Circadian variations of renal sodium handling in patients with orthostatic hypotension

Salt supplementation in the management of orthostatic intolerance: Vasovagal syncope and postural orthostatic tachycardia syndrome

Salt supplementation is a common non-pharmacological approach to the management of recurrent orthostatic syncope or presyncope, particularly for patients with vasovagal syncope (VVS) or postural orthostatic tachycardia syndrome (POTS), although there is limited consensus on the optimal dosage, formulation and duration of treatment. Accordingly, we reviewed the evidence for the use of salt supplementation to reduce susceptibility to syncope or presyncope in patients with VVS and POTS. We found that short-term (~3 months) salt supplementation improves susceptibility to VVS and associated symptoms, with little effect on supine blood pressure. In patients with VVS, salt supplementation is associated with increases in plasma volume, and an increase in the time taken to provoke a syncopal event during orthostatic tolerance testing, with smaller orthostatic heart rate increases, enhanced peripheral vascular responses to orthostatic stress, and improved cerebral autoregulation. Responses were most pronounced in those with a baseline sodium excretion <170 mmol/day. Salt supplementation also improved symptoms, plasma volume, and orthostatic responses in patients with POTS. Salt supplementation should be considered for individuals with recurrent and troublesome episodes of VVS or POTS without cardiovascular comorbidities, particularly if their typical urinary sodium excretion is low, and their supine blood pressure is not elevated. The efficacy of the response, in terms of the improvement in subjective and objective markers of orthostatic intolerance, and any potential deleterious effect on supine blood pressure, should be routinely monitored in individuals on high salt regimes.

Blood pressure regulation in autonomic failure by dietary sodium, blood volume and posture

In normal subjects natriuresis is tightly coupled to dietary salt ingestion to maintain sodium balance. Likewise, blood pressure remains unchanged over a wide range of salt intake because of pressure natriuresis, whereby an increase in blood pressure stimulates renal sodium excretion to restore homeostasis. These sodium handling mechanisms are impaired in autonomic failure. When exposed to salt restriction, autonomic failure patients are unable to reduce renal sodium excretion, and their orthostatic hypotension worsens. It follows that increased dietary salt would improve orthostatic tolerance. Indeed, most clinical practice guidelines emphasize a high salt intake (6-10 g/day) in the treatment of neurogenic orthostatic hypotension. This approach has been shown to improve other conditions such as syncope and postural tachycardia syndrome, but surprisingly there is no empirical evidence to support this recommendation in orthostatic hypotension. Even though there is expert opinion consensus in its favor, it would be reassuring if at least mechanistic proof of concept studies were available. Fludrocortisone is often added to a high salt diet to improve sodium retention and increase plasma volume, but these effects are transient. Fludrocortisone is contraindicated in patients with heart failure and should be used with caution, if at all, if supine hypertension is present. In patients with supine hypertension posture is an important determinant of sodium balance; blood pressure substantially increases while supine, triggering pressure natriuresis and extensive sodium loss. Thus, avoiding the supine posture may be as important as increasing dietary salt in the management of orthostatic hypotension.

Increased Salt Intake for Orthostatic Intolerance Syndromes: A Systematic Review and Meta-Analysis

BACKGROUND

Guidelines recommend increased salt intake as a first-line recommendation in the management of symptomatic orthostatic hypotension and recurrent syncope. There have been no systematic reviews of this intervention. We sought to summarize the evidence for increased salt intake in patients with orthostatic intolerance syndromes.

METHODS

We conducted a systematic review and meta-analysis of studies in PubMed, EMBASE, and CINAHL. Interventional studies that increased salt intake in individuals with orthostatic intolerance syndromes were included. Primary outcome measures included incidence of falls and injuries, and rates of syncope and presyncope. Secondary outcome measures included other orthostatic intolerance symptoms, blood pressure, and heart rate.

