Plasma ionized calcium during open-heart surgery

Con: calcium is not routinely indicated during separation from cardiopulmonary bypass

Severe hypocalcemia is uncommon in adult cardiac surgery patients; the nearly ubiquitous mild hypocalcemia does not impair myocardial performance. Clinicians should recognize that in certain circumstances, calcium may interact negatively with catecholamines such as epinephrine or dobutamine. Lastly, evidence suggests that calcium influx during ischemia-reperfusion contributes to myocardial dysfunction after CPB. Therefore, there appears to be no justification for the practice of routinely administering large doses of calcium salts to adult cardiac surgery patients after CPB.

Calcium administration for cardiovascular support in critically ill patients: when is it indicated?

Calcium has a fundamental role in the maintenance of myocardial function and vascular tone. The ionized form of calcium is the most important physiologically, and this form needs to be measured to assess physiologically active calcium levels. Ionized hypocalcemia can occur as a result of various pathophysiological disturbances, and it is seen frequently in critically ill patients. Several investigators have observed a poorer prognosis in those patients with ionized hypocalcemia. It is unclear whether calcium supplementation is beneficial in these patients. It may improve cardiovascular performance, but, in contrast, it may contribute to cellular damage (especially during hypoxia following cardiopulmonary resuscitation). In sepsis, there may be an increased cellular influx of calcium, which may be deleterious to cellular function; indeed, calcium entry blockers in this situation may be protective. We review the role of calcium as an inotropic agent, its interaction with other inotropic agents, and its use during blood transfusion and during cardiopulmonary resuscitation.

Changes in ionized calcium and related blood constituents in children after cardiopulmonary bypass

Calcium is necessary for myocardial muscle contraction. The ionized calcium fraction in blood is the physiologically active component and can be readily measured with calcium-selective electrodes. Cardiopulmonary bypass (CPB) alters the relationship between calcium and related blood constituents, depending on the composition of the pump-prime and the quantity of blood products administered peri-operatively. Serial measurements of total and ionized serum calcium concentrations and related biochemical parameters were made in 115 consecutive children over a 24-hour period before and after CPB. Total and ionized calcium concentrations were consistently elevated in the immediate postoperative period, and though infants may, theoretically, benefit from calcium infusion after CPB, the uncritical use of calcium salts in children after cardiac surgery is not justified.

Changes in calcium fractions during cardiopulmonary bypass in infants and neonates

The changes in total blood calcium fractions were investigated in 20 infants and neonates undergoing open-heart surgery for correction of congenital heart defects. On initiation of cardiopulmonary bypass (CPB), total serum calcium fell from 2.29 ± 0.05 mmol/I to 1.8 ± 0.04 mmol/I and there was a profound fall in biologically active ionized calcium from 1.26 ± 0.02 mmol/I to 0.49 ± 0.04 mmol/l. Protein-bound calcium also fell from 1.03 ± 0.07 mmol/I to 0.09 ± 0.03 mmol/l. Complexed calcium increased from 0.08 ± 0.02 mmol/I to 1.1 ± 0.05 mmol/l. The large fall in ionized calcium and increase in complexed calcium could be accounted for by the corresponding large increases in serum citrate (162.2 ± 29.8 μmol/l to 1689.8 ± 163.1 μmol/l) and serum lactate (1.27 ± 0.18 mmol/I to 7.79 ± 0.72 mmol/l). All measured fractions of calcium returned towards their pre-operative levels by the end of bypass. However, 13 of the 20 patients had less than optimal levels of ionized calcium when being weaned from bypass despite the empirical use of intravenous calcium supplements. This study demonstrates that ionized calcium can be quickly and easily measured in the peri-operative period, that there can be extremely large changes in calcium fractions during CPB in infants and neonates, and that ionized calcium can be unexpectedly low in the critical period when weaning patients from bypass. Total calcium measurements gave an inaccurate estimate of the ionized calcium fraction.

Parathyroid hormone responses to marked hypocalcemia in infants and young children undergoing repair of congenital heart disease

OBJECTIVES

The integrity of the parathyroid axis was tested in 18 infants and young children undergoing repair of congenital heart disease with cardiopulmonary bypass.

BACKGROUND

Infants are believed to have an immature parathyroid hormone response to hypocalcemia. Whereas adults are known to respond appropriately to hypocalcemia during cardiopulmonary bypass, children have not been studied carefully.

METHODS

Calcium, magnesium, parathyroid hormone, phosphate and total protein were measured in blood samples withdrawn at defined times before, during and after cardiopulmonary bypass.

RESULTS

At the initiation of cardiopulmonary bypass, ionized calcium decreased markedly in 12 infants less than or equal to 24 months old (mean +/- SEM 1.11 +/- 0.04 to 0.29 +/- 0.05 mM) and decreased significantly in 6 young children greater than 24 months old (1.19 +/- 0.02 to 0.42 +/- 0.12 mM). In response to hypocalcemia, parathyroid hormone concentration increased significantly in both the infants (from 42 +/- 8 to 103 +/- 29 and 85 +/- 22 pg/ml) and the young children (from 39 +/- 8 to 44 +/- 20 and 92 +/- 30 pg/ml). Before separation from cardiopulmonary bypass, increased parathyroid hormone concentration restored ionized calcium concentration to 0.75 +/- 0.03 mM in the infants and to 0.92 +/- 0.07 mM in the young children. There was no significant influence of either age or the use of deep hypothermia and circulatory arrest on either calcium or parathyroid hormone responses. Total magnesium and total protein concentrations decreased on initiation of cardiopulmonary bypass and thereafter remained stable. Phosphate concentrations were unchanged during the study.

CONCLUSIONS

In infants and young children undergoing cardiac surgery, the parathyroid hormone response to both hypocalcemia and to rising ionized calcium concentrations was at least as great as that of adults. Thus, the calcium-parathyroid-vitamin D axis functions in infants and young children as it does in adults.

Plasma ionized calcium during paediatric anaesthesia: effects of pH and succinylcholine

Variations of plasma ionized calcium and pH were studied in 16 children anaesthetized with halothane and succinylcholine for minor surgery. During anaesthesia, a close negative correlation was noted between pH and ionized calcium. Intubation was followed by a fall of the ionized calcium concentration. A mean decrease of 0.04 mmol X L-1 was noted, without pH change. The acute elevation of carbon dioxide following extubation allowed the in vivo effects of pH on ionized calcium to be studied. For each pH unit decrease, a mean rise of 0.60 mmol X L-1 of ionized calcium was found. During the study, no significant variations of plasma total calcium concentration were observed. The variations in the hydrogen ion concentration and the administration of succinylcholine were accompanied by changes of the free calcium ion concentration, due probably to alterations in calcium binding to proteins, and perhaps to the formation of a calcium complex with anions. Our results emphasize the importance of knowledge of the pH for the interpretation of ionized calcium variations in the presence of acid-base disturbances.

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.