Perfusion

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Cardio-Pulmonary Bypass:
The Perfusionist and the Heart Lung Machine

What part does the Heart-Lung machine play?

In order for the surgeon to actually do the repair, the heart needs to be quite empty, and in most cases, it needs to be stopped. This means that for a  period during the operation the blood which is normally circulated through the heart and lungs does not. This gives the surgeon the best conditions under which to operate. So, while the heart and lungs are out of action, the Heart-Lung Machine (HLM) takes over. This is called CARDIO-PULMONARY BYPASS.

What exactly doe the Heart-Lung machine do?

The HLM is a very sophisticated machine designed to:

What exactly does the Heart-Lung machine look like?

The machine itself has up to five precision blood pumps. In addition it has a heater / cooler device for temperature control and a series of individual microprocessor units which monitor and control patient temperatures, the gas supply, and the safety systems.

Below is a diagram of the circuit showing the position of the patient, the oxygenator, the pump and the filter.

The pump takes over the job of the heart, and pumps the blood through the oxygenator and back into the patient.

So that's the basic machine – what else is there?

Apart from the machine, we use a disposable oxygenator, and a disposable tubing circuit with the HLM. The oxygenator functions in a similar way to the patient's own lungs, and for a time during the operation will serve as the lungs. As blood flows through the oxygenator, oxygen can be added to the blood, and carbon dioxide can be removed, and the appropriate levels of each gas can be maintained.

The oxygenator also contains a heat exchanger, which allows the blood temperature, and therefore the patient's temperature to be altered.

The circuit is made up of a special plastic tubing, and is set up on the HLM. Blood travels from the patient, to a reservoir and then is pumped to a heat exchanger/oxygenator, and then is pumped back into the patient's circulation. The circuit also contains an arterial blood filter positioned after the oxygenator. We keep the circuit as short as possible, and so the HLM is right next to the operating table.

The patient is connected to the circuit through plastic cannulas, which the surgeon puts into the large veins (superior vena cava, right atrium, inferior vena cava) and an artery (aorta) near the heart at the beginning of surgery, and removes them at the end.

During cardio-pulmonary bypass, we need to be able to control the patient's temperature. In most cardiac operations we cool the patient in order to slow and  control the way the body uses oxygen. The level of cooling will be determined by the type of operation, and the body temperature is controlled by altering the blood temperature in the oxygenator. Toward the end of cardio-pulmonary bypass, we rewarm the patient back up to normal temperature.

So who is "driving" the Heart-Lung machine?

The perfusionist is the member of the team who is responsible for the operation of the HLM during cardiac surgery. The perfusionist is also responsible for the selection and set-up of the circuit components for the procedure.

The perfusionist works closely with the surgeon and anesthesiologist and is skilled  with tertiary qualifications which include human physiology and biochemistry, and also has more specialized training in the techniques of artificial blood circulation – commonly known as perfusion science.

How does the PERFUSIONIST keep track of what is happening to the patient's circulation?

We've mentioned the microprocessor control systems which display and aid in the regulation of the temperatures and pressures within the circuit. There is also a monitor which constantly displays the level of oxygen in the blood. The perfusionist takes regular blood samples for biochemical, and blood clotting tests. The analyzers are right next to the HLM. The patient's ECG and blood pressure are displayed on a monitor.

When the surgeon has finished the repair, and the team feels that the heart and lungs can take over from the HLM, the perfusionist slowly weans the patient, and the surgeon removes the cannulas.

HILP

Autotransfusion