Measurements during the catheterization include oxygen saturations
and pressures in the cardiac chambers and great vessels. This information
is used to calculate blood flows, intracardiac shunts, and vascular
resistances. Angiography in one or more chambers or vessels is then
done to define the cardiac anatomy.
A small blood sample for oximetry is taken from each of the major
vessels and cardiac chambers. Saturations are currently measured
by spectrophotometry based on the difference between the maximal
wavelength absorption of hemoglobin bound to oxygen (600 nm) versus
deoxygenated hemoglobin (506.5 nm). Shunts are diagnosed by comparing
oxygen saturations in great veins, chambers, and great vessels.
This information must be interpreted with caution, as streaming may
influence saturations, especially in the vena cavae and right atrium.
For example, hepatic venous blood with medium saturation mixes poorly
with highly saturated renal streams and less saturated femoral venous
streams in the inferior vena cava. The lowest oxygen concentrations
are found in coronary sinus blood, and because the coronary sinus
enters just above the tricuspid valve, a sample in the right ventricle may
be a little less saturated than one in the right atrium. Normally
saturations should vary little through the right heart, less than
4%, so any increase in saturation of greater than 5% indicates a
significant left to right shunt. An increase in oxygen saturation
at a given level may be the result of a shunt at that level, a shunt
at a more proximal level with streaming, or a shunt at a more distal
level with regurgitation.
Although noninvasive methods, including echocardiography with
color Doppler and nuclear medicine shunt studies, can be used to quantify
cardiac shunts, calculation from oxygen saturation data remains
the gold standard. Saturation data may be of limited use in quantifying
multiple cardiac shunts. A patient with a large left-to-right atrial
shunt has blood with very high oxygen saturation entering the right ventricle.
A second shunt of equal magnitude at the ventricular level produces
a relatively small additional increase in oxygen saturation. Consequently,
multiple cardiac shunts are best delineated by using both oxygen
saturation data and angiography.
Normal right-sided oxygen saturations range between 65% and
80%, depending on cardiac output and hemoglobin concentration.
In patients breathing room air, left-sided saturations are usually
95% to 98%. This mild left-sided desaturation
results from normal right-to-left shunting of desaturated pulmonary
bronchial blood into the pulmonary veins.
Flows and shunts may be calculated by the Fick (oxygen content) method
or by indicator dilution techniques that today are done with room
temperature saline and thermistor catheters. The thermodilution
technique is accurate and reliable except in patients with significant
pulmonary or tricuspid regurgitation, or a significant intracardiac
shunt. This technique is therefore primarily used in heart transplant
patients, those with isolated pulmonary hypertension or isolated
obstructive lesions, such as aortic stenosis or coarctation. Because
shunts and regurgitation are common in congenital heart disease,
the Fick method, using oxygen saturation and blood hemoglobin concentration,
is much more widely used. It makes use of the fact that flow through
an organ can be estimated by measuring the concentration of a substance
(indicator) in arterial blood flowing to that organ, the amount
of that substance added to or removed from blood as it passes through
that organ, and the concentration of the substance in venous blood
leaving that organ. For example, if blood entering the lung has
an oxygen content of 150 mL/L, and blood leaving the lung
has an oxygen content of 200 mL/L, then each liter of blood
passing through the lungs picks up 50 mL of oxygen. If the oxygen
consumption is 200 mL/min, then each minute 200 mL of oxygen
is taken up by the lungs and carried away by 200/50 = 4
L of blood. Hence the pulmonary flow (Q̇p) is
4 L/min, calculated as:
Where Q̇p is pulmonary flow (L/min), V̇O2 is
oxygen consumption (mL/min), Cpv is pulmonary
venous oxygen content (mL/L of blood), and Cpa is
pulmonary arterial oxygen content (mL/L of blood).
Note that the calculation uses oxygen content, not oxygen saturation.
Oxygen saturation refers to the percentage of hemoglobin
in blood that is combined with oxygen, and is independent of hemoglobin
concentration. In contrast, oxygen content refers
to the total amount of oxygen in a volume of blood and includes
physically dissolved oxygen in addition to that bound to hemoglobin.
