Learning Objectives
After reading this chapter you should be able to:
1. Describe the organization of the cardiovascular system.
2. Describe the sequence of events that occur during one cardiac cycle.
Explain how pressures and volumes within the heart chambers change.
3. Describe the pressure – volume loop.
4. List the approximate values for mean pressures found at various stages of
the cardiac cycle.
5. Explain how an increase in heart rate would affect various stages of the
cardiac cycle.
6. Describe and explain the atrial and central venous pressure waves.
7. Explain the ECG in terms of the cardiac cycle.
8. Describe and explain when sounds are heard during the cardiac cycle.
9. Explain why S2 is split.
10. List and explain the common types of heart murmurs.
To understand the Mechanical event you must recall:
Anatomy of the Heart
• The atrioventricular (AV) valves prevent from flow from the
ventricles back into the atria
• The pulmonary and aortic valves prevent backflow from the
pulmonary trunk into the right ventricle and from the aorta into the
left ventricle respectively
• Cardiac- muscle cells are joined by gap junctions that permit
action potentials to be conducted from cell to cell
• The myocardium also contains specialized muscle cells that
constitute the conducting system of the heart, initiating the cardiac
action potentials and speeding their spread through the heart
Cardiac cycle describes the sequence of
electrical and mechanical events that occur in
the heart during one single beat
• The cycle is divided into two major phases, both named
for events in the ventricles:the period of ventricular
contraction and blood ejection, systole, followed by the
period of ventricular relaxation and blood filling,
diastole.
• At an average heart rate of 72 beats/min, each cardiac
cycle lasts approximately 0.8 s, with 0.3 s in systole and
0.5 s in diastole
Cardiac valves operation. They open when pressure gradient across the valves is
increasing in the direction that blood normally flows (forward pressure gradient)
(A). A reverse pressure gradient (B) will force the valve closed, which prevents
reverse blood flow in response to reverse pressure gradient that occur as a result of the
pumping action. (C). A forward pressure gradient forces the semilunar valve to open.
Darker colors correspond to higher pressure
Fig. 7. Divisions of the cardiac cycle: (a) systole; (b) diastole
Cardiac pressures during phases of
cardiac cycle
Major difference between the
right and left side of
the heart is the pressure
values:
the right side operates at
much lower pressures
because the pulmonary
system is a low resistance
circulation.
7
Mechanical Events
Mechanical events of the cardiac cycle
1
START
5
4
Isovolumic ventricular
relaxation: as ventricles
relax, pressure in ventricles
falls, blood flows back into
cups of semilunar valves
and snaps them closed.
Ventricular ejection:
as ventricular pressure
rises and exceeds
pressure in the arteries,
the semilunar valves
open and blood is
ejected.
Late diastole: both sets of
chambers are relaxed and
ventricles fill passively.
Atrial systole: atrial contraction
forces a small amount of
additional blood into ventricles.
2
3
Isovolumic ventricular
contraction: first phase of
ventricular contraction pushes
AV valves closed but does not
create enough pressure to open
semilunar valves.
Mechanical Events
1
START
Late diastole: both sets of
chambers are relaxed and
ventricles fill passively.
1
START
Late diastole: both sets of
chambers are relaxed and
ventricles fill passively.
2
Atrial systole: atrial contraction
forces a small amount of
additional blood into ventricles.
1
START
Late diastole: both sets of
chambers are relaxed and
ventricles fill passively.
2
Atrial systole: atrial contraction
forces a small amount of
additional blood into ventricles.
Isovolumic ventricular
3 contraction: first phase of
ventricular contraction
pushes AV valves closed but
does not create enough
pressure to open
semilunar valves.
1
START
Late diastole: both sets of
chambers are relaxed and
ventricles fill passively.
Atrial systole: atrial contraction
forces a small amount of
additional blood into ventricles.
2
Ventricular ejection:
as ventricular pressure
rises and exceeds
pressure in the arteries,
the semilunar valves
open and blood is
ejected.
3
4
Isovolumic ventricular
contraction: first phase of
ventricular contraction pushes
AV valves closed but does not
create enough pressure to open
semilunar valves.
Isovolumic ventricular
relaxation: as ventricles
relax, pressure in ventricles
falls, blood flows back into
cups of semilunar valves
and snaps them closed.
5
4
START
1 Late diastole: both sets of
chambers are relaxed and
ventricles fill passively.
2 Atrial systole: atrial contraction
forces a small amount of
additional blood into ventricles.
Ventricular ejection:
as ventricular pressure
rises and exceeds
pressure in the arteries,
the semilunar valves
open and blood is
ejected.
3
Isovolumic ventricular
contraction: first phase of
ventricular contraction pushes
AV valves closed but does not
create enough pressure to open
semilunar valves.
The heart at rest:
atrial and ventricular diastole.
The atria are filling with blood from the veins,
and the ventricles are relaxing – the AV valves
between the atria and ventricles open. Blood
is flowing by gravity into the
ventricles. The relaxing ventricles expand to
accommodate the entering blood. At this
moment the ventricles are ~ 80 – 90% filled
with blood.
