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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
60
Tendon of Todaro
Oval fossa
Eustachian valve
Coronary sinus
Membranous septum
Hinge of tricuspid valve
Medial papillary muscle
Conduction axis
Figure 41. Macroscopic image depicting the location of the
atrioventricular conduction axis in a heart with concordant
atrioventricular connections. The right atrioventricular junction is
shown, in an attitudinally appropriate orientation, from the right side,
having opened the junction and reflected
of the axis is shown by the red line joining the apex of the triangle of
Koch with the medial papillary muscle.
its parietal wall. The location
ventricular septal components, nor in the setting of double-inlet left
ventricle or discordant atrioventricular connections.
The connecting node is found at the site of insertion of the malaligned
•
ventricular septum to the inferior atrioventricular junction when there
is straddling of the tricuspid valve.
The node is anterior and beneath the mouth of the right atrial
•
appendage in the setting of double-inlet left ventricle or discordant
atrioventricular connections.
Recommended reading
1. Anderson RH, Webb S, Brown NA, Lamers W, Moorman A. Development of the
heart: (2) septation of the atriums and ventricles.
2. Moorman A, Webb S, Brown NA, Lamers W, Anderson RH. Development of the
heart: (1) formation of the cardiac chambers and arterial trunks.
806-14.
3. Spicer DE, Bridgeman JM, Brown NA, Mohun TJ, Anderson RH. The anatomy and
development of the cardiac valves.
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eÉ~êí
2003; 89(8): 949-58.
eÉ~êí=
2014; 24(6): 1008-22.
2003; 89(7):
Chapter 2
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Congenital cardiac physiology
Carin van Doorn, Hatem Hosny
1 Describe the foetal circulation (Figure 1)
The fundamental difference between the foetal circulation and adult
•
circulation is that gas exchange occurs in the placenta and not in the
lungs.
The placenta receives the foetal deoxygenated blood via the umbilical
•
arteries and returns oxygen-rich blood via the umbilical vein to the
foetal circulation.
The foetal circulation flows as follows:
•
a) oxygenated blood returning from the placenta via the umbilical
vein bypasses the liver through the ductus venosus to drain
into the inferior vena cava (IVC), with oxygen saturation levels
of approximately 70%;
b) IVC blood streams by the Eustachian valve to cross from the
right atrium (RA) to the left atrium (LA) through the patent
foramen ovale;
c) LA blood passes into the left ventricle (LV), from where it is
pumped into the ascending aorta (supplying the coronary
arteries) and aortic arch (supplying the head vessels);
d) deoxygenated blood from the superior vena cava (SVC), with
oxygen saturation levels of approximately 40%, preferentially
enters the right ventricle (RV), and from there it is pumped into
the pulmonary artery (PA);
e) most of the blood from the PA is diverted through the ductus
arteriosus to the descending aorta (as the pulmonary vascular
resistance in the foetus is high) and returns to the placenta via
the umbilical arteries.
61
Most of the highly oxygenated blood is delivered to myocardium and
•
brain. This is achieved by preferential streaming, and intracardiac
and extracardiac shunting.
Key Questions in CONGENITAL CARDIAC SURGERY
62
High oxygen saturation
Medium oxygen saturation
Low oxygen saturation
Figure 1. Foetal circulation.
The foetal circulation allows shunting at four points, including the:
•
a) placenta — which has a low vascular resistance and receives
about 55% of the combined left and right ventricular foetal
cardiac output;
b) ductus venosus — which allows shunting from the umbilical
vein to the IVC, bypassing the liver;
c) foramen ovale — which allows unidirectional shunting of blood
from the RA to the LA by a valve-like flap;
2 Congenital cardiac physiology
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d) ductus arteriosus — which allows shunting from the PA to the
aorta.
2 Describe the circulatory changes that occur at birth
At birth, the neonate starts breathing that results in lung expansion,
•
which together with the pulmonary vasodilatation caused by a high
alveolar PO2, leads to a sharp decline in pulmonary vascular
resistance.
As pulmonary vascular resistance decreases, more blood from the
•
PA enters the lungs instead of the descending aorta (Figure 2).
63
Figure 2. Pressure, blood flow and resistance in the pulmonary
circulation before, at and after birth. The progressive decline in
pulmonary vascular resistance can be seen over the first 5-7 weeks after
birth.
