The Cardiac Cycle: A Comprehensive Study Guide

Introduction

The cardiac cycle is the sequence of events that occurs in the heart during a single heartbeat. It involves the contraction (systole) and relaxation (diastole) of the atria and ventricles, the opening and closing of the heart valves, and the flow of blood through the heart. The entire cycle for a normal heart rate of ~75 beats per minute (bpm) is approximately 0.8 seconds.

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Phases of the Cardiac Cycle

The cardiac cycle is primarily discussed in terms of ventricular activity because the ventricles are responsible for pumping blood to the lungs and the rest of the body.

1. Atrial Systole (0.1 seconds)

• Event: The atria contract.
• Action: This pushes the final ~30% of blood into the relaxed ventricles.
• Valves:
  • Atrioventricular (AV) valves (tricuspid and mitral) are open.
  • Semilunar valves (aortic and pulmonary) are closed.
• ECG Correlation: Occurs just after the P wave.

2. Ventricular Systole (0.3 seconds)

This phase is divided into two sub-phases:

a. Isovolumetric Contraction (0.05 seconds)

• Event: The ventricles begin to contract.
• Action: Ventricular pressure rises sharply, but there is no change in blood volume because all valves are closed.
• Valves: All valves are closed.
• Heart Sound: The first heart sound, S1 ("LUB"), is produced by the closure of the AV valves.

b. Ventricular Ejection (0.25 seconds)

• Event: Ventricular pressure becomes greater than the pressure in the aorta and pulmonary artery.
• Action: The semilunar valves open, and blood is forcefully ejected from the ventricles.
• Valves:
  • Semilunar valves are open.
  • AV valves are closed.
• ECG Correlation: Starts after the QRS complex.

3. Ventricular Diastole (0.5 seconds)

This is the relaxation phase of the ventricles and is divided into three sub-phases:

a. Isovolumetric Relaxation (0.08 seconds)

• Event: The ventricles relax.
• Action: Ventricular pressure falls rapidly, but the volume of blood in the ventricles remains unchanged.
• Valves: All valves are closed.
• Heart Sound: The second heart sound, S2 ("DUB"), is produced by the closure of the semilunar valves.

b. Rapid Filling Phase (0.11 seconds)

• Event: Ventricular pressure drops below atrial pressure.
• Action: The AV valves open, and blood that has been accumulating in the atria rushes into the ventricles.
• Valves: AV valves are open.

c. Slow Filling / Diastasis (0.19 seconds)

• Event: The filling of the ventricles continues, but at a slower rate.
• Action: Both the atria and ventricles are relaxed during this phase. This sets the stage for the next atrial systole.

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Key Concepts

Joint Diastole (Complete Diastole)

• What is it? The period when both the atria and ventricles are in a state of relaxation.
• Duration: Approximately 0.4 seconds, making it the longest phase of the cardiac cycle.
• When does it occur? It starts after the second heart sound (S2) and continues until the beginning of the next atrial systole.
• Importance: This is the primary "refill and reset" phase for the heart. It is also when the coronary arteries receive most of their blood flow.

Dicrotic Notch (Incisura)

• What is it? A small, brief dip in the aortic pressure curve that occurs shortly after the aortic valve closes.
• Cause: When the left ventricle relaxes, the pressure in the ventricle drops. The aortic valve snaps shut, causing a momentary backflow of blood that rebounds against the closed valve, creating a small increase in aortic pressure.
• Significance: It is a mechanical event that marks the closure of the aortic valve and is associated with the second heart sound (S2).

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Heart Sounds: "Lub" and "Dub"

The two main heart sounds are caused by the closure of the heart valves:

• S1 ("Lub"): The first heart sound is caused by the closure of the AV valves (mitral and tricuspid) at the beginning of ventricular systole.
• S2 ("Dub"): The second heart sound is caused by the closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole.

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The Aortic Pressure Curve

This graph shows the pressure changes within the aorta throughout the cardiac cycle.

• Y-axis: Pressure (in mmHg)
• X-axis: Time

Key Features of the Aortic Pressure Curve:

1. Peak Aortic Pressure: The highest pressure reached in the aorta during ventricular ejection, typically around 120 mmHg. This corresponds to systolic blood pressure (SBP).
2. Dicrotic Notch: The small dip in pressure that occurs when the aortic valve closes.
3. Diastolic Pressure: The lowest pressure in the aorta, just before the next ventricular ejection, typically around 80 mmHg. This corresponds to diastolic blood pressure (DBP).

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Summary Table of the Cardiac Cycle

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Clinical Correlations and Fun Facts

• Atrial contraction only contributes about 20-30% of the blood to the ventricles; the rest fills passively.
• The heart spends more time in diastole than in systole, which is crucial for the heart muscle to receive its own blood supply through the coronary arteries.
• During exercise, when the heart rate increases, the duration of diastole shortens more significantly than systole.