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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3612_Библиотеки_им_академика_М_И_Перельмана
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Key Questions in CONGENITAL CARDIAC SURGERY
Thus, any vasoactive drug should be used with care as the results
may be unpredictable.
22 What are the specific considerations for a patient with
a systemic-pulmonary shunt?
Patients with either a central or Blalock-Taussig type aortopulmonary
•
shunt depend on systemic arterial pressure to maintain pulmonary
blood flow via the shunt. In such patients, ensuing desaturation may
require systemic vasoconstriction. Additionally, any aggressive
ventilatory manoeuvres may reduce pulmonary blood flow.
Blood pressure measurement can be challenging, as patients who
•
have undergone a full repair following a previous classic or modified
Blalock-Taussig shunt, will have a reduced arterial pressure in the
ipsilateral arm.
Recommended reading
350
1. Price S, Jaggar SI, Jordan S, Trenfield S, Khan M, Sethia B, Shore D, Evans TW.
Adult congenital heart disease: intensive care management and outcome prediction.
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2. Baumgartner H, De Backer J, Babu-Narayan SV, Budts W, Chessa M, Diller GP, Lung
B, Kluin J, Lang IM, Meijboom F, Moons P, Mulder BJM, Oechslin E, Roos-Hesselink
JW, Schwerzmann M, Sondergaard L, Zeppenfeld K; ESC Scientific Document
Group. 2020 ESC Guidelines for the management of adult congenital heart disease.
bìê=eÉ~êí=g
3. Stout KK, Daniels CJ, Aboulhosn JA, Bozkurt B, Broberg CS, Colman JM, Crumb SR,
Dearani JA, Fuller S, Gurvitz M, Khairy P, Landzberg MJ, Saidi A, Valente AM, Van
Hare GF. 2018 AHA/ACC Guideline for the management of adults with congenital
heart disease: executive summary: a report of the American College of
Cardiology/American Heart Association Task Force on clinical practice guidelines.
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4. Engelfriet P, Boersma E, Oechslin E, Tijssen J, Gatzoulis MA, Thilén U, Kaemmerer H,
Moons P, Meijboom F, Popelová J, Laforest V, Hirsch R, Daliento L, Thaulow E, Mulder
B. The spectrum of adult congenital heart disease in Europe: morbidity and mortality
in a 5-year follow-up period. The Euro Heart Survey on adult congenital heart disease.
bìê=eÉ~êí=g
5. Lei Lei E, Ladha K, Mueller B, Roche L, Rao V, Hickey E, Heggie J. Noncardiac
determinants of death and intensive care morbidity in adult congenital heart disease
surgery.
2019; 139(14): e637-e97.
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2007; 33(4): 652-9.
2021; 42(6): 563-645.
2005; 26(21): 2325-33.
2020; 159(6): 2407-15.
Chapter 10
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Cardiopulmonary bypass
Sarah Goring, Lale Begum, Sian Jaggar
1 What are the aims of cardiopulmonary bypass (CPB)?
Cardiopulmonary bypass (CPB) is a complex circulation and
•
respiratory device that facilitates open heart surgery by:
a) completely replacing the function of the heart and lungs;
b) providing a clear operating field for the surgeon;
c) maintaining oxygen (O2) delivery to, and carbon dioxide (CO2)
removal from, the patient’s tissues;
d) reducing the workload of the heart by emptying, thus
minimising myocardial oxygen demand;
e) providing additional myocardial and end-organ protection
through:
i) cardioplegia solution to stop myocardial cellular
metabolism;
ii) reducing the patient’s temperature to decrease cellular
oxygen demand.
351
2 What are the critical components of a
cardiopulmonary bypass circuit (Figure 1)?
Venous line (blue) — which drains blood from the systemic venous
•
circulation, usually the superior and inferior vena cavae (SVC and
IVC), or less commonly in congenital surgery, the right atrium (RA) or
hepatic veins.
Reservoir — which is usually a hard-shell plastic container with filters
•
(to remove debris) and volume markings (to visualise changes in
venous return).
Pump — which can be either a roller or centrifugal pump.
•
Oxygenator — which contains thousands of microporous fibres
•
inside a plastic housing. Air and oxygen are circulated within the
fibres, whilst blood flows around them. Gas exchange occurs across
micropores, replacing the function of the lungs.
