Cardiac Cycle

What is the cardiac cycle?

The complete sequence of events between two consecutive heartbeats.

What initiates each heartbeat?

An action potential generated by the SA node.

What is the natural pacemaker of the heart?

The sinoatrial (SA) node.

Where is the SA node located?

In the posterior wall of the right atrium.

What is the role of the AV node?

Delays the electrical impulse before it reaches the ventricles.

How long is the AV nodal delay?

Approximately 100 ms.

Why is the AV nodal delay important?

It allows the atria to contract before the ventricles.

What is the conduction pathway through the heart?

SA node → Internodal pathways → AV node → Bundle of His → Bundle branches → Purkinje fibres.

What does the P wave represent?

Atrial depolarisation.

What does the QRS complex represent?

Ventricular depolarisation.

What does the T wave represent?

Ventricular repolarisation.

What does the U wave represent?

Papillary muscle repolarisation.

What determines arterial blood pressure?

Cardiac output × resistance.

State the equation for cardiac output.

CO = HR × SV.

Why do cardiac pressures fluctuate?

Because blood volume and flow change throughout the cardiac cycle.

What is a pressure waveform?

A graphical representation of pressure changes during the cardiac cycle.

What causes the left atrial v wave?

Filling of the left atrium during ventricular systole.

What can a large v wave indicate?

Mitral regurgitation, elevated atrial pressure, or atrial septal defect.

What causes the left atrial a wave?

Atrial contraction.

What can a large a wave indicate?

Mitral stenosis or reduced atrial compliance.

What causes the c wave in left atrial pressure?

Closure of the mitral valve.

What causes the x descent in atrial pressure?

Atrial relaxation.

What causes the y descent in atrial pressure?

Passive emptying of the atrium into the ventricle after mitral valve opening.

What is the normal mean left atrial pressure?

2–12 mmHg.

What is the normal left atrial a-wave pressure?

4–16 mmHg.

What is the normal left atrial v-wave pressure?

6–21 mmHg.

What is the normal left ventricular end-diastolic pressure?

5–12 mmHg.

What is the normal peak left ventricular systolic pressure?

90–140 mmHg.

When does the aortic valve open?

When left ventricular pressure exceeds aortic pressure.

When does the aortic valve close?

When aortic pressure exceeds left ventricular pressure.

Why does left ventricular pressure rise rapidly during systole?

Ventricular depolarisation triggers contraction.

Why does ventricular pressure rise only slightly during passive filling?

The ventricle is compliant and still relaxing.

What causes the a wave on the ventricular pressure trace?

Atrial contraction completing ventricular filling.

What causes the ascending limb of the aortic pressure waveform?

Blood ejection from the left ventricle.

What causes the dicrotic notch?

Closure of the aortic valve.

What is pulse pressure?

The difference between systolic and diastolic arterial pressure.

What does pulse pressure reflect?

Stroke volume and arterial compliance.

Why does aortic pressure remain relatively high during diastole?

Elastic recoil and vasomotor tone maintain pressure.

What are the six phases of the cardiac cycle?

Atrial contraction, isovolumetric contraction, ejection, isovolumetric relaxation, rapid filling, diastasis.

What happens during atrial contraction?

The atria contract and provide the final ~10% of ventricular filling.

How much ventricular filling occurs due to atrial contraction at rest?

Approximately 10%.

What happens during isovolumetric contraction?

Ventricles contract while all valves are closed.

Which valves are closed during isovolumetric contraction?

Mitral, tricuspid, aortic, and pulmonary valves.

Why is it called isovolumetric contraction?

Pressure rises but ventricular volume does not change.

What event marks the start of ejection?

Opening of the aortic and pulmonary valves.

What happens during ventricular ejection?

Blood is expelled into the aorta and pulmonary artery.

What happens during isovolumetric relaxation?

Ventricles relax while all valves are closed.

Why is ventricular volume unchanged during isovolumetric relaxation?

All valves remain closed.

What event marks the start of rapid filling?

Opening of the mitral and tricuspid valves.

What happens during rapid filling?

Blood accumulated in the atria rapidly enters the ventricles.

What is diastasis?

A phase of reduced ventricular filling after pressures equalise.

What happens to blood flow during diastasis?

Filling slows significantly.

Which phase follows diastasis?

Atrial contraction.

At point 1 on the pressure-volume diagram, what happens?

Mitral valve closes.

At point 2 on the pressure-volume diagram, what happens?

Aortic valve opens.

At point 3 on the pressure-volume diagram, what happens?

Aortic valve closes.

At point 4 on the pressure-volume diagram, what happens?

Mitral valve opens.

Between points 1 and 2, which phase occurs?

Isovolumetric contraction.

Between points 2 and 3, which phase occurs?

Ejection.

Between points 3 and 4, which phase occurs?

Isovolumetric relaxation.

Between points 4 and 1, which phase occurs?

Ventricular filling.

What happens to ventricular volume between points 1 and 2?

It remains constant.

What happens to ventricular volume between points 3 and 4?

It remains constant.

When does ventricular systole occur on the pressure diagram?

Between points 1 and 3.

When does ventricular diastole occur on the pressure diagram?

Between points 3 and 1.

Which valve closes at the beginning of systole?

Mitral valve.

Which valve opens during systole?

Aortic valve.

Which valve closes at the end of systole?

Aortic valve.

Which valve opens during diastole?

Mitral valve.

What is meant by isovolumetric?

Same volume.

Which cardiac cycle phase corresponds to the P wave?

Atrial contraction.

Which cardiac cycle phase begins shortly after the QRS complex?

Isovolumetric contraction.

Which phase occupies most of the ST segment?

Ventricular ejection.

Which ECG wave occurs during ventricular repolarisation?

T wave.

Which cardiac cycle phase follows the T wave?

Isovolumetric relaxation.

Why must atrial contraction occur before ventricular contraction?

To maximise ventricular filling.

Why is ventricular filling mostly passive?

Ventricular relaxation creates a pressure gradient favouring inflow.

What generates the left atrial v wave?

Venous filling against a closed mitral valve.

What generates the left atrial a wave?

Atrial systole.

What is the significance of a large left atrial a wave?

Increased resistance to ventricular filling.

What is the significance of a large left atrial v wave?

Increased atrial filling pressure or mitral regurgitation.

What keeps blood flowing during diastole despite no ventricular ejection?

Elastic recoil of arteries.

What happens first: atrial depolarisation or atrial contraction?

Atrial depolarisation.

What happens first: ventricular depolarisation or ventricular contraction?

Ventricular depolarisation.

What event causes mitral valve closure?

Left ventricular pressure exceeding left atrial pressure.

What event causes mitral valve opening?

Left atrial pressure exceeding left ventricular pressure.

What event causes aortic valve opening?

Left ventricular pressure exceeding aortic pressure.

What event causes aortic valve closure?

Aortic pressure exceeding left ventricular pressure.

Which side of the heart follows the same cardiac cycle pattern?

Both right and left sides.

Why are right-sided pressures lower than left-sided pressures?

The pulmonary circulation has much lower resistance than the systemic circulation.