Cardiac Cycle Flashcards
How long does the cardiac cycle last?
0.8 seconds
What are the two main phases of a heartbeat?
Diastole
Systole
What are the features of diastole?
Lasts approximately 2/3 of each beat
Ventricular relaxation
The ventricles fill with blood
Split into 4 distinct phases
What are the features of systole?
Lasts approximately 1/3 of each beat
Ventricular contraction
Ventricles generate pressure then eject blood into the arteries
Split into 3 distinct phases
What are the phases of the systole?
Isovolumetric contraction
Rapid ejection
Slow ejection
What is end-diastolic volume?
When the heart is filled
108ml
What is end-systolic volume?
Volume of blood after contraction
36ml
What is the stroke volume?
Difference between end-diastolic and end-systolic volume
72ml
What is ejection fraction?
Percentage of blood that is pumped into circulation
Describes the action and contractility of the heart
around 67%
What are the stages of diastole?
Atrial systole (Systole) Isovolumetric relaxation Rapid passive filling Slow passive filling
What does the cardiac cycle being with?
Atrial systole
What is atrial systole on an ECG?
P-wave
What occurs during atrial systole?
Atria already almost full from passive filling driven by pressure gradient. Atria contract to ‘top-up’ the volume of blood in ventricle
What heart sound is atrial systole responsible for?
4th heart sound
Abnormal
Would normally be silent
What does an abnormal 4th heart sounds indicate?
congestive heart failure pulmonary embolism
tricuspid incompetence
What is the second phase of the cardiac cycle?
Isovolumetric contaction
What is isovolumetric contraction on an ECG?
QRS complex marks the start of ventricular depolarisation
What occurs during isovolumetric contraction?
This is the interval between AV valves (tricuspid & mitral) closing and semi-lunar valves (pulmonary & aortic) opening
Contraction of ventricles with no change in volume
What heart sound is isovolumetric contraction responsible for?
1st heart sound (‘lub’) due to closure of AV valves and associated vibrations
What is the third phase of the cardiac cycle?
Rapid ejection
What marks the start of rapid ejection?
Opening of the aortic & pulmonary valves
What occurs during rapid ejection?
As ventricles contract pressure within them exceeds pressure in aorta and pulmonary arteries. Semilunar valves open, blood pumped out and the volumes of ventricles decrease.
What is the heart sound for rapid ejection?
No heart sounds are heard for this phase
What is the fourth phase of the cardiac cycle?
Reduced ejection
What does reduced ejection mark?
End of systole
What occurs during reduced ejection?
Reduced pressure gradient means aortic & pulmonary valves begin to close
Blood flow from ventricles decreases and ventricular volume decreases more slowly
As pressures in ventricles fall below that in arteries, blood begins to flow back causing semilunar valves to close
What is the fifth stage of the cardiac cycle?
Isovolumetric relaxation
What occurs during isovolumetric relaxation?
The aortic & pulmonary valves shut, but the AV valves remain closed until ventricular pressure drops below atrial pressure.
Atrial pressure continues to rise. Dichrotic notch (green line) caused by rebound pressure against aortic valve as distended aortic wall relaxes.
What heart sound is isovolumetric relaxation responsible for?
2nd heart sound (‘dub’) due to closure of semilunar and associated vibrations
What is the sixth phase of the cardiac cycle?
Reduced passive filling
What can reduced passive filling also be known as?
Diastasis
What occurs during reduced passive filling?
Ventricular volume fills more slowly
The ventricles are able to fill considerably without the contraction of the atria.
What are the difference between the left and right sides of the heart?
the pressures in the right heart and pulmonary circulation are much lower (peak of systole – 25mmHg in pulmonary artery)
Left: 120mmHg
What are the similarities between the left and right side of the heart?
The patterns of pressure changes in the right heart are essentially identical to those of the left
right ventricle ejects same volume of blood as left
What determines the preload that stretches the resting ventricular muscle?
Blood filling the ventricles during diastole
What represents afterload?
The blood pressure in the great vessels
aorta and pulmonary artery
What does increased preload result in?
Increased stroke volume
Frank-Starling relationship
What does increased afterload result in?
Decreased stroke volume
Decreased shortening
What is cardiac output equal to?
Heart rate x Stroke volume
What affects stroke volume?
Preload
Afterload
Contractility
Define contractility?
Contractile capability (or strength of contraction) of the heart
How can contractility be measured?
Ejection fraction
What increases contractility?
Sympathetic stimulation
More calcium release
Increases gradient of pressure and volume relationship
Increased force
What does hardening and narrowing of the aortic valve result in?
Reduces flow
Increases afterload
What does acute blood loss result in?
Reduces venous return
Decreases preload
What does exercise result in?
Venous return increases due to venoconstriction and skeletal muscle pump, and contractility is increased via sympathetic nervous system
