Calcium, magnesium and phosphate during and after hypothermic cardiopulmonary bypass without temperature correction of acid base status

Electrolyte considerations in targeted temperature management

PURPOSE

Targeted temperature management (TTM), including normothermia and therapeutic hypothermia, is used primarily for comatose patients with return of spontaneous circulation after cardiac arrest or following neurological injury. Despite the potential benefits of TTM, risks associated with physiological alterations, including electrolyte shifts, may require intervention.

SUMMARY

This review describes the normal physiological balance of electrolytes and temperature-related alterations as well as the impact of derangements on patient outcomes, providing general recommendations for repletion and monitoring of key electrolytes, including potassium, phosphate, and magnesium.

CONCLUSION

Frequent monitoring and consideration of patient variables such as renal function and other risk factors for adverse effects are important areas of awareness for clinicians caring for patients undergoing TTM.

Calcium Administration During Weaning From Cardiopulmonary Bypass: A Narrative Literature Review

The search for safe and effective patient management strategies during weaning from cardiopulmonary bypass is ongoing; intravenous calcium is occasionally used as a first-line drug. The physiologic role of calcium suggests that it can support the function of the cardiovascular system during this critical period. Patients may be mildly hypocalcemic after cardiopulmonary bypass; however, this degree of hypocalcemia does not significantly impair the cardiovascular system. The transient beneficial effects of calcium administration (increase in arterial blood pressure, systemic vascular resistance, cardiac index, stroke volume, and coronary perfusion pressure) might be helpful in cases of moderate contractility reduction or vasoplegia. Nonetheless, effects on clinically relevant endpoints are unknown, and possible systemic side effects, such as transient reduction in internal mammary artery graft flow, attenuation of the effects of β-sympathomimetics, "stone heart" phenomenon, and pancreatic cellular injury, may limit the use of calcium salts. Further studies are needed to expand the understanding of the effects of calcium administration on patient outcomes.

Accidental hypothermia and active rewarming: the metabolic and inflammatory changes observed above and below 32 degrees C

OBJECTIVES

In accidental hypothermia the underlying physiological mechanisms responsible for poor outcome during rewarming through 32 degrees C remain obscure, although possible associations include changes in acid-base balance, divalent cations, and inflammatory markers. This study investigated the metabolic and inflammatory changes that occur during the rewarming of hypothermic patients.

METHODS

Eight patients, four men and four women, age 45 to 85 years, admitted with core temperatures <35 degrees C were included in the study. Patients were rewarmed with dry warm blankets and fluid replaced by crystalloid at 40 degrees C. Bloods for pH, ionised calcium (Ca(2+)) and magnesium (Mg(2+)), parathyroid hormone (PTH), interleukin 1 (IL1), interleukin 6 (IL6), tissue necrosis factor alpha (TNFalpha), were collected at presentation, during rewarming, and at 24 hours.

RESULTS

Four patients were admitted with mild (32 degrees -35 degrees C) and four with moderate (28 degrees -32 degrees C) hypothermia. Rewarming to 32 degrees C had no significant effect on the presenting acidosis (p=0.1740), although above 32 degrees C pH increased with temperature (p<0.0001). There was a negative correlation between pH and both Ca(2+) (p=0.0005) and Mg(2+) (p=0.0488) below 32 degrees C; above this temperature the relation was significant only for Ca(2+) (p=0.0494). PTH and Ca(2+) correlated positively (p=0.0041) and negatively (p=0.0039) below and above 32 degrees C respectively. There was no relation between IL1 or TNFalpha with Ca(2+) during rewarming, but IL6 and Ca(2+) correlated positively (p=0.0039) and negatively (p=0.0018) when presentation temperature was below and above 32 degrees C respectively.

CONCLUSIONS

During rewarming pH remains unchanged until patient temperature approaches 32 degrees C. Ca(2+) and Mg(2+) decline is associated with the pH increase above 32 degrees C. Poor outcome is associated with presentation temperature (<32 degrees C), non-physiological correlation between IL6-PTH-Ca(2+), and age (>or=84 years).

