gravitational stress. It is known to be dependent on the degree of vasoconstriction and the magnitude of plasma volume, but the possible influence of packed cell volume (PCV) is unknown. High altitude residents have high haematocrits and probably high packed cell volumes. However, it is not known whether plasma volume and blood volume are affected, or whether their orthostatic tolerance is different from low altitude residents. In this study we determined plasma volume, PCV and orthostatic tolerance in a group of high altitude dwellers (HA), including a subgroup of highland dwellers with chronic mountain sickness (CMS) and extreme polycythaemia. Plasma volume and PCV were determined using Evans Blue dye dilution and peripheral haematocrit. Orthostatic tolerance was assessed as the time to presyncope in a test of head-up tilting and lower body suction. All studies were performed at 4338 m. Results showed that plasma volumes were not significantly different between CMS and HA, or in highland dwellers compared to those seen previously in lowlanders. PCV and haematocrit were greater in CMS than in HA. Orthostatic tolerance was high in both CMS and HA, although the heart rate responses to orthostasis were smaller in CMS than HA. Orthostatic tolerance was correlated with haematocrit (r= 0.57, P < 0.01) and PCV (r= 0.54, P < 0.01). This investigation has shown that although high altitude residents have large PCV, their plasma volumes were similar to lowland dwellers. The group with CMS have a particularly large PCV and also have a very high orthostatic tolerance, despite smaller heart rate responses. These results are compatible with the view that PCV is of importance in determining orthostatic tolerance.

Effects of simulated obstructive sleep apnoea on the human carotid baroreceptor-vascular resistance reflex

Obstructive sleep apnoea (OSA), which is characterized by periodic inspiratory obstruction, is associated with hypertension and possibly with changes in the baroreceptor reflex. In this investigation we induced changes in inspiratory resistance and in inspiratory oxygen and carbon dioxide content, which simulate some of the changes in OSA, to determine whether this caused changes in the gain or setting of the carotid baroreflex. In eight healthy subjects (aged 21-62 years) we changed the stimulus to carotid baroreceptors, using neck chambers and graded pressures of -40 to +60 mmHg, and assessed vascular resistance responses in the brachial artery from changes in blood pressure (Finapres) divided by brachial artery blood flow velocity (Doppler ultrasound). Stimulus-response curves were defined during (a) sham (no additional stimulus), (b) addition of an inspiratory resistance (inspiratory pressure -10 mmHg), (c) breathing asphyxic gas (12% O(2), 5% CO(2)), and (d) combined resistance and asphyxia. Sigmoid or polynomial functions were applied to the curves and maximum differentials (equivalent to peak gain) and the corresponding carotid pressures (equivalent to 'set point') were determined. The sham test had no effect on either gain or 'set point'. Inspiratory resistance alone had no effect on blood pressure and did not displace the curve. However, it reduced gain from -3.0 +/- 0.6 to -2.1 +/- 0.4 units (P < 0.05). Asphyxia alone did increase blood pressure (+7.0 +/- 1.1 mmHg, P < 0.0005) and displaced the curve to higher pressures by +16.8 +/- 2.1 mmHg (P < 0.0005). However, it did not affect gain. The combination of resistance and asphyxia both reduced gain and displaced the curve to higher pressures. These results suggest that inspiratory resistance and asphyxia cause changes in the baroreceptor reflex which could lead to an increase in blood pressure. These changes, if sustained, could provide a mechanism linking hypertension to obstructive sleep apnoea.

Cardiovascular regulation in the period preceding vasovagal syncope in conscious humans

To study cardiovascular control in the period leading to vasovagal syncope we monitored beat-to-beat blood pressure, heart rate (HR) and forearm blood flow in 14 patients with posturally related syncope, from supine through to tilt-induced pre-syncope. Signals of arterial blood pressure (BP) from a Finapres photoplethysmograph and an electrocardiograph (ECG) were fed into a NeuroScope system for continuous analysis. Non-invasive indices of cardiac vagal tone (CVT) and cardiac sensitivity to baroreflex (CSB) were derived on a beat-to-beat basis from these data. Brachial vascular resistance (VR) was assessed intermittently from brachial blood flow velocity (Doppler ultrasound) divided by mean arterial pressure (MAP). Patients underwent a progressive orthostatic stress test, which continued to pre-syncope and consisted of 20 min head-up tilt (HUT) at 60 deg, 10 min combined HUT and lower body suction (LBNP) at -20 mmHg followed by LBNP at -40 mmHg. Pre-syncope was defined as a fall in BP to below 80 mmHg systolic accompanied by symptoms. Baseline supine values were: MAP (means +/- S.E.M.) 84.9 +/- 3.2 mmHg; HR, 63.9 +/- 3.2 beats min-1; CVT, 10.8 +/- 2.6 (arbitrary units) and CSB, 8.2 +/- 1.6 ms mmHg-1. HUT alone provoked pre-syncope in 30 % of the patients whilst the remaining 70 % required LBNP. The cardiovascular responses leading to pre-syncope can be described in four phases. Phase 1, full compensation: where VR increased by 70.9 +/- 0.9 %, MAP was 89.2 +/- 3.8 mmHg and HR was 74.8 +/- 3.2 beats min-1 but CVT decreased to 3.5 +/- 0.5 units and CSB to 2.7 +/- 0.4 ms mmHg-1. Phase 2, tachycardia: a progressive increase in heart rate peaking at 104.2 +/- 5.1 beats min-1. Phase 3, instability: characterised by oscillations in BP and also often in HR; CVT and CSB also decreased to their lowest levels. Phase 4, pre-syncope: characterised by sudden decreases in arterial blood pressure and heart rate associated with intensification of the symptoms of pre-syncope. This study has given a clearer picture of the cardiovascular events leading up to pre-syncope. However, the mechanisms behind what causes a fully compensated system suddenly to become unstable remain unknown.

