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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
shunts, the venous return from one circulation has to flow to the other
circulation, with a Qp/Qs = 1. As there is no blood mixing, the
oxygen saturation in the aorta is 100%.
In some conditions, such as tricuspid atresia, the systemic and
•
pulmonary venous return drain to a functionally single ventricle, which
pumps the blood into both circulations. The circulations in such case
are arranged in parallel. The amount of blood flow to each circulation
is determined by its vascular resistance, in proportion to the other
circulation, and the presence of any obstruction to flow. Qp/Qs can
be >1 or <1. As there is total mixing of blood, the oxygen saturation
in the aorta is always <100%. The single ventricle is subjected to
volume overload because it receives the venous return from both
circulations.
21 What are the forces driving the systemic venous return
in a normal two-ventricle circulation?
Pumping action of the systemic ventricle (normally LV).
•
Active relaxation of the pulmonary ventricle (normally RV) draws the
•
80
blood from the atria and systemic veins.
Negative intrathoracic pressure.
•
Gravity — which aids venous return from the upper half of the body.
•
Muscle contraction in the lower limbs — which compresses the
•
venous sinusoids inside the muscles, together with the venous valves
in lower limb veins to direct the blood upwards towards the chest
(muscle pump).
22 What are the principles of single-ventricle circulation?
In patients with a functionally single ventricle, the systemic venous
•
return is connected to the pulmonary arteries without the
interposition of a ventricle.
In contemporary practice, this is most often achieved by sequentially
•
connecting the SVC and IVC to the PA (total cavopulmonary
connection — TCPC). Despite the absence of a pumping chamber
to drive the pulmonary circulation, the blood continues to flow to the
lungs driven by the systemic circulation. This transforms the
circulation (the Fontan circulation) to having the:
a) systemic and pulmonary circulations arranged in series without
a pumping chamber in between (Figure 10). The Qp/Qs is
almost 1;
2 Congenital cardiac physiology
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AB
Figure 10. A) Normal two-ventricle circulation. B) Single-ventricle
Fontan circulation. LV = left ventricle; Ao = aorta; S = systemic circulation;
RA = right atrium; RV = right ventricle; PA = pulmonary artery; PV =
pulmonary vein; LA = left atrium; LV = left ventricle; V = single ventricle;
CV = caval veins.
81
b) there is almost no mixing (except if there is a fenestration), so
oxygen saturation in the aorta is nearly 100%;
c) single ventricle subjected to less volume load (since it receives
only the pulmonary venous drainage) but more afterload (since
it has to support both circulations in series);
d) high pressure in the systemic veins (more than pulmonary
artery pressure), which in the long term may lead to:
i) protein-losing enteropathy and hypoalbuminaemia;
ii) pleural effusions due to high hydrostatic pressure and
low plasma oncotic pressure;
iii) plastic bronchitis;
iv) liver cirrhosis due to chronic parenchymal venous
congestion;
v) venous stasis that predisposes to subclinical thrombosis
and pulmonary embolism, which further increases the
pulmonary vascular resistance leading to more venous
stasis and a vicious circle.
23 What are the principles of a one-and-a-half-ventricle
circulation?
In patients with less than adequate RV size but with a normal LV, a
•
two-ventricle circulation is not possible because the RV is too small
Key Questions in CONGENITAL CARDIAC SURGERY
to pump the whole cardiac output, which is maintained by a right-toleft shunt at atrial level, at the cost of cyanosis.
In such patients, septation can still be achieved with a strategy of
•
one-and-a-half-ventricle circulation. This is established by connecting
the SVC to the PA (bi-directional Glenn shunt) after pulmonary
vascular resistance has fallen.
In this arrangement, the small RV only pumps the systemic venous
•
drainage from the lower half of the body, while the systemic venous
blood from the upper half of the body passes directly through the
Glenn shunt to the PA.
Other shunts (such as ASD and PDA) can then be closed.
•
Examples for this strategy include pulmonary atresia with intact
•
ventricular septum and small RV, and in some cases of Ebstein
anomaly with severe displacement of the septal leaflet of the tricuspid
valve and a small RV. Occasionally, a one-and-a-half-ventricle
strategy may be considered in patients with severe RV failure and
low PA pressure.
82
24 What are the effects of pressure load and volume load
on cardiac chambers and blood vessels?
Acute severe pressure or volume load is poorly tolerated by the
•
heart. The cardiac chambers acutely dilate, as the heart continues to
fill but cannot adequately empty. Contractility rapidly deteriorates, as
the acute increase in preload causes the myocardium to overstretch
(‘beyond the top of the Starling curve’).
