PHYSIOLOGY - The Cardiac Cycle Flashcards
What is the cardiac cycle?
The cardiac cycle refers to every mechanical and electrical event which occurs during every heart beat
Which chambers of the heart does the cardiac cycle graph represent?
The cardiac cycle graph represents the activity of all four chambers of the heart, even though it mainly focuses on the activity of the left atria and ventricle
What is systole?
Systole is the contraction phase of the cardiac cycle
What is diastole?
Diastole is the relaxation phase of the cardiac cycle
What are the three main phases of the cardiac cycle?
Atrial systole
Ventricular systole
Ventricular diastole
What are the two sub-phases of ventricular systole?
Isovolumetric contraction
Ventricular ejection
What are the two sub-phases of ventricular diastole?
Isovolumetric relaxation
Ventricular filling
Describe what happens during the atrial systole phase of the cardiac cycle
Refer to cardiac cycle graph
During atrial systole, the atria contract causing an increase in atrial pressure. Atrial contraction pushes the remaining blood that didn’t enter the ventricles during ventricular filling into the ventricles through the open atrioventricular valves, causing left ventricular pressure and left ventricular volume to increase
Which wave of an ECG corresponds to the atrial systole phase of the cardiac cycle? Why is this?
The P wave of an ECG corresponds to atrial systole because the P wave represents atrial depolarisation which leads to atrial contraction
Which heart sound can be heard during atrial systole? Why is this?
S4 heart sounds are a vibrating sound caused by atrial contraction moving blood into a stiff or non-compliant ventricle
Describe what happens during the isovolumetric contraction phase of the cardiac cycle
Refer to cardiac cycle graph
As the left ventricle fills with blood, the left ventricular pressure will exceed atrial pressure leading to the closure of the left atrioventricular (AV) valve to prevent the backflow of blood into the atrium. At this point, the AV and aortic valves will be closed, causing the left ventricular volume to remain the same. During isovolumetric contraction, the ventricular pressure will increase rapidly however will not exceed aortic pressure, so the aortic valve remains closed. This increase in ventricular pressure will cause the AV valves to buldge into the atria, causing atrial pressure to slightly increase. By the end of isovolumetric contraction, the ventricular pressure exceeds the aortic pressure, causing the aortic valve to open, marking the beginning of ventricular ejection
Which wave of an ECG corresponds to the isovolumetric contraction phase of the cardiac cycle? Why is this?
The QRS complex corresponds to isovolumetric contraction because this complex represents ventricular depolarisation, which leads to ventricular contraction
Which heart sound can be heard during isovolumetric contraction? Why is this?
S1 heart sounds are a ‘lub’ sound caused by the closure of the atrioventricular (AV) valves
Describe what happens during the ventricular ejection phase of the cardiac cycle
Refer to cardiac cycle graph
At the beginning of ventricular ejection, the ventricle contracts forcefully to push blood into the aorta, causing the left ventricular and aortic pressures to meet their maximum. Simultaneously, the left ventricular volume will rapidly decrease. As ventricular ejection continues, left ventricular and aortic pressure will begin to fall as there will be reduced blood entering the aorta from the left ventricle. This decreased left ventricular pressure allows the AV valves to return to a neutral postion, no longer buldging into the atria, causing the atrial pressure to slightly decrease. The atria will then begin to accumulate blood causing the atrial pressure to increase.
Which segment of the ECG corresponds with the beginning of the ventricular ejection phase of the cardiac cycle?
The ST segment of an ECG corresponds to the beginning of ventricular ejection as the ST segment represents the phase between ventricular depolarisation and repolarisation where the ventricle will be actively ejecting blood into the aorta before beginning relaxation
Which wave of an ECG corresponds with the end of the ventricular ejection phase of the cardiac cycle?
The T wave of an ECG corresponds to the end of ventricular ejection as the T wave represents ventricular repolarisation, which leads to ventricular relaxation which begins after the maximum ventricular pressure has been reached and the ventricular pressure begins to decrease
Describe what happens during the isovolumetric relaxation phase of the cardiac cycle
Refer to cardiac cycle graph
During isovolumetric relaxation, the left ventricular pressure will continue to fall as the left ventricle is relaxed. The aortic pressure will now exceed left ventricular pressure, causing a backflow of blood into the left ventricle, resulting in a sudden dip in aortic pressure known as the dicrotic notch. The aortic valve will then close to prevent further backflow of blood, causing the aortic pressure to initially rise followed by continuing to gradually decrease. At this point, the AV and aortic valves will all be closed, causing the left ventricular volume to remain the same. The atria are continuing to accumulate blood, causing the **atrial pressure to continue to increase **
Which heart sound can be heard during the isovolumetric relaxation phase?
S2 heart sounds are a ‘dub’ sound caused by the closure of the aortic and pulmonary valves
Describe what happens during the ventricular filling phase of the cardiac cycle
Refer to cardiac cycle graph
The atria have been accumulating blood causing the atrial pressure to exceed left ventricular pressure, causing the atrioventricular (AV) valve to open, causing blood to flow into the left ventricle, causing a decrease in atrial pressure. Simultaneously, the left ventricular pressure and volume will begin to increase. The aortic pressure will continue to decrease during ventricular filling
Which heart sound can be heard during the ventricular filling phase?
S3 heart sounds are a ‘shh’ sound caused by blood flowing into the ventricles
What is the end-diastolic volume?
The volume of blood in the left ventricle at the end of ventricular diastole (i.e. maximum volume of blood)
What is the end-systolic volume?
The volume of blood in the left ventricle at the end of ventricular systole (i.e. minimum volume of blood)
What is a pressure-volume (p-v) loop?
A pressure-volume (p-v) loop plots the left ventricular pressure on the y-axis and the left ventricular volume on the x-axis
Refer to your diagram to revise this