Syncope: what is the trigger?

Although a syncopal attack is frequently preceded by prodromal symptoms, sometimes the onset can be so abrupt that there is no warning at all. The switch in autonomic responses responsible for such an attack is quite rapid and dramatic, but the trigger for this remains one of the unresolved mysteries in cardiovascular physiology.

Effects of head-up tilting on baroreceptor control in subjects with different tolerances to orthostatic stress

During orthostatic stress, an increase in peripheral vascular resistance normally results in arterial blood pressure being well maintained, despite a decrease in cardiac output. The present study was undertaken to determine whether the sensitivity of the carotid baroreceptor reflex was increased during orthostatic stress and whether failure to develop this increase was associated with poor orthostatic tolerance. Three groups of subjects were studied: asymptomatic controls; patients investigated for suspected posturally related syncope but who had normal responses to an orthostatic stress test (normal patients); and patients who were shown to have low orthostatic tolerance (early fainters). We determined responses of R-R interval and forearm vascular resistance (mean arterial pressure/brachial artery velocity by Doppler ultrasonography) to the loading and unloading of carotid baroreceptors by application of pressures of -30 and +30 mmHg to a chamber fitted over the neck. Responses were determined after 20 min of supine rest and after 10 min of head-up tilt at 60 degrees. Responses of cardiac interval were not significantly different between the three groups, and they were not altered by the postural change. Vascular responses also did not differ between the groups during supine rest. However, in healthy volunteers and in normal patients, responses to both neck suction and pressure were significantly enhanced during head-up tilt. In controls, responses to suction were increased by tilt from 0.04+/-0.1 to -1.01+/-0.2%.mmHg(-1) (means+/-S.E.M.; P<0.001) and those to neck pressure from -0.6+/-0.3 to -3.1+/-1.1%.mmHg(-1) (P<0.05). In the normal patients, the corresponding changes were: during suction, from -0.2+/-0.1 to -0.7+/-0.1%.mmHg(-1) (P<0.05); during pressure, from -0.7+/-0.1 to -1.5+/-0.3%.mmHg(-1) (P<0.05). In contrast, in patients with low orthostatic tolerance, posture had no effect on the reflex (neck suction, from -0.3+/-0.1 to -0.3+/-0.1%.mmHg(-1); neck pressure, from -1.0+/-0.3 to -0.9+/-0.2%.mmHg(-1)). We suggest that an increase in the sensitivity of the carotid baroreceptor/vascular resistance reflex may be important in the maintenance of blood pressure during orthostatic stress, and that failure of this to occur in patients with posturally related syncope may go some way towards explaining their poor orthostatic tolerance.

Change in endothelial function in mesenteric arteries of Sprague-Dawley rats fed a high salt diet

A high salt diet in some species results in elevated arterial blood pressure and alterations in vascular smooth muscle responses to agonists. Weanling male Sprague-Dawley rats were given either a high salt diet containing 8 % or a low salt diet of 0.4 % sodium chloride for a period of 4 weeks. At the end of the feeding period, tail systolic pressure was higher in the high salt than in low salt rats. The rats were then killed and the intestines removed. Vascular smooth muscle (VSM) responses were estimated from the changes in lumenal diameter of pressurised second order mesenteric resistance arteries. High salt diet resulted in enhanced VSM responses to noradrenaline. The vessels dilated in response both to acetylcholine and to sodium nitroprusside and the responses were similar in vessels from both high and low salt rats. However, vessels from high salt rats were resistant to the blocking of endothelium derived nitric oxide (EDNO) with L-NAME and the responses were instead abolished by blocking endothelium derived hyperpolarising factor (EDHF) with apamin and charybdotoxin. These results show that in Sprague-Dawley rats, a high salt diet enhances the vasoconstriction in response to noradrenaline. The vasodilatory responses to acetylcholine were not significantly changed. However, they appeared to be mediated mainly by EDHF rather than by EDNO as in the low salt animals.