RESULTS

A total of 14 studies were eligible, including participants with orthostatic hypotension, syncope, postural orthostatic tachycardia syndrome, and idiopathic orthostatic tachycardia (n = 391). Mean age was 35.6 (± 15) years. All studies were small and short-term (<60 mins-90 days). No study reported on the effect of increased salt intake on falls or injuries. Meta-analysis demonstrated that during head-up tilt, mean time to presyncope with salt intake increased by 1.57 minutes (95% confidence interval [CI], 1.26-1.88), mean systolic blood pressure increased by 12.27 mm Hg (95% CI, 10.86-13.68), and mean heart rate decreased by -3.97 beats per minute (95% CI, -4.08 to -3.86), compared with control. Increased salt increased supine blood pressure by 1.03 mm Hg (95% CI, 0.81 to 1.25). Increased salt intake resulted in an improvement or resolution of symptoms in 62.3% (95% CI, 51.6 to 72.6) of participants in short-term follow-up studies (mean follow-up of 44.3 days, 6 studies; n=91). Methodological quality of studies were low with high statistical heterogeneity in all meta-analyses.

CONCLUSIONS

Our meta-analysis provides low-quality evidence of a short-term improvement in orthostatic intolerance with increased salt intake. There were no clinical trials demonstrating the efficacy and safety of increased salt intake on long-term clinical outcomes. Overall, there is a paucity of clinical trial evidence to support a cornerstone recommendation in the management of orthostatic intolerance syndromes.

How Protective Mechanisms Interact to Prevent Overnight Calcium Phosphate Precipitation - An Observational Study to Determine Factors Against Calcium Phosphate Lithogenesis in a Healthy Cohort

BACKGROUND/AIMS

As restful, non-interrupted sleep is essential for normal mental and physical functioning, the urine flow rate (UFR) overnight remains low. Due to this reduced UFR, the kidneys produce a lower urine volume, which may lead to supersaturation of lithogens in the renal collecting system. The protective mechanisms that prevent the rise in the concentration of the lithogenic substances in urine, such as calcium phosphate, are explored.

METHODS

Urine samples were collected from 26 subjects every 2-3 h during daylight with one nocturnal collection; the UFR was calculated in the median time for each collection period. Urinary constituents for calcium phosphate precipitation including electrolytes, calcium, phosphate, citrate, and pH were measured. Comparisons within individuals were done by paired t test.

RESULTS

The calcium excretion rate fell significantly overnight (from 2.4 ± 0.2 µmol/min during the daytime to 1.5 ± 0.3 µmol/min, p < 0.05), in parallel with sodium excretion (54 ± 16 µmol/min from its daytime 127 ± 12 µmol/min, p < 0.05), preventing nocturnal calcium concentration from increasing (3.0 ± 0.3 mmol/l daytime to 2.5 ± 0.5 mmol/l overnight), while citrate concentration did not change significantly. The total urine phosphate concentration rose significantly overnight (daytime 18.7 ± 1.4 µmol/min vs. nocturnal 20.9 ± 1.7 µmol/min), but the concentration of divalent phosphate did not increase in the overnight period.

CONCLUSIONS

Although the UFR was lower overnight, there was no evidence that the risk of calcium phosphate precipitate formation in healthy subjects was increased.

Circulatory responses to lower body negative pressure in young Afghans and Danes: implications for understanding ethnic effects on blood pressure regulation

PURPOSE

We have previously shown that Afghans residing in Denmark for at least 12 years exhibit a lower 24-h ambulatory blood pressure compared to Danes. The purpose of this study was to test the hypothesis that the lower blood pressure reflects attenuated compensatory baroreflex responses in the Afghans.

METHODS

On a controlled diet (2,822 cal/day, 55-75 mmol + 2 mmol Na+/kg/day), 12 young males of Afghan (Afghans) and 12 young males of Danish (Danes) origin were exposed to a two-step lower body negative pressure (LBNP) protocol of -20 and -50 mmHg, respectively, each of 10-min duration.

RESULTS

Afghans had lower 24-h systolic blood pressure compared to Danes (115 ± 2 vs. 123 ± 1 mmHg, p < 0.05). Cardiac output and stroke volume were significantly lower in Afghans compared to Danes prior to and during each level of LBNP. However, it decreased to the same extent in both groups during LBNP. During LBNP of -20 mmHg, plasma noradrenaline concentration and plasma renin activity (PRA) increased significantly only in the Afghans. At LBNP of -50 mmHg plasma noradrenaline concentration and PRA both increased significantly and similarly in the two groups.

