The effects of plasma volume variations on the calcium concentration during epidural anaesthesia

The plasma variations of the ionized and total calcium concentration in relation to protein concentration and plasma volume were studied in 12 patients during epidural anaesthesia for transurethral prostatectomy. The most significant changes were noted during the onset of anaesthesia. From arrival in the operating theatre to start of operation, the plasma concentration of ionized calcium decreased from 1.28 (s.d. = 0.05) mmol/l to 1.22 (s.d. = 0.04) mmol/l and total calcium decreased from 2.39 (s.d. = 0.11) mmol/l to 2.17 (s.d. = 0.10) mmol/l. At the same time, however, the decrease in ionized calcium concentration and the increase in plasma volume were associated with a rise of 10.3% in the pool of free calcium ion in plasma. After the start of surgery, the plasma ionized calcium concentration remained almost unaffected throughout the rest of the study period. The total plasma calcium concentration as well as the total protein concentration remained stable during operation, only increasing after the end of surgery. The preoperative changes in the plasma calcium concentration may have been the result of both haemodilution and mobilization of calcium ions from its bone stores. Postoperatively, an increased urinary loss of calcium must have followed the administration of furosemide at the end of surgery.