INTRODUCTION

Cardiopulmonary bypass (CPB) is a procedure in which venous blood is diverted away from the heart and lungs to an extracorporeal system that oxygenates and pumps blood back to the arterial side of the systemic circulation. CPB provides a motionless and bloodless operative field and time to perform precise cardiac repair. CPB has been used to treat a variety of cardiac conditions in dogs.^1-10 The feasibility of CPB in cats has been demonstrated experimentally, although its use in the clinical setting has not yet been pioneered.¹¹ Dogs undergoing open heart correction under CPB require all of the considerations for care after cardiac surgery outlined in Chapter 149, Postcardiac Surgery Management, as well as a full understanding of the pathophysiologic consequences and special considerations for supportive care imposed by CPB itself.

SYSTEMIC RESPONSE TO CARDIOPULMONARY BYPASS

CPB and open heart surgery expose the entire blood mass to biomaterials within the perfusion circuit and to nonendothelial surfaces within the wound. Further, CPB imposes the additional aberrations associated with significant dilution of the blood and whole-body hypothermia. Together these events initiate a systemic response mediated by contact activation of five blood cell types (neutrophils, platelets, endothelial cells, monocytes, lymphocytes) and five blood protein systems (intrinsic coagulation, extrinsic coagulation, fibrinolysis, kinin, and complement cascades).^12,13 These effects cause a pathophysiologic state of generalized nonseptic inflammation, consumptive coagulopathy, and increased vascular permeability collectively referred to as the systemic inflammatory response after bypass (SIRAB). The SIRAB response has pathologic similarities to the systemic inflammatory response syndrome (SIRS) associated with sepsis and other generalized inflammatory states. It is critically important that the role that SIRAB plays in post-CPB patients be understood and considered when providing supportive care after open heart surgery.

FLUID AND ELECTROLYTE THERAPY

Supportive fluid therapy after CPB should be guided by the central venous pressure (CVP). The optimal CVP after CPB is generally between 4 and 8 cm H₂O (3 and 7 mm Hg). Insight about the optimal CVP for individual patients is gained during weaning from CPB. As with all cardiac surgery patients, the goal of fluid therapy is to maintain a vascular volume adequate to support cardiac function without worsening or precipitating congestion. An additional important consideration is the very considerable vascular permeability issue caused by CPB. In fact, the most common error in managing dogs after open heart surgery and CPB is the overadministration of crystalloid fluids resulting in significant third spacing of water and diminishing pulmonary and organ function. As a general rule, crystalloid fluids should not be administered at greater than maintenance rates and volumes. If additional fluid volume is necessary, it should take the form of colloid-type fluids such as fresh frozen plasma, 5% albumin, stored red blood cells (RBCs), whole blood, or washed autogenous RBCs (see following discussion of coagulopathy and hemorrhage). Choice of colloid-type fluid support is determined by availability and guided by the therapeutic goals of maintaining the total plasma protein over 4 g/dl, colloid osmotic pressure over 12 mm Hg, and packed cell volume over 30%. Synthetic colloid solutions (e.g., hydroxyethyl starch [hetastarch]) should be avoided after CPB because of their effect on coagulation and tendency to leak out of the vascular space.

Electrolyte abnormalities are often present after CPB. Several factors contribute to these abnormalities, including hemodilution from CPB priming solutions, infusion of cardioplegia solutions into the bypass circuit, and hormonal aberrations caused by surgery, CPB, and heart failure. Hypocalcemia is invariably present in dogs during and after CPB, especially in small dogs. Hypocalcemia is corrected by intravenous bolus administration of calcium chloride (10 mg/kg) during weaning and during the first 1 to 2 hours after CPB. Ionized calcium should be maintained above 1 mmol/L. Supplementation of calcium beyond the first few hours after CPB is generally not necessary. Administration of calcium salts to boost cardiac function in dogs that are not hypocalcemic should be avoided.

Hypokalemia is often present after CPB in dogs despite the infusion of high-potassium cardioplegia solutions during bypass. Because of its proarrhythmic effects, hypokalemia should be corrected within the first few hours after CPB by constant rate infusion (CRI) of potassium chloride (0.25 mEq/kg/hr) until the serum potassium level is greater than 3.5 mEq/L. After the initial correction of hypokalemia, addition of potassium chloride to maintenance fluids (20 to 30 mEq/L) is generally adequate to maintain potassium levels.

Both hyponatremia and hypernatremia can occur in dogs after CPB. Both of these abnormalities are most likely to develop during the first 24 hours after CPB and thus sodium levels should be monitored every few hours during this period. Hypernatremia is more likely to develop in dogs undergoing mitral valve surgery that had severe preoperative congestive heart failure (CHF). If hypernatremia develops (sodium level >155 mEq/L), then a low-sodium crystalloid fluid (e.g., 0.45% saline) should be administered. Judicious administration of furosemide (0.1 to 0.2 mg/kg IV q2-4h) may also be necessary to counter inappropriate sodium retention. It may take several days for hypernatremia to resolve once it develops. Lastly, hypomagnesemia commonly occurs after CPB. Prophylactic treatment of this deficiency with intravenous magnesium supplementation (0.75 mEq/kg/day) decreases the likelihood of post-CPB atrial and ventricular arrhythmias.