Comparison of acid/base status in conscious and anaesthetized rats during acute hypothermia

Acute hypothermia was surface-induced in unrestrained conscious rats at two different levels, moderate (30 degrees C TB) and severe (20 degrees C TB). Data reflecting the acid/base status were determined. The values obtained for moderate hypothermia were compared with the acid/base pattern observed during hypothermia induced by two different anaesthetics, sodium pentobarbital and urethane, at room temperature. Conscious, hypothermic animals developed an apparent respiratory alkalosis, with an increase in pHa (from 7.476 to 7.546 in moderate hypothermia and from 7.484 to 7.563 in severe hypothermia) correlated with a decrease in arterial bicarbonate levels (from 22.9 to 16.8 mmol l-1 and from 20.7 to 14.9 mmol l-1 respectively). Lactate increased slightly in conscious, severely hypothermic rats (1.02 mmol l-1). This acid/base pattern was clearly different from that seen in sodium pentobarbital (mild respiratory acidosis) and urethane-induced hypothermia (metabolic acidosis). These results suggest that conscious rats follow a pattern closer to that underlying the relative alkalinity shown by many poikilotherms than to that underlying the constant pH shown in hibernating mammals. This latter pattern, nevertheless, approaches that observed during moderate pentobarbital hypothermia and the acid/base pattern during shallow hypothermia in birds. Anaesthesia may interfere with the development of the processes that lead to the acid/base pattern observed in conscious animals.

New whole blood analyzers and their impact on cardiac and critical care

Miniaturized whole blood biosensors, patient-focused hospitals, and rising expectations of patients and physicians are shifting laboratory diagnostics to the point of care. Expanding transplantation and intensive care are increasing the need for rapid test results. Whole blood analysis improves accuracy, eliminates centrifugation, reduces response time, and conserves blood volume. Several hand-held, and over 20 portable or transportable whole blood instruments are now available. Criteria for instrument evaluation include test menus, point-of-care features, analysis time, on-site performance, and information integration. Whole blood analyzers measure several vital indicators (pO2, pCO2, pH, hematocrit, K+, Ca2+, Na+, Cl-, glucose, and lactate) simultaneously in less than 2 min with less than 200 microliters of whole blood. Other in vitro tests are available (Mg2+, osmolality, CO2 content, urea nitrogen, beta-hydroxybutyrate, hemoglobin, coagulation) or under development (HCO3- phosphorus). Some can be monitored in vivo (O2 saturation, pO2, pCO2, pH, glucose) or ex vivo. The clinical impact is demonstrated by ionized calcium, now established in importance for cardiac and neurologic problems, and ionized magnesium, a promising new measurement. The hybrid laboratory (a composite of conventional clinical laboratory and patient-focused testing), performance maps, and quality paths facilitate implementation of new whole blood analyzers for optimal support of cardiac and critical care, and improved patient outcomes (prospects).

Effect of acid-base management with or without carbon dioxide on plasma phosphate concentration during and after hypothermic cardiopulmonary bypass

Variations of the phosphate concentration in plasma were studied in two groups of 12 patients undergoing cardiac surgery with hypothermic cardiopulmonary bypass (CPB). Management of the acid-base status differed between the groups, according to whether or not carbon dioxide was added to the anesthetic gas mixture during hypothermia ('pH-stat' vs. 'alpha-stat' mode) following correction vs. no correction of pCO2 and pH for body temperature. Phosphate variations throughout the study were mostly within normal limits. From the start to the end of CPB, the mean rise in phosphate levels was 70% in the pH-stat group and 37% in the alpha-stat group (p < 0.001). During 3 hours after CPB, the phosphate values continued to rise by a mean of 25% in the alpha-stat patients, but fell by a mean of 3% in the pH-stat patients (p < 0.001). Such different phosphate patterns during and immediately after CPB may reflect profound metabolic disturbances and may be related to the altering effects of CO2 addition and respiratory acidosis on intracellular metabolic activity and phosphate homeostasis.

Effects of erythrocytes, bicarbonate, temperature and albumin on in vitro ionized calcium variations with pH

Using a heart lung machine as an in vitro model, the relation between ionized calcium (cCa2+) and pH has been shown to depend on several variables such as erythrocytes, temperature and albumin as well as the total calcium and bicarbonate concentrations. The respiratory acid-base disturbances were simulated by changing the gas flow between 1.0 and 2.41 min-1 by adding CO2, to the machine at a concentration of 0 to 17%. When pCO2 was used to alter pH, cCa2+ varied from 0.16 mmol l-1 per pH unit to 0.52 mmol l-1. The regression slope of cCa2+ on pH was made steeper by decreasing erythrocyte volume fraction and by increasing temperature and the concentrations of HCO3, calcium or albumin. The metabolic acid-base alterations were produced by HCl or NaHCO3 at a constant gas flow, cCA2+ changes per pH unit were 0.70 mmol l-1 in plasma and 1.04 mmol l-1 in whole blood. The different results found in plasma and in erythrocyte fluids may be explained by their different buffering capacity. Haemoglobin may buffer hydrogen ions, and the formation of HCO3- is catalysed by carbonic anhydrase from the red cells.