66
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
and the ESV (50 mL) represents the stroke
volume (SV) of the ventricle, which is about
70 mL. In a normal ventricle, about 60% or
more of the EDV is ejected. The SV (EDV -
ESV) divided by the EDV is called the ejection
fraction (EF) of the ventricle, which normally
is >0.55 (or 55%). Although ventricular vol-
ume does not change during isovolumetric
relaxation, atrial volumes and pressures con-
tinue to increase owing to venous return.
Phase 6. Rapid Filling: AV Valves
Open; Aortic and Pulmonic
Valves Closed
When the ventricular pressures fall below
atrial pressures, the AV valves open and ven-
tricular filling begins. Initially, the ventricles
are still relaxing, which causes intraventricular
pressures to continue to fall by several mm Hg
despite ongoing ventricular filling. The rate of
initial filling is enhanced by the fact that atrial
volumes are maximal just prior to AV valve
opening. Once the valves open, the elevated
atrial pressures coupled with declining ventric-
ular pressures (ventricular diastolic suction)
and the low resistance of the opened AV valves
results in rapid, passive filling of the ventri-
cles. Once the ventricles are fully relaxed, their
pressure begins to rise as they fill.
The opening of the AV valves causes a rapid
fall in atrial pressures. The peak of the atrial pres-
sure just before the valve opens is the “v wave.”
This peak is followed by the “y descent” as
blood leaves the atria. The
v
wave and
y
descent
are transmitted into the proximal venous ves-
sels such as the jugular vein on the right side of
the heart and pulmonary veins on the left side.
Clinically, changes in atrial pressures and jugu-
lar pulses are useful in the diagnosis of altered
cardiac function (see Chapter 9).
If the AV valves are functioning normally, no
prominent sounds will be heard during filling.
When a Third Heart Sound (S3) is audible dur-
ing ventricular filling, it may represent tensing
of chordae tendineae and the AV ring, which is
the connective tissue support for the valve leaf-
lets. This S3 heart sound is normal in children,
but it is considered pathologic in adults because
it is often associated with ventricular dilation.
Phase 7. Reduced Filling:
AV Valves Open; Aortic and
Pulmonic Valves Closed
No clear demarcation exists between the phases
of rapid and reduced ventricular filling. The
reduced filling phase is the period during dias-
tole when passive ventricular filling is nearing
completion. This is sometimes referred to as
the period of ventricular diastasis. As the ven-
tricles continue to fill with blood and expand,
they become less compliant (i.e., “stiffer”).
This
causes
the
intraventricular
pressures
to rise, as described later in this chapter.
Increased intraventricular pressure reduces
the pressure gradient across the AV valve (the
pressure gradient is the difference between
the atrial and ventricular pressure) so that the
rate of filling declines, even though atrial pres-
sures continue to increase slightly as venous
blood continues to flow into the atria. Aortic
pressure and pulmonary arterial pressure con-
tinue to fall during this period as blood flows
into the systemic and pulmonary circulations.
It is important to note that Figure 4.2
depicts the cardiac cycle at a relatively low
heart rate (75 beats/min). At low heart rates,
the length of time allotted to diastole is rela-
tively long, which lengthens the time of the
reduced filling phase. High heart rates reduce
the overall cycle length and are associated
with reductions in the duration of both sys-
tole and diastole, although diastole shortens
much more than systole. Without compen-
satory mechanisms, this cycle length reduc-
tion would lead to less ventricular filling (i.e.,
reduced EDV).
Compensatory mechanisms
are important for maintaining adequate ven-
tricular filling during exercise (see Chapter 9).
Summary of Intracardiac
Pressures
It is important to know normal values of
intracardiac pressures, as well as the pressures
within the veins and arteries entering and
leaving the heart, because abnormal pressures
can be used to diagnose certain types of car-
diac disease and dysfunction. Figure 4.3 sum-
marizes normal, typical pressures in an adult
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