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

Differential contributions of cardiac, coronary and pulmonary artery vagal mechanoreceptors to reflex control of the circulation

Distinct populations of stretch‐sensitive mechanoreceptors attached to myelinated vagal afferents are found in the heart and adjoining coronary and pulmonary circulations. Receptors at atrio‐venous junctions appear to be involved in control of intravascular volume. These atrial receptors influence sympathetic control of the heart and kidney, but contribute little to reflex control of systemic vascular resistance. Baroreceptors at the origins of the coronary circulation elicit reflex vasodilatation, like feedback control from systemic arterial baroreceptors, as well as having characteristics that could contribute to regulation of mean pressure. In contrast, feedback from baroreceptors in the pulmonary artery and bifurcation is excitatory and elicits a pressor response. Elevation of pulmonary arterial pressure resets the vasomotor limb of the systemic arterial baroreflex, which could be relevant for control of sympathetic vasoconstrictor outflow during exercise and other states associated with elevated pulmonary arterial pressure. Ventricular receptors, situated mainly in the inferior posterior wall of the left ventricle, and attached to unmyelinated vagal afferents, are relatively inactive under basal conditions. However, a change to the biochemical environment of cardiac tissue surrounding these receptors elicits a depressor response. Some ventricular receptors respond, modestly, to mechanical distortion. Probably, ventricular receptors contribute little to tonic feedback control; however, reflex bradycardia and hypotension in response to chemical activation may decrease the work of the heart during myocardial ischaemia. Overall, greater awareness of heterogeneous reflex effects originating from cardiac, coronary and pulmonary artery mechanoreceptors is required for a better understanding of integrated neural control of circulatory function and arterial blood pressure.

Reflexes from pulmonary arterial baroreceptors in dogs: interaction with carotid sinus baroreceptors

In contrast to the reflex vasodilatation occurring in response to stimulation of baroreceptors in the aortic arch, carotid sinuses and coronary arteries, stimulation of receptors in the wall of pulmonary arteries results in reflex systemic vasoconstriction. It is rare for interventions to activate only one reflexogenic region, therefore we investigated how these two types of reflexes interact. In anaesthetized dogs connected to cardiopulmonary bypass, reflexogenic areas of the carotid sinuses, aortic arch and coronary arteries and the pulmonary artery were subjected to independently controlled pressures. Systemic perfusion pressure (SPP) measured in the descending aorta (constant flow) provided an index of systemic vascular resistance. In other experiments, sympathetic efferent neural activity was recorded in fibres dissected from the renal nerve (RSNA). Physiological increases in pulmonary arterial pressure (PAP) induced significant increases in SPP (+39.1 ± 10.4 mmHg) and RSNA (+17.6 ± 2.2 impulses s(−1)) whereas increases in carotid sinus pressure (CSP) induced significant decreases in SPP (−42.6 ± 10.8 mmHg) and RSNA (−42.8 ± 18.2 impulses s(−1)) (P < 0.05 for each comparison; paired t test). To examine possible interactions, PAP was changed at different levels of CSP in both studies. With CSP controlled at 124 ± 2 mmHg, the threshold, 'set point' and saturation pressures of the PAP–SPP relationship were higher than those with CSP at 60 ± 1 mmHg; this rightward shift was associated with a significant decrease in the reflex gain. Similarly, increasing CSP produced a rightward shift of the PAP–RSNA relationship, although the effect on reflex gain was inconsistent. Furthermore, the responses to changes in CSP were influenced by setting PAP at different levels; increasing the level of PAP from 5 ± 1 to 33 ± 3 mmHg significantly increased the set point and threshold pressures of the CSP–SPP relationship; the reflex gain was not affected. These results indicate the existence of interaction between pulmonary arterial and carotid sinus baroreceptor reflexes; physiological and pathological states that alter the stimulus to one may alter the reflex responses from the other.

Mechanosensitive cardiac C-fiber response to changes in left ventricular filling, coronary perfusion pressure, hemorrhage, and volume expansion in rats

Left ventricular (LV) end-diastolic pressure (LVEDP) increase due to volume expansion (VExp) enhances mechanosensitive vagal cardiac afferent C-fiber activity (CNFA), thus decreasing renal sympathetic nerve activity (RSNA). Hypotensive hemorrhage (hHem) attenuates RSNA despite decreased LVEDP. We hypothesized that CNFA increases with any change in LVEDP. Coronary perfusion pressure (CPP), supposedly affected in both conditions, might also be a stimulus of CNFA. VExp and hHem were performed in anesthetized male Sprague-Dawley rats while blood pressure, heart rate, and RSNA were measured. Cervical vagotomy abolished RSNA response in both reflex responses. Single-unit CNFA was recorded while LVEDP was changed. Rapid changes (± 4, ±6, ±8 mmHg) were obtained by graded occlusion of the caval vein and descending aorta. Prolonged changes were obtained by VExp and hHem. Furthermore, CNFA was recorded in a modified Langendorff heart while CPP was changed (70, 100, 40 mmHg). Rapid LVEDP changes increased CNFA [caval vein occlusion: +16 ± 3 Hz (approximately +602%); aortic occlusion: +15 ± 3 Hz (approximately +553%); 70 units; P < 0.05]. VExp and hHem ( n = 6) increased CNFA [VExp: +10 ± 4 Hz (approximately +1,033%); hHem: +10 ± 2 Hz (approximately +1,225%); P < 0.05]. An increase in CPP increased CNFA [+2 ± 1 Hz (approximately +225%); P < 0.05], whereas a decrease in CPP decreased CNFA [−0.8 ± 0.4 Hz (approximately −50%); P < 0.05]. All C fibers recorded originated from the LV. CNFA increased with any LVEDP change but changed equidirectionally with CPP. Thus neither LVEDP nor CPP fully accounts directly for afferent C-fiber and reflex sympathetic responses. The intrinsic afferent stimuli and receptive fields accounting for reflex sympathoinhibition still remain cryptic.

Syncope during exercise, documented with continuous blood pressure monitoring during ergometer testing

A 27-year old female had one episode of transient loss of consciousness and several of near-unconsciousness during strenuous exercise and sexual activity. Episodes started with abdominal discomfort or nausea and light headedness. Unconsciousness never exceeded one minute. When trying to stand up, she felt she would lose consciousness again. We performed a bicycle ergometer exercise test, continuously monitoring blood pressure via non-invasive finger photoplethysmography (Finometer, FMS, The Netherlands). Beat-to-beat changes in stroke volume, cardiac output and total peripheral resistance were calculated using Modelflow (FMS, The Netherlands). At a power of 140 W, the patient reported being near exhaustion; shortly after this she reported nausea. She stopped cycling 30 s later, then saw "black spots" and felt an oncoming loss of consciousness. Dismounting the ergometer and squatting provided immediate relief from symptoms. Symptoms during the test were similar to those during previous episodes. The diagnosis was exercise-induced vasovagal reactions. This is the first report that documents the beat-to-beat changes in blood pressure, stroke volume and total peripheral resistance during exercise-induced vasovagal syncope. It illustrates the usefulness of combining exercise testing with continuous non-invasive blood pressure monitoring in the diagnostic work-up of exercise-induced syncope, and shows the therapeutic value of squatting to prevent loss of consciousness in exercise-related vasovagal syncope.