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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
shear force, whilst being small enough to avoid completely
obstructing the vessel.
As resternotomy cases may be associated with a risk of direct
•
trauma to the heart and great vessels, cannulae may be placed in
alternate sites initially, such as the femoral or subclavian artery to
facilitate CPB prior to sternotomy.
Congenital cardiac anatomy shows greater inter-patient variability
•
and arterial cannulation needs to reflect this, such as for the:
a) arterial switch procedure — where distal placement is
necessary to avoid flow being directed away from the arterial
branches (Figure 4);
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AB
IVC cannula
SVC cannula
^ç
Figure 4. Cardiopulmonary bypass cannulation for a patient with
transposition of the great arteries and a ventricular septal defect,
including: A) the aortic cannulation placed in the aortic arch, with
inclination of the cannula at insertion to avoid damage to the posterior
wall; and B) bicaval venous cannulation to allow right atrial access to the
ventricular septal defect. Ao = aorta; SVC = superior vena cava; IVC =
inferior vena cava. Images courtesy of Mr. Antonio Ravaglioli, University
Hospital Southampton, UK.
10 Cardiopulmonary bypass
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b) stage I repair for hypoplastic left heart syndrome — where the
cannula is placed in the pulmonary artery, as the aorta is small
or absent;
c) complex aortic arch reconstructions — where two cannulae
may be required to perfuse the upper and lower body, and
account for anatomical anomalies of the epi-aortic branches
(Figure 5).
RA cannula
PDA cannula
RBCA cannula
Figure 5. Cardiopulmonary bypass set-up in a neonate with a Type B
interrupted aortic arch, where the proximal section of the aorta has been
cannulated via a Gore-Texinterposition graft anastomosed to the right
brachiocephalic artery (RBCA), thereby perfusing the right common
carotid artery, right subclavian artery and left common carotid artery.
The second arm of the arterial line has been used to cannulate the patent
ductus arteriosus (PDA) to perfuse the descending aorta and the left
subclavian artery. A single venous line is used to drain the right atrium
(RA). Image courtesy of Mr. Antonio Ravaglioli, University Hospital Southampton,
UK.
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362
Key Questions in CONGENITAL CARDIAC SURGERY
11 Describe the principles of venous cannulation for
cardiopulmonary bypass in congenital cardiac
surgery
Blood drains by gravity from the patient to the reservoir, secondary to
•
a siphon effect.
Separate cannulae in the IVC and SVC provide optimum drainage
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and an empty heart for surgical access.
Children provide particular challenges, with drainage more likely to
•
be compromised as vessels, cannulae and blood volumes are
smaller.
In addition, the small tubing used to reduce the prime volume also
•
reduces the gravity siphon drainage and therefore flow. This may
require augmentation by controlled vacuum-assisted suction.
In resternotomy cases, CPB may need to be initiated via the femoral
•
route, which may not provide adequate venous return, thereby also
requiring vacuum-assisted drainage.
Congenital patients may have unusual venous anatomy, such as left
•
or bilateral SVCs, azygos/hemiazygos continuation of the IVC,
necessitating more than two venous cannulae.
12 Describe the principles of venting the heart during
cardiopulmonary bypass
One or more of the suckers on the CPB circuit may be used as a
•
cardiac vent to:
a) clear the operative field;
b) prevent warming the heart — which increases myocardial
oxygen consumption;
c) avoid distending the arrested heart — which increases
ventricular wall tension, increases myocardial metabolic rate
and reduces subendocardial perfusion, thereby leaving the
myocardium more vulnerable to ischaemic damage;
d) de-air the heart at the completion of the procedure.
Venting is required more frequently than in acquired disease due to
•
more variable venous anatomy, with the increased likelihood that the
heart may not be fully emptied. Blood may return to the heart from
the:
a) Thebesian and bronchial veins — which occur in a normal
circulation;
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b) pathophysiological systemic-pulmonary shunts — such as
collateral vessels, patent ductus arteriosus;
c) abnormal anatomy — such as a persistent left SVC;
d) surgically implanted shunts — such as Blalock-Taussig shunt
or Potts shunt;
e) coronary sinus — following administration of cardioplegia.
Vents may be sited in the:
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a) left ventricle — antegrade via the right superior pulmonary vein
and left atrium, into the ventricle, or retrograde via the aortic
valve if the aorta is opened;
b) right ventricle — antegrade.
13 Describe how the required flow rate (cardiac output) is
calculated in adult and paediatric patients
Flow rate in litres per minute (LPM) can be calculated as body
•
surface area (BSA) multiplied by the desired cardiac index (CI):
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Flow (LPM) = BSA x CI
Although several calculations are available for BSA utilising height
•
(H) and weight (W), the most commonly used is the DuBois formula:
BSA = Height (cm) x Weight (kg)
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Adults usually require a cardiac index of 2.4, whereas in paediatrics,
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a CI of 2.6-2.8 is used due to their higher metabolic rate.