352
Key Questions in CONGENITAL CARDIAC SURGERY
Figure 1. Components of the cardiopulmonary bypass (CPB) circuit,
including the venous return component of the circuit (blue), arterial
component (red) and suction/venting system (green).
Heat exchanger — which is incorporated into the oxygenator
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housing. Water is pumped through the exchanger and allows precise
thermoregulation of the circulating blood.
Arterial line (red) — which returns blood to the arterial circulation
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(usually the aorta) providing antegrade flow. A variety of other
externally or internally accessible sites may be used, including the
subclavian artery, brachiocephalic artery and femoral artery (which
results in retrograde flow). This is particularly common in redo
surgery, which occurs more frequently in congenital procedures.
Cardiotomy suction (green) — which is controlled by roller pumps to
•
remove blood from the operative field and return it to the reservoir,
thereby keeping the surgical field clear.
10 Cardiopulmonary bypass
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3 What are the cardiopulmonary bypass components
made of?
The CPB circuit is single use and made of a variety of synthetic
•
polymers, with most tubing sets manufactured from polyvinyl chloride
(PVC) and plasticisers to increase flexibility.
Tubing contained in the raceway of the roller pumps and suckers are
•
usually made of silicone, which produces less haemolysis (in
comparison to PVC) when blood is repeatedly compressed by rollers.
Oxygenator fibres are made of polypropylene, stretched and heated
•
to form micropores.
The oxygenator accounts for the largest surface area within the CPB
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circuit and is often the only component of the circuit to be coated
with biocompatible substances, such as para-methoxy-Nethylamphetamine (PMEA) or heparin.
The rest of the circuit is less commonly coated, due to cost, though
•
evidence suggests fully coated circuits cause less inflammatory
response.
4 Describe the important safety features incorporated
into a cardiopulmonary bypass circuit
Intra-circuit air detectors — which minimise one of the main risks of
•
CPB, namely air entrapment within the circuit and its embolisation
into the patient, and include the following:
353
a) level detector attached to a reservoir — which stops the
arterial pump if venous return is reduced to the point of
emptying the reservoir. This would lead to gross air in the
circuit and lead to air bubbles in the system. It is most likely to
occur with surgical manipulation (inadvertently blocking venous
cannulae) or cannula displacement;
b) ultrasonic bubble alarm — which is attached to the arterial line
after the oxygenator. This can stop the pump if bubbles are
detected; for example, if negative pressure in the circuit draws
gas across the oxygenator or cavitates air;
c) one-way valves — which are often incorporated into suckers on
the neonatal and paediatric CPB circuits, and can prevent air
being blown into the patient if suckers are wrongly placed into the
pump raceway or the pump is operated in the wrong direction.
Arterial line pressure monitors — which alarm if the cannulae are
•
obstructed, thereby reducing the risk of circuit rupture. Inappropriate
positioning of the arterial lines may cause aortic dissection or poor
perfusion of the individual aortic branches. Cannulae are more easily
354
Key Questions in CONGENITAL CARDIAC SURGERY
displaced in neonatal and paediatric patients, due to their small size
and the size of the native vessels.
Filters — which are incorporated in the reservoir and oxygenator,
•
prevent gaseous or particulate microemboli reaching the patient.
In-line monitors — which allow constant monitoring of the metabolic
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status of the patient, including haematocrit, potassium, lactate and
O2levels.
Small-volume circuits — which reduce the amount of foreign material
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to which blood is exposed, thus reducing the associated
inflammatory response. They also reduce the risks of haemodilution
by reducing the volume of (mainly) clear prime fluid.
Hand crank — which is available on every CPB machine, for use in
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the event of electrical failure.
In addition, vigilance of the perfusionist is necessary to act upon
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safety features built into the circuit.
5 What are the different types of cardiopulmonary
bypass pumps?
In adults with acquired or congenital conditions, either a roller or
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centrifugal pump is routinely used.
In neonatal and infant patients, roller pumps are more common.
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Roller pumps (Figure 2) allow the desired flow to be set regardless
•
of the resistance to flow provided by the patient (afterload). The sole
constraints to this are:
a) adequate venous return;
b) excessive arterial line pressure.
With prolonged use, roller pumps may be associated with spallation
•
(breakdown of tubing) and particulate embolisation.
Centrifugal pumps (Figure 3) are non-occlusive and are more
•
commonly used in extracorporeal membrane oxygenation (ECMO)
circuits and ventricular assist devices (VADs).