This is the section to the left of
atrial systole on the diagram.
1. Atrial systole and completion of
ventricular filling.
The generation of an action potential by the SA
node results in a wave of depolarization
spreading through the atria P wave on the
ECG.
Atrial muscle contracts (atrial systole)
and atrial pressure rises (a wave).
A little more blood (10 – 20%), sometimes
called the‘atrial kick’ is pushed into the almost
full ventricles. However, a small amount of
blood is forced backwards into the great veins
(there are no oneway valves between the veins
and the atria) causing a similar a wave in the
central veins (vena cava). This wave can be
seen as a pulse in the jugular vein of a person
who is semirecumbent with the head and
chest elevated ~ 30 degrees .
If a pulse is seen higher up the jugular vein in a
person who is upright it indicates that the right
atrial pressure is higher than normal.
End Diastolic Volume, end-systolic volume and
stroke volume
• The amount of blood in the ventricles just before
systole is the end-diastolic volume. The volume
remaining after ejection is the end-systolic volume,
and the volume ejected is the stroke volume
• Pressure changes in the systemic and pulmonary
circulation have similar patterns, but the pulmonary
pressures are much lower
2. Ventricular systole
A. Isovolumetric Contraction
The wave of depolarization reaches
ventricles and we have the QRS complex
on the ECG, which denotes ventricular
depolarization.
Shortly thereafter the ventricles
contract and squeeze the blood upward
and towards the base. As soon as the
pressure in the ventricles rises above the
atrial pressure, the AV
valves close causing the first heart
sound.
Since the aortic and pulmonary valves are
already closed each ventricle is now a closed
chamber. They continue to contract and
because they are closed, the pressure within
them rises very steeply (more in the LV than
the RV). This is called the isovolumetric
contraction phase
-c wave in the RA pressure tracing.
• When Ventricular pressures
exceed aortic and pulmonary
trunk pressure, the aortic and
pulmonary valves open, and V
ejection of blood occurs
• When the ventricles relax at the
beginning of the diastole, the V
pressures fall significantly below
those in the aorta and pulmonary
trunk, and the aortic and
pulmonary valves close. Because
the AV valves are also still
closed, no change in V volume
occurs during this isovolumetric
ventricular relaxation
The vibrations due to closure of the semilunar
valves give rise to the second heart sound.
• When
ventricular pressures fall below the
pressures in the right and the left atria, the AV
valves open, and the ventricular filling phase of
diastole begins
• Filling occurs very rapidly at first so that atrial
contraction, which occurs at the very end of
diastole, usually adds only a small amount of
additional blood to the ventricles
Table of events in the cardiac cycle
Vascular events
Cardiac chamber events
Opening of AV valves
(tricuspid and mitral)
Rapid V filling
Decreased V filling. Diastasis
contraction (additional V filling)
Closing of AV valves
(tricuspid and mitral)
Phase
1
A 1
1
Diastole
Diastole
Diastole
Isometric V contraction (with all valves
closed)
2
Systole
Opening of semilunar
valves (pulmonary and
aortic)
Rapid V ejection fast muscle shortening
Decreased V ejection (slower muscle
shortening
3
3
Systole
Systole
Closing of semilunar
valves (pulmonary and
aortic)
Isovolumetric ventricular relaxation (with 4
all valves closed)
Diastole
Opening of AV valves
(tricuspid and mitral)
Ejection
Diastole
4
The Atrial and Central Venous Pressure (CVP) waves
• Since there are no valves between the jugular veins (JV), v. cavae
ant the RA, the right JV are communicated with the RA.
• Changes in pressure in the RA produces a series of pressure changes
which are reflected in the central veins and recorded from the JV: a, c
and v waves.
