Cardiopulmonary Bypass – what it is and why it’s used
Cardiopulmonary bypass (CPB) is used in open heart surgery. It involves a machine taking over the role of the heart and lungs by providing the body with oxygenated blood during a cardiac procedure. This means that if and when the heart is not beating there is an alternative reliable means of circulating blood.
The machine thus performs both a respiratory and a circulatory role. In very basic terms, the CPB machine is a pump. It is sometimes referred to as a “heart – lung machine”.
CPB facilitates a bloodless field for the performance of cardiac surgery. This is important because it allows the surgeon to see what s/he is doing.
The CPB machine allows de-oxygenated blood from the veins to be drained into a reservoir. A pump then moves the blood and it is then artificially oxygenated through a heat exchanger. The blood is then returned to the arterial circulation. Additionally, suckers are used to remove blood from where surgery is being performed. Vents decompress the heart.
Throughout the procedure, measurements are made of the oxygen saturation levels in the blood, haemoglobin levels, blood gasses and electrolytes (elements which are vital to physiological functioning – e.g. potassium). There are safety measures which include a bubble detector: bubbles in oxygenated blood are dangerous because they can cause blockages, usually either in the coronary (heart) or cerebral (brain) vessels. It is thus important to monitor the machinery accordingly. For the same reason, clotting is dangerous on CPB and anti-coagulation medication is normally given.
The patient’s body is connected to the machine via tubes known as cannulae. It is important to monitor these to ensure they do not become kinked, as that could impede bloodflow.
Meanwhile, the temperature of the patient is often reduced (“hypothermia”) whilst on CPB in order to preserve tissue and organs. The patient’s head may be packed in ice in order to reduce the risk of brain damage.
Any surgery that involves CPB is necessarily a team effort. In addition to scrub nurses, junior surgical colleagues and an anaesthetist, the surgeon will be joined by a perfusionist, whose job it is to look after the CPB machine and thus ensure a constant, regular supply of oxygenated blood to the patient throughout the procedure.
CPB is associated with a range of complications, and strategies to reduce these complications are an active area of academic research.
If the surgeon needs to operate inside the heart, then it will be necessary to clamp the aorta (the main systemic artery) and stop the heart from beating. Once the aorta is clamped, blood is removed from the heart.
A solution known as cardioplegia is then washed around the heart. This solution is aimed to protect the heart tissue both by reducing oxygen consumption by the tissues and providing nutrients to the tissue (potassium-based solutions are quite common for this reason). Warm or cold solutions can be given. The heart may also be cooled topically.
Following the conclusion of surgical repairs, vigorous de-airing of the heart must always be completed before the aortic cross-clamps are released. Failure to do so can led to bubbles of air escaping into the arterial circulation, which in turn can lead to stroke.
Problems associated with CPB
Temperature management is very important. As noted, hypothermia is commonly used but studies have shown that the rate of rewarming may be more important than the temperature to which patients are cooled in the prevention of brain injury.
There is a relatively high association (>10% in some reported case series) between CPB and a stroke, and thereafter developmental delay (i.e. it is unlikely that the effects of the stroke were transient), in infants.