Download Cardiac Cycle - Just In Time Medicine

OST 579
FS-15
Page 1 – Part G
Cardiac Cycle
Dr. Robert Stephenson
Cardiovascular
Part G
Preview
As you know, each heart beat involves the contraction of all the cardiac muscle cells in the right
and left atria, followed by a very brief pause (the AV delay), and then the synchronized
contraction of all the ventricular muscle cells. The cardiac ventricles constitute two pumps that
work together, side by side. In each cardiac cycle (heartbeat) the left ventricle takes in a volume of
blood from the pulmonary veins and left atrium, then ejects that same volume of blood into the
aorta. The right ventricle takes in a similar volume of blood from the systemic veins and right
atrium, then ejects it into the pulmonary artery. Then follows relaxation (and refilling), as the
cycle repeats.
Each cardiac cycle (heart beat) consists of ventricular diastole (relaxation), followed by ventricular
systole (contraction). Atrial systole (atrial contraction) occurs during ventricular diastole. Atrial
contraction is initiated by atrial depolarization, which is indicated by the P wave in the ECG.
Ventricular systole is initiated by ventricular depolarization, which is indicated by the QRS
complex in the ECG. Ventricular diastole (relaxation) is initiated by ventricular repolarization,
which is indicated by the T-wave. So, in reference to the ECG, ventricular diastole corresponds to
the period between a T wave and the next QRS complex. During this time, the ventricular cells
are at resting membrane potential . . . and therefore relaxed. Ventricular systole corresponds to
the period between a QRS complex and the next T-wave. During this time, the ventricular cells
are undergoing an action potential and are therefore contracting.
To anchor your study of the cardiac cycle, we will ask you to come to grips with a graphical
compilation of the blood pressures, blood flows, electrical events, heart sounds, and cardiac valve
openings and closings associated with one heart beat. Glance ahead to page 3 for a preview of
this "Diagram of the Cardiac Cycle", which is often called the "Wiggers Diagram". This diagram
has been inflicted on generations of medical students, ever since it was first concocted by Dr. Carl
Wiggers *. It is not inflicted upon you for punitive purposes, but rather because it unifies all the
fundamentals of cardiac function. It is no exaggeration to say that, if you understand this
diagram, you understand much of what you need to know about how the heart works. Our
instructional objective is that, after working through this lesson, you will be able to take a blank
sheet of paper and sketch a fairly accurate replication of this diagram . . . not because you
memorized it, but because everything on the diagram is a direct reflection of two very basic facts
of cardiac anatomy and physiology: (1) There are one-way valves between the atria and ventricles
and one-way valves within the outflow tracts of the ventricles, and (2) The heart pumps blood by
alternately contracting and then relaxing. In a very fundamental sense, that's all you need to
know.
* Professor and chair of the Department of Physiology at Case Western from 1918 to 1953.
Cardiac Cycle
Page 2 -- Part G
In Part I, after you've become comfortable with the Wiggers Diagram, we'll throw you a curve ball
by showing you a graph of left ventricular pressure vs. left ventricular volume during one cardiac
cycle (i.e. a "Pressure/Volume loop"). Once you grasp that this is just another way of plotting the
very same data that is in the Wiggers Diagram, you will be well prepared to think logically about
both normal cardiac function and cardiac dysfunction.
Topic Outline
Events of one heart beat (one cardiac cycle) – The Wiggers diagram
Electrocardiogram (ECG)
Ventricular systole and ventricular diastole
End-diastolic volume, end-systolic volume, and stroke volume
Role of atrial contraction in ventricular filling
Ejection fraction
Cardiac output
Atrial, Ventricular, and Aortic pressures
Opening and closing of cardiac valves in relation to phases of the cardiac cycle
Heart sounds
Venous pressure "waves"
Online Presentation and Self-Study
Begins on next page

Diagram on following page modified from
R. Stephenson. In: Cunningham's Textbook of
Veterinary Physiology 5/ed, Fig 21-1, pg 201.
Cardiac Cycle
Page 3 -- Part G
Events of one heart beat (one "cardiac cycle") – The Wiggers diagram
P-P interval =
Start
with
ECG:
So, HR =
See Note  (next page) re Ventricular
systole and Ventricular diastole.
Stroke volume (SV) = Volume of
blood ejected in one beat.
Ejection fraction (EF) = Fraction
of end-diastolic ventricular
volume ejected during systolic
contraction.
Normal EF is __________________.
