Magnesium Flashcards
1
Q
What is the function of Magnesium Metabolism?
A
After K+ is most abundant intracellular cation.Within cells Mg is co-factor in over 300 enzymatic reactions
- Energy metabolism
- Calcium & potassium channels
- Membrane stabilisation & neuromuscular excitability
- Protein & nucleic acid synthesis
- Oxidative phosphorylation
- Particular importance in formation & utilisation of ATP
2
Q
What is the ranges for Plasma Magnesium?
A
0.7 – 1.0 mmol / L
3
Q
What are plasma magnesium levels with in the blood?
A
- Less than 0.5% of total body Mg is in plasma
- 60% ionized form
- 15% complexed (phosphate, citrate or bicarbonate)
- 25% protein bound
Various forms can be effected by protein concentration & pH
4
Q
What is involved in Magnesium Homeostasis?
A
- Daily intake ranges from 6-20 mmol/day
- Green vegetables, hard water. Absorption in small bowel by active transport & passive diffusion
- Between 3-5% of filtered Mg appears in the urine. Evidence that kidneys can conserve Mg excretion when Mg is low
- Also intestinal adaptation when Mg is low
5
Q
How is Magnesium managed by the kidneys?
A
- Non–protein bound Mg2+ is filtered freely at the glomerulus, and the approximate percentages of filtered Mg2+ absorbed at different locations are shown.
- Under most physiologic conditions, about 10% of filtered Mg2+ is excreted.
- Final regulatory segment, the DCT, controls approximately 5% of filtered Mg2+, 25% in PCT and 60% in Loop of Henle
- Mg2+ is transported by both the paracellular and transcellular pathways.
6
Q
What are some congenital causes of magnesium disorders?
A
- Mutations in paracellin-1 and claudin-19 are involved in familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC).
- Mutations in TRPM6 are involved in HSH.
- Mutations in NaCl cotransporter are involved in Gitelman syndrome,
- Mutations in the γ subunit of Na,K-ATPase are involved in autosomal dominant renal hypomagnesemia with hypocalciuria (ADRHH).
7
Q
How are Mg and PTH related?
A
- Mg required for PTH release and action
- Mg can regulate PTH secretion in a manner similar to calcium but much less potent.
- PTH stimulated by modest hypoMg and suppressed by hyperMg
- Profound hypoMg inhibits PTH release
8
Q
What are symptoms of Hypomagnesaemia?
A
- Neuromuscular excitability with tetany and seizures
- Hypocalcaemic
- Anorexia & nausea
- Tremour
- Apathy
- Depression, agitation & confusion
- Hypokalaemic
9
Q
What cardiac effects of hypomagnesaemia?
A
- Striking relationship between moderate hypomagnesaemia and dysrhythmias
- Reduced levels of ATPase activity, loss of intracellular potassium and subsequent urinary loss
- Ratio of intra to extracellular is reduced so increased electrical excitability
- Some drug therapies for dysrhythmias are less responsive if patient hypomagnesaemic
10
Q
What are causes of hypomagnesaemia?
A
- Inadequate intake (Alcoholism, Protein Calorie Malnurition, Prologed Infusion or ingstion of low Mg diet)
- Malabsorption (Inflammatory bowel disease, Gluten enteropathy, Intestinal bypass, Radiation enteritis, Familial primary hypomagnesaemia)
- Intracellular shift (Post MI, Recovery from DKA, Refeeding syndrome, Acute pancreatitis)
- Renal tubular dysfunction
- Drugs (Proton Pump Inhibitors, Nephrotoxic drugs (amphotericin, cisplatin, laxatives), Diuretic (non-potassium sparing))
11
Q
What are conditions that benefit from magnesium supplementation?
A
- Acute myocardial infarction with: (recurrent ventricular fibrillation, recurrent ventricular tachycardia, complex ventricular tachycardia, supraventricular tachycardia)
- Digoxin intoxication with arrhythmia
- Torsades de pointes
- Low K+ unresponsive to K+ supplementation
- Low Ca++ unresponsive to Ca++ supplementation
- Diuretic therapy
12
Q
What types of patients do you expect to have hypomagnesaemia?
A
- Alcoholics
- Low Calcium
- Low Potassium
- Nutritionally Compromised
13
Q
What is refeeding syndrome?
A
- Chronic starvation leads to prolonged deficiency,
- Upon Calorie provision on feeding there is Insulin secretion
- This leads to synthesis of glycogen / fat / protein using Mg/ K/ PO4 and secretion drives Mg, K and PO4 in to cells
14
Q
What are causes of Hypermagnesaemia?
A
Usually only a problem in patients with kidney failure.
Iatrogenic
- Oral (generally + CRF) – i.e. laxatives, antacids
- Rectal - purgation
- Parenterally (Pre-eclampsia, Treatment of magnesium deficiency)
Renal failure
- Chronic (& admin Mg) – antacids, cathartic, enema, infusion, dialysis
- Acute - rhabdomyolysis
- Cardiac surgery
15
Q
What are symptoms of the Hypermagnesaemia?
A
- Asymptomatic up to 1.5 -2.5 mmol/L
- >2.5 mmol/L – areflexia, ECG changes (prolonged PR and QRS interval, peaked T waves)
- Higher levels – respiratory paralysis & cardiac arrest
- At very high levels Mg blocks acetylcholine release causing peripheral blockade