Key Questions in CONGENITAL CARDIAC SURGERY
As the intracardiac shunts close, the systemic and pulmonary
•
circulation are now separated, including:
a) closure of the ductus arteriosus, as a result of smooth muscle
contraction in its wall in response to increased arterial oxygen
saturation levels and decreased levels of circulating
prostaglandin (which came from the placenta);
b) clamping of the umbilical cord and the removal of the low
resistance placenta can result in increased systemic vascular
resistance and reduced return to the IVC and RA;
c) closure of the sphincter of the ductus venosus, which forces
blood to return via the liver to the RA;
d) closure of the flap valve foramen ovale as LA pressure rises,
as a result of the increased pulmonary blood flow.
The ductus arteriosus initially functionally closes due to smooth
•
muscle contraction. This becomes permanent after 3 weeks because
of anatomical changes in the endothelium and subintimal layers of the
64
ductus. Ductal closure can be manipulated in the initial period with
the use of prostaglandins.
The foramen ovale closes passively by the increased LA pressure
•
and is permanently closed in most infants within 6 months. In about
20% of cases, complete fusion of the foramen ovale flap does not
occur, leaving a patent foramen ovale (PFO), which does not
normally allow for significant shunting from left-to-right due to the
valve-like mechanism.
3 What is the ductus arteriosus?
The ductus arteriosus, which is also known as the ductus Botalli, is
•
a vascular channel connecting the main pulmonary vascular trunk
with the descending aorta.
During foetal life, the ductus arteriosus allows shunting of mostly
•
deoxygenated blood from the PA to the aorta.
Maintaining ductal patency after birth can be lifesaving as initial
•
palliation in neonates with duct-dependent congenital heart disease.
In patients with left-sided obstruction, such as severe aortic
•
coarctation, right-to-left shunting through the ductus maintains lower
body perfusion. Percutaneous oxygen measurement will show a
differential in oxygen saturation levels, with lower saturations in the
lower body compared to the upper body (duct-dependent systemic
circulation).
2 Congenital cardiac physiology
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In those with right-sided obstruction, such as pulmonary atresia,
•
retrograde flow through the ductus allows filling of the pulmonary
arteries (duct-dependent pulmonary circulation).
In transposition of the great arteries, where there are parallel
•
circulations, ductal patency aids mixing between the systemic and
pulmonary circulations (duct-dependent mixing).
4 How is ductal closure and opening regulated?
In full-term infants, the ductus closes 24-48 hours after birth due to
•
an increase in arterial pO2, breakdown of circulating PGE2in the
lungs, and lowering of intraluminal ductal pressure due to falling
pulmonary vascular resistance.
In preterm infants, the ductus is less responsive to oxygen, which
•
explains the higher incidence of duct patency in these young babies.
Prolonged patency of the ductus results in pulmonary overflow and
low systemic perfusion.
Ductal constriction can be promoted through inhibition of
•
prostaglandin synthesis via the cyclo-oxygenase pathway. The cyclooxygenase inhibitors, indomethacin and ibuprofen, are used in
clinical practice to promote ductal closure in premature infants.
To maintain ductal patency in a neonate with a duct-dependent
•
circulation, a continuous intravenous infusion of synthetic
prostaglandin E1is used (0.005-0.1µg/kg/min).
65
5 Describe the changes in pulmonary vascular
resistance that occur during normal development
In the full-term foetus, the lungs are collapsed and there is minimal
•
blood flow through the pulmonary vascular bed.
After birth, lung expansion and increased alveolar PO2cause a rapid
•
initial decline in the pulmonary vascular resistance (PVR). In the fullterm infant shortly after birth, the PVR is nearly equal to the systemic
vascular resistance.
During the first 6-8 weeks of life, there is a gradual decrease in PVR
•
with reduction in PA pressures. If any intracardiac communications
are present, there will be increasing shunting from the left to the right
side of the circulation, with resultant pulmonary overflow.
A further fall in PVR occurs after 2 years of age, when it will reach
•
levels that remain constant throughout adulthood.
Key Questions in CONGENITAL CARDIAC SURGERY
Persistent neonatal pulmonary hypertension, also called persistent
•
foetal circulation, can be caused by:
a) lack of normal physiological relaxation of the pulmonary
vascular bed, such as due to hypoxaemia or acidaemia;
b) increased vascular smooth muscle in the pulmonary vascular
bed;
c) inadequate number of blood vessels in the lung parenchyma.
Persistent neonatal pulmonary hypertension needs to be
•
distinguished from morphological cardiac defects that cause
pulmonary venous obstruction, such as obstruction of pulmonary
venous return, veno-occlusive disease or mitral stenosis.
6 Describe the factors that control pulmonary vascular
resistance
Oxygen, hypocapnia and alkalosis act as pulmonary arterial
•
66
vasodilators, whereas hypoxia, hypercapnia and acidosis are
associated with vasoconstriction.
