3.1.4. Cardiac Cycle and Introduction to Heart Sounds

Question 1

Explain the conduction of the heart system from the SA node to ventricular contraction.

Answer 1

To initiate a contraction, the SA nodes fires at an intrinsically. The impulse travels through the atrial myocardium and arrives at the AV node, located at the base of the right atrium. The impulse stalls at the AV node, then propagates through the His bundle along the interventricular septum, into bifurcating paths called the right bundle branch (RBB) and left bundle branch (LBB). These bundles subdivide into Purkinje fibers that relay the impulse to the ventricular myocardium. The Purkinje fibers conduct the most quickly, to enable a rapid and synchronized ventricular contraction.

Question 2

When does the mitral valve open?

Answer 2

During diastole.

Question 3

Explain isovolumentric contraction and when it occurs.

Answer 3

It is the period between MV closure and aortic valve opening. Since the MV and AoV are closed, the LV is a closed chamber and contracts under a constant volume. Eventually, LV pressure is greater than that in the aorta and the AoV opens It corresponds to the QRS on ECG and is the period of highest O2 consumption. This occurs during diastole

Question 4

What is diastole?

Answer 4

The period of ventricular filling or “relaxation”. i. Mitral valve opens, and the ventricle rapidly fills until the pressure of the LV is ii. Isovolumetric contraction is the period between MV closure and aortic valve greater than that of the LA and the MV closes -Corresponds to P and PR intervals on ECG opening. Since the MV and AoV are closed, the LV is a closed chamber and contracts under a constant volume. Eventually, LV pressure is greater than that in the aorta and the AoV opens

Question 5

Explain systole.

Answer 5

The period of ventricular emptying or “contraction”. The cardiac cycle is the sequence of electrical and mechanical events that occur between one heartbeat and the next. It is the combination of systole and diastole.

Question 6

Explain the four periods of systole.

Answer 6

1. Systolic ejection: period b/w AoV opening and closing. Volume ejected from LV in this phase is the SV (width of pressure volume loop. LV pressure decreases and the AoV closes (ST on ECG) 2. Isovolumetric relaxation: period b/w AoV closing and MV opening. LV is a closed chamber with both the MV and AoV closed. LV relaxes until pressure in the LV drops to that of the LA.MV opens, and blood moves from LA to the LV. 3. Rapid filling: period just after MV opening 4. Slow filling: period just before MV closure

Question 7

What allows the atria to contract before the ventricles?

Answer 7

A delay in the signal transmission of the SA node.

Question 8

Explain what is happening in the shadded box.

Answer 8

Isovolumetric contraction

• The start of ventricular systole
• Abrupt rise in ventricular pressure
• No change in volume

Question 9

Explain what is occurring in the shaded box.

Answer 9

Period of rapid ejection

• Begins when the pressure in the ventricle is greater than the pressure in the artery
• About first 1/3 of ejection time; About 70% of blood exits ventricle.

Question 10

Explain what is happening in the shaded box.

Answer 10

Period of reduced ejection

• About 30% of empyting in the last 2/3 of systole
• Ventricular pressure begins to slightly less than arterial pressure
• Ends in closure of aortic/pulmonary valves.

Question 11

Explain what is happening in the shaded box.

Answer 11

Isovolumetric contraction

• Rapid relaxation and decrease in ventricular pressure
• Ends when ventricular pressure falls to less than the corresponding atrium

Question 12

What occurs during rapid ventricular filling?

Answer 12

• Atrial volumes and pressures increase.
• AV valves open
• Ventricles relax
• Most filling is in the first 1/3 of diastole

Question 13

What happens when pressure in the ventricle is greater than pressure in the artery?

Answer 13

Once Pressure in the left/right ventricle is greater than the aorta/pulmonary artery, arterial wall pressure increases. This occurs around the same time as the period of rapid ejection of the ventricles.

Question 14

What is elastic recoil?

Answer 14

It occurs in the aorta when the kinetic energy of the blood’s momentum is converted into aortic presssure.

Question 15

What is the dicrotic notch?

Answer 15

A sudden decrease and increase in aortic pressure that occurs around 90-100 mmHg. This is a short period of blood backflow followed by a sudden cessation of aortic valve closure.

Question 16

Describe left atrial pressure during each phase of the cardiac cycle.

Answer 16

• The a wave follows the P wave of the ECG and is caused by atrial contraction
• The c wave denotes the onset of ventricular contraction and is caused by a backward

bulge of the AV valves.

• The x descent is a rapid fall in atrial pressure due to ventricular emptying.
• The v wave is a slight rise in atrial pressure during flow of blood into the atria from veins. This occurs during the end of the ventricular systole when the AV valves are closed.
• The y descent occurs due to a rapid fall in atrial pressure due to drainage into the

ventricles.

Question 18

Identify each of the four major heart sounds and associate them with a functional cause for the

sound.

Answer 18

S1 is the closing of the AV valves. Slow and low pitched.

S2 is closing of the aortic and pulmonary valves. Rapid and high frequency.

S3 is the vibration caused by rapid ventricular filling. Not normally heard.

S4 is atrial contraction. Not normally heard.