Augmented Cardiopulmonary Baroreflex Sensitivity in Intradialytic Hypertension

Unraveling autonomic cardiovascular control complexity during orthostatic stress: Insights from a mathematical model

Understanding cardiovascular control mediated by the autonomic system remains challenging due to its inherent complexity. Consequently, syndromes such as orthostatic intolerance continue to evoke debates regarding the underlying pathophysiological mechanisms. This study develops a comprehensive mathematical model simulating the control of the sympathetic branch of the cardiovascular system in individuals with normal and abnormal responses to the head-up-tilt test. We recruited four young women: one control, one with vasovagal syncope, one with orthostatic hypertension, and one with orthostatic hypotension, exposing them to an orthostatic head-up tilt test (HUTT) employing non-invasive methods to measure electrocardiography and continuous blood pressure. Our work encompasses a compartmental model formulated using a system of ordinary differential equations. Using heart rate as input, we predict blood pressure, flow, and volume in compartments representing the veins, arteries, heart, and the sympathetic branch of the baroreflex control system. The latter is modulated by high- and low-pressure baroreceptor afferents activated by changes in blood pressure induced by the HUTT. Sensitivity analysis, parameter subset selection, and optimization are employed to estimate patient-specific parameters associated with autonomic performance. The model has seven sensitive and identifiable parameters with significant physiological relevance that can serve as biomarkers for patient classification. Results show that the model can reproduce a spectrum of blood pressure responses successfully, fitting the trajectory displayed by the experimental data. The controller exhibits behavior that emulates the operation of the sympathetic system. These encouraging findings underscore the potential of computational methods in evaluating pathologies associated with autonomic nervous system control, warranting further exploration and novel approaches.

Increased respiratory modulation of cardiovascular control reflects improved blood pressure regulation in pregnancy

Hypertensive pregnancy disorders put the maternal-fetal dyad at risk and are one of the leading causes of morbidity and mortality during pregnancy. Multiple efforts have been made to understand the physiological mechanisms behind changes in blood pressure. Still, to date, no study has focused on analyzing the dynamics of the interactions between the systems involved in blood pressure control. In this work, we aim to address this question by evaluating the phase coherence between different signals using wavelet phase coherence. Electrocardiogram, continuous blood pressure, electrocardiogram-derived respiration, and muscle sympathetic nerve activity signals were obtained from ten normotensive pregnant women, ten normotensive non-pregnant women, and ten pregnant women with preeclampsia during rest and cold pressor test. At rest, normotensive pregnant women showed higher phase coherence in the high-frequency band (0.15-0.4 Hz) between muscle sympathetic nerve activity and the RR interval, blood pressure, and respiration compared to non-pregnant normotensive women. Although normotensive pregnant women showed no phase coherence differences with respect to hypertensive pregnant women at rest, higher phase coherence between the same pairs of variables was found during the cold pressor test. These results suggest that, in addition to the increased sympathetic tone of normotensive pregnant women widely described in the existing literature, there is an increase in cardiac parasympathetic modulation and respiratory-driven modulation of muscle sympathetic nerve activity and blood pressure that could compensate sympathetic increase and make blood pressure control more efficient to maintain it in normal ranges. Moreover, blunted modulation could prevent its buffer effect and produce an increase in blood pressure levels, as observed in the hypertensive women in this study. This initial exploration of cardiorespiratory coupling in pregnancy opens the opportunity to follow up on more in-depth analyses and determine causal influences.

Management of intradialytic hypertension: current evidence and future perspectives

Intradialytic hypertension (IDH), that is, a paradoxical rise in blood pressure (BP) during or immediately after a hemodialysis session, affects approximately 10-15% of the hemodialysis population. It is currently recognized as a phenomenon of major clinical significance as recent studies have shown that BP elevation extends to the whole interdialytic interval and associates with increased cardiovascular and all-cause mortality. The pathophysiology of IDH is complex involving volume and sodium overload, endothelial dysfunction, excess renin-angiotensin-aldosterone system and sympathetic nervous system activation, and other mechanisms. For several years, there was a scarcity of studies regarding IDH treatment; recently, however, several attempts to examine the effect of nonpharmacological and pharmacological measures on BP levels in IDH are made. This review attempts to summarize this latest evidence in the field of management of IDH and discuss areas for future research.

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.

Intradialytic hypertension: epidemiology and pathophysiology of a silent killer

The term intradialytic hypertension (IDH) describes a paradoxical rise in blood pressure (BP) during or immediately after the hemodialysis session. Although it was formerly considered a phenomenon without clinical implications, current evidence suggests that IDH may affect up to 15% of hemodialysis patients and exhibit independent associations with future cardiovascular events and all-cause mortality. Furthermore, during the last decade, several studies have tried to elucidate the complex pathophysiological mechanisms responsible for this phenomenon. Volume overload, intradialytic sodium gain, overactivity of the sympathetic-nervous-system and renin-angiotensin-aldosterone system, endothelial dysfunction and dialysis-related electrolyte disturbances have been proposed to be involved in the pathogenesis of the BP increase during hemodialysis. This review attempts to summarize existing evidence on the epidemiology, pathophysiology and clinical characteristics of the distinct phenomenon of IDH.

Ischemic heart disease cause of intradialytic hypertension in a patient with diabetic nephropathy

A 73-year-old Japanese man was admitted with extreme intradialytic hypertension of four months' duration that was refractory to antihypertensive agents. He had started hemodialysis five years previously because of diabetic nephropathy. Coronary angiography revealed coronary artery disease with significant stenosis of the left main trunk and the right coronary artery, and he underwent a coronary artery bypass graft. Thereafter, the intradialytic hypertension disappeared. Ischemic heart disease appears to be one cause of intradialytic hypertension. <Learning objective: Intradialytic hypertension occurs in some patients with hemodialysis. Such patients are at increased risk for mortality and adverse cardiovascular events. Although the pathophysiological mechanisms remain unknown, ischemic heart disease appears to be one cause of intradialytic hypertension.>.

Cerebral blood flow regulation in end-stage kidney disease

Patients with chronic kidney disease (CKD) and end-stage kidney disease (ESKD) experience an increased risk of cerebrovascular disease and cognitive dysfunction. Hemodialysis (HD), a major modality of renal replacement therapy in ESKD, can cause rapid changes in blood pressure, osmolality, and acid-base balance that collectively present a unique stress to the cerebral vasculature. This review presents an update regarding cerebral blood flow (CBF) regulation in CKD and ESKD and how the maintenance of cerebral oxygenation may be compromised during HD. Patients with ESKD exhibit decreased cerebral oxygen delivery due to anemia, despite cerebral hyperperfusion at rest. Cerebral oxygenation further declines during HD due to reductions in CBF, and this may induce cerebral ischemia or "stunning." Intradialytic reductions in CBF are driven by decreases in cerebral perfusion pressure that may be partially opposed by bicarbonate shifts during dialysis. Intradialytic reductions in CBF have been related to several variables that are routinely measured in clinical practice including ultrafiltration rate and blood pressure. However, the role of compensatory cerebrovascular regulatory mechanisms during HD remains relatively unexplored. In particular, cerebral autoregulation can oppose reductions in CBF driven by reductions in systemic blood pressure, while cerebrovascular reactivity to CO₂ may attenuate intradialytic reductions in CBF through promoting cerebral vasodilation. However, whether these mechanisms are effective in ESKD and during HD remain relatively unexplored. Important areas for future work include investigating potential alterations in cerebrovascular regulation in CKD and ESKD and how key regulatory mechanisms are engaged and integrated during HD to modulate intradialytic declines in CBF.