Randomized controlled trial of home-based exercise training to evaluate cardiac functional gains

There is evidence that multiple benefits can be obtained through exercise training that leads to increases in peak oxygen consumption (V(O(2))). It is unclear whether significant improvements can also be achieved through unsupervised low-budget home-based training regimes, especially in terms of cardiac functional gains. A randomized cross-over trial was conducted to investigate the effects of a home-based unsupervised exercise training programme of moderate intensity on aerobic capacity, cardiac reserve and peak cardiac power output in healthy middle-aged volunteers. Nine subjects with no known cardiovascular diseases performed symptom-limited treadmill cardiopulmonary exercise tests after an 8-week period of exercise training, and results were compared with those obtained after a similar 'non-exercising' control period. Cardiac output was measured non-invasively during exercise tests using the CO(2)-rebreathing method. With exercise training, resting heart rate decreased significantly from 88.3+/-3.4 to 78.7+/-3.2 beats.min(-1) (P<0.05), heart rate at a submaximal workload (V(O(2))=1.5 litres.min(-1)) decreased from 125.5+/-2.4 to 115.5+/-1.6 beats.min(-1), and peak V(O(2)) increased by 9% from 2.62+/-0.19 to 2.85+/-0.18 litres.min(-1) (P<0.01). Baseline cardiac power output was 1.11+/-0.05 W, and this remained unchanged with training. Peak cardiac power output increased by 16% from 4.1+/-0.3 to 4.7+/-0.3 W (P<0.001), and cardiac reserve increased by 21% (P<0.01). A major contribution to these increases was from the 11% increase in stroke volume, from 100.1+/-5.3 to 111.2+/-6.2 ml (P<0.001). All subjects reported more positive perceptions of their health (P<0.05), fitness (P<0.01) and levels of activity (P<0.01) after the training period. These results show that motivated subjects undergoing low-budget unsupervised home-based exercise training of moderate intensity can derive benefit in terms of symptoms, aerobic capacity and cardiac functional reserve.

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.

Responses to stimulation of coronary and carotid baroreceptors and the coronary chemoreflex at different ventricular distending pressures in anaesthetised dogs

Stimulation of left ventricular mechanoreceptors was believed not only to exert important effects on the circulation, but also to influence the responses to baroreceptor reflexes. However, most previous work is flawed due to inadequate localisation of stimuli to specific reflexogenic areas. In this study, we applied a discrete stimulus to left ventricular mechanoreceptors to examine other reflexes known to effect the circulation. Dogs were anaesthetised, artificially ventilated and a cardiopulmonary bypass established. The pressure distending the left ventricle was controlled through an apical cannula with the aortic valve obstructed by a balloon. Changes in ventricular systolic and end-diastolic pressure had only a small effect on vascular resistance, assessed as perfusion pressure in the systemic circulation (flow constant). Responses to changes in carotid or coronary pressure or to stimulation of chemosensitive afferents by injecting veratridine into the coronary circulation were always much larger. Responses to stimulation of these reflexes were little affected by the level of stimulus to the ventricular receptors. These experiments confirm that responses to stimulation of ventricular mechanoreceptors are very small and show that they remain small at different levels of input to other baroreceptive regions. There was no evidence of interaction between ventricular mechanoreceptor reflexes and carotid or coronary baroreceptors or ventricular chemosensitive reflexes.

Reflex vascular responses to independent changes in left ventricular end-diastolic and peak systolic pressures and inotropic state in anaesthetised dogs

1. Ventricular mechanoreceptors are known to exist and can when stimulated induce reflex vasodilatation, but the nature of the effective stimuli and the physiological role of the reflex remain to be established. 2. Dogs were anaesthetised with chloralose and a cardiopulmonary bypass established. Ventricular pressures were separated from those in the aortic root and coronary arteries by a balloon inflated in the ventricular outflow tract. Ventricular filling was controlled by adjusting the rate of inflow of blood through an apical cannula and peak pressure by regulating the outflow pressure from the same cannula. Carotid and aortic pressures were also controlled and vascular resistance was assessed from changes in perfusion pressure (constant flow conditions) to the descending abdominal aorta. 3. Increased coronary or carotid sinus pressure induced a significant vasodilatation. Changes in ventricular peak systolic pressure, without associated changes in end-diastolic pressure, had no significant effect on vascular resistance. In contrast, changes in end-diastolic pressure did induce vasodilatation that, although small, was proportional to the magnitude of the end-diastolic pressure change. 4. Changes in ventricular inotropic state induced by dobutamine infusion or by stimulation of efferent cardiac sympathetic nerves did not induce significant responses. Furthermore, the combined effects of reduced ventricular filling and increased inotropic state were also ineffective in inducing responses. 5. We conclude that, to induce reflex responses, the only effective stimulus to ventricular mechanoreceptors was an increase in filling. Compared with other mechanoreflexes, however, responses to ventricular distension were small and seem unlikely to be of importance except perhaps during abnormal ventricular distension.