CONCLUSION

The lower 24-h ambulatory blood pressure in the Afghans is probably caused by a lower stroke volume, which augmented the circulatory and vasoactive hormonal responses to LBNP in the Afghans. The lower stroke volume in Afghans residing in Denmark compared to that of matched native Danes remains to be explained.

The primary vascular dysregulation syndrome: implications for eye diseases

Vascular dysregulation refers to the regulation of blood flow that is not adapted to the needs of the respective tissue. We distinguish primary vascular dysregulation (PVD, formerly called vasospastic syndrome) and secondary vascular dysregulation (SVD). Subjects with PVD tend to have cold extremities, low blood pressure, reduced feeling of thirst, altered drug sensitivity, increased pain sensitivity, prolonged sleep onset time, altered gene expression in the lymphocytes, signs of oxidative stress, slightly increased endothelin-1 plasma level, low body mass index and often diffuse and fluctuating visual field defects. Coldness, emotional or mechanical stress and starving can provoke symptoms. Virtually all organs, particularly the eye, can be involved. In subjects with PVD, retinal vessels are stiffer and more irregular, and both neurovascular coupling and autoregulation capacity are reduced while retinal venous pressure is often increased. Subjects with PVD have increased risk for normal-tension glaucoma, optic nerve compartment syndrome, central serous choroidopathy, Susac syndrome, retinal artery and vein occlusions and anterior ischaemic neuropathy without atherosclerosis. Further characteristics are their weaker blood–brain and blood-retinal barriers and the higher prevalence of optic disc haemorrhages and activated astrocytes. Subjects with PVD tend to suffer more often from tinnitus, muscle cramps, migraine with aura and silent myocardial ischaemic and are at greater risk for altitude sickness. While the main cause of vascular dysregulation is vascular endotheliopathy, dysfunction of the autonomic nervous system is also involved. In contrast, SVD occurs in the context of other diseases such as multiple sclerosis, retrobulbar neuritis, rheumatoid arthritis, fibromyalgia and giant cell arteritis. Taking into consideration the high prevalence of PVD in the population and potentially linked pathologies, in the current article, the authors provide recommendations on how to effectively promote the field in order to create innovative diagnostic tools to predict the pathology and develop more efficient treatment approaches tailored to the person.

The circadian clock modulates renal sodium handling

The circadian clock contributes to the control of BP, but the underlying mechanisms remain unclear. We analyzed circadian rhythms in kidneys of wild-type mice and mice lacking the circadian transcriptional activator clock gene. Mice deficient in clock exhibited dramatic changes in the circadian rhythm of renal sodium excretion. In parallel, these mice lost the normal circadian rhythm of plasma aldosterone levels. Analysis of renal circadian transcriptomes demonstrated changes in multiple mechanisms involved in maintaining sodium balance. Pathway analysis revealed the strongest effect on the enzymatic system involved in the formation of 20-HETE, a powerful regulator of renal sodium excretion, renal vascular tone, and BP. This correlated with a significant decrease in the renal and urinary content of 20-HETE in clock-deficient mice. In summary, this study demonstrates that the circadian clock modulates renal function and identifies the 20-HETE synthesis pathway as one of its principal renal targets. It also suggests that the circadian clock affects BP, at least in part, by exerting dynamic control over renal sodium handling.

Targeted preventive measures and advanced approaches in personalised treatment of glaucoma neuropathy

Glaucoma is a major cause of vision loss worldwide with nearly 8 million people bilaterally blind from the disease. This number is estimated to increase over the next 10 years. The key to preventing blindness from glaucoma is effective diagnosis and treatment. The classical glaucoma treatment focuses on intraocular pressure (IOP) reduction. Better knowledge of the pathogenesis has opened up additional therapeutical approaches often called non-IOP lowering treatment. Whilst most of these new avenues of treatment are still in the experimental phase, others are already used by some physicians. These new therapeutic approaches allow a more personalised patient treatment. Non-IOP lowering treatment includes improvements of ocular blood flow, particularly blood flow regulation. This can be achieved by improving the regulation of ocular blood flow (improving autoregulation) by drugs such as carbonic anhydrase inhibitors, magnesium or calcium channel blockers. It can also be improved by decreasing blood pressure over-dips. Blood pressure can be increased by an increase in salt intake or in rare cases by treatment with fludrocortisone. Experimentally, glaucomatous optic neuropathy can be prevented by inhibition of astrocyte activation, either by blockage of epidermal growth factor receptor or by counteracting Endothelin. Glaucomatous optic neuropathy can also be prevented by nitric oxide-2 synthase inhibition. Suppression of matrix metalloproteinase-9 inhibits apoptosis of retinal ganglion cells and tissue remodelling. Upregulation of heat shock proteins protects the retinal ganglion cells and the optic nerve head. Reduction of oxidative stress especially at the level of mitochondria also seems to be protective. This can be achieved by gingko, dark chocolate, polyphenolic flavonoids occurring in tea, coffee or red wine and anthocyanosides found in bilberries as well as by ubiquinone and melatonin. This review describes the individual mechanisms which may be targeted by non-IOP lowering treatment based on our pathogenic scheme.