Pharmacological treatment of arterial pulmonary hypertension
•
involves manipulation of endothelium-derived vasodilators (such as
nitric oxide) and vasoconstrictors (such as endothelin-1).
Nitric oxide (NO):
•
a) endothelium-derived NO — which is produced locally in the
lungs and has profound effects on smooth muscle relaxation
and proliferation;
b) exogenous NO — which can be administered in closed
ventilation circuits at 1-20 parts per million for use as a short-
acting pulmonary vasodilator.
Phosphodiesterase (PDE) inhibitors:
•
a) milrinone — which is a PDE Type 3 inhibitor that increases the
bioavailability of cyclic adenosine monophosphate (cAMP),
resulting in smooth muscle relaxation, with resultant pulmonary
and systemic vasodilatation. Intravenous milrinone is widely
used as an inodilator in neonates;
b) sildenafil — which is a PDE Type 5 inhibitor that prevents the
hydrolysis of cyclic guanosine monophosphate (cGMP), the
second messenger of nitric oxide, allowing a more sustained
effect of endogenous nitric oxide.
2 Congenital cardiac physiology
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Prostacyclines:
•
a) prostacyclin — which is a naturally occurring prostaglandin
synthesised in the vascular endothelium from arachnidonic
acid via the cyclo-oxygenase pathway. It is a short-acting,
potent vasodilator throughout the vascular system, and also
has antiplatelet effects;
b) synthetic prostacyclin analogues — which can be delivered via
a continuous intravenous infusion, such as epoprostenol, or
intermittent inhalation, such as iloprost.
Endothelin receptor antagonists (ERA) — which are used to block
•
the action of endothelin-1, a peptide that is produced by the vascular
endothelium with potent vasoconstrictive and proliferative paracrine
actions on vascular smooth muscle cells. An example is bosentan,
which is administered orally.
7 What are the normal values for vital signs in the
paediatric population (Table 1)?
Table 1. Normal values for heart rate, respiratory rate and blood pressure,
according to age.
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Key Questions in CONGENITAL CARDIAC SURGERY
8 What are the normal values for weight, height and body
surface area in the paediatric population (Table 2)?
Table 2. Normal values for weight, height and body surface area,
according to age *.
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9 What is the average circulating blood volume in the
paediatric population?
Premature neonates 95mL/kg.
•
Full term neonates 85mL/kg.
•
Infants 80mL/kg.
•
Adult male 75mL/kg.
•
Adult female 65mL/kg.
•
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2 Congenital cardiac physiology
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10 Describe the relationship between blood flow and
vascular resistance
Blood flow (Q) is the amount of blood volume passing through a
•
vessel per unit time and is often expressed as mL/min.
Blood flow is directly proportionate to the pressure drop (P) over
•
the vessel and inversely proportionate to the vascular resistance (R),
as per the equation:
Q = P
R
In the normal adult circulation, the pulmonary vascular resistance is
•
much lower than the systemic vascular resistance, hence the
pressure required for the blood to cross the pulmonary circulation is
much lower than that required for the same amount of blood for the
systemic circulation.
This is also reflected in that the muscle mass of the right ventricle is
•
substantially less than that of the left ventricle.
In the neonate, as the pulmonary vascular resistance is high and
•
equals systemic vascular resistance, both the right and left ventricles
need to generate high pressure. As the pulmonary vascular
resistance drops in the early weeks of life, there is a concomitant
reduction in right ventricular pressure, and this is accompanied by a
reduction in the right ventricular mass. In patients with transposition
of the great arteries, in whom the morphological left ventricle is the
subpulmonary ventricle, it is the left ventricle that will reduce in mass.
69
11 What are the anatomical characteristics of the normal
adult circulation?
The normal circulation consists of a systemic circulation and a
•
pulmonary circulation, each supported by a dedicated pumping
chamber (i.e. a biventricular circulation).
All the usual cardiovascular structures are present and of normal
•
size, and are connected in normal sequence, with the SVC and IVC
connecting to the right atrium, then right ventricle, pulmonary artery,
pulmonary vascular bed, left atrium, pulmonary veins, left ventricle,
aorta and systemic vascular bed.
The systemic and pulmonary circulations are separated, with no
•
intracardiac communications.
The systemic and pulmonary blood flow are in series, where the blood
•
goes sequentially through the systemic then pulmonary vascular bed.
Blood that leaves the heart via the aorta is distributed to major organ
•
arteries, including the carotid, brachial, superior mesenteric, renal
and iliac arteries, which are in parallel with each other.
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