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Key Questions in CONGENITAL CARDIAC SURGERY
The exception are the gut and hepatic circulations, which are partly
•
in series as the venous drainage from the intestines drains into the
portal vein that supplies most of the inflow to the liver.
Within each organ, there is a microcirculation arranged as a series of
•
in-parallel and in-series vessels (Figure 3).
70
Figure 3. Normal adult circulation.
12 What are the principles of a biventricular circulation?
A biventricular circulation has two adequately sized functioning
•
ventricles.
One ventricle supports the pulmonary circulation, the other supports
•
the systemic circulation and, in the absence of any intracardiac
shunts, these circulations are connected in series.
Usually, the morphological right ventricle is the subpulmonary
•
ventricle and the morphological left ventricle is the subaortic
ventricle.
Each ventricle provides a step-up in blood pressure that allows the
•
propulsion of blood through the downstream vascular bed.
2 Congenital cardiac physiology
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As the systemic circulation normally has a much higher resistance
•
than the pulmonary circulation, the subaortic ventricle needs to
generate a much higher step-up in blood pressure than the
subpulmonary ventricle (Figure 4).
71
Figure 4. Step-up in systemic and pulmonary blood pressure by the left
and right ventricles.
13 Describe the principles of calculating systemic and
pulmonary vascular resistance
Vascular resistance represents the opposition to blood flow in the
•
circulation.
Although resistance cannot be directly measured, vascular
•
resistance is calculated as:
R = P
Q
Key Questions in CONGENITAL CARDIAC SURGERY
R = resistance; P = change in pressure across the circulation loop
from its beginning (immediately after exiting the ventricle) to its end
(entering the atrium); Q = flow through the vasculature.
Systemic vascular resistance (SVR) can be calculated as:
•
SVR (dynes.sec/cm5) = (MAP – CVP) x 80
CO
SVR (Wood units) = (MAP – CVP)
CO
MAP = mean arterial pressure (mmHg); CVP = central venous
pressure (mmHg); CO = cardiac output (L/min).
Normal SVR is 770-1500 dynes.sec/cm
Pulmonary vascular resistance (PVR) can be calculated as:
•
PVR (dynes.sec/cm5) = (MPAP – LAP) x 80
CO
5
or 10-20 WU.
72
PVR (Wood units) = (MPAP – LAP)
CO
MPAP = mean pulmonary arterial pressure (mmHg); LAP = left atrial
pressure (or pulmonary venous wedge pressure) (mmHg); CO =
cardiac output (L/min).
Normal PVR is 20-120 dynes.sec/cm5or 0.25-1.5 WU.
14 Discuss the regulation of peripheral vascular resistance
in the systemic circulation
Within the body as a whole, and in individual organs, the circulation
•
is made up of both in-series and in-parallel vascular elements.
The major distributing arteries from the aorta (e.g. carotid, brachial,
•
superior mesenteric, renal, iliac) are in parallel with each other. The
notable exception is the liver, which is partly in series with the gut.
Within individual organs, the arteries branch out, terminating in
•
microvascular vascular beds comprising of small arteries (A),
arterioles (a), capillaries (c), venules (v) and veins (V) (Figure 5).
The Poiseuille equation (R = 8Lη/.r4) dictates that resistance in an
•
individual segment of vessel (R) is directly proportional to the length
(L) of the vessel and inversely proportional to the radius to the fourth
power (r4).
2 Congenital cardiac physiology
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73
Figure 5. Distribution of systemic blood flow.
When vascular segments are connected in series, the total vascular
•
resistance in this system equals the sum of the individual resistances.
In the typical microvascular bed, the relative contributions to the total
•
resistance (RT) of the bed are approximately RA = 20%, Ra = 50%,
Rc = 20%, Rv = 6%, RV = 4% (Figure 6).
Small arteries and arterioles comprise approximately 70% of the total
•
resistance in most organs, and changes in the diameter, and
therefore resistance, of these arteries are the major determinants of
the vascular resistance in an organ.
For body arteries that are in a parallel arrangement, including the
•
distribution of the aortic blood flow, the total resistance of such
vascular networks can be calculated (Figure 7).
