Role of dietary magnesium deficiency in the pressor and arrhythmogenic response to epinephrine in the intact dog

Effect of Magnesium on Ventricular Extrasystoles in Children

Magnesium (Mg) is a crucial element for cardiovascular system and its deficiency results in a variety of cardiac arrhythmias. The aim of this study is to determine the effect of oral Mg supplementation on the frequency of ventricular extrasystoles (VES) in children. Magnesium supplementation was given to 42 children who had VES without structural heart disease. Clinical, electrocardiographic, and Holter monitoring studies were reviewed. The mean baseline 24 h VES burden on Holter monitoring was 10.26% ± 4.13% and it was decreased to 6.62% ± 3.88% after. There was no significant difference between the pre-treatment serum Mg levels and the decrease in the frequency of VES. In conclusion, oral Mg therapy was found to be effective at suppressing VES in children regardless of serum Mg levels. Large and randomized studies are needed to demonstrate the effect of magnesium on VES suppression.

Vitamin-mineral supplements do not guarantee the minimum recommendations and may imply risks of mercury poisoning in dogs and cats

Given the importance of using the vitamin-mineral supplements to guarantee the minimum nutritional recommendations for pets in homemade foods, and hypothesizing that these products may contribute to toxic metals contamination, the present study aimed to determine the concentrations of essential minerals and toxic metals in vitamin-mineral supplements available in the Brazilian market and calculate if the amount recommended by the manufacturer guarantees the minimum recommendations of NRC (2006) and FEDIAF (2020), as well as calculating the amount of toxic metals that animals would consume, according to the amounts recommended by the manufacturer. Seven vitamin-mineral supplements were analyzed. The determination of essential minerals and toxic metals was performed using ICP-OES. Comparisons were made with the minimum recommendations for essential minerals, and with the maximum tolerated levels of toxic metals established by the FDA (2011), descriptively. Most of the vitamin-mineral supplements, in the quantities recommended by the manufacturers, do not guarantee the minimum recommendations of NCR (2006) and FEDIAF (2020) for the following elements: calcium, potassium, magnesium, sodium, phosphorus, selenium, and zinc. Only one supplement had detectable selenium concentrations. Three supplements provided more than 0.02mg of mercury/kg of body weight, the safe upper limit used to establish the maximum tolerated level of this element. It is concluded that most vitamin-mineral supplements do not meet the minimum recommendations for most essential minerals and, if formulated by untrained professionals, even with supplementation, homemade foods may still be nutritionally deficient. Furthermore, some vitamin-mineral supplements analyzed may imply risks of mercury poisoning in pets.

The nerve-heart connection in the pro-oxidant response to Mg-deficiency

Magnesium is a micronutrient essential for the normal functioning of the cardiovascular system, and Mg deficiency (MgD) is frequently associated in the clinical setting with chronic pathologies such as CHF, diabetes, hypertension, and other pathologies. Animal models of MgD have demonstrated a systemic pro-inflammatory/pro-oxidant state, involving multiple tissues/organs including neuronal, hematopoietic, cardiovascular, and gastrointestinal systems; during later stages of MgD, a cardiomyopathy develops which may result from a cascade of inflammatory events. In rodent models of dietary MgD, a significant rise in circulating levels of proinflammatory neuropeptides such as substance P (SP) and calcitonin gene-related peptide among others, was observed within days (1-7) of initiating the Mg-restricted diet, and implicated a neurogenic trigger for the subsequent inflammatory events; this early "neurogenic inflammation" phase may be mediated in part, by the Mg-gated N: -methyl-D-aspartate (NMDA) receptor/channel complex. Deregulation of the NMDA receptor may trigger the abrupt release of neuronal SP from the sensory-motor C-fibers to promote the subsequent pro-inflammatory changes: elevations in circulating inflammatory cells, inflammatory cytokines, histamine, and PGE(2) levels, as well as formation of nitric oxide, reactive oxygen species, lipid peroxidation products, and depletion of key endogenous antioxidants. Concurrent elevations of tissue CD14, a high affinity receptor for lipopolyssacharide, suggest that intestinal permeability may be compromised leading to endotoxemia. If exposure to these early (1-3 weeks MgD) inflammatory/pro-oxidant events becomes prolonged, this might lead to impaired cardiac function, and when co-existing with other pathologies, may enhance the risk of developing chronic heart failure.