Spectral and cross-spectral autoregressive analysis of cardiovascular variables in subjects with different degrees of orthostatic tolerance

The mechanisms leading to vasovagal syncope are still unclear. A simple discriminating test for the identification of syncope-prone subjects is not presently available. Fifty-two subjects had a stepwise orthostatic test with 60 degrees tilt and -20 and -40 mm Hg lower-body negative pressure before the appearance of impending syncope symptoms. Spectral and cross-spectral analyses of heart period and systolic pressure time series were performed to estimate the power of the high-frequency (approximately equals 0.25 Hz) and low-frequency (approximately equals 0.1 Hz) oscillations, the coherence between heart period and systolic pressure, and the mean low-frequency and high-frequency central frequency, phase shift, and transfer function at maximal coherence. According to time to presyncope, the 52 subjects were divided into two groups: 25 with normal orthostatic tolerance, and 27 with poor orthostatic tolerance. In the supine positions, the mean central low-frequency was significantly lower in poor-tolerance group than in normal-tolerance group, discriminating poor from normal orthostatic tolerance with 80% specificity and 83% sensitivity, and was significantly correlated to time to presyncope. In the 2 to 3 minutes preceding syncope, subjects with poor orthostatic tolerance had less tachycardia, lower low-frequency power of systolic pressure, higher respiratory frequency, and a less negative phase shift in high-frequency range. In presyncope, sympathetic activation is reduced in subjects with poor orthostatic tolerance. In addition, the higher breathing frequency and the smaller negativity of phase shift in high-frequency range, which may indicate an inadequate engagement of the baroreflex, suggest a causal role of respiration in the development of syncope. Supine central values of low frequency may be proposed as a valuable clinical index of orthostatic intolerance.

Reflex effects of independent stimulation of coronary and left ventricular mechanoreceptors in anaesthetised dogs

Previous studies which have indicated that the stimulation of ventricular mechanoreceptors induces significant reflex responses can be criticised because of the likelihood of concomitant stimulation of coronary arterial baroreceptors. We therefore undertook this investigation to examine the coronary and ventricular mechanoreflexes in a preparation in which the pressure stimuli to each region were effectively separated. Dogs were anaesthetised, artificially ventilated and placed on cardiopulmonary bypass. A balloon at the ventricular outflow separated pressure in the left ventricle from that perfusing the coronary arteries. Ventricular pressures were changed by varying inflow and outflow of blood entering and leaving the ventricle through an apical cannula, and coronary pressure by changing pressure in a reservoir connected to a cannula tied in the aortic root. Pressures distending carotid and aortic baroreceptors were controlled. Changes in descending aortic perfusion pressure (flow constant) were used to assess systemic vascular responses. Large changes in carotid sinus and coronary pressures decreased vascular resistance by 35+/-1.9 and 40+/-2.5%, respectively. Intracoronary injections of veratridine (30-60 microg) decreased vascular resistance by 31+/-2.5%. However, large increases in ventricular pressure decreased resistance by only 9+/-2.2%. Significant changes in vascular resistance were obtained with increases in coronary arterial pressure from 60 to 90 mmHg. However, ventricular pressures had to increase to 152/18 mmHg (systolic/end-diastolic) before there was a significant response. These results show that coronary mechanoreceptors are likely to play an important role in cardiovascular control. If ventricular receptors have any function at all, it is as a protective mechanism during gross distension, possibly associated with myocardial ischaemia.

Vascular responses to orthostatic stress in patients with postural tachycardia syndrome (POTS), in patients with low orthostatic tolerance, and in asymptomatic controls

Patients with postural tachycardia syndrome (POTS) are characterized by development of symptoms of orthostatic intolerance during standing that are not because of hypotension but are associated with tachycardia. The goal of this study was to compare the cardiac and vascular responses to orthostatic stress for patients with POTS (n = 8) with those for patients with low orthostatic tolerance (n = 29) and for healthy control subjects (n = 12). Responses of heart rate (ECG), arterial blood pressure, and brachial artery blood velocity (Doppler) were determined during a progressive orthostatic stress test of head-up tilt and lower body suction. Changes in forearm vascular resistance (mean arterial pressure/brachial velocity) were less for patients with POTS than for healthy persons, and also less than for most of the patients with low orthostatic tolerance. However, patients with POTS did not have a low tolerance to orthostatic stress (measured as time to discontinuation of the test). For the patients with POTS, the test was discontinued often because of symptoms associated with tachycardia but not hypotension, whereas for the other two groups, the test was discontinued because of hypotension. This study shows that for patients with POTS, abnormal sympathetic responses exist, with an increased sympathetic drive to the heart but deficient peripheral vascular responses, as measured in the forearm. This supports the hypothesis that patients with POTS may have a selective peripheral neuropathy, with small responses in some regions being compensated by overactivity in other regions.

