Demystifying hyponatremia: A clinical guide to evaluation and management.

Hyponatremia (serum sodium <135 mEq/L) is a frequent electrolyte abnormality complicating the clinical care of hospitalized patients. Hyponatremia has been associated with an increased risk of mortality. Hyponatremia can be seen in patients with euvolemia, hypovolemia, or hypervolemia. Evaluation of hyponatremia relies on clinical assessment and estimation of serum sodium, urine electrolytes, and serum and urine osmolality in addition to other case-specific laboratory parameters. In addition, point-of-care ultrasonography is an important adjunct to physical assessment in estimation of volume status. Understanding the pathophysiology of the underlying process can lead to a timely diagnosis and appropriate management of hyponatremia.

AKI in Hospitalized Patients with COVID-19 and Seasonal Influenza: A Comparative Analysis.

Background: Coronavirus disease 2019 (COVID-19) is often compared with seasonal influenza and the two diseases have similarities, including the risk of systemic manifestations such as AKI. The aim of this study was to perform a comparative analysis of the prevalence, risk factors, and outcomes of AKI in patients who were hospitalized with COVID-19 and influenza. Methods: Retrospective cohort study of patients who were hospitalized with COVID-19 (n=325) or seasonal influenza (n=433). AKI was defined by Kidney Disease: Improving Global Outcomes (KDIGO) criteria. Baseline characteristics and hospitalization data were collected, and multivariable analysis was performed to determine the independent predictors for AKI. Results: AKI occurred in 33% of COVID-19 hospitalizations (COV-AKI) and 33% of influenza hospitalizations (FLU-AKI). After adjusting for age, sex, and comorbidity count, the risk of stage 3 AKI was significantly higher in COV-AKI (OR, 3.46; 95% CI, 1.63 to 7.37). Pre-existing CKD was associated with a six- to seven-fold increased likelihood for FLU-AKI and COV-AKI. Mechanical ventilation was associated with a higher likelihood of developing AKI in the COVID-19 cohort (OR, 5.85; 95% CI, 2.30 to 15.63). Black race, after adjustment for comorbidities, was an independent risk for COV-AKI. Conclusions: Pre-existing CKD was a major risk factor for AKI in both cohorts. Black race (independent of comorbidities) and mechanical ventilation were associated with a higher risk of developing COV-AKI, which is characterized by a higher burden of stage 3 AKI and overall poorer prognosis.

ß1-Adrenergic and M2 Muscarinic Autoantibodies and Thyroid Hormone Facilitate Induction of Atrial Fibrillation in Male Rabbits.

Activating autoantibodies to the ß1-adrenergic and M2 muscarinic receptors are present in a very high percentage of patients with Graves' disease and atrial fibrillation (AF). The objective of this study was to develop a reproducible animal model and thereby to examine the impact of these endocrine-like autoantibodies alone and with thyroid hormone on induction of thyroid-associated atrial tachyarrhythmias. Five New Zealand white rabbits were coimmunized with peptides from the second extracellular loops of the ß1-adrenergic and M2 muscarinic receptors to produce both sympathomimetic and parasympathomimetic antibodies. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization and subsequent treatment with thyroid hormone. Antibody expression facilitated the induction of sustained sinus, junctional and atrial tachycardias, but not AF. Addition of excessive thyroid hormone resulted in induced sustained AF in all animals. AF induction was blocked acutely by the neutralization of these antibodies with immunogenic peptides despite continued hyperthyroidism. The measured atrial effective refractory period as one parameter of AF propensity shortened significantly after immunization and was acutely reversed by peptide neutralization. No further decrease in the effective refractory period was observed after the addition of thyroid hormone, suggesting other cardiac effects of thyroid hormone may contribute to its role in AF induction. This study demonstrates autonomic autoantibodies and thyroid hormone potentiate the vulnerability of the heart to AF, which can be reversed by decoy peptide therapy. These data help fulfill Witebsky's postulates for an increased autoimmune/endocrine basis for Graves' hyperthyroidism and AF.

A peptidomimetic inhibitor suppresses the inducibility of ß1-adrenergic autoantibody-mediated cardiac arrhythmias in the rabbit.

PURPOSE: Previous studies demonstrated that burst pacing and subthreshold infusion of acetylcholine in ß1-adrenergic receptor (ß1AR)-immunized rabbits induced sustained sinus tachycardia. The aim of this study was to examine the anti-arrhythmogenic effect of a newly designed retro-inverso (RI) peptidomimetic inhibitor that specifically targets the ß1AR antibodies in the rabbit. METHODS: Six New Zealand white rabbits were immunized with a ß1AR second extracellular loop peptide to produce sympathomimetic ß1AR antibodies. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization and subsequent treatment with the RI peptide inhibitor. Each rabbit served as its own control. RESULTS: No sustained arrhythmias were induced at preimmune baseline. At 6 weeks after immunization, there was a marked increase in induced sustained tachyarrhythmias, predominantly sinus tachycardia, which was largely suppressed by the RI peptide. The atrial effective refractory period was shortened significantly in immunized rabbits compared to their preimmune state. The RI peptide reversed and prolonged this shortening. ß1AR antibody levels were negatively correlated with the atrial effective refractory period. Postimmune sera-induced ß1AR activation in transfected cells in vitro was also blocked by the RI peptide. CONCLUSIONS: ß1AR-activating autoantibodies are associated with reduction of the atrial effective refractory period and facilitate arrhythmia induction in this model. The RI peptide reversal may have important therapeutic implications in subjects who harbor these autoantibodies.