Key Questions in CONGENITAL CARDIAC SURGERY
RT= RA+ Ra+ Rc+ Rv+ R
V
Figure 6. Calculation of the vascular resistance with vessels in series.
74
Figure 7. Calculation of vascular resistance with vessels in parallel.
In principle, adding a vascular bed in series to a circulation increases
•
its total vascular resistance, while adding a vascular bed in parallel
decreases the total resistance.
For the body as a whole, the large distributing arteries comprise only
•
approximately 1% of the total vascular resistance, and therefore,
unlike arterioles, changes in the diameter have a relatively small
effect on total resistance. Although a 50% reduction in radius should
2 Congenital cardiac physiology
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increase the resistance in the individual vessel 16-fold according to
the Poiseuille equation, the overall resistance will only increase by
about 15% as the relative contribution of the vessel to the overall
resistance is small.
15 What is collateral circulation?
A collateral circulation is an alternate circulation, which develops
•
around an obstruction or occasionally lack of development, of an
artery or vein. Some examples encountered in congenital cardiac
surgery include the:
a) collateral arterial circulation that develops via intercostal
arteries in patients with coarctation of the aorta, which
provides blood flow to organs distal to the coarctation;
b) veno-veno collateral vessels in patients with a superior vena
cava to pulmonary artery shunt (Glenn shunt), with blood
flowing from the higher pressure SVC to the lower pressure
IVC territory. The resultant bypassing of the pulmonary
vascular bed results in desaturation;
c) major aortopulmonary collateral arteries (MAPCAs), which are
systemic collateral arteries from the aorta or subclavian artery,
that perfuse the lung parenchyma if the central pulmonary
arteries are underdeveloped or absent, thereby enabling blood
delivery for pulmonary gas exchange. Longstanding highpressure MAPCAs lead to the development of pulmonary
vascular disease;
d) arteriovenous malformations in the lung, which are abnormal
connections between pulmonary arteries and veins bypassing
the capillary system and leading to central cyanosis.
75
The collateral circulation connects a high-pressure proximal vascular
•
bed to a lower-pressure vascular bed. In the event of an arterial
occlusion, it helps to provide oxygenated blood to the downstream
area.
The collateral blood flow may occur via pre-existing redundancy or
•
via new branches formed between adjacent blood vessels
(neovascularisation).
Collateral blood flow may result in significant volume loading of the
•
circulation.
Key Questions in CONGENITAL CARDIAC SURGERY
16 What is cardiac output?
Cardiac output (CO) is the volume of blood ejected by the heart per
•
unit of time, and is usually expressed as litres/minute (L/min).
Cardiac output can be calculated as:
•
CO = Stroke volume x Heart rate.
Stroke volume is the volume of blood ejected by the ventricle in a
•
single beat.
In neonates, as ventricular volumes are small and the ventricles are
•
poorly compliant, the stroke volume is fixed within narrow margins.
Hence, any decrease in heart rate will result in a decrease in cardiac
output.
In adults, however, the ventricles are much more compliant. When
•
the heart rate decreases, there will be more blood entering the heart,
and thus stroke volume will increase (according to Starling’s law) and
cardiac output remains unaffected (within limits).
76
17 What are the principles of cardiac shunting?
In the normally connected circulation, the pulmonary and systemic
•
blood flow are separated and arranged in series. The blood must
pass through the pulmonary vascular bed to reach the systemic
circulation, and vice versa.
In the absence of shunts, the amount of pulmonary blood flow (Qp)
•
is equal to that of systemic blood flow (Qs), hence the ratio of
pulmonary to systemic flow (Qp/Qs) = 1.
Shunting of blood between the systemic and pulmonary circulations
•
occurs when the normal flow of blood is diverted because of an
intracardiac communication and the blood goes back to the same
capillary bed that it came from.
Some cardiac shunting may occur in normal states, including:
•
a) anatomic shunting — which occurs when bronchial arterial
blood returns to the left atrium;
b) physiological shunting — which occurs in the lungs due to the
effect of gravity, where more blood flows to the lung bases,
which are not well ventilated, and bypasses the alveoli.
