Magnesium Disorders Flashcards
Clinical Practice Review JVECC 2015
Magnesium physiology and clinical therapy in veterinary critical care
WHat is the suspected insidence of hypomagnesemia in critically ill canine and feline patients?
Dog: 54%
Cat: 50% hypo or hyper (didn’t distinguish)
Where is the majority of magnesium found?
Intracellular - 99%
In what forms does serum magnesium take?
Protein bound (30-40%), complexed to anions (4-6%), or ionised (55-65%)
Ionised is physiologically active
In what tissue is most magnesium found
Bone
What is the normal cytosolic magnesium conc and what are the three mechanisms that maintain it?
0.5-1mmol/L
1. Intracellular protein binding, 2. influx and efflux of magnesium, 3. Sequestration
What is the affect of increased cAMP on mg levels in the cell
May be increased by catecholamines or by calcium/magnesium sensing receptor –> causes release from organelles, increasing intracellular mg, which then may cross into the ECF via trPM7.
Cellular Functions of MG
What are the functions of Mg2+
controlling ox phos
Na/K and ca ATPases require Mg as a cofactor
involved in t cell activation
depolarisation of cardiac and neuronal cells
vascular contractility
regulate calcium movement as both often bind to same divalent cation receptors
required for antioxidant synthesis
Mg2+ Absorption and Excretion
What is total body magnesium dependent on ?
Intestinal and renal absorption and excretion
Through what mechanisms may magnesium be absorbed?
Transcellular or paracellular (paracellular is primary)
Transcellular occurs due to divalent cation receptors
By how much can intestinal absorption be increased in times of need?
6x
Approximately how much renally filtered mag is resorbed
> 95%
Where does most renal absorption of mg occur?
Loop of henle - 80%
Proximal tubules 5-15%
DCT 5-10%
What is the more ideal way to measure mg?
Ionised however given so much is intracellular while ionised is thought to reflect total body mg status it may not be entirely accurate
Mg2+ Disorders
What are the two most common causes of mg2+ excess
iatrogenic and renal failure
What Csx may be seen with hypermg
Hypotension - due to excess Mg2+ binding to Ca receptors in vascular smooth muscle inhibiting contraction
may also see prolonged P-R interval and bradycardia
Neuro –> decreased deep tendon reflexes, mental depression, flaccid paralysis
GI: V+ and Ileus
What are examples of causes of hypomag
Decreased intake - anorexia
Decreased GI absorption - V+, D+, IBD, EPI
Body compartment loss - Insulin, catecholamine excess, glucose admin, pancreatits
Renal loss - diuretics, saline
Renal disease - post-obstructive diuresis, dialysis, nephritis or RTA
Endocrine - HyperA, Hypercalm Hyperthyroid,
Drugs - ACEi, citrate, beta agonists, insulin, mannitol
Other - lactation, pregnancy, burns, growth
Clinical Signs of hypomagnesemia?
Resp - bronchoconstriction
Cardiovascular - vasospasm, hypertension
ECG - VPCs, Vtach, Afib
Neuromuscular - Tetany, muscle spasm, MSK weakness, Seizures,
WHy does hypomag result in refractory hypokalaemia?
Impaired magnesium-dependent potassium channels result in continuous loss of potassium into the ECF and renal wasting
Why is hypomag associated with impaired insulin sensitivity?
magnesium serves as a cofactor for insulin release,
also maintains normal cellular response to insulin
How may magnesium be involved in analgesia?
NMDA antagonism and inhibition of substance P release
How may hypomagnesemia affect the immune system?
Substance P activation
less antiox
increased inflammatory mediators
TNFa
How may hypermagnesemia be treated?
Magnesium free IVFT
Calcium gluconate
mannitol or diuretics
Dialysis
How may hypomagnesemia be treated?
Oral or IV dosing
Dosing is pretty unknown
Recommended at 0.1-0.15mmol/kg (0.2-0.3mEq/kg) for acute
CRI 0.1-0.5mmol/kg/day (0.2-1mEq/kg/day) may be required
What fluids are incompatible with mag ?
Ca, Bicarb and lactate containing fluids
What should be monitored during magnesium infusion in case of hypermagnesemia?
GI signs - V+, lethargy, diarrhoea
CV - hypotension, bradycardia, prolonged Q-T, PR prolongation, QRS widening
Magnesium as Adjuct to Therapy
How may magnesium be neuroprotective?
Shown to significantly improve survival in people suffering from TBI, modulates several pathologic pathways in secondary brain injury, NMDA antagonism