It is expressed as either milliliters of oxygen per deciliter (dL
or 100 mL) or liter of blood or as volume percent. Clearly, the
oxygen content of blood with 50% oxygen saturation and
a hemoglobin concentration of 20 g/dL is nearly twice that
of blood with 50% oxygen saturation and a hemoglobin concentration
of 10 g/dL.
In patients breathing room air, the physically dissolved oxygen
(0.3 mL/dL) is negligible in comparison with the oxygen
bound to hemoglobin (13.6 mL/dL at 100% saturation,
even in an anemic patient with a hemoglobin of 10g/dL).
The oxygen capacity, or the total content of oxygen bound
to hemoglobin when 100% saturated, is calculated by multiplying
the patient’s hemoglobin concentration in gm/dL
by the constant 1.36 mL O2/gm of hemoglobin times
10 to convert dL to liters. A close approximation of the arterial-venous (AV)
difference in oxygen content may be obtained by multiplying the
arterial-venous difference in oxygen saturation by the oxygen capacity.
where satPV is pulmonary venous oxygen saturation expressed
as a fraction of 1, and satpa is pulmonary arterial oxygen
saturation, and capacity = Hgb (in gm/dl) × 1.36 × 10.
With no right-to-left shunting, an aortic sample is preferable
to a pulmonary venous sample because it is a mixture of blood from
all four pulmonary veins. Similarly, with no left-to-right shunting,
a pulmonary arterial sample is the best representative mixture of
systemic venous blood.
In a steady state, the amount of oxygen utilized in the tissues
is equal to the amount taken into the lungs, and systemic flow is
calculated with the same equation and oxygen consumption. Without
shunts the systemic arterial-venous (AV) oxygen difference is the
same as the pulmonary AV oxygen difference, and the calculated pulmonary
flow (Q̇p) equals the systemic flow (Q̇s)
or cardiac output. If there are shunts, then pulmonary and systemic
flows must be calculated separately, the former from oxygen contents
in pulmonary veins and pulmonary artery, the latter from aortic
and mixed systemic venous blood.
Flow and shunt calculations are shown in eFigures
496.2, 496.3, 496.4, and 496.5. In each, the oxygen
consumption (V̇O2) is assumed to be 150
mL/min/m2, and the hemoglobin (Hb) concentration
is assumed to be 14.7 g/dL blood or 147 g/L of
blood. Because 1 g of Hb combines with 1.36 mL O2, then
Schematic of patient with ventricular septal defect (VSD)
showing saturations indicating a left to right shunt with a Qp:Qs
Schematic of patient with ventricular septal defect (VSD)
and pulmonary hypertension showing saturations indicating a right
to left shunt with a pulmonary:systemic blood flow (Qp:Qs) of 0.67.
Schematic of patient with VSD and pulmonary hypertension
showing saturations indicating bidirectional shunt with a Qp:Qs
In the equations, saturations are given as fractions of unity.
Thus, 96% is given as 0.96.
Example 1: Left‐to-Right
The most distal saturation that is proximal to the left-to-right
shunt is used in the calculation, as it is usually the best-mixed
venous sample available. Left-to-right shunt = Q̇p – Q̇s = 3.75
Example 2: Right‐to-Left
Right-to-left shunt = Qs – Qp = 1.25
Example 3: Bidirectional
Calculation of bidirectional shunts requires the concept of effective
pulmonary flow (Qep), defined as the volume of systemic
venous blood per minute that flows through the lungs. This volume
does not include blood that is shunted in either direction because shunted
blood is not effective; it neither picks up oxygen nor delivers
it. In a patient with only a left-to-right shunt, effective flow equals
systemic flow (Q̇p – Q̇ep the
left-to-right shunt), and in a patient with only a right-to-left shunt,
effective flow equals pulmonary flow (Q̇s – Q̇ep is
the right-to-left shunt). In patients with bidirectional shunting, the
arteriovenous difference used to calculate effective pulmonary flow
uses the mixed systemic venous and mixed pulmonary venous oxygen
contents. So effective pulmonary flow is calculated by
Left-to-right shunt Q̇p– Q̇ep = 0.75
L/min/m2; right-to-left shunt = Q̇s – Q̇ep = 2.0