The Cardiac Cycle Flashcards
What is the cardiac cycle?
Sequence of mechanical events that occur during a single heart beat
When are all the chambers of the heart relaxed?
Towards the end of diastole
Why do the valves between the atria and ventricles open?
Because atrial pressure remains slightly greater than ventricular pressure until the ventricles are fully distended
Why are the pulmonary and aortic outflow valves closed?
As pulmonary artery and aortic pressures are greater than ventricular pressure
When does the cardiac cycle begin?
When sinoatrial node initiates the heartbeat
What happens in late diastole?
Both sets of chambers are relaxed and ventricles fill passively
What happens during atrial systole?
Atrial contraction forces a small amount of additional blood into the ventricles
What happens during isovolumic ventricular contraction?
First phase of ventricular contraction, pushes atrioventricular valves closed but does not create enough pressure to open the semilunar valves
What happens during ventricular ejection?
As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected
What happens during isovolumic ventricular relaxation?
As ventricles relax, pressure in the ventricles falls, blood flows back into the cusps of the semilunar valves and snaps them closed
What completes ventricular filling?
Contraction of the atria
At rest, the atria contribute less than 20% of ventricular volume, how does this change with heart rate?
This proportion increases with heart rate as diastole shortens and there is less time for ventricular filling
Why does some blood regurgitate into the veins during atrial systole?
Because there are no valves between the veins and atria
What reflects atrial systole?
The a-wave of atrial and venous pressure traces
What is the name given to ventricular volume after filling? What is the normal volume for this?
End diastolic volume (EDV)
120-140mL
What is the value of end diastolic pressure (EDP)?
Less than 10mmHg
Where is end diastolic pressure higher?
Higher in the left ventricle than in the right due to the more muscular left ventricular wall
What is EDV an important determinant of?
The strength of the subsequent contraction
What does ventricular contraction cause?
Sharp rise in ventricular pressure
Atrioventricular valves close once this exceeds atrial pressure
What causes the first heart sounds?
Closure of atrioventricular valves
What does ventricular depolarisation correspond to on the ECG?
QRS complex
Why do the outflow valves remain closed during the initial phase of ventricular contraction?
Pressure is less than that in the pulmonary artery and aorta so outflow valves remain closed
What does increasing pressure during ventricular systole cause?
AV valves to bulge into the atria resulting in the small atrial pressure wave (c wave) followed by a fall
When do outflow valves open?
When pressure in the ventricle exceeds that in its respective artery
What is the difference between pulmonary artery pressure and the pressure in the aorta?
Pulmonary artery pressure is considerably less than that in the aorta - 15mmHg vs 80mmHg
What is the change in flow into the arteries during ejection?
Flow into the arteries is initially very rapid but as contraction wanes, ejection is reduced
What might rapid ejection be heard as?
A murmur
When does active contraction cease?
During the second half of ejection
Ventricular pressure during the reduced ejection phase is slightly less than in the artery, why does blood continue to flow out of the ventricle?
Due to momentum
What causes the second heart sound?
Closure of the semilunar valves
What causes closure of the outflow valve?
Brief reversal of flow during ejection
What is the amount of blood ejected by the ventricle in one beat known as? What is the usual value for this?
Stroke volume
70mL
What is the name given to the amount of blood left int he ventricles at the end of systole? What is the usual volume for this?
End systolic volume
About 50mL
What is the name given to the proportion of end diastolic volume that is ejected?
Ejection fraction (SV/EDV)
Why does atrial pressure rise during the last 2/3 of systole?
As a result of filling from the veins
What happens to the ventricles following the closure of the outflow valves?
Ventricles are rapidly relaxing
Why do AV valves remain closed during diastole?
As ventricular pressure is still greater than atrial pressure
What happens when ventricular pressure falls below atrial pressure?
AV valves open and atrial pressure falls as the ventricles refill
What is the the refilling of the ventricles assisted by?
Elastic recoil of the ventricular walls
When might a third heart sound be heart?
In young people or when EDP is high
What happens to refilling as the ventricles relax completely?
Refilling slows
What happens to diastole during exercise and with increasing heart rate?
Diastole at rest is twice the length of systole, this decreases proportionally during exercise and as heart rate increases
What generates the pressure-volume loop?
Ventricular pressure plotted against volume
What is the shape of the pressure-volume loop affected by?
Contractility of the ventricles and factors that alter refilling or ejection
What does the bottom dotted line of the pressure-volume loop show?
Passive elastic properties of the ventricle
What would happen to the curve of the pressure-volume loop is compliance was decreased e.g. as a result of fibrotic damage following an infarct?
The curve would be steeper
What is the area of the pressure-volume loop an indicator of?
Cardiac function
What is a phonocardiogram?
Chart/record of the sounds made by the heart
What causes the first heart sound?
Closure of the atrioventricular valves (mitral and tricuspid)
What causes the second heart sound?
Closure of the semilunar valves (aortic and pulmonary)
When does the third heart sound occur? What causes this?
Occurs early in diastole
Normal phenomenon in young people and athletes
In older adults it indicates the presence of congestive heart failure
Caused by a sudden deceleration of blood flow into the left ventricle from the left atrium
When does the fourth heart sound occur? What causes this?
Extra heart sound occurring in late diastole
Due to atrial contraction inducing ventricular filling
Caused by a greatly thickened left ventricular wall e.g. due to hypertension or aortic stenosis, may be an indicator of coronary heart disease
