Feb 5 - The Cardiac Cycle Flashcards
What are the two alternating phases of the cardiac cycle?
Systole
Diastole
Describe systole
Contraction of the heart and ejection of blood
Occurs due to depolarization of the cardiac muscle
Describe diastole
Relaxation and refilling of the heart
Occurs due to repolarization of the cardiac muscle
Describe the contractions of the right and left atria and ventricles
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
What is heart block
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
Why does atrial pressure initially increase?
Atrial pressure increases due to continuous passive filling of blood into the atria
How does atrial pressure contribute to the cardiac cycle?
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
What does the P wave represent on an ECG?
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.
What follows atrial contraction?
Ventricular pressure increases as ventricular blood volume increases (in part due to atrial contraction)
What is the end-diastolic volume?
Aka EDV.
The volume at the end of ventricular diastole
EDV is approximately 135 ml
What does the QRS complex on an ECG represent?
Ventricles become depolarized, initiating contraction
What happens to the AV valves when the ventricles contract?
The ventricular pressure exceeds the atrial pressure, causing the closure of the AV valves
What is the isovolumetric ventricular contraction?
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
What follows isovolumetric ventricular contraction?
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
What is end-systolic volume?
The volume of blood at the end of systole
Aka ESV
ESV is approximately 65 ml
What is stroke volume?
Aka SV
Blood volume ejected by each ventricle with each contraction
SV = EDV-ESV = 135-65 = 70 ml
What is the ejection fraction?
Aka EF
The proportion of the blood volume ejected by each ventricle with each contraction
EF = SV/EDV = 70/135 = 0.52
What does the T wave represent on an ECG?
Ventricles become repolarized
What follows ventricular repolarization?
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
What is isovolumetric ventricular relaxation?
The period where all the valves are closed and the ventricles remain as closed chambers. Ventricular pressure falls and ventricular volume does not change
What follow the isovolumetric ventricular relaxation?
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.
How does an increased preload affect the cardiac cycle?
An increased preload triggers the Frank-Starling mechanism, resulting in increased stroke volume
How does an increased afterload affect the cardiac cycle?
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
How can the cardiac cycle be represented on a xy axis?
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
How does exercise affect the heart rate and the cardiac cycle?
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
Describe the first heart sound
Low pitch, soft, long duration (“LUB”)
Linked to AV valve closure
Signals the onset of ventricular systole
Describe the second heart sound
Higher pitch, sharp shorter duration (“DUB”)
Linked to semilunar valve closure
Signals the onset of ventricular diastole
What causes abnormal heart sounds?
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
How can the timing of the murmur help identify the type of valvular defect?
Between 1st and 2nd sound = systolic
Between 2nd and 1st sound = diastolic
Describe a stenotic valve
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
Describe an insufficient valve
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
What causes valve malfunction?
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
What is the result of rheumatic fever?
Valves become thick, stiff and scarred because of large hemorrhagic fibrinous lesions which form along inflamed valve edges
Besides rheumatic fever, what can cause valve defects?
Valve defects can also be congenital, i.e. a result of improper formation during embryonic development
What happens if the valve dysfunction is severe?
The valve will have to surgically replaced. The new valve may be artificial, or may be obtained from cadavers
