The intention of this study is to determine whether the precision of the measure of cardiac output can be optimized by conducting the measurement while the participant is instructed to exhale slowly. This will be compared to measurements done at random to respiration and timed with the participant's spontaneous expiration.

Inclusion Criteria:

• Every patient that receives a pulmonary artery catheter (PAC) related to elective cardiac surgeries,
• informed and written consent to participation in the study in accordance with the Helsinki declaration
• hemodynamic stability

Exclusion Criteria:

• Hemodynamical instability
• atrial fibrillation
• tricuspid insufficiency ≥ grade 2
• hemodialysis or other conditions where extra volume load can be negative,
• lack of ability to give a written consent to participate in the study