Cardiac cycle

Question 1

Describe the sequence of electrical vs. mechanical cardiac cycle

Answer 1

Question 2

What are the 2 main events for a complete cardiac cycle

Answer 2

Ventricular systole (contraction)
1. Isovolumic ventricular contraction
2. Ventricular ejection
-Rapid ejection phase
-Reduced ejection phase

Ventricular diastole relaxation
1. Isovolumic ventricular relaxation
2. Ventricular filling
-Rapid filling phase
-Reduced filling fase

Question 3

Explain the each step of the cardiac cycle

Answer 3

1. Ventricular systole:

Isovolumic ventricular contraction:
 Blood volume in the ventricle during this period is known as end-diastolic volume (EDV), commonly refers to as preload
 Begins with the closure of AV (mitral) valve
 Generates 1st low- pitched heart sound
 With no change in ventricular volume

Ventricular ejection:
 Begins with the opening of the semilunar valve (AV valves are now closed)
 Sharp increase in the ventricular and aortic pressure (rapid ejection phase, sharp increase in dp/dt )
 There is a sudden drop of left atrial pressure (Mitral valve is now closed)
 Even with continuation of left atrial filling from the pulmonary veins
 Follows by reduced ejection phase (associated with the onset of T-wave)
-Peak of left ventricular pressure coincides with arterial systolic pressure
-Ends with the closure of the semilunar valves
-Ventricular volume at the end of ejection phase in known as end systolic volume

2. Ventricular diastole:

Isovolumic ventricular relaxation:
-Blood volume in the ventricle during this period is known as end-systolic volume (ESV)
-Begin with the closure of semilunar (aortic) valve
-Generate 2nd higher pitched heart sound
-Dicrotic notch (an increase in aortic pressure) is observed
-Stroke volume (SV) is the difference between EDV and ESV
-SV = EDV - ESV

Ventricular filling:
-Beings with the opening of the AV (mitral valve) semilunar are now closed
-A slight drop of atrial pressure is observed
-Majority of ventricular passing filling occurs in this phase (70%) (rapid filling phase)
-Follow by reduced filling phase with atrial contraction (also known as active ventricular filling phase)

Question 4

Explain the ventricular pressure volume loop

Answer 4

A-B segment (late diastole):
-Starts at ESV (end systolic volume; not all blood is pumped out during ventricle contraction, the volume of blood left over after contraction is ESV)
-Pressure in ventricle is lower than atria and the AV valve opens causing the ventricle to passively fill with blood (majority is passive)
-Pressure drop during filling is due to ventricle still relaxing causing a suction effect until mitral valve opens

B-C segment (atrial systole)
-Atria contracts forcing more blood into the ventricle slightly increasing volume and pressure
-At the end of segment B-C the maximal amount of blood is in the ventricles, this occurs at the end of ventricular diastole and is termed the end diastolic volume (EDV)

C-D segment (isovolumetric contraction):
-The ventricle begins contracting closing AV valve, continued contraction causes a large increase in pressure within the ventricle

D-F segment (ventricular ejection)
-Once pressure in ventricle rises above 80mmHg, it exceeds the aorta and the aortic valve opens causing a rapid ejection of blood.
-Pressure still rises as the ventricle continues to contract
-Part way through this segment the ventricle begins to relax and pressure begins to drop but blood still flows due to the inertia

FA segment (isovolumetric relaxation)
-Pressure in aorta begins to exceed ventricle causing semi-lunar valve to close, ventricle continues to relax