See Note  (next page) re role of
Atrial systole in ventricular filling.
Cardiac output (CO):
Pressures (see Note )
Left ventricular systolic pressure:
Left ventricular end-diastolic
pressure:
Heart sounds – What causes
them? (see Note )
Cardiac Cycle
Page 4 -- Part G
Notes on Cardiac Cycle
 Each Heart Beat is divided into Ventricular systole and Ventricular diastole.
•
Ventricular systole (contraction) – Initiated by ventricular depolarization, which is
"signaled" by QRS complex in ECG.
Ejection happens during ventricular systole.
During ejection, ventricular volume decreases to end-systolic volume (ESV).
•
Ventricular diastole (relaxation) – Initiated by ventricular repolarization, which is
"signaled" by end of T-wave in ECG.
Ventricular filling happens during ventricular diastole.
During filling, ventricular volume increases to end-diastolic volume (EDV).
 Role of atrial systole (contraction) in ventricular filling:
Minor role in normal persons, when heart rate is low (at rest)
There is plenty of time between systoles for ventricular filling.
Ventricles "almost full" by the time atria contract.
Atrial contraction just "tops up" the almost-full ventricles.
More important if heart rate is high (e.g. during exercise)
Not enough time between systoles for normal ventricular filling.
Atrial contraction adds significantly to EDV.
Also important in persons with valve defects or myocardial failure.
More about this later.
 Atrial pressure, Ventricular pressure, and Aortic pressure:
First, draw in the correct locations of the cardiac valves, and label with them with
correct anatomical names:
Then proceed to the notes on next page.
Cardiac Cycle
Page 5 -- Part G
Refer to the letter labels (A, B, C, D) on the cardiac cycle graphs
(Note: The following commentary describes changes in the left heart, but corresponding events
are occurring simultaneously in the right heart.)
•
Left ventricle fills between Point D and Point A: Note that ventricular pressure very
low (normally 0 - 5 mm Hg) during filling, because the ventricular muscle is relaxed,
and relaxed ventricles are normally fairly compliant (stretchy).
•
Point A: Start of ventricular systole (contraction). As ventricular pressure starts to
rise, there is a momentary backflow of blood across the A-V valve (mitral valve in left
heart), which swings the valve shut. The sudden cessation of backflow causes a
momentary tubulence and vibration . . . heard as the First heart sound ("lub"), which
marks the start of systole.
•
Between A and B: Isovolumetric contraction. Both the mitral and aortic valves are
closed, so although the ventricle is contracting and pressurizing the blood within, there
is no ejection of blood and therefore no change in ventricular volume . . . hence iso
(same) volumetric (volume) contraction.
•
Point B: The rapidly rising ventricular pressure reaches (and slightly exceeds) the level
of aortic pressure. Therefore, the aortic valve swings open and ejection of blood into
the aorta begins.
•
Between B and C: Ejection. The outflow of blood from the ventricle into the aorta is
rapid at first (Rapid ejection) and then tapers off (Reduced ejection). Coincident with
this, ventricular pressure peaks, then falls.
•
Point C: Ventricular pressure falls below aortic pressure. There is a momentary
backflow of blood across the aortic valve, which swings the valve shut. The sudden
cessation of this backflow causes a momentary tubulence and vibration . . . heard as
the Second heart sound ("dub"), which marks the end of systole.
•
Between C and D: Isovolumetric relaxation. The ventricular muscle is relaxing, so
pressure within the ventricle drops rapidly. However, both the mitral and aortic valves
are closed, so there is neither ejection nor filling . . . hence iso (same) volumetric
(volume) relaxation.
•
Point D: The rapidly falling ventricular pressure reaches (and falls just below) the level
of atrial pressure. Therefore, the mitral valve swings open and ventricular filling
begins.
•
Back to start.
 What makes the first and second heart sounds (S1 and S2)?
It is NOT the valve leaflets "slamming shut"! (Normal valves are light and flexible to make
a sound, even if forcefully closed.)
Instead, it is the turbulence of blood flow and the vibration of the cardiac walls caused
when the momentary backflow of blood comes to a sudden stop against the closing valves.
Punch line: Heart sounds are caused by _________________________________________
and by __________________________________________________.
Cardiac Cycle
Page 6 -- Part G
Additional things to notice about the Cardiac Cycle
•
State the valve event that occurs at each of the labeled points:
 Point A - Point B - Point C - Point D --
•
Heart sounds S1 and S2 can be ausculted in all normal patients. In some normal
persons, two additional, fainter heart sounds (S3 and S4) can be ausculted.
