0630 - Cardiac Cycle - RM

Question 1

What electrical events are used to time the cardiac cycle?

Answer 1

SA node fires
Atrial depolarisation (P-wave)
AV node fires
Septal and Ventricular depolarisation (QRS)
Ventricular repolarisation (T-wave)

Question 2

What mechanical events are used to time the cardiac cycle?

Answer 2

Systole - Period between S1 (closure of mitral and tricuspid valves) and S2 (closure of aortic and pulmonary valves). Contraction and ejection period. AV valves are shut for all of systole and the beginning of Diastole. Arterial valves are shut for the beginning of systole, then open, and close at the end of systole.
Diastole - Period between S2 (closure of the Aortic and Pulmonary valves) and S1 (Closure of mitral and tricuspid valves). Relaxation and filling period. AV valves are shut for the beginning of diastole, then open until the end. Arterial valves are shut throughout diastole.

Question 3

What are the 7 phases of the cardiac cycle?

Answer 3

1. Isovolumetric contraction
2. fast ejection
3. slow ejection
4. isovolumetric relaxation
5. fast ventricular filling
6. slow ventricular filling
7. atrial contraction

Question 4

What causes the heart sounds S1 and S2? What is their role?

Answer 4

Delimit systole and diastole (systole is between S1 and S2).

S1 - Closure of AV valves
S2 - Closure of Semilunar valves.

AV valves shut through systole, plus a bit after.
Semilunar valves shut through diastole, plus a bit after.
Systole is relatively constant length - diastole varies with HR.

Question 5

Be able to DRAW the important features of pressure, volume, and flow changes during cardiac cycle in ventricles, atria and arteries

Answer 5

Need to work on this one

Question 6

How and when are the ventricles filled?

Answer 6

Filled during diastole.
While atria are filled during systole (horror vacui displaces the valvular plane, causing suction into the atria) as ventricular contraction physically pulls the atria towards the apex, ventricles are filled during diastole. There is a fast filling (aided by reversion of the valvular plane), slow filling (passive) and finally ‘last little push’ of filling from atrial contraction (almost insignificant at rest - though this becomes more important as HR rises).

Question 7

How is stroke volume determined by the pressure difference between systole and diastole?

Key Concept for Exam

Answer 7

The pressure difference between systolic and diastolic pressure is a key determinant of SV. Lower diastolic pressure in the aorta means that the valve will open earlier and close later, prolonging ejection and thus increasing stroke volume (Key Concept!)

Question 8

What is the relative timing between the left and right atria and ventricles?

Answer 8

The cardiac cycle starts in RA (SA node), progresses to LA, and is delayed at AV node.

Progresses down septum, LV contracts first (bigger), and RV contracts slightly afterwards, as it needs a stable septum to contract against.