Common questions and their answers
We have written this
booklet in order to provide some information and answer some
questions regarding one aspect of your child's upcoming heart
surgery..
Some parents may have heard of, or know a little about, the
heart-lung machine and cardio-pulmonary bypass. This booklet should
provide some more details of this integral part of the child's
operation, and hopefully, you will be able to understand your
child's operation a little better. This is your booklet to keep.
Maybe other family members or friends may benefit from reading the
booklet, and your child may be interested in it, when he or she is
old enough.
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 for a while. This means that for
a short period, during the operation the blood which normally
circulated through the heart and lungs, cannot do so. 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:
- take over the job of the heart, and so it pumps blood around
the body;
- take over the job of the lungs, so it adds oxygen to the blood,
and removes carbon dioxide
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 the oxygenator,
and then is pumped back into the patient's circulation. The circuit
also contains a blood filter. We keep the circuit very short, and
so the HLM is right next to the operating table.
The patient is connected to the circuit through plastic
cannulae, which the surgeon puts into the large veins and an artery
near the heart at the beginning of surgery, and removes them at the
end.
What kind of things will happen while
connected to the Heart-Lung machine?
Firstly, we fill the tubing circuit with a special solution
called the priming solution. The priming solution removes all the
air in the circuit, oxygenator and filter, and makes it bubble
free. The priming solution is a mixture of donor blood and a type
of saline solution. The donor blood is necessary because of the
size of children who need cardio-pulmonary bypass and also the
volume required to fill the circuit. Almost all children who weigh
less than 16 Kg, will need some donor blood (generally, about 500
mls) to prime the tubing circuit.
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 to 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 warm the patient back up to normal
temperature.
So who is "driving" the Heart-Lung
machine?
Let's introduce the
Perfusionist.
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 anaesthetist
and is a skilled scientist, with tertiary qualifications which
include human physiology and biochemistry, and also has more
specialised training in the techniques of artificial blood
circulation , commonly known as perfusion science. All our
perfusionists are certified by the Australasian Board of
Cardio-Vascular Perfusion, and are members of the Australasian
Society of Cardio-Vascular Perfusion.
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 analysers are right
next to the HLM. The patient's ECG and blood pressure are displayed
on a television-type monitor, mounted on the HLM. The whole set-up
is similar to a cockpit of an airplane.
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
cannulae.
Now you know a little more about cardio-pulmonary bypass, and
your child's operations. We perform between 400 & 500
operations requiring cardio-pulmonary bypass every year.