From WardWiki - Foundation Doctor Helper
Magnesium is the second most common intracellular cation. Its extracellular concentration is of great clinical importance but continues to be poorly measure by medical and surgical teams alike. Its main store is in the bones but its mobility is rather limited, being largely dependent on co-resorbtion with calcium which is tightly regulated by the parathyroid gland. It is usually raised only when supplemented excessively (tumour lysis syndrome can raise magnesium); low magnesium levels are much more common.
It is collected in a lithium heparin bottle and therefore it is usual to take a U&E sample at the same time. Blood will start to coagulate is is drawn from the body and will separte further on centrifuge to leave a serum layer to be analysed.
Hypomagnesaemia is an avoidable, treatable cause for arrhythmias and general weakness in hospital inpatients. 40% of hypokalaemic patients have hypomagnesaemia and will not be able to retain potassium until magnesium is supplemented.
Treatment of hypomagnesaemia
Each ward will have its own infusion policy, sadly it is not easy to prescribe an infusion faster than the ward limit. Bolus injection can cause flushing and hypotension and the transiently high magnesium level may lead to renal excretion before full utility can be gained. it is limited to emergency situations.
|Infusion (8mmol/hour)||Infusion (1mmol/hour)|
|8mmol in 250ml normal saline||8-12mmol in 1 litre normal saline|
Measure serum magnesium one hour after infusion has finished if further supplementation is required.