 Look up the timing and cause of S3 and S4 (online or in textbook of internal
medicine or cardiology), and make note of this below:
Cause of S3 –
Cause of S4 -
•
Also, add S3 and S4 to the timeline of heart sounds on the Cardiac Cycle
Diagram (on page 3).
Aortic pressure is clearly pulsatile, as indicated on the cardiac cycle diagram.
(Likewise, pulmonary artery pressure is also pulsatile.)
 On the graph of aortic pressure, find and label the exact points of maximum
pressure (aortic systolic pressure) and minimum pressure (aortic diastolic
pressure). Record the approximate values (including units) here:
Aortic systolic pressure ~ _______________
Aortic diastolic pressure ~ ______________
•
Note that there are small pulsations in atrial pressure. Since there are no valves
between the veins and the atria, these changes in atrial pressure occur also in the
large veins (pulmonary on the left and vena cavae on the right). The resulting venous
pulsations can be measured with a venous catheter, or (in a normal, recumbant
person) can be observed visually in the jugular veins. These "venous pulsations" are
designated as the a, c, and v waves.
 Look online, or look at Figure 13.3 in your Rhoades & Bell text, and find the a,
c, and v waves on a diagram of the cardiac cycle.
 Label these waves on the cardiac cycle diagram (on page 3).
 Look online, or read on page 252 of Rhoades & Bell, to determine what causes
these venous waves (pulsations) and summarize here:
Cause of a wave:
Cause of c wave:
Cause of v wave:
Cardiac Cycle
Page 7 -- Part G
Cardiac Cycle Quiz
NOTE: Unless otherwise specified, "systole" means ventricular systole and "diastole" means
ventricular diastole. This convention is followed generally by both physiologists and by clinicians.
TRUE OR FALSE ??? (for a normal heart in a resting person)
1.
The aortic and mitral valves are never open at the same time.
2.
The first heart sound is heard at the beginning of ventricular systole.
3.
The first heart sound is caused by the rapid ejection of blood from the ventricles.
4.
The end of systole is the beginning of diastole.
5.
Left ventricular pressure is lower at the end of diastole than it is at the beginning of
diastole.
6.
Ventricular end-diastolic volume is greater than end-systolic volume.
7.
Atrial systole occurs during ventricular diastole.
8.
The ventricles eject blood during part (but not all) of systole
9.
The aortic valve is closed throughout diastole.
10. The mitral valve is open throughout diastole.
11. Aortic pressure reaches its minimum level during ventricular systole.
12. Aortic pressure reaches its maximum level during ventricular systole.
13. Left ventricular pressure is always less than aortic pressure.
14. Left ventricular pressure exceeds left atrial pressure during systole.
15. Ventricular filling only occurs during ventricular diastole.
16. Ventricular filling occurs primarily during atrial systole.
17. The second heart sound is associated with the opening of the aortic valve.
18. The only moments of turbulent blood flow coincide with the S3 and S4 heart sounds.
19. Left atrial pressure steadily increased throughout ventricular diastole.
Answers to Cardiac Cycle Quiz:
1-T
2-T
3-F
4-T
5-T
6-T
13-F
14-T
15-T
16-F
17-F
18-F
7-T
19 - F
8-T
9-T
10-F
11-T
12-T
Cardiac Cycle
Page 8 -- Part G
Suggestions to Confirm Your Understanding of the Cardiac Cycle
1.
Fill in any gaps in the notes on pages 3-6.
2.
Correctly answer each question in the "Cardiac Cycle Quiz" on page 7, and be able to explain
your answers.
3.
Practice sketching the Wiggers Diagram, as stipulated in Learning Objective #2 on page 13.
Remember to extend your diagram for two successive heart beats (cardiac cycles), as
suggested during the online lecture.
REMINDER: Every feature of the volume, flow, and pressure graphs on the Wiggers Diagram is a
direct consequence of two facts:
(1) There are one-way valves between the atria and ventricles, and one-way valves in the
outflow tracts of the two ventricles.
(2) The heart pumps blood by alternately relaxing and filling with blood, and then contracting
and ejecting some of that blood.
If you know only those two facts (and then think about it), you should be able to draw the
Wiggers Diagram from scratch . . . not because you memorized it, but because it makes sense.
If you can do that, then you understand how the heart works. If you can't do that, then you don't
understand how the heart works . . . and you'll be "fuzzy" about cardiac function and dysfunction
for the rest of this course and for the rest of your career. Doing this work now will pay big
dividends.
Suggestion: Don't try to master this in one sitting; "sleeping on it" is very likely to help. Then, try
to explain the cardiac cycle to a colleague, or quiz a colleague as he/she explains the cardiac
cycle to you. And remember, this really does make sense!
Once you feel like you've "got it": Work through the following set of "Confirm Your
Understanding Exercises", which provide a good recap of the Cardiac Cycle.
Self-Assessment: Confirm your Understanding of the Cardiac Cycle.
[Set of 20 Study Questions. Answers follow]
Fill in the blank:
1.
During isovolumetric contraction, the AV valves are _______________.
2.
During ventricular ejection, the AV valves are _______________..
3.
During isovolumetric contraction, the aortic and pulmonary valves are _______________.
4.
During ventricular ejection, the aortic and pulmonary valves are _______________..
5.
During isovolumetric relaxation, the AV valves are _______________.
6.
During ventricular filling, the AV valves are _______________.
7.
During atrial contraction the AV valves are _____________.
8.
During isovolumetric relaxation, the aortic and pulmonary valves are _______________.
9.
During ventricular filling, the aortic and pulmonary valves are _______________.
Cardiac Cycle
Page 9 -- Part G
10. During atrial contraction the aortic and pulmonary valves are _______________.
11. Are the AV valves ever open at a time when the aortic and pulmonary are also open? _______
12. The volume of blood ejected from a ventricle in a single beat is the ____________________.
13. The volume of blood in a ventricle at the end of diastole is the _________________________.
14. The volume of blood in a ventricle at the end of systole is the ___________________________.
15. Stroke volume equals end-_________________ volume minus end-________________ volume.
16. Ejection fraction equals stroke volume divided by __________________________________.
17. Cardiac output equals _______________________________ × _____________________________.
The next three multiple-course questions are based on the following graph, which shows
aortic pressure, left ventricular pressure, and atrial pressure during two successive cardiac
cycles.
18. In the above figure ventricular repolarization occurs within the time period between:
A. L and M
B. M and N
C. N and P
D. P and R
19. In the figure above, isovolumetric relaxation occurs between points:
A. L and M
B. M and N
C. N and P
D. P and R
20. In the figure above, rapid ventricular filling would occur within the time period between:
A. L and M
B. M and N
C. N and P
D. P and R
E. R and S
Cardiac Cycle
Page 10 -- Part G
Answers to the twenty-question review of Cardiac Cycle:
1. closed
2. closed
3. closed
4. open
5. closed
6. open
7. open
8. closed
9. closed
10. closed
11. No
12. stroke volume
13. end-diastolic volume
14. end-systolic volume
15. diastolic . . . systolic
16. end-diastolic volume
17. stroke volume × heart rate
18. B – Check the position of the T wave in the Wiggers Diagram in your Course Pack (page 3 of
Part G) or in Figure 13.3 of your Rhoades & Bell physiology text.
19. C
20. D
Cardiac Cycle
Page 11 -- Part G
Supplementary Material
OPTIONAL – Only if you need clarification
If any of the foregoing material was unclear to you, it may help to consult pages 251-252 in
your Rhoades & Bell textbook. Similar discussions are presented in all other medical
physiology textbooks as well.
Or, for an approximate restatement of the online presentation, you may consult the following
optional narrative regarding the Cardiac Cycle:
Each cardiac cycle (heart beat) consists of ventricular diastole (relaxation), followed by
ventricular systole (contraction) and. Atrial systole (atrial contraction) occurs during
ventricular diastole. Atrial contraction is initiated by atrial depolarization, which is
indicated by the P wave in the ECG. Ventricular systole is initiated by ventricular
depolarization, which is indicated by the QRS complex. Ventricular diastole (relaxation) is
initiated by ventricular repolarization, which is indicated by the T-wave. So, ventricular
diastole corresponds to the period between a T wave and the subsequent QRS complex.
During this time, the ventricular cells are at resting membrane potential.
The volume of blood ejected from one ventricle in one beat is called stroke volume,
expressed as follows:
Stroke volume = end-diastolic volume – end-systolic volume
The ventricles do not empty completely during systole. The fraction of end-diastolic
volume that is ejected during ventricular systole is called the ejection fraction, as follows:
Ejection fraction =
Stroke volume
End - diastolic volume
Values of ejection fraction between 50% and 65% are typical for resting subjects.
As shown in the cardiac cycle diagram, left ventricular pressure is low at the beginning of
ventricular systole, but the powerful contraction of the ventricular muscle causes the
ventricular pressure to increase rapidly. The increase in left ventricular pressure causes a
momentary backflow of blood from the left ventricle to the left atrium, which closes the left
atrioventricular (AV) valve (the mitral valve). Blood is not immediately ejected from the left
ventricle into the aorta at the beginning of systole, because the aortic valve remains closed
until the left ventricular pressure exceeds the aortic pressure. Therefore, ventricular
volume remains unchanged during this first phase of systole, which is aptly named
isovolumetric contraction.
When left ventricular pressure does rise above aortic pressure, the aortic valve is pushed
open, and there is a rapid ejection of blood into the aorta. Rapid ejection is followed by a
phase of reduced ejection of blood as both ventricular pressure and aortic pressure pass
their peak (systolic) values and begin to decrease. (During the period of reduced ejection,
the ventricular pressure actually falls below the aortic pressure, but ejection continues for
a few moments, because the blood flowing out of the ventricle is carried along by the
momentum imparted to it during rapid ejection.) As the ventricular pressure continues to
decrease, ejection comes to an end. A momentary backflow of blood from the aorta into the
left ventricle closes the aortic valve. The closure of the aortic valve demarcates the end of
ventricular systole and the beginning of ventricular diastole.
Cardiac Cycle
Page 12 -- Part G
During the first phase of ventricular diastole, the ventricular muscle relaxes, and left
ventricular pressure declines from a value near aortic pressure to a value near left atrial
pressure. However, no filling of the ventricle can occur because the mitral valve remains
closed until left ventricular pressure drops below left atrial pressure. This first phase of
ventricular diastole is called isovolumetric relaxation because there is neither filling nor
emptying of the ventricle.
When left ventricular pressure does fall below left atrial pressure, the mitral valve is
pushed open, as blood begins to flow from the atrium into the ventricle. First, there is a
period of rapid ventricular filling, which is followed by a phase of reduced ventricular filling
(diastasis). Diastasis persists until the sinoatrial node cells initiate an atrial action
potential and atrial contraction (atrial systole). As depicted in the cardiac cycle diagram,
ventricular volume is nearly at its end-diastolic level even before atrial systole. Typically,
80% to 90% of ventricular filling occurs before atrial systole. Atrial systole simply “tops up”
the almost-full ventricles. An important clinical consequence of this fact is that the
ventricles in a resting animal can pump a nearly normal stroke volume even in the
absence of properly timed atrial contractions (e.g., during atrial fibrillation). During
exercise, however, atrial contractions make a relatively greater contribution to ventricular
filling because the rapid heart rate in exercise leaves a shorter time for diastolic filling.
Therefore, animals with atrial fibrillation typically exhibit exercise intolerance. Ventricular
filling also becomes more dependent on atrial systole in patients with certain valve defects,
such as narrowing of the mitral valve (mitral stenosis).
At the end of atrial systole, the atria begin to relax. The left atrial pressure drops slightly.
Then, as the ventricles begin to contract, there is a momentary backflow of blood from the
left ventricle to the left atrium. The backflow closes the mitral valve, which marks the end
of ventricular diastole and the beginning of another left ventricular systole.
By definition, the cardiac cycle is divided into ventricular systole and ventricular diastole.
Closure of the mitral valve marks the beginning of ventricular systole. Closure of the aortic
valve marks the beginning of ventricular diastole. Note that atrial systole takes place
during ventricular diastole.
The preceding six paragraphs discussed pressure changes in the left atrium, left ventricle,
and aorta. However, all the events of the cardiac cycle also take place on the right side of
the heart. Therefore, all the statements made about the left side of the heart also hold true
for the right side of the heart; simply substitute “pulmonary artery” for “aorta,” “pulmonic
valve” for “aortic valve,” and “tricuspid valve” for “mitral valve.” As indicated in the cardiac
cycle diagram, the ventricular volumes are similar for the left and right sides, and so are
the blood flow rates. The pressures, however, differ greatly on the two sides. Systolic (peak)
pressure in the right ventricle and pulmonary artery is only about 20 mm Hg, whereas
systolic pressure on the left side of the heart reaches 120 mm Hg. This explains why there
are different scales on the pressure axes for the left and right hearts in the cardiac cycle
diagram presented earlier in these notes.
End of Optional Supplementary Material
"Once more through the Cardiac Cycle"