14 Describe the principles of gas exchange in the
cardiopulmonary bypass circuits
Gases diffuse in and out of blood across the micropores in the
•
oxygenator fibres in accordance with their concentration gradients
(Figure 6).
Microporous fibres are orientated in such a way that gas flow is
•
perpendicular (or counter-current) to blood flow, which increases the
efficiency of gas transfer.
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Key Questions in CONGENITAL CARDIAC SURGERY
Figure 6. Membrane oxygenator with a hydrophobic membrane that
presents a large surface area for gas exchange.
Fick’s law of diffusion states that the amount of gas that diffuses
•
increases as concentration gradients increase. Thus, to increase:
a) O2transfer into blood — the concentration of O2in the fibres
is increased via a blender;
b) CO2removal from blood — the rate of gas flow (‘sweep’)
through the fibres is increased.
15 What are the principles of myocardial protection in
patients undergoing surgery for congenital cardiac
disease?
It is important to match O2supply with demand of the myocardium to
•
provide myocardial protection by ensuring:
a) adequate aortic root (driving) pressure;
b) minimal ventricular wall tension with an empty ventricle;
c) optimal Hct for carriage of O2(high);
d) adequate flow (low viscosity) through small coronary vessels.
Myocardial O2demand can be reduced by means of:
•
a) temperature — with a reduction associated with decreased
metabolic rate;
10 Cardiopulmonary bypass
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b) pharmacologically — using cardioplegia to paralyse the
myocardium in a flaccid state by increasing extracellular
potassium concentration, thus blocking contraction and
minimising cellular metabolism (Figure 7).
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Figure 7. Antegrade crystalloid cardioplegia delivered under pressure,
usually via a roller pump.
Cardioplegia is often mixed with arterial blood, containing O2bound
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to haemoglobin, in older children and adults. The ratios vary but 4:1
(blood: cardioplegia) and 1:1 are the most common.
In neonates and infants, crystalloid solutions avoid highly viscous
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blood failing to perfuse tiny coronary arteries at low temperatures.
As any collateral circulation may cause washout of cardioplegia and
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compromise myocardial protection, the ECG must be monitored to
check for signs of electrical activity at all times.
Where there is ventricular hypertrophy, such as in patients with aortic
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stenosis, higher doses of cardioplegia may be required.
16 When is cardiac arrest required for congenital cardiac
surgery?
Arresting the heart with cardioplegia represents myocardial
•
ischaemic time and therefore is only performed when:
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Key Questions in CONGENITAL CARDIAC SURGERY
a) complex surgery is made easier on a still heart;
b) surgery is needed on the myocardium of the very young;
c) there is a risk of generating systemic air in patients with an
intracardiac shunt, such as an ASD or VSD, access to leftsided structures is required, or when operating on patients
with a single ventricle.
The heart may not need to be arrested for:
•
a) surgery for an extracardiac shunt, such as a Blalock-Taussig
shunt;
b) a right-sided abnormality, such as right ventricular outflow tract
obstruction, provided intracardiac communications have been
excluded.
17 What are the principles of myocardial arrest in
congenital cardiac surgery?
Cardioplegic arrest is the mechanism by which depolarisation (with
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subsequent prevention of repolarisation) or hyperpolarisation
(preventing depolarisation) of the myocyte membrane is obtained in
order to stop the release of calcium into the myocellular cytoplasm
thus preventing contraction.
High-concentration solutions (such as St. Thomas’ solution) that
•
induce membrane depolarisation by means of:
a) potassium chloride in high concentrations — which alters the
resting membrane potential (-90mV) and ionic gradients. The
availability of rapid opening sodium channels is reduced
thereby maintaining diastolic arrest. The side effects include
vasoconstriction (with consequent variable myocardial
delivery) and myocyte calcium loading (with increased
repolarisation injury);
b) magnesium — which inhibits myosin phosphorylase,
protecting ATP reserves for post-ischaemic activity, and also
reduces vasoconstriction and intracellular calcium loading.
Depleted solutions (such as Bretschneider solution) that induce
•
membrane hyperpolarisation by means of:
a) sodium depletion — which induces diastolic cardiac arrest by
eliminating the sodium differential across the cell membrane;
b) calcium depletion — which prevents the contractile force from
being generated;
10 Cardiopulmonary bypass
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c) lidocaine — which prevents depolarisation by entering the
myocyte and blocking the sodium channels on the cytoplasmic
side of the membrane (local anaesthetic action). Associated
vasodilatation promotes even distribution, thereby overcoming
the coronary vasoconstriction action of potassium;
d) adenosine — which is believed to enhance recovery with small
doses.
