CB [034] Cardiac cycle 2024-2025

Cardiac Cycle

Overview of the Function of the Heart

The cardiac cycle is a comprehensive process detailing the function of the heart over one complete heartbeat, essential for understanding cardiovascular physiology. It consists of two primary stages: diastole (the relaxation phase) and systole (the contraction phase). Each phase plays a crucial role in maintaining proper blood circulation throughout the body.

Objectives of the Lecture

• Identify the various phases of atrial and ventricular systole and diastole.

• Understand the fluctuations in pressure and volume during each phase of the cardiac cycle.

• Recognize the changes in electrocardiogram (ECG) readings that correspond with each phase.

• Determine the status of vital cardiac valves (atrioventricular and semilunar) during each phase of the cycle.

• Learn the causes behind heart sounds, including normal and abnormal murmurs.

Definition of the Cardiac Cycle

The cardiac cycle refers to the period from the start of one heartbeat to the initiation of the next. It includes:

• Diastole: A phase of relaxation in which the heart muscles relax and the heart fills with blood from the veins.

• Systole: A phase of contraction where cardiac muscles contract to pump blood out of the heart to various organs.

• Atrial systole takes place right before ventricular systole due to the delay occurring at the atrioventricular (AV) node, ensuring efficient blood flow from the atria to the ventricles.

Duration of the Cardiac Cycle

In a healthy adult, the normal heart rate is approximately 75 beats per minute, resulting in a complete cardiac cycle duration of 0.8 seconds. The breakdown of durations for the phases is as follows:

• Atrial systole: 0.1 seconds.

• Atrial diastole: 0.7 seconds.

• Ventricular systole: 0.3 seconds.

• Ventricular diastole: 0.5 seconds. Most coronary artery blood flow occurs during diastole, highlighting the importance of this phase. Additionally, an increased heart rate primarily shortens diastole, which can affect cardiac efficiency.

Phases of the Cardiac Cycle

• Atrial Diastole: Begins during ventricular systole and continues into early ventricular diastole, allowing atria to fill with blood from the veins.

• Atrial Systole: Occurs during late ventricular diastole, topping off the ventricles with an additional volume of blood prior to contraction.

• Ventricular Systole: Comprised of three stages: isovolumetric contraction, where ventricles contract but volume remains constant; maximum ejection, where blood is rapidly ejected; and reduced ejection, where ejection slows down towards the end of systole.

• Ventricular Diastole: Comprises isovolumetric relaxation, maximum filling, and reduced filling phases, highlighting a crucial rest period where the ventricles fill with blood. Importantly, the atria and ventricles never undergo systole simultaneously, maintaining a coordinated contraction-relaxation cycle.

Major Mechanical and Electrical Events

The cardiac cycle is marked by significant mechanical and electrical events that influence:

• Duration of each phase, affecting overall cardiac performance.

• ECG records, providing visual representation of the heart's electrical activity.

• Atrial and ventricular pressures, which determine the efficiency of blood flow.

• Valve openings and closures, crucial for maintaining unidirectional blood flow through the heart.

• Heart sounds correspond directly with valve closures:

  • First Heart Sound (S1): Closure of the AV valves at the start of ventricular systole, typically low pitch and longest in duration.

  • Second Heart Sound (S2): Closure of the semilunar valves at the end of ventricular systole, characterized by a high pitch and shorter duration.

Atrial Diastole

• Duration: 0.7 seconds.

• ECG: Beginning of P wave formation indicating atrial depolarization.

• Atrial pressure remains low, enabling the filling of ventricles. During this time, ventricular pressure may slightly rise due to passive filling, but it stays lower than atrial pressure—facilitating the opening of AV valves.

Atrial Systole

• Duration: 0.1 seconds.

• ECG: Following the P wave, denotes active contraction of the atria.

• This phase contributes around 20% of the blood volume in the ventricles through active atrial contraction. As a result, ventricular volume reaches end-diastolic volume (120-130 ml).

Ventricular Systole: Isovolumetric Contraction

• Duration: 0.05 seconds.

• ECG: Follows the QRS complex, marking ventricular depolarization.

• During this phase, ventricular pressure rises sharply while volume remains constant since all valves are closed. Its completion results in the closure of the AV valves, producing the first heart sound.

Ventricular Systole: Ejection Phases

1. Maximum Ejection Phase: Duration 0.15 seconds, characterized by rapid ejection of blood into the aorta and pulmonary artery.

2. Reduced Ejection Phase: Duration 0.1 seconds, where blood ejection slows as the pressure difference decreases.

• ECG: T wave corresponds with the completion of ventricular repolarization and reflects this phase. Peak ventricular pressure can reach approximately 120 mmHg. The end-systolic volume of blood remaining in the ventricles is about 50 ml.

Ventricular Diastole: Isovolumetric Relaxation

• Duration: 0.1 seconds.

• ECG: T wave ends, indicating ventricular relaxation.

• Ventricular pressure significantly decreases while volume remains unchanged due to all valves being closed. The closure of semilunar valves takes place during this phase, producing the second heart sound.

Ventricular Diastole: Filling Phases

1. Rapid Filling Phase: Duration 0.1 seconds, marked by significant passive filling of the ventricles due to low pressure in the ventricles.

2. Reduced Filling Phase: Duration 0.2 seconds, where filling slows as pressures begin to equilibrate.

• Ventricular volume increases to reach end-diastolic volume.

Heart Sounds

• 1st Heart Sound (S1): Closure of AV valves, perceived as low pitch and lasting the longest duration among heart sounds.

• 2nd Heart Sound (S2): Closure of semilunar valves, perceived as high pitch but with a shorter duration.

• Intervals between heart sounds are crucial for distinguishing between systole and diastole.

Abnormal Heart Sounds and Murmurs

• Murmur: An abnormal heart sound often arising from turbulent blood flow, indicting underlying heart conditions.

• Stenosis: A pathological narrowing of the valve that typically produces harsh murmurs.

• Regurgitation: Occurs when valves do not close properly, leading to soft murmurs and possible backflow of blood.

References

• Adapted from "Human Physiology" by Pocock, Richards & Richards, 4th edition.

• Guyton & Hall, "Textbook of Medical Physiology", 11th edition.