Heart-Lung Machine: Impact of the Priming Solution on Acid-Base Balance, Electrolytes and Outcome on Patients Undergoing Cardiac Surgery

Cardiopulmonary bypass (CBP) with a heart-lung machine is mandatory for a vast majority of cardiac surgeries. Briefly the disposables in a CPB circuit consists of a reservoir, an oxygenator and tubings.

The role of CPB can be summarized as follows:

• Empty the heart. Drain out the blood (achieved via venous cannulas).
• Oxygenate the blood and remove carbon dioxide. Thus, the lungs are not participating during CPB.
• Maintain homeostasis and adjust chemical and electrolyte blood composition.
• Maintain or adjust the body's temperature via a heat exchanger, which is a part of the heart-lung machine.
• Return oxygenated blood to the patient via arterial cannula.

Heart-lung machine provides even the following:

• Salvage blood loss during surgery via cardiotomy suckers and return it to the patient.
• Prevent distention of the heart during surgery via cardiac vents.
• Deliver cardioplegia which stops the heart, providing myocardial protection and condition to perform cardiac surgery.
• Additional treatments, such as hemofiltration and removing av cytokines.

Before connecting the CPB circuit to the patient's circulation it needs to be deaired. This is achieved by priming, i.e. filling the circuit with a liquid solution. The 2024 EACTS (European Association for Cardio-Thoracic Surgery)/EACTA (European Association of Cardiothoracic Anaesthesiology)/EBCP (European Board of Cardiovascular Perfusin) Guidelines on CPB in adult cardiac surgery point out that despite a wealth of studies, no consensus has been reached on the optimal composition of the priming solution. According to recent surveys, balanced crystalloids are the preferred priming solution, but there are no recommendations of the composition in the solution. The standard prime solution at our institution consists of Ringer Acetate which is a common crystalloid fluid in Sweden used intravenously for fluid and volume loss. Due to its composition, i.e. lower pH and sodium and higher chloride concentrations than the human blood, it is sometimes necessary to add electrolytes and other components to the circuit either in advance or after initiation of CPB. Previous studies have investigated various prime compositions and plasma osmolality. The crystalloid priming fluid used in these studies contained mannitol, which has a high osmolality. A former study from our group showed a significant decrease in blood sodium with a mannitol-containing prime, but no effects on plasma osmolality. Highly osmolar priming solution has further been studied, also containing mannitol, and found a steep rise in plasma osmolality. These studies used priming fluids with an addition of 80 mmol NaCl and 160 mmol NaCl, respectively. Plasmalyte is a newer crystalloid solution with a composition of sodium and chloride more similar to human plasma which could be of advantage. Plasmalyte is used for CPB priming in some cardiac centers. Studies on Plasmalyte as a priming solution have shown less metabolic acidosis compared to Ringer Lactate. However, it has not been used in our institution for this purpose and there are no studies comparing Plasmalyte with Ringer Acetate. The literature search has further not found any studies investigating non-mannitol CPB priming solutions with differing additions of NaCl. There is a need to fill this knowledge gap to proceed in developing of the optimal priming solution, tailored to variouse preexisting diseases or conditions.

Scientific questions The overall aim of the project is to investigate if clinical outcome after cardiac surgery can be improved by optimizing the priming solution.

The sub-projects will answer the questions:

• Do the different priming solutions: Ringer Acetate alone, Ringer Acetate with added NaCl and Plasmalyte alone affect the acid-base balance and composition of electrolytes in the blood?
• Do the different priming solutions affect the need to correct the acid-base balance during the heart-lung machine period?
• Do the different priming solutions affect the total fluid balance in connection with cardiac surgery?
• Do the different priming solutions affect the osmolality in the blood per- and postoperatively?
• Do the different priming solutions affect postoperative renal function? Do the different priming solutions affect neurological outcome?

Inclusion criteria

• Patients 18 years and above
• Undergoing coronary artery bypass graft (CABG) surgery as single surgery
• Undergoing aortic valve replacement (AVR) as single surgery (AtriClip is allowed)
• Given consent to participate, both verbal and written

Exclusion criteria

• Subnormal heart function (defined as an ejection fraction <45%), and no signs of heart failure (edema).
• Body weight <60 kg or >120 kg
• Preoperative hemoglobin <120 g/L,
• Subnormal kidney function (defined as GFR <30 ml/min),
• Blood sodium outside normal range (135-145 mmol/l),
• Need of acute surgery
• AVR due to aortic valve insufficiency
• Changes in operating method or addition of intraoperative procedures.