Cardiac shunts are common in congenital heart disease. They can
•
present as communications at the level of the:
a) atria — atrial septal defect (ASD);
2 Congenital cardiac physiology
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b) ventricles — ventricular septal defect (VSD);
c) great arteries — patent ductus arteriosus (PDA).
The blood flow across the shunt is pressure driven. In the case of a
•
VSD, this is directly related to the pressure difference between the
ventricles at any time during the cardiac cycle, whereas for an ASD,
the pressure difference between the atria is governed by the
ventricular end-diastolic pressure. Since this pressure is normally
higher in the thick-walled, relatively non-compliant left ventricle, an
ASD will usually shunt from the left to right atrium.
In patients with a cardiac communication and otherwise normally
•
connected circulation, oxygenated blood will shunt from the highpressure left side to the low-pressure right side, representing a leftto-right shunt, where the patient is not cyanosed (pink).
If a VSD is associated with severe pulmonary stenosis, such as in
•
tetralogy of Fallot, the obstruction to pulmonary blood flow forces the
blood to pass through the VSD from the RV to the LV, representing
a right-to-left shunt, where the patient is cyanosed (blue).
77
18 Describe the changing physiology of a cardiac shunt
in the first few months following birth
In patients with congenital heart disease, changes in pulmonary
•
vascular resistance in the neonatal period affect the direction and
magnitude of the shunt flow.
Immediately after birth, as pulmonary vascular resistance is high and
•
there is little difference between pulmonary and systemic pressure,
there will be minimal overall blood flow across the cardiac
communication.
After the pulmonary vascular resistance drops in the first few weeks
•
of life, the pressure difference between the systemic and pulmonary
circulations increases, causing an increase in flow from left to right
across the shunt, with a Qp/Qs ratio of >1.
Patients with a right-to-left shunt have lower pulmonary flow than
•
systemic flow, with a Qp/Qs ratio of <1.
19 What is the difference between cardiac streaming and
cardiac mixing?
Cardiac streaming refers to the preferential flow of blood when the
•
systemic and pulmonary circulations meet without a physical barrier
within the cardiac chambers. In this case, as complete mixing of
blood does not take place, oxygen saturation levels at the outlets
(great arteries) will be different and determined by the specific
anatomical setting.
Key Questions in CONGENITAL CARDIAC SURGERY
For example, in the foetal circulation when oxygenated blood from
•
the ductus venosus enters the right atrium, it preferentially flows to
the left atrium. Most of the deoxygenated blood returning from the
SVC will enter the right atrium.
Streaming is common in patients with double-outlet right ventricle, a
•
condition where both great arteries arise completely or
predominantly from the right ventricle. If the VSD is subaortic, the
oxygenated blood from the LV will be ejected into the aorta just above
it, and the deoxygenated blood will enter the pulmonary artery (Figure
8A). If the VSD is subpulmonic (Taussig-Bing anomaly), however, the
oxygenated blood from the LV will predominantly enter the pulmonary
artery, while the deoxygenated blood will enter the aorta and the
patient will be cyanosed (Figure 8B). This is a similar clinical picture
to transposition of the great arteries and VSD, except in this
condition the aorta comes from the RV and the PA from the LV.
AB
78
Figure 8. Streaming in double-outlet right ventricle, with a: A) subaortic
ventricular septal defect; and B) subpulmonic ventricular septal defect.
2 Congenital cardiac physiology
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In cardiac mixing, the systemic and pulmonary circulations totally
•
blend when they meet inside the cardiac chambers, and the oxygen
saturations will be the same in both great arteries. An example of this
is tricuspid atresia, where there is no connection between the RA
and RV, and all systemic venous return crosses an ASD to enter the
LA and mixes with pulmonary venous return (Figure 9).
79
Figure 9. Total mixing in tricuspid atresia.
20 Describe the difference between the arrangement of
systemic and pulmonary circulations in series and in
parallel
Normally, the systemic and pulmonary circulations are arranged in
•
series, each supported by a pumping ventricle. In the absence of
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