These solutions may be delivered:
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a) antegradely via the aortic root — which requires a competent
aortic valve;
b) antegradely directly into coronary vessels — where care must
be taken to avoid causing trauma, especially with small
coronary ostia (Figure 8);
AB
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Figure 8. Cardioplegia delivery in neonatal surgery: A) the
cardioplegia cannula is inserted at an angle and caudally to the aorta to
avoid perforation of the posterior wall and injury to the aortic valve; and
B) following cross-clamp particular attention is given to avoid left
ventricular distension and high aortic root pressure.
Antonio Ravaglioli, University Hospital Southampton, UK.
c) retrogradely via the coronary sinus — which is uncommonly
used due to possible malformations in patients with congenital
heart disease.
Images courtesy of Mr.
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Key Questions in CONGENITAL CARDIAC SURGERY
Hypothermia:
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a) local hypothermia to the heart (such as using ice or a cooling
jacket) — which reduces phosphate utilisation and decreases
basal O2consumption by approximately 50% for every 10°C
drop;
b) systemic cooling — which decreases the heat delivered to the
heart by blood returning via:
i) normal body drainage — via the venae cavae (congenital
patients may have an additional persistent left SVC);
ii) normal heart drainage — via the coronary sinus and
Thebesian veins;
iii) lungs — via bronchial veins or anomalous pulmonary
venous drainage;
iv) collateral vessels — which may require active venting, if
significant.
Hypothermic fibrillation — which provides a still field for surgery
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using systemic hypothermia (<24°C), at which temperature the heart
fibrillates. If the systemic pressure is maintained >70mmHg, the
myocardium remains perfused by blood passing antegradely through
coronary vessels, potentially providing protection for an hour or
more. It is useful (and commonly reported) in patients undergoing
redo surgery where the risk of injury to the coronary arteries must be
avoided. Problems, however, may occur with:
a) impaired coronary flow — if the fibrillating myocardium
compresses the coronary arteries;
b) incompetent aortic valve — which may cause ventricular
distension and coronary malperfusion.
Cross-clamp fibrillation — which involves:
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a) the heart electrically fibrillated, thus reducing the myocardial
load, although it does induce global ischaemia and anoxic
arrest;
b) hypothermia (32-34°C), to reduce myocyte O2demand and
improve myocardial protection;
c) decompression of the ventricles by intermittently using a cross-
clamp applied to the aorta, to avoid distension;
d) limiting each fibrillating episode to 10-15 minutes, with at least
3-4 minutes of coronary reperfusion time required between
clamping periods.
10 Cardiopulmonary bypass
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18 What are the important complications of
cardiopulmonary bypass in patients with congenital
cardiac disease?
In this patient population, the immature organs are more vulnerable
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to damage from CPB.
The foetal lungs have a high pulmonary vascular resistance (PVR)
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that progressively drops during the first few weeks of life. The use of
CPB in neonates, particularly those with a persistently high PVR,
increases the incidence of lung injury and further dysfunction, from
both inflammation (secondary to surface contact with the circuit) and
ischaemia-reperfusion injury (following periods of inadequate flow).
Neurological complications, including cerebrovascular accident,
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seizures, intracranial haemorrhage, and delayed choreoathetoid
syndrome have been described.
The use of deep hypothermic circulatory arrest (DHCA) increases
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the risk of complications, including major gastrointestinal
complications, such as necrotising enterocolitis, perforation and
bowel ischaemia.
Patients with cyanosis or a hypertrophied myocardium are
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particularly challenging to protect and more sensitive to ischaemicreperfusion injury.
Acute kidney injury is common in children following CPB.
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Chronic hypoxia and high blood viscosity in older, severely cyanotic
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patients may further impact organ function.
Surgery in patients with adult congenital heart disease (ACHD) is
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associated with a higher morbidity compared to patients with
acquired cardiac disease, as these patients have:
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a) an increased risk of catastrophic surgical bleeding during redo
procedures;
b) pre-existing coagulopathy, often due to varying degrees of liver
failure and exacerbated by dilution of clotting factors and
platelets during CPB, thereby increasing the risk of
haemorrhage;
c) concurrent renal abnormalities, caused by repeated CPB and
chronic hypoxia, which may further increase the risk of acute
kidney injury (AKI) and long-term damage.
Pre-existing abnormalities make it difficult to attribute these solely to
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CPB, but hypoxia, temperature-related cerebral flow changes and
embolism are potential mechanisms of damage.
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