• CVP is the pressure in the vein at the entrance of the RA
• a wave is due to increase in pressure caused by atrial systole
• av descent (minimum) is due to relaxation of the right atrium and
closure of the tricuspid valve
• c wave is caused in the RA by the tricuspid valve bulging back into
the atrial chamber as it closes. In the internal JV the c wave (c =
carotid) is caused partly by expansions of the carotid artery
• X descent is a sharp fall in the pressure caused by atrial relaxation
• v wave. As the atria fill, A pressure rises producing v wave (v =
ventricular systole which is occurring at the same time)
• Y descent is a fall in pressure due to the rapid emptying of the
atria after the AV valve opens
Fig. 13. Jugular venous pressure changes caused by cardiac cycle
Clinical examination of the JVP
• JP of the internal jugular vein can be assessed by
expecting the right side of the neck of a recumbent
subject. Two sudden venous collapses (the X and Y
descent) should be seen and measured externally on the
right side of the neck- positive JVP
• Certain cardiac diseases produce characteristic
abnormalities in the JVP, e.g. tricuspid incompetence
produces exaggerated v waves in the neck, because V
systole forces blood back into the RA & JV
• In right-side cardiac failure there is also a positive JVP
due to accumulation of blood into the failing RV and RA
Fig. 8. Pressure-volume loop of the cardiac V at rest and during
exercise
Fig. 9. Events in the left atrium, left V, and aorta during the cardiac cycle
Fig. 10. Pressures in the right ventricle and pulmonary artery during the
cardiac cycle. The fig. is done in the same scale as the previous to facilitate
comparison
Fig.11. Stroke volume. R. Rhodes & R. Pflanzer, Human Physiology
Fig. 12. Cardiac cycle again
with identification of the
jugular venous pulse (a, c and v
waves)
Heart Sounds. The common heart sound
are:
• The first heart sound is due to the closing
of the AV valves
• The second heart sound is due to the
closing of the aortic and pulmonary valves
Heart Sounds
Sound Characteristics
Associated events
S1
First heart sound (sounds like “lub”)
Two bursts, a mitral M1 and a tricuspid T1
components
Closure of mitral &
tricuspid valves
S2
Second heart sound (sounds like “dub”)
An aortic A2 and a pulmonary P2 component
Closure of aortic and
pulmonary valves
OS
Opening Snap
Opening of a stenotic
mitral valve
S3
Third heard sound
Diastolic filling gallop or
V or protodiastolic gallop
S4
Fourth heart sound
Atrial sound that creates
an atrial or presystolic
gallop
Note that a physiological S3 sound is present in some normal individuals,
particularly children. Occurs in early diastole with rapid filling of the ventricles;
When present S4 coincides with atrial contraction but usually it is abnormal
Location of the sounds on the chest
Each valve is best heard by a stethoscope from 4 distinct
areas:
Mitral valve: Mid clavicular line of the 5th left intercostal
space
Tricuspid valve: 5th interspace at the left sternal edge
Aortic valve: 2nd interspace at the right sternal edge
Pulmonary valve: 2nd interspace at the left sternal edge
Heart Murmurs: Abnormal heart sounds heard on
auscultation which are due to faulty valves.
• Incompetence: Failure of the valve to seal properly
(valve may be torn, perforated, affected by rheumatic fever
or a failing heart may be enlarged) such that it becomes
leaky allowing blood to regurgitate through it
• Stenosis: The open valve is narrowed so that a higher
pressure gradient is needed to drive blood through
(cicatrization after rheumatic or other infection)
• Defective valves can be congenital or acquired. Abnormal valve
causes blood turbulence which sets up high frequency vibrations
which are heard as murmurs through the stetoscope
Fig. 14. Common valvular abnormalities
Heart Murmurs (cont.)
• Benign Systolic Murmur is common in the young.
Caused by turbulence in the ventricular outflow tract. Also
during pregnancy, strenuous exercise and anemi
• Aortic stenosis: Systolic murmur. Due to narrowing of
the aortic valve when the flow during ejection becomes
turbulent. Heard during ejection (systolic murmur) as
ejection waxes and wanes (a crescendo – decrescendo
murmur). Loudest over aortic area
• Mitral incompetence: Pan systolic murmur. During V
systole blood regurgitates through the mitral valve back
into LA resulting a murmur that extends throughout
ventricular contraction
• Aortic Incompetence: Diastolic murmur
When aortic valve does not close completely blood
regurgitates back into the V during diastole. The
turbulence is upstream of the aortic valve and the murmur
begins at the time of S2 and lasts through the early part of
diastole
• Mitral Stenosis: Diastolic murmur
Blood is forced through the narrowed mitral valve during
the phase of ventricular filling – ventricular diastole
• Listen to these murmurs at various websites e.g.
www.med.ucla.edu/wilkes/intro.html
Summary of the Cardiac Cycle
Assessed at the bedside by noting:
• Peripheral pulse at radial artery (heart rate and force)
• Systolic and diastolic blood pressure (will be discussed later)
• Jugular venous pulse observation
• Apex beat (displacement on the left identifies left V hypertrophy)
• Heart sounds
When pathology is suspected more specialized tests are carried out:
• Echocardiography (non-invasive): Observing movement of the
valves and walls of the heart (valve lesions, myocardial infarction,
cardiac hypertrophy of different origin)
• Cardiac catheterization (invasive)
Valve positions during the Cardiac cycle
Cardiac Cycle
Phase
Duration Inlet (A-V)
valves
Outlet (semilunar)
valves
Ventricular filling
0.5 s
Open
Closed
Isovolumetric
contraction
0.05 s
Closed
Closed
Ejection
0.3 s
Closed
Open
Isovolumetric
relaxation
0.08 s
Closed
Closed
Mean Pressures During Cardiac Cycle
Pulmonary
Circulation
mm Hg Systemic Circulation mm Hg
Right Atrium
3
Left Atrium
8
RV: Peak systole
End of diastole
25
4
LV: Peal systole
End of diastole
120 9
25
10
Aorta:
Systolic
Diastolic
Pulmonary artery:
Systolic
Diastolic
120
80