Is there more to glaucoma treatment than lowering IOP?

Classic glaucoma treatment focuses on intraocular pressure (IOP) reduction. Better knowledge of the pathogenesis of the disease has opened up new therapeutical approaches. Whereas most of these new avenues of treatment are still in the experimental phase, others, such as magnesium, gingko, salt and fludrocortisone, are already used by some physicians. Blood pressure dips can be avoided by intake of salt or fludrocortisone. Vascular regulation can be improved locally by carbonic anhydrase inhibitors, and systemically with magnesium or with low doses of calcium channel blockers. Experimentally, glaucomatous optic neuropathy can be prevented by inhibition of astrocyte activation, either by blockage of epidermal growth factor receptor or by counteracting endothelin. Glaucomatous optic neuropathy can also be prevented by nitric oxide-2 synthase inhibition. Inhibition of matrix metalloproteinase-9 inhibits apoptosis of retinal ganglion cells and tissue remodeling. Upregulation of heat shock proteins protects the retinal ganglion cells and the optic nerve head. Reduction of oxidative stress especially at the level of mitochondria also seems to be protective. This can be achieved by gingko; dark chocolate; polyphenolic flavonoids occurring in tea, coffee, or red wine; anthocyanosides found in bilberries; as well as by ubiquinone and melatonin.

What is the link between vascular dysregulation and glaucoma?

The need of blood flow to different organs varies rapidly over time which is why there is sophisticated local regulation of blood flow. The term dysregulation simply means that blood flow is not properly adapted to this need. Dysregulative mechanisms can lead to an over- or underperfusion. A steady overperfusion may be less critical for long-term damage. A constant underperfusion, however, can lead to some tissue atrophy or in extreme situations to infarction. Unstable perfusion (underperfusion followed by reperfusion) leads to oxidative stress. There are a number of causes that lead to local or systemic vascular dysregulation. Systemic dysregulation can be primary or secondary of nature. A secondary dysregulation is due to other autoimmune diseases such as rheumatoid arthritis, giant cell arteritis, systemic lupus erythematodes, multiple sclerosis, colitis ulcerosa, or Crohns disease. Patients with a secondary vascular dysregulation normally have a high level of circulating endothelin-1 (ET-1). This increased level of ET-1 leads to a reduction of blood flow both in the choroid and the optic nerve head but has little influence on autoregulation. In contrast, primary vascular dysregulation has little influence on baseline ocular blood flow but interferes with autoregulation. This, in turn, leads to unstable oxygen supply, which seems to be a relevant component in the pathogenesis of glaucomatous optic neuropathy.

Phase of the menstrual cycle does not affect orthostatic tolerance in healthy women

