Magnesium depletion in patients treated with therapeutic hypothermia after cardiac arrest

Magnesium (Mg(2+)) depletion can have detrimental effects in postcardiac arrest patients through multiple potential mechanisms. Therapeutic hypothermia (TH) produces a Mg(2+) diuresis, but the effects of postcardiac arrest TH on serum Mg(2+) levels in patients with postcardiac arrest syndrome (PCAS) are yet to be systematically quantified. We conducted a retrospective chart review of 119 consecutive comatose PCAS patients treated with TH between 2005 and 2010 and compared them to 33 matched historic controls (HCs) seen at the same institution between 2002 and 2005 who were not treated with TH. We abstracted data from the first 96 hours postarrest, including date, time, and value of serum Mg(2+) levels and date, time, and amount of Mg(2+) repletion, along with outcomes at discharge. The median Mg(2+) level of TH patients was 2.0 mg/dL [interquartile range (IQR), 1.9-2.2 mg/dL] (0.82 mmol/L [IQR, 0.78-0.90 mmol/L]) versus 2.2 mg/dL [IQR, 1.9-2.4 mg/dL] (0.90 mmol/L [IQR, 0.82-0.99 mmol/L]) (p=0.2) in HCs. In addition, 42.9% (520/1214) of Mg(2+) levels in TH patients versus 31.9% (43/135) (p=0.014) in HC patients were below 2.0 mg/dL [0.82 mmol/L]. The average number of times the Mg(2+) level was checked in TH patients was 10.2 (range 1-18) versus 4.1 (range 1-10) in HCs. The TH patients were more likely to receive supplemental Mg(2+) than HCs (81.5% [97/119] vs. 27.3% [9/33] [p<0.01]). The mean supplemental Mg(2+) dose was 1.9 g for TH patients versus 0.5 g for HC patients. Mortality in patients treated with TH was 53.1% (60/113) versus 78.6% (22/28) (p=0.014) in HCs. Low serum Mg(2+) levels with subsequent Mg(2+) supplementation were more common in comatose patients with PCAS treated with TH compared to normothermic HC patients. The effect of untreated hypomagnesemia on postcardiac arrest outcomes remains to be determined.