Magnesium deficiency alters the threshold for epinephrine-induced arrhythmias during halothane or sevoflurane anesthesia in the rat

OBJECTIVE

To determine the effect of chronic magnesium (Mg2+) deficiency on the relative arrhythmogenicity of halothane and sevoflurane in the rat.

DESIGN

Prospective, randomized, nonblinded study.

SETTING

University laboratory.

PARTICIPANTS

Male Sprague-Dawley rats (n = 48).

INTERVENTIONS

Rats were maintained on a Mg2+-deficient or control diet for 14 days, at which time they were anesthetized with halothane or sevoflurane, a tracheostomy was performed, and the lungs were ventilated to maintain normocapnia. Catheters were inserted into a femoral vein and carotid artery. Lead II of the electrocardiogram was monitored to determine the threshold for epinephrine-induced arrhythmias.

MEASUREMENTS AND MAIN RESULTS

Chronic Mg2+ deficiency significantly decreased the dose of epinephrine required for arrhythmias (ADE). The reduction in the ADE was approximately one third during halothane anesthesia (p < 0.05) and one fifth during sevoflurane anesthesia (p < 0.001). Infusion of magnesium sulphate completely reversed the reduction in ADE. In normomagnesemic rats, the halothane ADE was significantly less than the sevoflurane ADE (mean difference = 6.0 microg/kg, 95% confidence interval of the difference = 3.6 to 8.4 microg/kg) (p < 0.005). Mg2+ deficiency significantly attenuated the difference between the halothane ADE and the sevoflurane ADE (mean difference in the Mg2+-deficient group = 0.6 microg/kg, 95% confidence interval of the difference = -0.2 to 1.5 microg/kg).

CONCLUSION

Chronic Mg2+ deficiency decreased the threshold for epinephrine-induced arrhythmias and attenuated differences between the arrhythmogenic potential of halothane and sevoflurane, suggesting that arrhythmias are as likely to develop with sevoflurane as with halothane in the presence of coexisting magnesium deficiency and elevated catecholamines.

Electrolyte Disturbances and Cardiac Arrhythmias in a Dog Following Pamidronate, Calcitonin, and Furosemide Administration for Hypercalcemia of Malignancy

A 13-year-old dog was diagnosed with hypercalcemia of malignancy associated with adenocarcinoma of the anal sacs. Hypercalcemia was treated with intravenous (IV) 0.9% sodium chloride (NaCl), furosemide, calcitonin, and pamidronate. Hypomagnesemia was documented by 72 hours following a single, IV dose of pamidronate. The dog subsequently underwent surgery to remove the primary tumors, and multiple cardiac arrhythmias occurred during anesthesia. This case documents electrolyte abnormalities in a dog following treatment with pamidronate in conjunction with other therapies used to manage hypercalcemia. The authors postulate that hypomagnesemia may have contributed to the arrhythmias that occurred during anesthesia. Electrolyte abnormalities should be anticipated and corrected following pamidronate therapy in canine patients.

Effect of magnesium deficiency on autonomic circulatory regulation in conscious rats

A close relationship between magnesium and cardiovascular function has been reported; however, the effect of magnesium deficiency on autonomic cardiovascular regulation has not been clarified. We investigated the effect of magnesium deficiency on the autonomic regulation of oscillations of the R-R interval, arterial blood pressure (BP), and renal sympathetic nerve activity (RSNA) by using the maximum entropy method in conscious rats. Its effect on baroreflex control of RSNA and heart rate were also investigated with a logistic function curve. Mean BP in magnesium-deficient rats was higher than that in control rats (mean+/-SE, 114.0+/-4.3 versus 101.6+/-3.4 mm Hg; P<0.05), and urinary excretion of catecholamine was increased by 2.4-fold. The fraction of low-frequency oscillation of RSNA was reduced (31.7+/-0.9% versus 36.2+/-1.5%, P<0.05) and the correlation between low-frequency oscillations of BP and RSNA was weakened in magnesium-deficient rats. There was no difference in high-frequency oscillation of the R-R interval, which is related to vagal tone, whereas sympathetic tone became dominant (square root of low-frequency/high-frequency ratio of R-R interval, 1.00+/-0.05 versus 0.67+/-0.05, P<0.0001) in magnesium-deficient rats. The maximal gain in the BP-RSNA relation tended to be reduced in magnesium-deficient rats (-7.7+/-1.1% versus -12.2+/-1.9%/mm Hg, P=0. 07); however, that in the BP-heart rate relation was increased (-8. 1+/-0.7 versus -4.5+/-0.5 bpm/mm Hg, P<0.01). These results suggest that magnesium deficiency induces sympathetic excitation, which results in hypertension but attenuates the baroreflex-related response of sympathetic nerves, whereas magnesium deficiency enhances the sensitivity of the sinus node to autonomic regulation.

