2. Extracorporeal Circulation
ECC or perfusion is defined as a procedure
carried out with the aid of HLM
In the early stages, cardiac surgery was
mainly concerned with the treatment of
congenital heart defects. Today, the main
focus is on acquired heart defects.
Access path to the operation area is kept
as small as possible to reduce the patient’s
surgical trauma this surgical technique
usually has a direct influence on the
configuration of the HLM and on the way
the perfusion is carried out
OPCAB (off-pump coronary artery bypass)
refer to cardiac surgeries carried out
without the support of an HLM
The HLM can never be looked at in
isolation additional components, mainly in
the form of sterile disposables such as an
oxygenator, heat exchanger, or reservoir,
take on other important functions during
ECC
3. Structure and Function of the Heart–Lung Machine
The HLM is the basis of ECC and cover
two important organ functions :-
1. Pump function of the heart
2. Gas exchange function of the lungs
It must ensure :-
A sufficient perfusion volume that
corresponds to the normal cardiac
output of the patient under
anesthetic
An adequate perfusion pressure (50–
90 mmHg)
It must also ensure sufficient
oxygenation, elimination of CO2, and
control of the blood temperature
The following components make up the
basic equipment of the ECC that is used
during modern cardiac surgery:
1. Blood pumps
2. Oxygenator
3. Tubing system with various tubing
diameters
4. Blood filters with various functions
5. Cardiotomy reservoir
6. Cannulae and intracardiac suction
tubes
4. Blood Pumps and Their Function
Blood pumps can generally be
categorized into two groups, depending
on their function:
1. Roller pumps
2. Centrifugal pumps
Essentially, both pump types should
meet certain criteria when used with
the HLM:
1. Dosed delivery of liquids, with
precise display of the actual flow rate
and delivered volume
2. External control mechanism (e.g.
monitoring functions)
3. Sufficient pressure or vacuum
generation
5. Minimal blood damage
6. Adjustable occlusion settings
(roller pumps only)
7. Pulsatile flow generation
8. High efficiency
9. High reliability and safety
10 . Option for emergency
operation (e.g., manual
operation)
5. Blood Pumps and Their Function
Roller Pumps:-
De Bakey blood pumps are based on the
displacement principle and deliver blood
through a tubing segment from the pump
housing using rotating rollers
The roller pump consists of :-
1. Rotating pump arm with two attached
cylindrical rollers
2. Pump housing into which a semicircular
silicone tubing segment is inserted and
then secured using special tubing
inserts
The rotating rollers alternately compress
the tubing segment and deliver the liquid
contained in the tubing in accordance
with the rotational speed and direction
The exact adjustment of the two rollers
determines the delivery
rollers move outward symmetrically and
block the inserted tube evenly to reduces
erythrocyte damage caused by shear
stress or direct crushing.
6. Blood Pumps and Their Function
Centrifugal Pumps:-
Such pumps use centrifugal
forces to transport the blood instead
of tubing compression.
Because of the technical principle of
operation, the centrifugal pump has
a limited application.
Centrifugal pumps have the
advantage of :-
delivering limited amounts of air
causing less blood damage in the
long run.
An additional flow meter is required
to determine the delivered flow is a
disadvantage.
Centrifugal Pump from www.britannica.com
7. Oxygenator and Gas Exchange Function
Oxygenator:-
The artificial lung, also called the
oxygenator, takes on the lung function
during ECC and is therefore responsible for
the exchange of vital gases.
The membrane oxygenators:-
Are now used as standard.
They contain a semipermeable membrane
in the shape of a microporous hollow fiber.
This liquid-impermeable membrane
separates the gas side from the
bloodstream.
Due to the partial pressure gradients, O2
and CO2 diffuse through the microporous
membrane
Today, most membranes are made of
polypropylene or polyethylene and permit
operating times of 6–8 h
The oxygenator must be able to
oxygenate about 5 litres per minute
of venous blood from 65% oxygen
saturation to above 95% oxygen
saturation before the blood enters
the body systems.
oxygenator
8. The oxygenator
The oxygenator consists of :-
1. A cylindrical glass vessel of 15 cm
internal diameter, 38 cm long
2. 80–100 stainless steel discs of 0.6 mm
thickness and 14 cm diameter are
mounted axially with 3 mm spacers
between discs.