AT2R autoantibodies block angiotensin II and AT1R autoantibody-induced vasoconstriction.

Activating autoantibodies to the angiotensin type 1 receptor (AT1R) are associated with hypertensive disorders. The angiotensin type 2 receptor (AT2R) is known to counter-regulate the actions of AT1R. We investigated whether AT2R autoantibodies produced in immunized rabbits will activate AT2R and suppress the vasopressor responses to angiotensin II and AT1R-activating autoantibodies. Five rabbits immunized with a peptide corresponding to the second extracellular loop of AT2R developed high AT2R antibody titers. Rabbit anti-AT2R sera failed to directly dilate isolated rat cremaster arterioles; however, when co-perfused with angiotensin II or AT1R-activating autoantibodies, the anti-AT2R sera significantly inhibited their contractile effects. Rabbit anti-AT2R sera recognized a predominant sequence near the N-terminus of the AT2R second extracellular loop. A decoy peptide based on this sequence effectively reversed the opposing effect of the anti-AT2R sera on angiotensin II-induced contraction of rat cremaster arterioles. A similar blockade of the anti-AT2R sera effect was observed with the AT2R antagonist PD 123319 and the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Rabbit anti-AT2R sera reacted specifically with AT2R. No cross-reactivity with AT1R was observed. Blood pressure did not change in immunized animals. However, the pressor responses to incremental angiotensin II infusions were blunted in immunized animals. Thirteen subjects with primary aldosteronism demonstrated increased AT2R autoantibody levels compared with normal controls. In conclusion, AT2R autoantibodies produced in immunized rabbits have the ability to activate AT2R and counteract the AT1R-mediated vasoconstriction. These autoantibodies provide useful and selective tools for the study of their roles in blood pressure regulation and possible therapeutic intervention.

Prevalence of angiotensin II type 1 receptor (AT1R)-activating autoantibodies in primary aldosteronism.

Autoantibodies to the angiotensin II type 1 receptor (AT1R) have been reported in patients with primary aldosteronism, including aldosterone producing adenoma (APA) and idiopathic adrenal hyperplasia (IAH). Sera from 25 primary aldosteronism subjects (12 with IAH and 13 with APA) and 15 normotensive control subjects were assayed for AT1R autoantibodies by enzyme-linked immunosorbent assay and an AT1R-transfected cell-based bioassay. Nine of 12 IAH subjects (75%) and six of 13 APA subjects (46%) were positive for AT1R autoantibodies in the bioactivity assay. The mean AT1R autoantibody activity for the IAH and APA subjects was significantly greater than controls (P < .001 and P < .01, respectively), and this in vitro activity was suppressed by the AT1R blocker losartan. None of the controls had significant AT1R autoantibody activity. Enzyme-linked immunosorbent assay values were less sensitive but were positive in some subjects with IAH and APA. The mean arterial pressure of these primary aldosteronism subjects correlated modestly with AT1R autoantibody activity. These data confirm the presence of active AT1R autoantibodies in a high percentage of subjects with primary aldosteronism irrespective of their underlying etiology. These observations have both pathophysiological and clinical implications.

Novel retro-inverso peptide inhibitor reverses angiotensin receptor autoantibody-induced hypertension in the rabbit.

Activating autoantibodies to the angiotensin II type 1 receptor (AT1R) have been implicated in hypertensive disorders. We investigated whether AT1R antibodies produced in immunized rabbits will activate AT1R and contribute to hypertension by a direct contractile effect on the vasculature and whether they can be blocked by a novel decoy peptide. A multiple antigenic peptide containing the AT1R epitope AFHYESQ, which is the receptor-binding epitope of AT1R-activating autoantibodies, was used to immunize 6 rabbits. AT1R antibody activity was analyzed in AT1R-transfected cells, and their contractile effects were assayed using isolated perfused rat cremaster resistance arterioles. A retro-inverso D-amino acid epitope-mimetic peptide was tested for AT1R antibody inhibition in vitro and in vivo. All immunized animals produced high AT1R antibody titers and developed elevated blood pressure. No changes in measured blood chemistry values were observed after immunization. Rabbit anti-AT1R sera induced significant AT1R activation in transfected cells and vasoconstriction in the arteriole assay, both of which were blocked by losartan and the retro-inverso D-amino acid peptide. A single intravenous bolus injection of the retro-inverso d-amino acid peptide (1 mg/kg) into immunized rabbits dropped the mean arterial pressure from 122±11 to 82±6 mm Hg. Rabbit anti-AT1R sera partially suppressed angiotensin II-induced contraction of isolated rat cremaster arterioles, and the pressor response to angiotensin II infusion was attenuated in immunized animals. In conclusion, AT1R-activating autoantibodies and the retro-inverso d-amino acid peptide, respectively, have important etiologic and therapeutic implications in hypertensive subjects who harbor these autoantibodies.