Many methods
have been proposed for the estimation of pulmonary arterial pressures.
Unit 2 discusses the method based on the presence of tricuspid regurgitation.
Other methods are based on the time to peak velocity of the pulmonary
arterial flow velocity recording. These methods only work when there
is no evidence for pulmonary stenosis.
Methods for estimating pulmonary arterial pressures are based on
alterations in the capacity of the pulmonary vasculature to accept
forward systolic flow. In normal individuals without pulmonary hypertension,
the pulmonary vasculature is a very low resistance circuit and has
a great capacity to accept the sudden increase in volume. The vessels
are quite distensible and as blood is ejected from the right ventricle
the time to peak velocity and acceleration time are accordingly
relatively slow.
In pulmonary hypertension, however, resistance rises as blood vessels
thicken and become less distensible. This results in a diminished
capacity to accept the forward systolic flow out of the right ventricle.
The sudden rush of blood into the main pulmonary artery in this
setting results in a more rapid time to peak velocity as well as
a more rapid acceleration time.
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| Fig.3.9 |
These relationships are illustrated in Figure 3.9, where idealized
plots of mean pulmonary artery pressure and time to peak velocity
are shown. Note that these relationships are curvilinear, making
estimates of very high, or very low mean pulmonary pressures difficult.
To overcome this problem, several investigators have pointed out
that plotting time to peak velocity against the logarithm of the
mean pulmonary artery pressures makes the correlations much better.
The clinical application of this approach for estimation of mean
pulmonary arterial pressure remains controversial and many methods
have been proposed. Our purpose is to present the general concept
of these relationships and the reader should consult current literature
for more detailed descriptions of the continuing evolution of this
principle. Many factors must be taken into account, an important
one being heart rate. Adult patients with pulmonary hypertension
may have normal heart rates of 60-70 beats/min, and infants and
children >140 beats/min; this may significantly shorten measurements
of time to peak velocity or acceleration times, and affect the reliability
of these estimates of pressure.
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| Fig.3.10 |
What is most important is that time to peak velocity is significantly
shortened in patients with pulmonary hypertension. Figure 3.10 demonstrates
both normal and rapid time to peak velocities in two idealized spectral
recordings.
2004