Lecture 4- Cardiac Cycle Flashcards
What is the cardiac cycle?
Sequence of pressure flow changes and valve operations that occur with each heartbeat
Give a brief description of what happens in the cardiac cycle.
During diastole atrial pressure begins to exceed ventricular pressure. This causes the bicuspid and tricuspid valves to open and blood flows into the ventricles and fills them. At the point where intra ventricular pressure matches that in the atria blood stops flowing into the ventricles. Atrial systole now occurs which forces a small amount more blood into the ventricles.
Now systole occurs where the ventricles contract and force blood into the atria which causes turbulence and the closing of the atrio ventricular valves. Ventricles then contract isovolumetrically as all valves are closed. Th pressure generated exceeds the diastolic pressure in the arteries and so the aortic and pulmonary valves are forced open. Towards the end of systole intra ventricular pressure falls and the pulmonary and aortic valves close. The atria then fill up again and the process starts over
What are the heart sounds?
s1 happens upon the closing of the atrio ventricular valves and s2 occurs on the closing of the pulmonary and aortic valves.
Systole happens between s1 and s2
Why are resistance vessels important?
Create blood pressure which allows blood to perfuse areas of the body that are typically hard to perfuse
What is systole?
Contraction and ejection of blood from ventricles
What is diastole?
Relaxation and filling of ventricles
What is typical stroke volume?
70ml per beat
What is standard heart rate
70 bpm
What is average cardiac output?
70ml by 70bpm= 4.9 litres
How long is a cardiac action potential?
280ms (duration of a single contraction)
What way to the mitral and tricuspid valves point and why is this relevant to prolapse?
Point down into ventricles and during prolapse ventricular contraction and subsequent force of blood forces them up into the atria
What prevents inversion (prolapse) of the atrio ventricular valves during systole?
Papillary muscles which are attached to the valves through chordae tendineae
Easy way of remembering valve direction?
They follow the flow of blood like the valves in veins
How do pacemaker cells in SA node initiate action potential?
Have ion channels that depolarise automatically
Why is there a delay of 120ms when action potential reaches AV node?
Allows the atria to contract before the ventricles
Atrial vs ventricular fibrillation?
Atrial fibrillation means blood isn’t sent to ventricles properly and not life threatening while ventricular fibrillation is life threatening.
Atrial contraction only accounts for 10-20% of ventricular volume anyway
How long do systole and diastole last?
Diastole= 0.55 seconds
Systole= 0.35 seconds
During exercise diastole is reduced while systole doesn’t change
What are the 7 phases of the cardiac cycle?
Atrial contraction
Isovolumetric contraction
Rapid ejection
Reduced ejection
Isovolumetric relaxation
Rapid filling
Reduced filling
What is a wiggers diagram?
Diagram of time against aortic pressure, left ventricular pressure, left ventricular volume, an electrocardiograph and a phonocardiogram
Is a wiggers diagram for the left or right side of the heart?
Typically the left, right would be similar but with lower pressures
How much is end diastolic volume?
Around 120ml
What happens in atrial contraction phase 1?
Atrial pressure rises known as the A wave
P wave in ECG signifies onset of atrial depolarisation
Volume of left ventricle increases by around 10%
What happens in phase 2 isovolumetric contraction?
Mitral valve closes which increases atrial pressure known as C wave.
Volume of left ventricle remains the same as all valves are closed
QRS complex in ECG signifies onset of ventricular depolarisation
Closing of mitral and tricuspid valves cause s1 sound
Mitral valve closes due to ventricular pressure exceeding atrial pressure
Pressure in left ventricle rises rapidly
What happens in phase three rapid ejection?
Ejection begins when ventricular pressure exceeds aortic pressure.
During contraction atria are pulled down which initially lowers pressure known as the X descent
Rapid decrease in ventricular blood volume
What happens in phase 4 reduced ejection?
Ventricle is depolarised, pressure is reduced and rate of ejection falls
Atrial pressure rises as they are filled known as V wave
Ventricular depolarisation known as T wave on ECG
What happens in phase 5 isovolumetric relaxation?
Brief backflow of blood when ventricular pressure falls below aortic pressure causes aortic valve too close
This valve closure results in a dicrotic notch in aortic pressure
Ventricular pressure falls but volume remains the same due to all valves being closed
End systolic volume is reached and gives a stroke volume of around 70ml
Closure of aortic valve gives s2 heart sound
What happens in phase 6 rapid filling?
Fall in atrial pressure known as Y descent
When intraventricular pressure falls below atrial pressure the mitral valve opens and rapid ventricular filling begins
This is normally silent but s3 sound sometimes heard in children and is normal but is a sign of pathology in adults
What happens in reduced filling stage 7?
Ventricular pressure is close to reaching atrial pressure and so filling slows. Atrial contraction will be responsible for final volume
What are the two main abnormal valve functions?
Stenosis where the valve doesnt open enough and blood flow is obstructed
Regurgitation where valve doesnt close all the way so there is leakage
What causes aortic valve stenosis?
Degenerative (senile calcification or fibrosis)
Congenital (bicuspid valve)
Chronic rheumatic fever leading to inflammation
What are the problems associated with aortic valve stenosis?
Blood struggles to get through valve which means heart must pump harder eating to hypertrophy.
It can also lead to left sided heart failure if sever enough which can result in angina as the cardiac muscle lacks blood supply through coronary arteries and syncope (fainting) as brain lacks oxygen
It can also lead to anaemia if the space is small enough as RBC’s het sheared (microangiopathic haemolytic anaemia)
What causes aortic valve regurgitation?
Aortic root dilation (leaflets pulled apart)
Valvular damage (endocarditis rheumatic fever)
What happens in aortic valve regurgitation?
During diastole blood flows into ventricle which increases the stroke volume and increases systolic pressure while decreasing diastolic pressure.
Results in a Bounding pulse, head bobbing and quinkes sign where nail beds go flush and pale in time with pulse
Causes left ventricular hypertrophy
What happens in mitral valve regurgitation?
Prolapse can occur due to myxomatous degeneration, damage to papillary muscle after heart attack, left side heart failure dilation of LV and stretching of valve and rheumatic fever.
Blood leaks back into atria which causes greater subsequent preload in ventricle and results in LV hypertrophy
Mitral valve stenosis?
Almost all cases caused by rheumatic fever.
Valve leaflets fuse and inhibit flow from LA to LV.
Increased LA pressure leads to pulmonary oedema, dyspnea (shortness of breath) and pulmonary hypertension and RV hypertrophy as it struggles to pump blood to lungs.
Can also lead to LA dilation which can cause atrial fibrillation and thrombus formation and oesophagus compression and dysphagia