Absence of reflex vascular responses from the intrapulmonary circulation in anaesthetised dogs

The aim of this investigation was to determine whether reflex cardiovascular responses were obtained to localised distension of the intrapulmonary arterial and venous circulations in a preparation in which the stimuli to other major reflexogenic areas were controlled and the lung was shown to possess reflex activity. Dogs were anaesthetised with -chloralose, artificially ventilated, the chests widely opened and a cardiopulmonary bypass established. The intrapulmonary region of the left lung was isolated and perfused through the left pulmonary artery and drained through cannulae in the left pulmonary veins via a Starling resistance. Intrapulmonary arterial and venous pressures were controlled by the rate of inflow of blood and the pressure applied to the Starling resistance. Pressures to the carotid, aortic and coronary baroreceptors and heart chambers were controlled. Responses of vascular resistance were assessed from changes in perfusion pressures to a vascularly isolated hind limb and to the remainder of the subdiaphragmatic circulation (flows constant). The reactivity of the preparation was demonstrated by observing decreases in vascular resistance to large step changes in carotid sinus pressure (systemic vascular resistance decreased by -40 +/- 5%), chemical stimulation of lung receptors by injection into the pulmonary circulation of veratridine or capsaicin (resistance decreased by -32 +/- 4%) and, in the four dogs tested, increasing pulmonary stroke volume to 450 ml (resistance decreased by -24 +/- 6%). However, despite this evidence that the lung was innervated, increases in intrapulmonary arterial pressure from 14 +/- 1 to 43 +/- 3 mmHg or in intrapulmonary venous pressure from 5 +/- 2 to 34 +/- 2 mmHg or both did not result in any consistent changes in systemic or limb vascular resistances. In two animals tested, however, there were marked decreases in efferent phrenic nerve activity. These results indicate that increases in pressure confined to the intrapulmonary arterial and venous circulations do not cause consistent reflex vascular responses, even though the preparation was shown to be reflexly active and the lung was shown to be innervated.

Reflex responses from the main pulmonary artery and bifurcation in anaesthetised dogs

This study was undertaken to determine the reflex cardiovascular and respiratory responses to discrete stimulation of pulmonary arterial baroreceptors using a preparation in which secondary modulation of responses from other reflexes was prevented. Dogs were anaesthetised with -chloralose, artificially ventilated, the chests widely opened and a cardiopulmonary bypass established. The main pulmonary arterial trunk, bifurcation and extrapulmonary arteries as far as the first lobar arteries on each side were vascularly isolated and perfused through the left pulmonary artery and drained via the right artery through a Starling resistance which controlled pulmonary arterial pressure. Pressures distending systemic baroreceptors and reflexogenic regions in the heart were controlled. Reflex vascular responses were assessed from changes in perfusion pressures to a vascularly isolated hind limb and to the remainder of the subdiaphragmatic systemic circulation, both of which were perfused at constant flows. Respiratory responses were assessed from recordings of efferent phrenic nerve activity. Increases in pulmonary arterial pressure consistently evoked increases in both perfusion pressures and in phrenic nerve activity. Both vascular and respiratory responses were obtained when pulmonary arterial pressure was increased to above about 30 mmHg. Responses increased at higher levels of pulmonary arterial pressures. In 13 dogs increases in pulmonary arterial pressure to 45 mmHg increased systemic perfusion pressure by 24 +/- 7 mmHg (mean +/- S.E.M.) from 162 +/- 11 mmHg. Setting carotid sinus pressure at different levels did not influence the vascular response to changes in pulmonary arterial pressure. The presence of a negative intrathoracic pressure of -20 mmHg resulted in larger vascular responses being obtained at lower levels of pulmonary arterial pressure. This indicates that the reflex may be more effective in the intact closed-chest animal. These results demonstrate that stimulation of pulmonary arterial baroreceptors evokes a pressor reflex and augments respiratory drive. This reflex is likely to be elicited in circumstances where pulmonary arterial pressure increases and the negative excursions of intrathoracic pressure become greater. They are likely, therefore, to be involved in the cardio-respiratory response to exercise as well as in pathological states such as pulmonary hypertension or restrictive or obstructive lung disease.

