About 6000 heart operations are performed in Sweden every year. A heart-lung machine is used almost exclusively in all heart operations. This machine fills the role of heart and lungs during surgery while the heart is stopped. The extra corporeal circuit (ECC) prime results in hemodilution, as assessed from the decrease in haematocrit, electrolyte concentration and total protein content. This hemodilution is an unavoidable consequence of the use of a heart-lung machine with nonblood ECC prime. The alteration of the patient´s blood volume and electrolytes is affected by the prime solution and can be maintained within normal limits. There are no clear recommendation regarding prime components and numerous prime solutions are in use worldwide.

The aim of this study is to investigate whether the use of mannitol in heart-lung machine prime has an effect on electrolytes levels and osmolality when compared with patients who receive no Mannitol.

Inclusion Criteria:

• adult patients of both sexes who will undergo elective isolated coronary artery bypass grafting at the Department of Cardiothoracic Surgery, Skåne University Hospital, Lund

Exclusion Criteria:

• heart failure with left ventricular ejection fraction below 50%
• small size (defined as bodyweight less than 50 kg)
• anaemia with haematocrit less than 24%
• patients with previous cardiac surgery
• patients who receive other fluids or more than 1000 ml additional Ringers's Acetate during cardiopulmonary bypass
• patients with peroperative complications including massive peroperative fluid transfusions