Aldosterone escape during angiotensin-converting enzyme inhibitor therapy in chronic heart failure

In chronic heart failure, angiotensin-converting enzyme inhibitors produce an acute decrease in aldosterone levels. Long-term angiotensin-converting enzyme inhibition is, however, associated with aldosterone suppression that is weak, variable, and unsustained (ie, aldosterone escapes). The possible harmful effects of this residual aldosterone are multiple Magnesium loss caused by aldosterone and by diuretics could contribute to coronary artery spasm and arrhythmias. Aldosterone blocks norepinephrine uptake by the myocardium; extracellular catecholamines may, therefore, lead to arrhythmias and ischemia. Aldosterone has been shown to have an acute arrhythmogenic effect as well as a detrimental effect on parasympathetic and baroreflex function. Both angiotensin II and aldosterone stimulate myocardial fibrosis, which may lead to a higher incidence of malignant ventricular arrhythmias. Spironolactone therapy added to the regimen of an angiotensin-converting enzyme inhibitor and diuretic has been shown to cause natriuresis, magnesium retention, increased myocardial norepinephrine uptake, and reduced incidence of ventricular arrhythmias. It may well be that residual aldosterone mediates many harmful effects in chronic heart failure and that to optimize the benefit of blocking the renin-angiotensin-aldosterone system may require specific blockade of residual aldosterone as well as traditional angiotensin-converting enzyme inhibition.

Noninvasive measurement of tissue magnesium and correlation with cardiac levels

BACKGROUND

Intracellular magnesium ([Mg]i) plays an important role in the regulation of myocardial metabolism, contractility, and the maintenance of transsarcolemmal and intracellular ionic gradients. An understanding of the role of magnesium in the clinical setting, however, is hampered by the lack of an assay of intracellular tissue magnesium levels.

METHODS AND RESULTS

We used energy-dispersive x-ray analysis to measure [Mg]i in sublingual epithelial cells and to correlate the level with those in atrial biopsy specimens from the same patients during cardiopulmonary bypass. Levels were also measured in acute myocardial infarction (AMI) patients before and after intravenous magnesium sulfate administration and compared with those from intensive care unit (ICU) patients and healthy individuals. A strong correlation between sublingual epithelial cell (mean, 32.1 +/- 0.3 mEq/L) and atrial tissue (mean, 32.1 +/- 0.3 mEq/L) [Mg]i was present in 18 cardiac surgery patients (r = .68, P < .002). Epithelial and atrial [Mg]i levels were lower than in healthy individuals (33.7 +/- 0.5 mEq/L, P < .01) studied at that time and correlated poorly with serum magnesium. Mean [Mg]i in 22 AMI patients was 30.7 +/- 0.4 mEq/L, which was significantly lower than in 21 ICU patients and 15 healthy individuals (35.0 +/- 0.5 mEq/L and 34.5 +/- 0.7 mEq/L, respectively, P < .001). Intravenous magnesium sulfate was administered to most of the AMI patients (mean dose, 36 +/- 6 mmol). [Mg]i rose significantly in the AMI patients over the first 24 hours, and the magnitude of the increase was greater in those who received higher doses of intravenous magnesium sulfate.

CONCLUSIONS

Sublingual epithelial cell [Mg]i correlates well with atrial [Mg]i but not with serum magnesium. [Mg]i levels are low in patients undergoing cardiac surgery and those with AMI. Intravenous magnesium sulfate corrects low [Mg]i levels in AMI patients. Energy-dispersive x-ray analysis determination of sublingual cell [Mg]i may expedite the investigation of the role of magnesium deficiency in heart disease.