Forearm vascular responses during orthostatic stress in control subjects and patients with posturally related syncope

The aim of this study was to compare the changes in forearm vascular resistance that occurred during orthostatic stress in asymptomatic volunteer subjects with those in patients with posturally related syncope. The authors hoped firstly that it would indicate the importance of vasoconstriction in the maintenance of blood pressure, and secondly that it might have diagnostic value if there were differences between symptomatic patients and asymptomatic volunteers. Twelve volunteers and 67 patients with unexplained syncope were classified as early or late fainters, based on their endurance of a test of combined head-up tilting and lower-body suction. Responses of vascular resistance were assessed from the ratio of arterial blood pressure (Finapres) to brachial artery blood velocity (Doppler). Changes in vascular resistance were greater in volunteers at all stages of the procedure than in patients. There was, however, no significant difference between the responses of early and late-fainting volunteers. These results demonstrate the importance of vasoconstriction in the resistance to posturally related syncope, and they indicate that assessments of responses of vascular resistance may improve the accuracy of the diagnosis.

Assessment of capillary fluid shifts during orthostatic stress in normal subjects and subjects with orthostatic intolerance

Orthostatic stress causes, in addition to venous pooling, a loss of plasma fluid from capillaries to the dependent tissues. The rate of this loss may be one of the factors determining orthostatic tolerance. In this study we assessed the use of a multichannel impedance plethysmograph for determining changes in volume in the calf, thigh, and abdominal segments, in asymptomatic volunteers and in patients shown to have poor tolerance to orthostatic stress. Impedance plethysmography showed, for leg segments, that following head-up tilt there was an initial rapid change in volume followed after 2 to 4 minutes by an almost linear change. Results from the abdominal segment were more variable. The rate of change of leg (thigh + calf) volume was significantly correlated with the estimated loss of plasma volume derived from the changes in the concentration of plasma protein, using Evans Blue dye as the marker. Comparison of results of leg filtration rates between patients and volunteers indicated that some of the patients had abnormally high filtration rates and suggests that impedance plethysmography may have a role in assessing the possible reasons for orthostatic intolerance.

Effects of moderate exercise training on plasma volume, baroreceptor sensitivity and orthostatic tolerance in healthy subjects

The effect of physical training on an individual's ability to withstand an orthostatic stress is unclear. This study was undertaken to determine the effects on orthostatic tolerance in healthy volunteers of training at a level appropriate for unfit subjects and cardiorespiratory patients. In 11 asymptomatic, untrained subjects the following assessments were made: plasma volume by Evans Blue dye dilution and blood volume derived from haematocrit; carotid baroreceptor sensitivity from the slope of the regression of change in cardiac interval against pressure applied to a neck chamber; orthostatic tolerance as time to presyncope in a test of head-up tilting combined with lower body suction; exercise test relating heart rate to oxygen consumption. Subjects were then given a training schedule (5BX/XBX, Royal Canadian Air Force) involving 11-12 min of mixed exercises per day until an age/sex related 'target' was reached. Following training all subjects showed evidence of improved fitness, seen as decreases in heart rate at an oxygen uptake (Vo2) of 1.5 1 min-1 and in the elevation of the regression line between heart rate and Vo2. All also had increases in plasma and blood volumes and decreases in baroreceptor sensitivity. Seven of the subjects showed increases in orthostatic tolerance. Improvement in orthostatic tolerance was related to a low initial tolerance, and was associated with increases in plasma volume and decreases in baroreceptor sensitivity. These results show that moderate exercise training increases orthostatic tolerance in subjects who do not already have a high initial tolerance and suggest that training may be of value in the management of untrained patients with attacks of syncope due to orthostatic intolerance.

Increased orthostatic tolerance following moderate exercise training in patients with unexplained syncope

OBJECTIVE

To determine whether a programme of simple, moderate exercise training increases blood volume and improves orthostatic tolerance in patients with attacks of syncope or near syncope related to orthostatic stress.

DESIGN

An open study in 14 patients referred with unexplained attacks of syncope, who were shown to have a low tolerance to an orthostatic stress test. Measurements were made of plasma and blood volumes, orthostatic tolerance to a test of combined head up tilt and lower body suction, and baroreceptor sensitivity by applying subatmospheric pressures to a chamber over the neck. Cardiorespiratory fitness was assessed from the relation between heart rate and oxygen uptake during a graded treadmill exercise test. Assessments were made before and after undertaking an exercise training programme (Canadian Air Force 5BX/XBX).

RESULTS

After the training period, 12 of the 14 patients showed evidence of improved cardiorespiratory fitness. All 12 patients were symptomatically improved; they showed increases in plasma and blood volumes and in orthostatic tolerance, and decreases in baroreceptor sensitivity. Despite the improved orthostatic tolerance, values of blood pressure both while supine and initially following tilting were lower than before training.

CONCLUSIONS

Exercise training has a role in the management of patients with syncope and poor orthostatic tolerance. It improves symptoms and increases orthostatic tolerance without increasing resting blood pressure.

