Pulmonary hypertensive crisis is a life-threatening condition, in which the blood pressure in the pulmonary artery, vein, and capillaries. Infusion of hypertonic saline solutions expand the circulating volume, thus allowing more blood to flow and reducing pressure in the artery, vein, and capillaries. Furthermore, infusion of hypertonic saline has been shown to reduce both systemic and pulmonary vascular resistances in adults. If the pulmonary vascular resistance decreases more or to the same degree as the systemic resistance, infusion of hypertonic saline may prove beneficial in the treatment of pulmonary hypertensive crisis. The primary objective of this study is to investigate how a clinically relevant dose of hypertonic saline affects the systemic and pulmonary circulations in children undergoing cardiac catheterization during general anesthesia. This study hypothesizes that an infusion of hypertonic saline over 10 minutes will reduce the pulmonary vascular resistance more than the systemic vascular resistance.

Inclusion Criteria:

• Children who have had a heart transplant and are scheduled for elective endomyocardial biopsies during general anaesthesia
• Age > 2 years

Exclusion Criteria:

• Plasma Na < 130 mmol/l or >150 mmol/l
• Refusal of consent
• An estimated pulmonary arterial pressure which is greater than or equal to 66% of systemic blood pressure
• Children with heart failure (right and/or left), i.e clinical suspicion by either a cardiologist or an anesthesiologist of inability to tolerate a fluid bolus of 3 ml/kg 7.2% NaCl corresponding to an expansion of the circulatory volume by approximately 9% (see the section on safety), which is the equivalent of infusion approximately 20 ml/kg 0.9% NaCl over 10 minutes.