Open heart surgery can’t take place without heart lung machine. Have a look over this incredible machine that revolutionized the outlook over treating heart diseases.
What is Heart Lung Machine ?
- Heart-Lung Machine is a blood pumping machine that takes over the functions of the heart and lungs during surgery (i.e. open-heart surgery).
- It is most commonly used to perform a cardiopulmonary bypass (CPB), which is the technique whereby blood is totally or partially diverted from the heart into a machine with the gas exchange capacity and subsequently returned to the arterial circulation at appropriate pressures & flow rates.
- CPB allows for the heart to stop beating as its function is taken over by Heart Lung Machine, which makes it easier to operate on, and surgeons can operate in a blood-free area.
What functions are taken over by Heart Lung Machine ?
- RESPIRATION: Within which it includes Ventilation and Oxygenation.
- CIRCULATION: Maintaining circulation at appropriate pressures and flow rates.
- TEMP. REGULATION: It involves controlled hypothermia.
CPB allows for supporting the circulation of the blood; induce total body hypothermia, where the body can be maintained for up to 45 minutes without perfusion (blood flow).
The purpose of using temporary hypothermia is to provide a degree protection and safety margin during CPB surgery to body organs by reducing metabolic rate and oxygen consumption of the body. It also helps to preserve high-energy phosphate stores and reduces excitatory neurotransmitter release, which is especially important to central nervous system protection.
CPB in conjunction with systemic hypothermia allows lower pump flows, better myocardial protection, less blood trauma, and better organ protection than does normothermic perfusion.
Note: Hypothermia is the condition in which body’s physiology becomes abnormal So it should be controlled efficiently by minimizing its the negative effects. Generally perfusionist ensures this process.
What are the Components of Heart Lung Machine ?
- PUMPS: The pumps are designed to minimize the damage to blood cells and effective in pumping within physiological range.
- MEMBRANE OXYGENATOR: It imitates the function of lungs. Membrane oxygenator are more common now a days.
- Here O2 & CO2 Exchange takes place.
- Gas exchange take place by the process of diffusion across a thin membrane separating blood and gas made of highly permeable silicon rubber or microporous polypropylene,Teflon & polyacrylamide.
- Current designs of oxygenator for short term life support typically use an extraluminal flow regime, where the blood flows outside the gas filled woven strands of hollow fibers, Oxygen enters one end of the bundles of hollow fibers and exits at the opposite end. Oxygen and carbon dioxide diffuse in opposite directions across the aggregate large surface of the hollow fibers.
- HEAT EXCHANGER: Heat exchangers control body temperature by heating or cooling blood passing through the perfusion circuit.
- Arterial filter/bubble trap: It is used to filter small air bubbles that may have entered, or been generated by the machine.
- Aortic/atrial/vena caval cannulae through which blood is taken and returned to body.
- Tubing in the circuit which should be non thrombogenic, Chemically Inert to prevent any blood cell damage.
What is the work flow of Heart Lung Machine ?
- It Composed of: a chamber that receives all the blood from the body (right atrium of the heart), pumps that move the blood through an oxygenator (that mimic function right ventricle), oxygenator removes the carbon dioxide and adds oxygen to the blood (mimic lungs).
- Machine continues by pumping the oxygenated blood back to the body (that mimic function of left atrium and ventricle) using a series of tubes.
- The heart-lung machine itself is operated by perfusionists during surgery, and at the end of an operation, the surgeon gradually lets the patient’s heart resume its normal functions.
Advantage of using Heart Lung machine: The ability of a surgeon to perform an open-heart surgery in a blood-free zone while the heart is not beating.
It also allows for medications and anesthetics to be administered directly into the blood, adding them to the blood in the heart-lung reservoir, arriving immediately to the patient.
Chemical sensors placed in the blood path are able to detect the amount of oxygen bound to hemoglobin. Other, detailed sensors can constantly trend the blood pH, partial pressure of oxygen and carbon dioxide, and electrolytes. This constant trending can quickly analyze the metabolic demands of the body and correction to be applied in case of any deviation to normal range.
Sensors that communicate system pressures are also very important. These sensors consists of transducers that are placed in areas where pressure is high, after the pump. Readings outside of normal ranges often alert the operator to abnormal pressure ranges or obstructions in the blood-flow path.
References and Further reading:
- Principles of cardiopulmonary bypass by David Machin, BSc ACP and Chris Allsager, MB ChB FRCA