Transient dilutional acidosis but no lactic acidosis upon cardiopulmonary bypass in patients undergoing coronary artery bypass grafting

Abstract

Introduction: Dilutional acidosis may result from the introduction of a large fluid volume into the patients' systemic circulation, resulting in a considerable dilution of endogenous bicarbonate in the presence of a constant carbon dioxide partial pressure. Its significance or even existence, however, has been strongly questioned. Blood gas samples of patients operated on with standard cardiopulmonary bypass (CPB) were analyzed in order to provide further evidence for the existence of dilutional acidosis.

Material and methods: Between 07/2014 and 10/2014, a total of 25 consecutive patients scheduled for elective isolated coronary artery bypass grafting with CPB were enrolled in this prospective observational study. Blood gas samples taken regularly after CPB initiation were analyzed for dilutional effects and acid-base changes.

Results: After CPB initiation, hemoglobin concentration dropped from an average initial value of 12.8 g/dl to 8.8 g/dl. Before the beginning of CPB, the mean value of the patients' pH and base excess (BE) value averaged 7.41 and 0.5 mEq/l, respectively. After the onset of CPB, pH and BE values significantly dropped to a mean value of 7.33 (p < 0.0001) and -3.3 mEq/l (p < 0.0001), respectively, within the first 20 min. In the following period during CPB they recovered to 7.38 and -0.5 mEq/l, respectively, on average. Patients did not show overt lactic acidosis.

Conclusions: The present data underline the general existence of dilutional acidosis, albeit very limited in its duration. In patients undergoing coronary artery bypass grafting it seems to be the only obvious disturbance in acid-base homeostasis during CPB.

Keywords: cardioplegia; cardioplegic solution; metabolic acidosis; priming solution.

Figure 1

Changes in pH, BE, pCO₂ and lactate from the beginning of the operation until the end of CPB. A – pH, B – BE, C – pCO₂, D – lactate. Documentation of parameters begins with the start of the operation. Time point zero characterizes onset of CPB. The period of CPB is divided into two intervals: 0–20 min (shaded in grey) and the remaining time (20 min until the end of CPB). The red line represents the mean value in the respective interval

Figure 2

Changes in pH and BE in a typical patient from the beginning of the operation until the end of CPB. With the onset of the heart-lung machine at time point zero, the patient received 1.1 l of NaCl as priming fluid. Subsequently, he received 1.8 l of Bretschneider solution for cardioplegic arrest. For neutralization of dilutional acidosis, bicarbonate was administered four times in the amounts of 50 mmol, 40 mmol, 20 mmol and 30 mmol