Systemic Absorption of Magnesium Citrate: A Cause for Preoperative Lethargy?
Julia Bauerfeind, MD and R. Victor Zhang, MD,
PhD
Department of Anesthesiology, University of Florida, Gainesville, Florida
Introduction: Magnesium citrate is commonly used as a laxative or for
preoperative cleansing of the bowel for patients undergoing gastrointestinal
(GI) surgery. Patients scheduled for GI surgery often present with altered
GI function, e.g. delayed gastric emptying, ileus, and mucosal irritation.
These alterations make it difficult to predict the amount of systemic
magnesium uptake that takes place across the injured barrier. Therefore,
close attention should be paid to recognize clinical symptoms of
hypermagnesemia preoperatively, such as weakness, lethargy, hypotension and
bradycardia, as these may further progress to neuromuscular blockade and
heart block, imposing an increased risk for general anesthesia.
In this report we discuss a case of hypermagnesemia, preoperatively
diagnosed because of a decrease in the patient’s vigilance in the
preoperative holding area. Clinical findings correlated with laboratory
results that showed acutely an increased concentration of magnesium and
resulted in postponement of the surgery until magnesium concentrations had
normalized.
Clinical Course: A 58-year-old woman was scheduled for total gastrectomy for
a large stromal cell tumor involving the fundus and posterior wall of the
stomach with an adjacent ulcer. A CT scan showed no evidence of metastasis.
Medical history included hypertension, hypothyroidism and newly diagnosed
diabetes mellitus. The patient denied having heart or renal disease in the
past. In the weeks prior to surgery she had received a blood transfusion for
a GI bleed that had decreased her hematocrit from 32% to 26%.
Preoperative pertinent laboratory values showed hematocrit 32%, BUN 14
mg/dl, creatinine 1.0 mg/dl, calcium 7.8 mg/dl, and magnesium 2.1 mg/dl; all
other laboratory values were within normal limits. The day prior to surgery,
the patient had received 120 ml of magnesium citrate as a bowel prep
beginning at noon. On the morning of surgery the patient was found to be
weak and lethargic, an obvious change from the day before. Blood pressure
and heart rate were decreased by 30%. Laboratory results showed a critically
high concentration of magnesium, 3.7 mg/dl. All other parameters were
unchanged. The surgery was postponed until clinical symptoms subsided and
magnesium concentrations were on a downward trend. The patient remained
clinically stable and was taken to the OR later that afternoon.
Discussion: Hypermagnesemia in the perioperative setting puts patients at
risk for an array of complications that may manifest during general
anesthesia. Most cases of hypermagnesemia (> 2.8 mg/dl), seen especially in
patients with impaired renal function, are iatrogenic, resulting from the
administration of magnesium in antacids, enemas or parenteral nutrition. In
our case, magnesium most likely was absorbed through highly vascularized and
altered mucosa in the presence of a gastric tumor. The fact that the
patient’s clinical symptoms were recognized early and correlated with
abnormal laboratory values, resulted in intentional delay of surgery and
subsequently an uncomplicated anesthetic course. This unique clinical
scenario has not previously been described.