Women of child-bearing age have a lower orthostatic tolerance (OT) than older women or men, and women suffering from frequent syncopal episodes often comment that their symptoms occur at certain times of the menstrual cycle. However, it is not known whether, in asymptomatic women, OT varies at different phases of the menstrual cycle. We studied 8 healthy asymptomatic women aged 26.8 +/- 3.4 years. We determined OT using a test of combined head-up tilting and lower body suction. We continuously monitored beat-to-beat blood pressure (Finapres), heart rate (ECG), and cerebral and forearm blood flow velocities (Doppler ultrasound). On each test day we assessed carotid baroreceptor sensitivity from suction/pressure applied to a neck chamber. We also determined estradiol and progesterone levels from a venous blood sample. Tests were performed in early follicular and late luteal phases, and during ovulation. Serum concentrations of estradiol (pmol x l(-1)) and progesterone (nmol x l(-1)) were in follicular phase 464.1 +/- 63 and 6.3 +/- 2.8; ovulation 941.6 +/- 298 and 5.8 +/- 1.2; luteal phase 698 +/- 188 and 32.3 +/- 9.6. Progesterone levels were significantly higher in the luteal phase (p < 0.001). OT was not different on any test day: follicular 31.9 +/- 1.6 min, ovulation 31.3 +/- 0.7 min; luteal 31.1 +/- 2.2 min. Supine and tilted heart rates and blood pressures, the maximum heart rate, and the cerebral autoregulatory and forearm vascular resistance responses to the orthostatic stress were similar during all studies. Both cardiac and vascular resistance carotid baroreceptor sensitivities were also similar on all test days. These results suggest that there is no difference in either OT or cardiovascular control at the tested phases of the menstrual cycle in healthy women.

Renal sodium handling in patients with normal pressure glaucoma

Low BP (blood pressure) is a recognized risk factor for some patients with NPG (normal pressure glaucoma). We have shown previously that patients with orthostasis have impaired circadian renal handling of sodium, which may contribute to the low BP. Therefore the aim of the present study was to examine the renal handling of sodium, the circadian variations in BP and the neurohormonal response to an orthostatic test in a selected subpopulation of 18 patients with NPG with vasospastic and orthostatic symptoms, and in 24 healthy control subjects. The variations in BP and renal tubular sodium handling were evaluated using 24 h ambulatory BP recordings, 24 h urine collections and determination of endogenous lithium clearance as a marker of proximal sodium reabsorption. The neurohormonal and BP responses to changes in posture were also determined in a 30 min orthostatic test. This selected group of patients with NPG had lower 24 h ambulatory BPs (P<0.001), and a more pronounced fall in BP when assuming an upright position (P<0.001) compared with controls. FE(Li) (fractional excretion of lithium) was higher in patients with NPG than controls during the day (36.6+/-21.8 compared with 20.4+/-8.7% respectively; P<0.01; values are means+/-S.D.) as well as during the night (38.8+/-41.9 compared with 19.7+/-10.8% respectively; P<0.02), suggesting a reduced reabsorption of sodium in the proximal tubule. This was compensated for by an increased distal reabsorption of sodium in patients with NPG (P<0.01). These data demonstrate that patients with vasospastic NPG have a high excretion of lithium, suggesting reduced sodium reabsorption in the proximal tubule, in spite of a low BP. The abnormal renal sodium handling might contribute to the maintenance of arterial hypotension and progression of the optic nerve damage in these patients.

Salt wasting, hypotension, polydipsia, and hyponatremia and the level of spinal cord injury

STUDY DESIGN

Case control.

OBJECTIVE

To test the reported correlation of hypotension, polydipsia, and hyponatremia with higher levels of spinal cord injury (SCI).

SETTING

A Veterans Administration Hospital, USA.

METHODS

The records of men who were paralyzed owing to trauma at any spinal cord level with motor complete lesions (ASIA A or B) and who received an annual physical and laboratory examination were reviewed for age, duration of paralysis, level of paralysis, blood pressure (BP), serum sodium, and 24 h urinary volume, creatinine, and sodium. Creatinine clearance and fractional excretion of sodium (FcNa) were calculated. Spearman rank-order correlations (r (s)) were carried out.

RESULTS

Patients were aged 25 to 88 years, median 56 years, paralyzed 2-61 years, median 26 years, with levels of paralysis ranging from C2 to L4, median T4, n=111. From lower to higher levels of paralysis FcNa increased (0.4-7.3%), mean BP diminished (132-66 mmHg), urine volume increased (600-5400 ml), and serum sodium was reduced (148-129 mEq/l) - r (s)=0.29, 0.49, -0.22, and 0.23, respectively. Increasing 24 h urinary volumes correlated with lower serum sodium concentrations but higher creatinine clearance, r (s)=-0.28, 0.24. Increasing 24 h urinary sodium improved creatinine clearance, r (s)=0.37. P-values ranged from <0.05 to <0.001.

CONCLUSION

Higher levels of SCI correlate with reduced sodium conservation, hypotension, polydipsia, and hyponatremia. Greater water intake raises creatinine clearance but lowers serum sodium. Greater salt intake increases creatinine clearance.

