DEPT OF ERROR

The patient whose hypocalcaemia worsened after prompt
intravenous calcium replacement therapy
Piero Stratta, Giorgio Soragna, Veronica Morellini, Massimo Gai, Daria Motta,Elisa Lazzarich, Maddalena Brustia, Marco Quaglia,Caterina Canavese
LANCET JAN. 21, 2006
As a junior in the emergency department, I saw a 30-year-old man with severe numbness, tingling in the fingers and toes and perioral area, muscle cramps, and tetany; he had chronic hypoparathyroidism, due to thyroidectomy, which was being treated with oral replacement of thyroid hormones, calcium, and vitamin D. He had been unable to take his regular treatment over the past few weeks because of gastroenteritis.

Blood tests showed a low total calcium of 1·125 mmol/L ( ie equal to 4.4 mg/dl in American terms: kk) (free ionised portion 0·51 mmol/L), phosphate 1·8 mmol/L,creatinine 88·4 mol/L, urea 3·57 mmol/L, sodium139 mmol/L, potassium 4·5 mmol/L, blood bicarbonate29 mmol/L, and pH 7·4.

Because of the severe hypocalcaemia, I chose intravenous calcium replacement according to the formula: 1–2 mg/kg per h (body-weight 50 kg) of calcium. Using ampoules of 10 mL10% calcium gluconate, I assumed that each ampoule contained 1 g (1000 mg) of calcium. I diluted the 10 mL ampoule into 500 mL 5% dextrose, calculating that each mL of this solution contained almost 2 mg of calcium(1000 mg in 510 mL=1·96 mg/mL). The infusion was scheduled at 50 mL/h for the next 10 h, delivering 98 mgof calcium per h (1·96 mg/mL at 50 mL/h); this schedule met the requirement of 1–2 mg/kg per h, as the patient weighed 50 kg.

3 h later, my consultant took me to the patient’s bedside; the young man had complained of carpopedal spasm, laryngeal spasm, and bronchospasm, and 1 h earlier he had had other typical signs of hypocal-caemia—namely, a focal seizure, arrhythmia, and prolongation of the Q-T interval. His ionised calcium concentration had decreased further to 0·49 mmol/L.

Unfortunately, my calculation of calcium replacement had been incorrect: the replacement assumed that calcium gluconate was constituted only by elemental calcium; in the formula the replacement therapy is expressed as mg of elemental calcium and not mg of calcium salt. Calcium gluconate is the calcium salt of gluconic acid, and contains only 9 mg/mL elemental calcium (table). Therefore, each ampoule of 10%calcium gluconate contained 1000 mg of calcium gluconate, but only 90 mg of elemental calcium.

100–360 mg of elemental calcium should be given over5–10 min in cases of life-threatening hypocalcaemia(free ionised calcium 0·59 mmol/L), followed by a1–2 mg/kg per h of elemental calcium. This means that one to four 10 mL ampoules of 10% calcium gluconate should have been used in our patient, as this dose raises the concentration of ionised calcium by 0·5 mmol/L. We applied this dosing-schedule to our patient, followed by an infusion of 50 mL (five ampoules) of 10% calcium gluconate diluted in 500 mL 5% dextrose (450 mg elemental calcium); the 550 mL solution contained0·8 mg of elemental calcium per mL of solution.Therefore, an infusion of 100 mL/h provided 80 mg/h of elemental calcium—equivalent to 1·6 mg/h of elemental calcium for each kg of bodyweight. IE: five ampoules in 500 ml to run 100 cc an hour that is how you treat life threatening hypocalcemia- kk- The patient’s condition gradually improved over 24 h.

Of note, calcium chloride salt is second line choice for treatment of hypocalcaemia, unless there is severe alka-losis, as it causes more tissue necrosis if extravasated.

My mistake will haunt me for the rest of my professional life. When replacing electrolytes, it is important to bear in mind the absolute need to understand basic chemistry, calculate replacement in Standard International Units, replace element and not salts, and to refer to the hospital formulary when in doubt. Lancet 2006; 367: 273

Nephrology andTransplantation, Departmentof Nephro-Urology, AmedeoAvogadro University, OspedaleMaggiore della Carità, CorsoMazzini 18, 28100 Novara,Italy (Prof P Stratta,V Morellini MD,E Lazzarich MD,M Brustia MD, M Quaglia MD,C Canavese MD); andDepartment of InternalMedicine, Section ofNephrology, University ofTorino, Molinette Hospital,Corso Bramante 88, 10126Torino (G Soragna MD, M Gai MD,D Motta MD)Correspondence to:Prof P Stratta strattanefro@hotmail.comThe patient whose hypocalcaemia worsened after promptintravenous calcium replacement therapyPiero Stratta, Giorgio Soragna, Veronica Morellini, Massimo Gai, Daria Motta,Elisa Lazzarich, Maddalena Brustia, Marco Quaglia,Caterina Canavese

Calcium salt Formula Molecular Elemental calcium Elemental Calcium weight(Ca2) mg(Ca2) mmol

Calcium gluconate C12H22CaO14430·3890 mg per2·3 mmol per10% injection 10 mL10 mL10 mL ampouleampouleampoule(1000 mg per 10 mL)/ but only 90 mg per ampoule
Calcium chlorideCaCl2110·98270 mg per6·8 mmol per10% anhydrous injection 10 mL 10 mL10 mL ampoule(1000 mg per 10 mL) but
rino, Molinette Hospital,Corso Bramante 88, 10126Torino (G Soragna MD, M Gai MD,D Motta MD)Correspondence to:Prof P Stratta strattanefro@hotmail.com
7:11 AM