In centrifugal pumps, a vortex is generated by spinning magnets,
•
creating a pressure gradient between the inlet (A) and outlet (B) of
the pump. The spinning action sucks venous blood in, imparts kinetic
energy to it, and expels it to the patient. It is important to note that:
a) flow generated is entirely dependent on preload and afterload;
b) backflow may occur, particularly at low flows;
c) in children with a small cardiac output and high afterload, a
disproportionately large drop in flow may occur.
10 Cardiopulmonary bypass
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Figure 2. A roller pump, which is repeatedly
compressed to propel the blood and consists of
tubing manufactured from silicone to reduce the
risk of spallation.
355
A
B
Figure 3. A centrifugal pump with red arrows that
demonstrate the inlet (A) and outlet (B) pipes.
356
Key Questions in CONGENITAL CARDIAC SURGERY
6 What are the typical priming constituents for
cardiopulmonary bypass circuits in congenital heart
surgery?
Crystalloid prime (such as Hartmann’s solution, Plasma-Lyte™) —
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which mimics physiological conditions and maintains physiological
tonicity, thus minimising red cell lysis and unwanted fluid shifts.
Packed red blood cells — which are often added to the prime volume
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in neonates and infants because even minimised CPB volumes risk
excessive haemodilution. Although haematocrit (Hct) levels >24% on
CPB are associated with reduced lactate and better psychomotor
development scores in infants, increases in Hct above this level have
not been shown to produce better outcomes. In cyanotic patients,
who may have a much higher Hct, reducing this may not be tolerated
if a mixed circulation remains postoperatively.
Colloids (such as gelatins, hetastarch or albumin) — which may be
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added to increase the osmotic pressure of the prime, thereby
theoretically reducing oedema, although this has not been shown to
affect outcomes in adult or paediatric patients.
Mannitol — which may be added to produce an osmotic diuretic
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effect, stabilise cell membranes, scavenge oxygen free radicals and
reduce cerebral oedema. Alterations in outcome, however, are yet to
be consistently demonstrated.
Sodium bicarbonate or tromethamine (THAM) — may be added to
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the prime to correct a base deficit.
Fresh frozen plasma (FFP) — which is added to prime in some
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paediatric and neonatal centres, and is thought to improve fibrinogen
levels post-CPB. Evidence of clinical benefit is inconclusive.
Aprotinin — which is frequently added to prime for redo surgery, due
•
to increased bleeding risk. Its use has varied over time and with
some evidence available on advantages and disadvantages.
7 What parameters are monitored on cardiopulmonary
bypass during congenital heart disease surgery?
Monitoring during CPB assesses the three main domains involved,
•
namely the CPB circuit, fluid within the circuit and the patient.
Within the CPB circuit, a number of parameters are monitored,
•
including the:
a) reservoir volume — to ensure the pump output matches the
return;
b) venous return — to ensure there is no reduction, such as
following snaring of the venous cannulae or associated with
10 Cardiopulmonary bypass
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kinking of the venous lines, which may cause cerebral
hypoperfusion due to increased central venous (CVP) and
intracranial (ICP) pressures;
c) arterial flow — to ensure adequate cardiac output is delivered;
d) line pressures (such as arterial and cardioplegia) — to avoid
circuit rupture if the lines become kinked or are otherwise
obstructed. It also gives an idea of the shear forces exerted on
blood, and pressure being applied to, for example, the
coronary sinus during retrograde cardioplegia delivery. High
pressures (300-350mmHg) cause the pump to stop.
Monitoring cardioplegic line pressure is critical in avoiding
hypertensive flow into the neonatal myocardium.
As regards fluid within the bypass circuit, many values are
•
continuously monitored in-line, including:
a) total gas flow and fractional oxygen delivery through the
oxygenator — which are manipulated to maintain O2and CO
levels within physiological values;
b) arterial oxygen pressure (PaO2) — which is usually maintained
between 20-30kPa. Although O2demand is higher in children,
hyperoxic bypass has been associated with increased
myocardial injury and systemic stress in cyanotic patients;
c) venous oxygen saturation (SvO2) — which is an important
indicator of systemic perfusion. Its interpretation, however, is
complicated in patients with systemic-pulmonary collateral
vessels, when high venous saturation may occur despite organ
hypoperfusion. During deep hypothermic CPB, a higher
proportion of O2delivery is provided by O2dissolved in
plasma, and less from that carried by haemoglobin (due to the
Bohr effect and reduced dissociation with decreasing
temperature);
d) arterial carbon dioxide pressure (PaCO2) — which allows
appropriate addition of CO2during paediatric CPB, where
excessively efficient removal can result in a respiratory
alkalosis;
e) haematocrit (Hct) — which allows efficient delivery of oxygen.