Acute congestion occurs in the cardiovascular bed upstream from
•
the haemodynamic load, such as acute liver congestion in a
pulmonary hypertensive crisis. The cardiovascular bed downstream
from the load will be underfilled, with associated hypotension.
More gradual onset of pressure and volume overload will lead to
•
adaptive changes in the heart. Volume load, such as from a shunt and
valvar regurgitation, leads to dilatation of the cardiac chambers, while
pressure load, such as from an obstruction to flow and increased
resistance, leads to hypertrophy.
For a cardiac chamber to dilate because of volume load, the excess
•
volume needs to stay long enough in that chamber to stretch its
walls. For example, in the presence of a VSD, excess blood volume
continuously reaches the left atrium but can only be released to the
left ventricle during diastole. As a result, there will be dilatation of the
left atrium. In the presence of a large ASD, however, the excess
blood volume will continuously be offloaded to the right atrium and
the left atrium will not dilate.
2 Congenital cardiac physiology
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Both volume and pressure load lead to dilatation of blood vessels,
•
such as post-stenotic dilatation of the aorta, associated with aortic
stenosis or coarctation.
25 What is compliance?
Compliance (C) is used to describe how easily a chamber of the
•
heart or the lumen of a blood vessel adapts volume of blood (V)
without an increase in pressure (P), where:
C = V
P
Compliance decreases with increased wall thickness. So, normally,
•
LV compliance is lower than RV compliance. Therefore, LV enddiastolic pressure (LVEDP) is normally higher than RV end-diastolic
pressure (RVEDP).
In the neonatal heart, both ventricles are thick and non-compliant,
•
whereas in adulthood, LV compliance reduces with advancing age.
As shunting through an ASD is driven by ventricular compliance, a
similar sized ASD will result in a higher left-to-right shunt with older
age.
83
26 Describe the compliance of the arterial bed and
venous bed
At higher pressure and volume, the compliance of blood vessels
•
decreases, with the vessels becoming stiffer (Figure 11).
The compliance of arteries is less than that of veins. At lower
•
pressures (venous pressure is usually less than 15mmHg), the
compliance of a vein is about 10 to 20 times greater than an artery.
Therefore, veins can accommodate large changes in blood volume
with only small changes in pressure.
The compliance is affected by the vascular tone in the vessel.
•
Hence, the higher vascular tone in small arteries and arterioles
(resistance vessels) determines the vascular resistance and
cardiac afterload, whilst the lower vascular tone in small and large
veins (capacitance vessels) determine the venous pressure and
cardiac preload.
Key Questions in CONGENITAL CARDIAC SURGERY
AB
84
Figure 11. The volume-pressure relationship (compliance) for an artery
and a vein: A) an increase in volume in the compliant venous system is
accommodated by a much smaller rise in pressure compared to that of
the arterial system; B) the effect of increasing vascular tone (reducing
compliance) in the arterial and venous systems.
27 What is the difference between a restrictive and non-
restrictive cardiac defect?
A non-restrictive defect is one that is large enough so that it does not
•
restrict blood flow across it. The defect allows equalisation of
pressures on both sides, with no pressure drop (gradient). For
example, a large VSD allows communication between the left and the
right ventricles, and since the defect offers no resistance to flow,
pressures equalise on both sides of the interventricular septum.
A restrictive defect, however, is small enough to restrict blood flow
•
across it, so that there is a significant pressure gradient across it. For
example, a small VSD restricts blood flow, so the pressure in the
right ventricle is still significantly lower than that in the left ventricle.
2 Congenital cardiac physiology
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28 What are the symptoms and clinical signs of a
significant left-to-right shunt in infants?
Increased pulmonary blood flow leads to increased hydrostatic
•
pressure in the pulmonary capillaries, which leads to increased
filtration of fluid in the pulmonary interstitial space and impaired
exchange of gases across the alveolar-capillary membrane, resulting
in dyspnoea and tachypnoea.
This dyspnoea causes young infants to interrupt feeding. In addition,
•
the combination of increased effort of breathing and inadequate
systemic circulation, due to steal into the pulmonary circulation, can
lead to a failure to thrive.
The majority of infants with a large left-to-right shunt and congestive
•
heart failure are stunted in growth, if not malnourished, due to a
combination of high energy expenditure that exceeds caloric intake
and reduced absorption due to venous congestion of the gut.
