European Journal of Cardiovascular Medicine

Electrolytes such as potassium, magnesium, calcium, and phosphate play an important role in  cell membrane potential regulation. A severe arrhythmia and neuromuscular dysfunction results from the depletion of these electrolytes.  Dyselectrolemia in the setting of cardiac diseases may be life threatening [1]. Hypokalemia causes cardiac arrhythmias, especially in ischemic heart disease and left ventricular hypertrophy. Hence monitoring potassium levels is of tremendous value in most intensive care units (ICUs). Electrolytes such as magnesium, calcium and phosphate depletion may adversely affect outcomes – especially in cardiovascular patients. Studies have even linked hypomagnesemia to increased mortality both in ICUs [2,3] and general wards[3].

Hypomagnesaemia results in adverse outcomes in patients with acute/chronic coronary syndromes [4], and supplementing magnesium has been demonstrated to reduce mortality [5]. Hypomagnesemia causes cardiac arrhythmias, vasoconstriction and impaired insulin sensitivity [6,7]. Moreover, magnesium is a free radical scavenger preventing reperfusion injury [8], which has been implicated in myocardial injury in perioperative coronary bypass surgery [9].

Hypophosphatemia disrupts numerous enzymes involved in energy metabolism and lowers intracellular ATP concentrations. Myopathy, respiratory failure, susceptibility to respiratory infections, myocardial depression and a reduced cardiac output are associated with hypophosphatemia [7][10,11]. Hypophosphatemia causes ventricular tachyarrhythmia in ischemic heart disease patients [12].

Hypocalcaemia is implicated in cardiovascular depression [13]. It causes congestive heart failure that do not respond to inotropic agents, in patients with cardiomyopathies [14]. Hypomagnesemia and concomitant hypokalemia results in additive effect causing tachyarrhythmia and hemodynamic compromise during perioperative period [15].

Hypothermia during cardiac surgery also leads to dyselectrolemia.

We measured serum levels of magnesium, phosphate, potassium, calcium and sodium in 100 consecutive patients. 50 patients undergoing cardiac surgical interventions (group 1) and the rest 50 underwent lung/peripheral vascular surgery. For our laboratory's reference values, the following mmol/l were established: magnesium 0.8–1.1, phosphate 0.7–1.2, potassium 3.8–4.8, calcium 2.20–2.60 and sodium 135–145.

All patients (n=50) in group 1 underwent on pump cardiac surgery procedures with cardioplegic arrest using cold crystalloid solution St. Thomas.

Fifty patients who had other major elective operations (lung surgery and repair of abdominal aortic aneurysms) were used as the control group (group 2). Since this study was conducted in Dept. of CTVS, patients undergoing other surgical procedures were deliberately excluded [16]. Patients with emergency surgical procedures were excluded from the study.

Patients in Group 1 were administered low-dose dopamine/dobutamine/norepinephrine/epinephrine and nitroglycerin according to clinical situation intra and postoperative period. MgSO4 and phosphate containing fluids were also introduced after blood aspiration for all participants of this group. Simultaneously, potassium was initiated within the first group during the surgery, which continued and increased for those admitted to intensive care unit

Tables 1 and 2 summarize the results. Group 1 (cardiac surgery patients) had levels of potassium, magnesium, phosphate and calcium that were significantly lower than group 2 (control individuals). Even with substantial electrolyte correction, the differences in potassium levels between the two groups were starkly visible; 10.4 ± 4.6 mmol/ hour for group 1 versus only 1.6 ± 1.4 mmol/hour for group 2 (P < 0.001). A similar observation was observed for magnesium, with 38 patients in group 1 receiving an average amount of 2.1 g due to arrhythmias as opposed to only one patient in group 2 (P < 0.001). 8 patients in the cardiac surgery group received calcium as compared to one in the control group (P < 0.001).

TABLE 1 Comparison of patients undergoing on pump arrested heart cardiac surgery (group 1) with those of patients undergoing lung / vascular surgeries off cardiopulmonary bypass (group 2)

TABLE 2 Comparison of electrolyte levels among patients undergoing on pump arrested heart cardiac surgery (group 1) with those of patients undergoing lung / vascular surgeries off cardiopulmonary bypass (group 2)

In Group 1, 59% patients had magnesium levels below 0.70 mmol/l, compared to 9 % of patients in group 2 (p < 0.001). In group 1, 88% had phosphate levels lower than 0.60 mmol, compared to 10% in group 2 (P < 0.001). Moreover, 45% patients in group 1 experienced moderate hypokalemia (potassium <3.6 mmol/l) even with potassium supplementation which was significantly higher compared to 8% patients in group 2 (P < 0.001).

This study proves that patients undergoing extracorporeal cardiac surgery are at a high risk of electrolyte depletion. The factors responsible are increased urine excretion and trans cellular shifts due to the cardiopulmonary bypass and hypothermia during surgery. It appears that shear forces at play are an additional mechanism responsible for electrolyte depletion.

 Several potential mechanisms could be the cause of intracellular changes seen in our findings. Literature review indicate that hypomagnesemia, hypophosphatemia, hypocalcaemia in patients undergoing on pump open heart surgery can be attributed to hemodilution [17]. Most patients needed potassium during surgery and many had at least one dose of magnesium. Clinical criteria were also observed despite the extra potassium supplements being given.

 Hypomagnesemia causes vasoconstriction leading to hypertension, coronary arteries spasm, and cardiac arrhythmias [6, 7][18]. Magnesium appears acts as physiological calcium channel blocker, but there are no negative contractile effects associated with it [18, 19]. Blood vessels (including coronary arteries and possibly mammary arteries, which are commonly used in mammary bypass surgery) become more susceptible to vasoconstrictors when hypomagnesemia is present [20, 21].The majority of the heart surgery patients we examined showed that their phosphate levels were low. This was likely due to the fact that no phosphate had been added during surgery and increased intracellular transport may have occurred. Hypophosphatemia has already been noted as an after effect of cardiac surgeries, and this study noted alleviation through blood transfusion. The anticoagulant solutions used for stored blood contain a relatively high amount of phosphate, which could explain this. As previously mentioned in the introduction, hypophosphatemia can cause adverse effects on myocardial and respiratory function, and cited studies suggest that it can negatively affect cardiac surgery outcomes. The clinical problems associated with hypocalcemia are also briefly described in the introduction above. Usually, mild hypocalcaemia is asymptomatic, but it depends on the level of other electrolyte disturbances as well as how quickly it develops. We observed mild hypocalcaemia in our patients, possibly due to magnesium deficiency (a common cause of hypocalcaemia). We observed no visible signs of hypocalcaemia, such as stiffness.

In patients undergoing cardiopulmonary bypass cardiac surgery, electrolyte depletion was a high risk. Due to a combination of intraoperative hypothermia and extracorporeal circulation, the mechanism is most likely a combination of increased urinary excretion and intracellular migration. Our findings may partly explain the elevated risk of tachyarrhythmia in patients who undergo cardiac surgery. Additionally, electrolyte depletion might heighten this risk. Thus, we advise frequent measurement of magnesium, potassium, phosphorus and calcium levels during and after heart surgery. We suggest that careful monitoring and correction of electrolytes shall be beneficial for the prevention of postoperative tachyarrhythmia and for optimizing outcomes in those undergoing cardiac surgery

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