Lesson 4 - Cardiac Cycle

Question 1

What is represented by the P-wave on the ECG complex?

Answer 1

Electrical impulses originate from the SA-node. These impulses are spread to the atrial
myocardium and result in atrial depolarization.

Question 2

What is represented by a period of electrical silence on the ECG complex known as the PR-segment?

Answer 2

The impulses are rapidly conducted to the AV-node, where it is delayed before passing to the bundle of His. The delay is to allow adequate time for ventricular filling.

Question 3

What is represented by the QRS-complex on the ECG?

Answer 3

Ventricular depolarization. From the bundle of His, impulses are conducted to the left and right bundle branches and then the Purkinje fibers causing ventricular depolarization.

Question 4

What is represented by the T-wave on the ECG?

Answer 4

Ventricular repolarization. Depolarization is followed by repolarization wherein the muscles relax.

Question 5

What is not distinctly
represented on an ECG because of its small electrical signal?

Answer 5

Atrial repolarization

Question 6

What is represented by the U-wave?

Answer 6

An extension of ventricular repolarization (which may or may not be visible).

Question 7

What does the cardiac cycle refer to?

Answer 7

The cardiac cycle refers to one complete cycle of atrial and ventricular contraction and
relaxation. It describes the systolic and diastolic activities of the heart, pressure changes
within the chambers, action of the valves, and blood volume.

Question 8

Describe Atrial Systole.

Answer 8

• the first stage of the cardiac cycle and is initiated by the P-wave of the ECG complex.
• As the atria contracts, the pressure in the atrial chambers increase, and forces more blood
  from the atria passed the atrioventricular valves and into the ventricles.
• Atrial contraction forces the last 10% of blood from the atria to the ventricles as most of the blood flows down passively. This ejection of the last 10% of blood is commonly referred to as the atrial kick.
• At this stage, the atrioventricular valves are open and the semilunar valves are closed.

Question 9

Describe Isovolumetric Ventricular Contraction (or Early Ventricular Systole).

Answer 9

• At this stage, ventricular pressure increases and exceeds the atrial pressure. This stage
  produces a preliminary ventricular contraction that is strong enough to close the AV-valves, but not strong enough to open the semilunar valves.
• The volume of blood in the ventricles remains the same and all valves are closed. The volume of blood in the ventricles at this
  time is referred to as the end-diastolic volume (EDV), which is synonymous with preload.
• End-diastolic volume is therefore defined as the volume of blood in the ventricles before
  ventricular ejection.
• This stage produces the S1 heart sound.

Question 10

Describe Ventricular Ejection (or Late Ventricular Systole).

Answer 10

• During this stage, the atria relax and the ventricles contract. Ejection begins when the
  pressure within the ventricles exceeds the pressure in the aorta and pulmonary artery,
  forcing the semilunar valves open and blood is discharged out of the aorta and pulmonary
  artery.
• Maximal outflow velocity is reached early in this phase. During this stage, the AV valves are closed and the semilunar valves are open.

Question 11

Describe Isovolumetric Ventricular Relaxation (or Early Ventricular Diastole).

Answer 11

• As blood leaves the ventricle, the intraventricular pressure starts to fall. This causes the tail
  end of blood in the aorta and pulmonary artery to flow back into the ventricles. The sinus of
  Valsalva catches the tail end of blood, causing the semilunar valves to close.
• The coronary arteries fill with blood at this time and all valves are closed during this stage. The atria start to receive blood from their venous tributaries but not all of the blood in the ventricles is ejected. Some blood is left over in the ventricles after systole and is referred to as the endsystolic volume (ESV).
• The difference between the end-diastolic volume and end-systolic volume is known as the stroke volume (SV). Stroke volume can be therefore defined as the amount of blood ejected from the ventricles per beat.
• Ejection fraction (EF) represents the volumetric volume of blood pumped out from the right or left ventricles with each heartbeat. It is calculated by dividing the stroke volume by the end-diastolic volume (EF = SV/EDV).
• Normal ejection fraction is ~55-75%.
• This stage produces the S2 heart sound.

Question 12

Describe Rapid Ventricular Filling.

Answer 12

• As the ventricles continue to relax, the intraventricular pressure falls below the atrial
  pressure. The weight of the blood in the atria causes the AV-valves to open and ventricular
  filling begins. As the ventricles fill with blood, the intraventricular pressure gradually rises.
• During this stage, the AV-valves are open and the semilunar valves are closed.

Question 13

Describe Diastasis (or Late Diastole).

Answer 13

• During this stage, the ventricles have passively received 90% of the blood from the atria.
  The ventricles expand and the intraventricular pressure rises.
• The aortic and pulmonary
  pressure continues to fall at this point.
• During this stage, the AV-valves are open and the
  semilunar valves are close.