The increased fluid in the pulmonary interstitial space also
•
predisposes to recurrent chest infections.
29 Describe the pathophysiology of an atrial septal defect
In most cases of an ASD, there is a net left-to-right shunt from the LA
•
to the RA. The extra volume of blood passes to the RV, then the PA
and back via the pulmonary veins to the LA.
The volume overload caused by the ASD results in dilatation of the
•
RA, RV and PA, as these chambers and vessels operate as the
compliant reservoir of volume overload.
The dominant effect of dilatation is usually on the right ventricle, as
•
the right atrium is able to quickly discharge the overload as a
compliant chamber (Figure 12). When ventricular dilatation reaches
a point of decreased compliance and end-diastolic pressure
increases, the right atrium starts dilating. This process, which could
lead to a net reduction of the shunt once compliance on the right side
equates to that of the left side, is usually very slow and can take 5-6
decades.
Conditions with an increased degree of shunting, such as ASD
•
associated with partial pulmonary venous return, produce right
ventricular dilatation earlier and to a more severe degree. Although
the volume of blood that enters the LA is increased, the LA does not
dilate because the increased volume does not stay in the LA long
enough to stretch its walls, as it gets directly shunted through the
ASD to the RA.
85
Key Questions in CONGENITAL CARDIAC SURGERY
AoSVC
RA
IVC
86
LA
PA
LV
RV
Figure 12. Cardiac chambers affected by volume overload, secondary
to an atrial septal defect. The right atrium empties into a low-resistance
chamber, thus delaying its dilatation, with the right ventricle becoming
the capacitant chamber.
30 Describe the pathophysiology of a ventricular septal
defect
A VSD is associated with a left-to-right shunt from the LV to the RV.
•
The extra volume of blood passes to the PA and back via pulmonary
veins to the LA then the LV.
The volume overload resulting from the VSD results in dilatation of
•
the PA, pulmonary veins, LA and LV. Although the volume of blood
that enters the RV increases, the RV does not dilate because the
shunting occurs in systole, when the ventricle is contracting. Thus,
the blood passes through the RV into the PA, without causing it to
dilate (Figure 13).
2 Congenital cardiac physiology
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87
Figure 13. Left ventricular overload secondary to a ventricular septal
defect. As the blood is immediately ejected into the pulmonary
circulation, the right ventricle (RV) is rarely affected. The increased
preload, however, results in dilatation of the left atrium (LA), left ventricle
(LV), and pulmonary artery (PA). Ao = aorta; RA = right atrium.
If pulmonary vascular disease with pulmonary hypertension develops
•
secondary to the VSD, then the RV becomes hypertrophic (not
dilated).
31 Describe the pathophysiology of a patent ductus
arteriosus
A PDA is associated with a left-to-right shunt from the aorta to the
•
PA. The extra volume of blood passes to the PA branches and back
via the pulmonary veins to the LA, then the LV and the aorta.
The volume overload resulting from the PDA results in dilatation of
•
the PA, pulmonary veins, LA, LV and aorta, proximal to the PDA
(Figure 14).
In summary, in left-to-right shunts, the PA and pulmonary veins are
•
dilated. In ASD, the right side of the heart is dilated, while in VSD and
PDA the left side of the heart is dilated. It is important to remember
that patients may have multiple congenital cardiac defects.
Key Questions in CONGENITAL CARDIAC SURGERY
88
Figure 14. Mechanism of progressive congestive heart failure in a
patient with a high-flow patent ductus arteriosus.
32 Describe the findings of a left-to-right shunt on non-
invasive cardiac imaging
Chest radiograph (Figure 15) — which may demonstrate:
•
a) cardiomegaly;
b) enlarged PA shadow;
c) pulmonary plethora.
Echocardiography (Figure 16) — which may demonstrate:
•
a) chamber dilatation (RA and RV in ASD; LA and LV in VSD; LA
and LV in PDA);
2 Congenital cardiac physiology
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Figure 15. Chest radiograph of a patient with a non-
restrictive ventricular septal defect causing a significant
left-to-right shunt, demonstrating cardiomegaly, an
enlarged pulmonary artery shadow and pulmonary
plethora.
89
AB
RVOT
RA
Ao
LA
Figure 16. Parasternal short-axis echocardiographic view
demonstrating: A) a non-restrictive ventricular septal defect (arrow); and
B) a left-to-right shunt across the ventricular septal defect on the
corresponding colour flow Doppler image.
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