Optimum levels, however, remain somewhat controversial. At
low temperatures, some haemodilution may improve the
microcirculation by reducing viscosity and may be better
tolerated due to the increased O2content of plasma. A higher
Hct strategy during low-flow hypothermic CPB in infants,
however, has been associated with better short- (lower lactate
2
357
358
Key Questions in CONGENITAL CARDIAC SURGERY
and higher cardiac index postoperatively) and long-term
(higher psychomotor development index scores) outcomes.
Optimisation of Hct above 40 in neonates with single-ventricle
circulation has been associated with better outcomes,
especially following stage I procedures;
f) pH, potassium and lactate — which are calibrated from blood
gases. Acidosis and increased lactate suggest inadequate
perfusion, whilst potassium may rise during CPB following
cardioplegia delivery;
g) blood glucose — which tends to rise during CPB as part of an
inflammatory response.
Patient parameters that are monitored include the:
•
a) arterial pressure — which can be adjusted mechanically by
changing pump flow or pharmacologically (such as with
metaraminol, phenylephrine or glyceryl trinitrate) to increase or
decrease the systemic vascular resistance (SVR), to ensure
perfusion of tissues on CPB;
b) right atrial pressure (RAP) — which should be zero on CPB if
venous drainage is optimal, thereby avoiding raised ICP, and
helps to guide filling when weaning from CPB;
c) core and peripheral temperature — to ensure even cooling and
warming, and avoid large temperature gradients between the
heat exchanger and blood. Inadequate cooling may be
associated with inadequate protection, whereas over-warming
is linked to neurological damage;
d) cerebral oxygenation by near-infrared spectroscopy (NIRS) —
as an indication of cerebral perfusion, particularly in paediatric
patients.
8 What are the major differences in cardiopulmonary
bypass between patients with congenital and
acquired cardiac disease?
Patients with congenital heart disease may range from preterm
•
neonates to adult. Although the adult congenital heart disease
(ACHD) population is increasing, it remains much smaller than the
population with acquired disease.
Patients require an appropriate sized circuit, as even with small
•
circuits for infants, the extent of haemodilution is greater than that for
adults.
10 Cardiopulmonary bypass
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Many congenital conditions require multiple staged or repeat surgical
•
procedures over a lifetime and the risk of catastrophic injury to
mediastinal structures when opening the sternum increases with the
number of procedures. In these cases, CPB may be initiated via
femoral cannulation whilst the sternum is opened, although in
children, femoral cannulae size may be inadequate to provide the
required flow.
Abnormal systemic-pulmonary collateral connections are common in
•
congenital patients. These may shunt blood away from the systemic
circulation making it difficult to assess the efficacy of tissue O
delivery. In addition, they produce a significant increase in blood in
the operative field, which must be managed with appropriate venting.
Complicated anatomy requiring extensive open-heart access, and
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long CPB and cross-clamp durations, are associated with higher
morbidity.
The need for deep hypothermic circulatory arrest (DHCA) is greater
•
in patients with congenital heart disease. This is associated with
worse outcomes.
2
359
9 Describe the anticoagulant regimens used for
cardiopulmonary bypass in patients with congenital
heart disease
Heparin is added to the CPB prime (in addition to an intravenous
•
bolus) to avoid clotting of blood on contact with the foreign surfaces
of the CPB circuit.
A target activated clotting time (ACT) of 400-480 seconds is
•
maintained throughout CPB, with additional heparin as needed.
As neonates and infants have more variable clotting factors and
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immature drug metabolism, the efficacy and duration of effect are
less predictable.
Congenital heart disease patients have repeated exposure to heparin
•
from recurrent procedures and although heparin-induced
thrombocytopenia (HIT) has been reported as rare, it may well be
under-recognised in these patients.
10 Describe the principles of arterial cannulation for
cardiopulmonary bypass in congenital cardiac
surgery
Although children have small vessels, the cannulae must be large
•
enough to provide adequate flow without excessive pressure or
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