Study background: Free or ionized calcium (Ca+2) is known to play a critical role in normal cardiovascular function, and Ca+2 administration in the setting of ionized hypocalcemia has been shown to improve indexes of cardiac function. The value of Ca+2 administration in the setting of cardiac arrest and resuscitation is unproven and controversial, in large part because ionized Ca+2 levels during cardiac arrest and resuscitation have not been adequately studied and exogenous calcium therapy may worsen ischemic cellular injury.
Study purpose: To measure free calcium during prolonged cardiac arrest and CPR in a canine model.
Methods and measurements: Central arterial and venous catheters were positioned in nine dogs, and ventricular fibrillation (VF) was induced electrically. After seven and one-half minutes of VF, countershocks were administered, and CPR was initiated and performed in accordance with current recommendations for 20 minutes. At five-minute intervals during resuscitation efforts, arterial pH, ionized Ca+2, and lactate as well as aortic pressure were measured.
Results: During resuscitation, average systolic arterial pressure was 50 mm Hg. Within five minutes of instituting CPR, ionized Ca+2 significantly decreased from control values (5.1 +/- 0.1 at control to 4.0 +/- 0.1 mg/dL); after 20 minutes of attempted resuscitation, it averaged 3.2 +/- 0.2 mg/dL (P less than .05 vs control). There was no change in total Ca+2 during the arrest period (9.2 +/- 0.5 at control to 8.6 +/- 0.8 mg/dL at 27.5 minutes). Arterial lactate significantly increased throughout the arrest and resuscitation period (1.9 +/- 0.2 at control to 7.5 +/- 0.4 mM/L at 27.5 minutes). A significant correlation was demonstrated between ionized Ca+2 and lactate concentrations (r = -.72, P less than .001) but not between ionized calcium and pH (r = -.22, P greater than .20).
Conclusion: Ionized hypocalcemia occurs during prolonged cardiac arrest and resuscitation, and ionized hypocalcemia during prolonged arrest and resuscitation may be due to binding by lactate, as has been demonstrated in vitro.