Absence of early resetting of coronary baroreceptors in anaesthetized dogs

1. Both carotid and aortic arch baroreceptors have been shown to reset after as little as 20 min exposure to a different conditioning pressure; the mid-point of the stimulus-response curve is displaced towards the conditioning pressure. 2. Coronary baroreceptors operate over much lower pressures and induce slower reflex vasoconstriction than the other baroreceptors and this investigation was designed to determine whether their resetting characteristics are also different. 3. In chloralose anaesthetized dogs, a perfusion circuit allowed independent control of pressures distending carotid, aortic and coronary baroreceptors. Stimulus-response curves were obtained for carotid and coronary baroreceptors after maintaining the distending pressure at 60 or 180 mmHg for 20 min. 4. Neither the magnitude of the responses nor the baroreceptor pressure corresponding to 50 % of the response (BP50) of the coronary curves was changed by the conditioning regime. In contrast, conditioning carotid baroreceptors with the same regime produced significant shifts in the BP50 towards the conditioning pressure. 5. No changes were obtained after conditioning the coronary baroreceptors at 60 or 120 mmHg for 40 min. 6. These results confirm early resetting of carotid baroreceptors but show that coronary baroreceptors do not reset over a period of at least 40 min.

Reflex control of splanchnic blood volume in anaesthetized dogs

1. In chloralose-anaesthetized, artificially ventilated dogs, the splenic pedicle was tied and the carotid sinuses were vascularly isolated and perfused at controlled pressures. In Series 1 experiments, the hepatosplanchnic circulation was perfused through the abdominal aorta with a tie on the aorta separating it from the caudal circulation, which was perfused through the femoral arteries. The two circulations were drained from cannulae in the inferior vena cava and the femoral veins, with a tie on the inferior vena cava separating the two. In Series 2, the splanchnic circulation drained from the portal vein. In both series, inflows and outflows were measured and integrated to derive volume changes. Capacitance responses were assessed during constant flow, and capacitance plus passive responses were obtained during constant pressure perfusion. 2. In Series 1, an increase in carotid sinus pressure (from 8 to 26 kPa) during constant flow and constant pressure perfusion increased hepatosplanchnic volume by 2.5 and 5.7 ml (kg body weight)-1, respectively. The volume of the subdiaphragmatic circulation did not increase during constant flow, but during constant pressure it increased by 2.0 ml (kg body weight)-1. 3. In Series 2, increasing carotid pressure during constant flow and constant pressure increased the volume of the splanchnic circulation by 0.5 and 4.2 ml (kg body weight)-1, respectively. 4. These results confirm that carotid baroreceptor stimulation causes larger volume changes during constant pressure perfusion than during constant flow perfusion. Also, the active capacitance change in the splanchnic circulation is small in relation to the passive response. We propose that in dogs (following splenic ligation), the major active capacitance control is from the liver. However, large passive changes in splanchnic volume occur due to changes in flow.

Reflex vascular responses in the anesthetized dog to large rapid changes in carotid sinus pressure

This study examined reflex vascular responses to large rapid increases and decreases in carotid sinus pressure to determine whether delayed or inappropriate vascular responses might be obtained that, if they occurred in people, could lead to hypotension during exposure to rapidly alternating gravitational forces. In chloralose-anesthetized open-chest dogs, a perfusion circuit controlled carotid sinus and thoracic aortic pressures and blood flows to both the vascularly isolated abdominal circulation and a hindlimb (perfusion pressure changes denoted resistance). When carotid pressure was increased and decreased over the range of 60-180 mmHg, the resulting reflex vasodilatation occurred significantly more rapidly than the vasoconstriction (P < 0.001). In the abdominal vascular bed, time constants for vasodilatation and vasoconstriction were 4.2 +/- 0.5 and 7.5 +/- 1.0 s, respectively. Decreases in carotid pressure in pulses of 10-s duration or less failed to elicit maximal vasoconstriction, whereas increases in carotid pressure lasting as little as 5 s did elicit maximal vasodilatation. "Square-wave" alternations in carotid pressure with periods of 10 s or less (5 s high, 5 s low) resulted in attenuation of the vasoconstriction, and at a 4-s period, both vascular beds remained almost maximally vasodilated throughout. The failure of vascular resistance to follow carotid pressure changes was not due to a failure of the response of sympathetic efferent activity, since the time constants for the reduction and increase in discharge were much shorter at 0.56 +/- 0.13 and 0.43 +/- 0.10 s, respectively. These results indicate that rapid changes in carotid pressure could result in inappropriate vasodilatation and hypotension and might, in some circumstances, such as in pilots flying high-performance aircraft, predispose to syncope.