The shaft on which the discs are mounted
is hollow and is supported by three ball
bearings at the ends.
The oxygen is fed through the axis of the
shaft and it enters the oxygenator through
the distributing apertures on the
circumference of the shaft.
The oxygen feed system permits use of disc
diameters close to the diameter of the
oxygenator cylinder so that for a given
blood priming volume maximum utilization
of the available surface area is made.
The optimum blood level in the oxygenator
that provides the maximum film area for a
given volume is 0.7 R where R is the radius
of the disc
9. Tubing Systems for Extracorporeal Circulation
The tubing system is used to connect the
individual components of the
extracorporeal system and forms a closed
circuit with the vascular system of the
patient.
Depending on the location, either PVC or
silicone tubing is used
Tubes are available in different diameters
and wall thicknesses.
Tubing sizes range from a diameter of
1/8" and a wall thickness of 1/16" to
3/16" and 1/4 " for pediatric applications
(with correspondingly low flow rates).
Tubing for adult applications is usually
3/8 " or 1/2 " with a wall thickness of
3/32", facilitating flow rates of more than
10 l/min
Completely preassembled tubing
system with infant oxygenator
(Sorin Group S.p.A., Milan)
10. Blood Filters
Blood filters are integrated into the
ECC system mainly to avoid micro-
embolisms caused by autologous
effect , foreign particles, and
microbubbles
Depth Filters
made of Dacron wool or
polyurethane foam, are inserted into
cardiotomy reservoirs and are used
mainly for particle filtration.
The pore size can vary from 80 -
100μm (coarse separation of
particles) to 20–40μm (micropore
range).
Mesh Filters:-
Made of a mesh of woven polyester
strands.
Used as arterial blood filters and work
on the principle of sieves.
The pore size of mesh filters ranges
from 20 to 40μm.
Unlike depth filters, mesh filters have
little adhesive force.
Their air retention properties are
excellent as air bubbles can only pass
the filter medium when a certain
pressure difference (bubble point
pressure) is reached.
11. Cardiotomy Reservoir
The cardiotomy reservoir functions:-
1. Collects and filters the blood
aspirated from the operation area
and feeds it back into the ECC as
required.
2. Provides volume storage
3. The transparent housing facilitates
continuous level control
A detailed scale makes it easy to
quantify the level and detect
corresponding changes in volume
A minimum residual volume always
remains in the reservoir during the
entire ECC to prevent air delivery
into the extracorporeal system
12. Cannulae
Cannulae are the interface between the
ECC system and the vascular system of the
patient.
They are inserted into the relevant vessels
by a surgeon, where they are secured and
deaerated before their sterile connection
to the tubing system of the HLM.
Two types of cannulae:--
1. Arterial Cannulation
2. Venous Cannulation
Arterial Cannulation:-
Arterial cannulae are used to return the
oxygenated blood to the systemic
circulation of the patient
The aorta ascendens is the most
frequently used site.
The type and size of the selected cannula
depends on both the required blood flow
and the anatomic conditions
Venous Cannulation:-
Venous cannulae drain the patient’s blood
from the venous vascular system to the
HLM.
The type and size of the selected cannula is
determined by conditions that are similar
to those on the arterial side.
Vent Catheters:-
Inserted to protect the temporarily
arrested heart and especially the left
ventricle from overextension due to blood
flowing back from the bronchial
circulation.
