92
Dynamics
of
the Vascular System
oscillations superimposed on the diastolic pressure and the decay is not
necessarily mono-exponential.
The right ventricle and the pulmonary arterial system
is
known as the
“low pressure” system and the left ventricle and the systemic arterial
system
is
known as the “high pressure” system. This implies that
pulmonary aorta has a much lower pressure amplitude that that of the
aorta. The stroke volume is the same, therefore, the flow magnitudes are
similar, although their waveforms can be quite different.
Fig.
4.2.2
illustrates the differences. The pulmonary aorta is much more compliant
than the aorta and the pressure waveform tends
to
be more closer in
morphology to the flow waveform (Van den Bos et al.
,
1982).
This has
been attributed to the comparatively smaller amount of wave reflections
in the pulmonary system.
This is due to the spatial distribution of
compliances that are greater in the pulmonary arteries than the stiffer
systemic arteries. Major reflections do not reach the proximal aortas in
early ejection phase.
Thus, the time courses of pressure and flow
waveforms are similar during this interval.
Blood pressure waveforms are periodical, as illustrated in Fig.
4.2.3
which displays pressure waveforms recorded in the ascending aorta,
descending thoracic aorta, the abdominal aorta and the iliac artery, when
the catheter-tip pressure transducer is slowly withdrawn away from the
heart.
The progressive increases in pulse pressure amplitudes with
increasing systolic pressure and decreasing diastolic pressure can be
observed. The electrocardiogram
(ECG)
is normally recorded
as
timing
reference and for the calculation of cardiac period (T). The bottom
tracing is the ascending aortic flow measured with an electromagnetic
flow probe. The waveforms are reproducible following each heart beat.
This is normally true under steady state conditions.
Respiratory influence on the pressure waveforms is easily observed
when consecutive beats are recorded over a respiratory cycle. The
normal ratio
of
heart frequency to respiratory frequency is about
4:l.
The changes induced by respiration are usually small, except in some
pathological conditions,
such as in pulsus paradoxus where blood
pressure change is excessive during both inspiration and expiration.
