Bypassing their way into your heart: considerations for the cardiothoracic patient

Patients undergoing complex open-heart surgery will often be placed on cardiopulmonary bypass, which is a technique where a machine temporarily takes over the job of the heart and lungs, allowing a bloodless surgical field. Staff at the Royal Veterinary College have been performing open heart surgery since 2005 for correction of: pulmonic stenosis, double chambered right ventricle, atrial septal defect, ventricular septal defect, atrioventricular septal defect, Tetralogy of Fallot and mitral and tricuspid valve disease.

Cardiopulmonary bypass is a form of extracorporeal circulation where venous blood is drained via cannulation of the right atrium and right auricular appendage into a reservoir, oxygenated and returned to the body via a pump (Figure 1). Cardiopulmonary bypass is used for both beating and non-beating cardiac surgery.

Therapeutic hypothermia is applied via cooling of the blood in the cardiopulmonary bypass machine (Kanemoto, 2014). This reduces the metabolic rate and oxygen demand from the tissues reducing risk of ischaemic injury, and it also allows the use of slower artificial cardiac output settings, which is advantageous in decreasing air bubbles and therefore reducing the risk of embolic events and limiting trauma to blood cells. The core body temperature is usually only cooled by up to 8°C based on the patient's initial body temperature and is measured at various points in the body and closely monitored throughout surgery, to limit the risk of complications that can arise secondary to hypothermia which include: