Feb 5 - The Cardiac Cycle

Question 1

What are the two alternating phases of the cardiac cycle?

Answer 1

Systole

Diastole

Question 2

Describe systole

Answer 2

Contraction of the heart and ejection of blood

Occurs due to depolarization of the cardiac muscle

Question 3

Describe diastole

Answer 3

Relaxation and refilling of the heart

Occurs due to repolarization of the cardiac muscle

Question 4

Describe the contractions of the right and left atria and ventricles

Answer 4

The atria and the ventricles go through separate cycles of systole and diastole, with the atria contracting first to help move blood into the ventricles. The right atria and left atria contract at the same time as one another; similarly, the right and left ventricles also contract at the same time as one another

Question 5

What is heart block

Answer 5

In patients with heart block (an arrhythmia), this automatic rhythm is disrupted, leading to fainting and dizziness. In severe cases, block can result in death

Question 6

Why does atrial pressure initially increase?

Answer 6

Atrial pressure increases due to continuous passive filling of blood into the atria

Question 7

How does atrial pressure contribute to the cardiac cycle?

Answer 7

Atrial pressure exceeds ventricular pressure, causing the opening of the left/right atrioventricular valves. Ventricular volume increases as blood flows into the ventricles from the atria

Question 8

What does the P wave represent on an ECG?

Answer 8

Atria become depolarized, represented as the P wave on the ECG. The atria contract and squeeze blood into the ventricles causing an increase in atrial pressure.

Question 9

What follows atrial contraction?

Answer 9

Ventricular pressure increases as ventricular blood volume increases (in part due to atrial contraction)

Question 10

What is the end-diastolic volume?

Answer 10

Aka EDV.
The volume at the end of ventricular diastole
EDV is approximately 135 ml

Question 11

What does the QRS complex on an ECG represent?

Answer 11

Ventricles become depolarized, initiating contraction

Question 12

What happens to the AV valves when the ventricles contract?

Answer 12

The ventricular pressure exceeds the atrial pressure, causing the closure of the AV valves

Question 13

What is the isovolumetric ventricular contraction?

Answer 13

The period of at which all the valves are closed and the ventricle remains a closed chamber. Ventricular pressure rises, but volume does not change

Question 14

What follows isovolumetric ventricular contraction?

Answer 14

Ventricular pressure exceeds aortic/pulmonary pressure causing opening of aortic/pulmonary valves and the ventricles eject blood. Aortic/pulmonary pressure increases due to blood forced into the aorta/pulmonary artery. Ventricular volume reduces significantly

Question 15

What is end-systolic volume?

Answer 15

The volume of blood at the end of systole
Aka ESV
ESV is approximately 65 ml

Question 16

What is stroke volume?

Answer 16

Aka SV
Blood volume ejected by each ventricle with each contraction
SV = EDV-ESV = 135-65 = 70 ml

Question 17

What is the ejection fraction?

Answer 17

Aka EF
The proportion of the blood volume ejected by each ventricle with each contraction
EF = SV/EDV = 70/135 = 0.52

Question 18

What does the T wave represent on an ECG?

Answer 18

Ventricles become repolarized

Question 19

What follows ventricular repolarization?

Answer 19

As the ventricles relax, ventricular pressure falls below aortic/pulmonary pressure causing the aortic/pulmonary valves to shut. Semilunar valve closure causes a disturbance and is seen as a notch on the semilunar pressure curve

Question 20

What is isovolumetric ventricular relaxation?

Answer 20

The period where all the valves are closed and the ventricles remain as closed chambers. Ventricular pressure falls and ventricular volume does not change

Question 21

What follow the isovolumetric ventricular relaxation?

Answer 21

The ventricular pressure falls below atrial pressure and the AV valves open. Passive filling of the blood in the atria results in an increase in atrial pressure.

Question 22

How does an increased preload affect the cardiac cycle?

Answer 22

An increased preload triggers the Frank-Starling mechanism, resulting in increased stroke volume

Question 23

How does an increased afterload affect the cardiac cycle?

Answer 23

Increased afterload results in increased pressure development in the ventricle before the aortic valve opens; the ventricle empties less efficiently, resulting in decreased stroke volume

Question 24

How can the cardiac cycle be represented on a xy axis?

Answer 24

The cardiac cycle can be depicted as a pressure/volume loop. The area bounded by the curve is a measure of work. The shape and/or area of the PV loop can change in characteristic ways in disease

Question 25

How does exercise affect the heart rate and the cardiac cycle?

Answer 25

During exercise, heart rate may increase from resting rate of 75 beats per minute to 180 beats per minute. When this occurs, the length of ventricular diastole is reduced by 25% and subsequently there is less ventricular filling. However since much of the filling of the ventricles occurs in the rapid filling phase, a shortened diastole does not cause a significant drop in cardiac output

Question 26

Describe the first heart sound

Answer 26

Low pitch, soft, long duration (“LUB”)
Linked to AV valve closure
Signals the onset of ventricular systole

Question 27

Describe the second heart sound

Answer 27

Higher pitch, sharp shorter duration (“DUB”)
Linked to semilunar valve closure
Signals the onset of ventricular diastole

Question 28

What causes abnormal heart sounds?

Answer 28

Abnormal heart sounds arise when blood flow which is normally laminar (layers of fluid sliding over each other smoothly) becomes turbulent. Abnormal heart sounds may occur as a consequence of stenotic or insufficient valves

Question 29

How can the timing of the murmur help identify the type of valvular defect?

Answer 29

Between 1st and 2nd sound = systolic

Between 2nd and 1st sound = diastolic

Question 30

Describe a stenotic valve

Answer 30

Does not open completely due to stiffness and narrowing
When blood is forced through a stenotic valve, there is turbulence which results in an abnormal sound resembling whistling

Question 31

Describe an insufficient valve

Answer 31

Is scarred, resulting in poor apposition of the leaflets and preventing complete closure of the valve
Allows blood to flow backwards (regurgitation)
Blood flowing backward collides with forward-moving blood, producing a swishing sound

Question 32

What causes valve malfunction?

Answer 32

Valve malfunction is mostly caused by rheumatic fever, an autoimmune disease caused by streptococcal bacterial infection. In rheumatic fever, antibodies produced against bacterial toxins attack many of the body’s own tissues, the valves of the heart being a major target

Question 33

What is the result of rheumatic fever?

Answer 33

Valves become thick, stiff and scarred because of large hemorrhagic fibrinous lesions which form along inflamed valve edges

Question 34

Besides rheumatic fever, what can cause valve defects?

Answer 34

Valve defects can also be congenital, i.e. a result of improper formation during embryonic development

Question 35

What happens if the valve dysfunction is severe?

Answer 35

The valve will have to surgically replaced. The new valve may be artificial, or may be obtained from cadavers