Early effects of oral salt on plasma volume, orthostatic tolerance, and baroreceptor sensitivity in patients with syncope

Salt loading of patients with orthostatic-related syncope has been shown to increase plasma volume and orthostatic tolerance and to decrease baroreceptor sensitivity. This study aims to define the time course of these changes and reveal indications as to the likely mechanisms involved. We measured plasma volume by Evans blue dilution, orthostatic tolerance by time to presyncope in a test of combined head-up tilt and lower body suction, and baroreceptor sensitivity as the cardiac interval prolongation to neck suction. Tests were carried out in patients with poor orthostatic tolerance and low initial urinary salt excretion, before and after 7 or 3 days of salt loading (120 mmol/d). In both groups after salt loading plasma volume and orthostatic tolerance increased and baroreceptor sensitivity decreased. These results confirm the beneficial effects of salt in some patients and show that all these changes occur within 3 days.

Blood mobilization from the liver of the anaesthetized dog

The abdominal circulation contains a high proportion of the total blood volume and this can change either passively in response to changes in vascular distending pressure or actively (termed a capacitance response) to changes in sympathetic nervous activity. The liver is the largest abdominal organ and this study was designed to evaluate its potential contribution to overall vascular capacitance and compliance. In chloralose anaesthetized dogs, the liver was vascularly isolated, perfused through the portal vein and hepatic artery at either constant pressures or constant flows and drained from the hepatic veins at constant pressure. Changes in vascular resistance were assessed from changes in inflow pressures or flows and hepatic blood volume was determined by differences between net inflow and outflow. During constant flow perfusion the change in hepatic volume (capacitance change) in response to supramaximal stimulation of sympathetic nerves at 16 Hz was (mean +/- S.E.M.) -2.40 +/- 0.61 ml (kg body weight)-1. This response was not significantly different during constant pressure perfusion. The changes in portal venous and hepatic arterial pressures during stimulation at constant flow perfusion were +0.67 +/- 0.13 and +4.92 +/- 0.67 kPa, respectively. The compliance of the liver, assessed as the change in volume to a change in hepatic venous pressure, was +5.44 +/- 0.18 ml kg-1 kPa-1. These results indicate that the liver has a major capacitance role, comparable to that of the canine spleen and, in addition, is highly compliant. No evidence was found to suggest that a sphincter on the hepatic outflow exists. Assuming similar responses occur in humans, who do not possess a large contractile spleen, the liver would be the most important controllable blood reservoir in the body.

Physiological cardiac reserve: development of a non-invasive method and first estimates in man

OBJECTIVE

To investigate whether physiological cardiac reserve can be measured in man without invasive procedures and whether it is a major determinant of exercise capacity.

DESIGN

Development of method of measurement and an observational study.

SETTING

A regional cardiothoracic centre.

SUBJECTS

70 subjects with a wide range of cardiac function, from heart failure patients to athletes.

METHODS

Subjects underwent treadmill, symptom limited cardiopulmonary exercise tests to measure aerobic exercise capacity (represented by VO2max) and cardiac reserve. Cardiac output was measured non-invasively using the CO2 rebreathing technique.

RESULTS

Cardiac power output (CPOmax) at peak exercise was found to be significantly related to aerobic capacity: CPOmax (W) = 0.35 + 1.5 VO2max (1/min), r = 0.87, p < 0.001. It also correlated well with exercise duration (r = 0.62, p < 0.001), suggesting that cardiac reserve is a major determinant of exercise capacity. In the study, cardiac reserve ranged from 0.27 to 5.65 W, indicating a 20-fold difference between the most impaired cardiac function and that of the fittest subject.

CONCLUSIONS

A non-invasive method of estimating physiological cardiac reserve was developed. The reserve was found to be a major determinant of exercise capacity in a population of normal subjects and patients with heart disease. This method may thus be used to provide a clearer definition of the extent of cardiac impairment in patients with heart failure.

Reflex vascular responses to alterations in abdominal arterial pressure and flow in anaesthetized dogs

The existence of abdominal arterial baroreceptors has long been controversial. Previously difficulties have been encountered in localizing a stimulus to abdominal arteries without affecting reflexogenic areas elsewhere. In these experiments, using anaesthetized dogs, the abdomen was vascularly isolated at the level of the diaphragm, perfused through the aorta, and drained from the inferior vena cava to a reservoir. Changes in abdominal arterial pressure were effected by changing the perfusion pump speed. During this procedure the flow back to the animal from the venous outflow reservoir was held constant. Increases and decreases in abdominal arterial pressure resulted, respectively, in decreases and increases in perfusion pressure to a vascularly isolated hind-limb and in some dogs also a forelimb. Responses were significantly larger when carotid sinus pressure was high (120-180 mmHg) than when it was low (60 mmHg). Responses were still obtained after cutting vagus, phrenic and splanchnic nerves, but were abolished by spinal cord lesion at T12. These experiments provide evidence for the existence of abdominal arterial baroreceptors. The afferent pathway for the reflex vasodilatation appears to run in the spinal cord.

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.