Characterization of tubular functional capacity in humans using para-aminohippurate and famotidine

BACKGROUND

Renal drug excretion by glomerular filtration and active tubular secretion may be altered by factors such as acute and chronic renal disease, nephrotoxins, and drug interactions. Thus, accurate and reproducible methods for quantitation of glomerular filtration rate (GFR) and tubular functional capacity are critical.

METHODS

We utilized a four-step sequential infusion method to characterize anionic [para-aminohippurate (PAH)] and cationic (famotidine) tubular functional capacity in healthy volunteers. Filtration and secretion rates were quantitated from renal clearance and iothalamate-derived GFR determinations.

RESULTS

Concentration-dependent renal clearance of PAH was observed at plasma concentrations> 100 mg/L; renal clearances were 442 +/- 131 (mean +/- SD), 423 +/- 94, 233 +/- 45, and 152 +/- 18 mL/min/1.73 m2 at plasma concentrations of 18 +/- 2, 92 +/- 5, 291 +/- 47 and 789 +/- 28 mg/L, respectively. The apparent affinity (Km) and maximum secretory capacity (TmPAH) were 141 +/- 70 mg/L and 71 +/- 16 mg/min/1.73 m2, respectively. The unbound renal clearance and tubular secretory clearance of famotidine were 384 +/- 70 and 329 +/- 78 mL/min/1.73 m2, respectively, and were not significantly correlated with the unbound plasma concentrations, which ranged from 126 to 2659 ng/mL. The rate of tubular secretion was linear at unbound plasma concentrations up to 2659 ng/mL.

CONCLUSIONS

These data indicate that a sequential infusion method using PAH may be used to characterize the anionic secretory component of proximal tubular function. The tubular clearance of famotidine may be a suitable index of the cationic secretory capacity of the proximal tubule in humans. Saturation of the cationic secretory pathway was not observed, and further investigation into parallel pathways of cationic secretion, such as p-glycoprotein, may be warranted.

Low LBNP tolerance in men is associated with attenuated activation of the renin-angiotensin system

Plasma vasoactive hormone concentrations [epinephrine (p(Epi)), norepinephrine (p(NE)), ANG II (p(ANG II)), vasopressin (p(VP)), endothelin-1 (p(ET-1))] and plasma renin activity (p(RA)) were measured periodically and compared during lower body negative pressure (LBNP) to test the hypothesis that responsiveness of the renin-angiotensin system, the latter being one of the most powerful vasoconstrictors in the body, is of major importance for LBNP tolerance. Healthy men on a controlled diet (2,822 cal/day, 2 mmol. kg(-1). day(-1) Na(+)) were exposed to 30 min of LBNP from -15 to -50 mmHg. LBNP was uneventful for seven men [25 +/- 2 yr, high-tolerance (HiTol) group], but eight men (26 +/- 3 yr) reached presyncope after 11 +/- 1 min [P < 0.001, low-tolerance (LoTol) group]. Mean arterial pressure (MAP) did not change measurably, but central venous pressure and left atrial diameter decreased similarly in both groups (5-6 mmHg, by approximately 30%, P < 0.05). Control (0 mmHg LBNP) hormone concentrations were similar between groups, however, p(RA) differed between them (LoTol 0.6 +/- 0.1, HiTol 1.2 +/- 0.1 ng ANG I. ml(-1). h(-1), P < 0.05). LBNP increased (P < 0. 05) p(RA) and p(ANG II), respectively, more in the HiTol group (9.9 +/- 2.2 ng ANG I. ml(-1). h(-1) and 58 +/- 12 pg/ml) than in LoTol subjects (4.3 +/- 0.9 ng ANG I. ml(-1). h(-1) and 28 +/- 6 pg/ml). In contrast, the increase in p(VP) was higher (P < 0.05) in the LoTol than in the HiTol group. The increases (P < 0.05) for p(NE) were nonsignificant between groups, and p(ET-1) remained unchanged. Thus there may be a causal relationship between attenuated activation of p(RA) and p(ANG II) and presyncope, with p(VP) being a possible cofactor. Measurement of resting p(RA) may be of predictive value for those with lower hypotensive tolerance.