Cardiac Cycle Flashcards
What is the cardiac cycle?
It is all the events associated with the flow of blood through the heart during one heart beat
What are diastole and systole in the cardiac cycle?
Diastole - where the heart fills with blood
Systole - where the myocardium contracts and the blood is ejected
How long does one cardiac cycle last?
What is normal resting heart rate?
One cardiac cycle lasts 0.8 seconds
A normal resting heart rate is 55 - 60 beats per minute
What are the three phases of the cardiac cycle?
- diastole
- isovolumetric contraction
- systole
What are the stages involved in diastole?
It is the phase where the ventricles fill with blood:
- isovolumic relaxation
- rapid inflow
- slow inflow (diastasis)
- atrial systole
What is the role of the valves in diastole?
They prevent backflow of blood and ensure that blood goes to the correct place
What are the 4 heart valves?
- pulmonary valve
- aortic valve
- tricuspid valve
- mitral (bicuspid) valve
What is involved in diastole?
The filling of both the left and right ventricles with blood
How does blood flow into the ventricles change as diastole progresses?
- passive filling as blood from the atria enters the ventricles
- atrial systole involves contraction of the atria to ensure all blood enters the ventricles
What is heart sound 1 of the heart beat?
When does it occur?
Closing of the mitral/tricuspid valve
It occurs at the beginning of ventricular systole
What is heart sound 2 of the heart beat?
When does it occur?
Closing of the aortic and pulmonary valves
It occurs at the beginning of diastole
When does a split heart sound occur?
When there is increased venous return into the right atrium and ventricle
What is a split heart sound and why does it occur?
During systole, it takes longer for all of the blood to move from the RV into the pulmonary artery
The pulmonary valve shuts slightly later than the aortic valve
What structures prevent the heart valves from inverting?
The papillary muscles in the ventricles attach to the cusps of the valves via chordae tendineae
What is the consequence to the heart valves when damage to the myocardium causes tissue loss?
The papillary muscle may be damaged or the chordae tendineae may be ruptured
How is the movement of the tendinous cords influenced in atrial systole?
Blood flows down a pressure gradient from the atria into the ventricles
The tendinous cords are relaxed as the pressure is low
What happens to the tendinous cords as the ventricles contract and why?
As ventricles contract, valves are pushed upwards to prevent blood flowing back into the atria
The increase in blood pressure in the ventricles pulls the tendinous cords taut
What cardiovascular condition usually leads to damage to the chordae tendineae?
myocardial infarction
When does mitral regurgitation occur and what does it allow for?
It occurs when blood leaks backwards through the mitral valve each time the LV contracts
This allows blood to flow in 2 directions during contraction
In an ECG, what does ST elevation and ST depression show?
ST elevation is typical of a myocardial infarction
ST depression shows myocardial ischaemia
What is the frequency of fibrillation of the atria and ventricles?
Why are they different?
Atria can fibrillate up to 300 beats per minute
The ventricles can fibrillate up to 110-120 beats per minute due to the delay in the AV node
What happens in a pressure-volume loop during the P wave?
P wave shows atrial depolarisation as the atria contract
Atrial pressure increases
Blood flows from the atria into the ventricle, leading to a quick increase in ventricular volume
What happens in a pressure-volume loop during the QRS complex?
QRS complex shows ventricular depolarisation as the ventricles contract
There is an increase in ventricular pressure and decrease in ventricular volume
This is due to blood being ejected out of the ventricles
When does an isovolumetric contraction occur?
When the pressure increases but the volume stays the same
What are the different stages shown on a pressure-volume loop?
- isovolumic contraction
- ejection
- isovolumic relaxation
- rapid inflow
- diastasis
- atrial systole
What happens during the isovolumic contraction stage of the pressure-volume loop?
For a brief period of time, all of the valves are shut
When all the valves are shut, the pressure is increasing but the volume stays the same
What value does ventricular pressure build up to in the isovolumic contraction phase?
What is significant about this value?
80 mmHg
This is the point at which ventricular pressure rises above aortic pressure
What happens once ventricular pressure rises above the pressure in the aorta?
The aortic valve opens and the ejection phase begins
What occurs during isovolumetric relaxation?
All of the valves are shut
The pressure is decreasing but the volume stays the same
What does the T wave show on an ECG?
Ventricle repolarisation as the ventricles start to fill with blood again
What stage is shown during the T-wave on the pressure-volume loop?
The rapid inflow stage
This is where ventricular pressure decreases and ventricular volume increases
What happens during diastasis and when does it occur?
It occurs mid-diastole
The initial passive filling of the ventricles has slowed down, but the atria have not yet contracted to complete active filling
What is stroke volume?
The volume of blood ejected from the left ventricle with each cardiac cycle
At rest, what is average stroke volume?
around 70 ml
What is cardiac output?
The volume of blood ejected from the left ventricle each minute
What is normal cardiac output at rest?
Around 5 L/min
What is the equation for cardiac output?
cardiac output = heart rate x stroke volume
What is blood pressure?
the pressure of the blood in the circulatory system
Why is blood pressure often measured for diagnosis?
It is closely related to the force and rate of the heart beat
It is also related to the diameter and elasticity of the arterial walls
What is the equation for blood pressure?
Blood pressure =
heart rate x stroke volume x TPR
This is the same as
cardiac output x TPR
What is total peripheral resistance?
The resistance offered by the systemic circulation
This is the resistance of the arteries to the flow of blood
What is the resistance offered by the pulmonary circulation?
pulmonary vascular resistance (PVR)
How long does the PR interval last for?
from 0.12 to 0.20 seconds
How long does the QRS complex last for?
from 0.08 to 0.12 seconds
How long does the QT interval last for?
0.35 to 0.43 seconds