Using this vent suction tube, excessive
blood is drained usually via the auricle of
the left atrium or into the cardiotomy
reservoir of the HLM
13. Components of the Heart–Lung Machine
HLM Basic Components:-
1. Mobile console for mounting
multiple pumps
2. Adjustable mast system for
mounting Blood pumps
3. Control and monitoring devices
4. Display and control panel
5. Electronic or mechanical gas
blender
6. Anesthetic gas vaporizer
7. Electronic documentation system
Additional HLM Components:-
1. water mattress
2. Heater–Cooler Device
During ECC, the heat exchanger of the
oxygenator indirectly regulates the
patient’s body temperature by heating or
cooling the blood flow. Patients can also
be placed on a water mattress that
increases or decreases body temperature
14. Components of the Heart–Lung Machine
Mobile console:-
for mounting :-
1. Multiple pumps
2. The power supply
3. Emergency power supply
4. Electronics parts
Adjustable mast system:-
for mounting the holders for:-
1. The oxygenator
2. Filters
3. Cardiotomy reservoirs
4. External pumps, and additional
devices
Figure : shows a modern HLM.
15. Blood pumps:-
Large roller pump:
pump with a longer tubing path in the
pump housing that produces higher flow
rates, required for the arterial blood flow,
suction, or vent pump
Small roller pump:
pump with a shorter tubing path in the
pump housing for lower flow rates, e.g.,
for the perfusion of infants and children
or for the administration of cardioplegic
solutions, usually a double pump
Centrifugal pumps:
pumps for the arterial blood flow.
Components of the Heart–Lung Machine
16. Components of the Heart–Lung Machine
Control and monitoring devices:-
Include :-
1. Pressure monitor:- including
sensors, for measuring different
pressures in the extracorporeal
system and for controlling the flow
rates.
2. Temperature monitor:- including
sensors, for measuring and
displaying different system
temperatures and, if required,
patient temperatures.
3. Level monitor :- including sensors,
for measuring and controlling the
volume level in the cardiotomy
reservoir.
4. Bubble monitor :- in the form of
ultrasonic sensors that regulate the
flow rate of the affected pump
when air is detected in the system.
5. Pulsatile flow control :-for creating
and controlling a pulsatile flow
profile.
6. Cardioplegia control:- including pressure
and bubble sensor, for controlling
cardioplegia delivery.
7. Timer for measuring important times
and intervals during perfusion such as total
perfusion time, aortic clamping time, and
reperfusion time
17. Display and control panel:-
providing access to all display and
control elements described above as
well as to additional system
information and alarm management
Electronic or mechanical gas
blender:-
1. For controlling and displaying the
ventilation gases (air, O2, and CO2)
that are delivered to the gas side of
the oxygenator; vacuum controller.
2. For the precise control and display
of a permanent vacuum on the
venous reservoir
Components of the Heart–Lung Machine
Figure : shows a modern HLM.
18. Components of the Heart–Lung Machine
Anesthetic gas vaporizer:-
Allowing and displaying the
administration of a precisely dosed
amount of anesthetic gas to the
oxygenator.
Electronic documentation system
That displays and stores all relevant data
during perfusion.
All data are stored centrally and
displayed instantly as a perfusion report
which allows for data evaluation at a later
stage for statistical and scientific
purposes.
perfusion report
19. Additional HLM Components:-
Heater–Cooler Device (Heat exchanger ):-
The heater–cooler device delivers water with
a temperature ranging from 2 to 42°C.
The Heat exchanger contain:-
1. Two chamber pumps ensure that the
delivered water does not cause a
buildup of overpressure in the circuits.
2. Two circuits supply the oxygenator and
the heating/cooling mattress
3. Third circuit is connected to the heat
exchanger in the cardioplegia system.
4. Two independent tanks with different
temperatures, which allows instant
switching from cold to warm or vice
versa
Modern heater–cooler devices feature
several circuits with different
temperatures
Heater–cooler devices have always presented
a hygiene challenge because
1. The system parts that transport water
inevitably become contaminated by
germs.
Regular water changes and the use of
appropriate chemical cleaning agents are
the two most effective cleaning and
maintenance strategies.
Components of the Heart–Lung Machine
20. References
Springer Handbook of Medical Technology
Rüdiger Kramme, Klaus-Peter Hoffmann,
Robert S. Pozos
Springer-Verlag Berlin Heidelberg 2011
