176
Dynamics
of
the
Vascular
System
6.2
Pressure
and
Flow
Waveforms
in
Veins
6.2.1
The Normal Pressure and Flow Waveforms
in
Veins
Since veins are the major conduits that return blood to the heart, their
pressure and flow waveforms must be intimated connected to cardiac
function.
This view has been deemed important particularly when
considering large veins, such as the jugular vein, the main pulmonary
vein and the vena cava. Veins are larger, but thinner, than their
companion arteries. The relatively lower elastic modulus and greater
compliance are reflected in the mechanical properties of the veins when
stress-strain relations are examined. Since veins and arteries are normally
structured in parallel and in close proximity to each other, there
is
usually cross-talk in pulsations, particularly the influence of arterial
pressure oscillations on venous pulse waveform. This
is
seen in the
jugular venous pulse, which is often compounded with the high pressure
carotid artery pulse. The second major influence of the pulse waveform
comes from cardiac chamber pressures, particularly that of the right and
left atria. Thus, central venous pulses are important indicators of cardiac
function.
Unlike arterial pressure and flow waveforms, those of veins can vary
considerably, subjecting to its collapsibility and external factors. Several
features can be identified, however, from a typical venous pulse. Figure
6.2.1 illustrates the jugular pressure and flow waveforms recorded in a
normal subject (Kalmanson and Veyrat,
1978).
The
a
wave reflects right
atria contraction. This is followed by the
x
decent or valley, due to atrial
relaxation. The
c
wave is attributed to the carotid artery cross-talk or the
systolic bulging of the tricuspid valve during the onset of ventricular
systole. The
v
wave occurs during ventricular relaxation. They decent or
valley is during the onset of the rapid filling phase. These
acxvy
landmarks in jugular venous pressure waveform can be easily altered
with small perturbations, and significantly
so
in diseased conditions.
The jugular venous waveform reflects events occurring in the right
atrium and right ventricle. Frequently, landmarks on the recording of the
phasic waveform of the jugular venous pulse are often similar to the
pressure waveforms of the jugular vein, superior vena cava, and the right
atrium.
