Cardiac Cycle

The cardiac cycle is the sequence of events that occur in the heart during one complete heartbeat, starting with the contraction (systole) of the ventricles, followed by the relaxation (diastole) of the ventricles. The cycle includes the filling of the ventricles with blood during diastole and the ejection of blood into the circulatory system during systole. The cardiac cycle is regulated by electrical impulses in the heart and is essential for providing the body with a continuous supply of oxygenated blood.

Systole

Systole refers to the phase of the cardiac cycle where the heart muscle contracts and pumps blood out of the heart. During systole, the ventricles of the heart (the lower chambers) contract and squeeze the blood they contain into the aorta and pulmonary artery, which then distribute the blood to the rest of the body. Systole is the active phase of the cardiac cycle, and it is followed by diastole, the period of relaxation and filling of the heart with blood. The alternating cycle of systole and diastole constitutes one complete heartbeat.

Diastole

Diastole refers to the phase of the cardiac cycle where the heart muscle relaxes and fills with blood. During diastole, the ventricles of the heart (the lower chambers) expand and receive blood from the atria (the upper chambers) and the blood vessels that enter the heart. Diastole is the passive phase of the cardiac cycle, and it follows systole, the period of contraction and pumping of the heart. The duration and pressure of diastole determine the amount of blood that is filled in the heart and subsequently pumped out during systole.

Electrocardiogram (ECG)

The cardiac cycle can be represented on an electrocardiogram (ECG) as a series of waves and deflections that reflect the electrical activity of the heart. The ECG provides a visual representation of the depolarisation and repolarisation of the heart during each heartbeat, allowing for the assessment of the heart’s rhythm and function.

The ECG trace of a typical cardiac cycle consists of five waves and intervals, including:

The P wave, which represents the depolarisation of the atria and the start of atrial contraction.
The PR interval, which represents the time it takes for the electrical signal to travel from the atria to the ventricles.
The QRS complex, which represents the depolarisation of the ventricles and the start of ventricular contraction. The QRS complex is typically the largest waveform on the ECG trace.
The ST segment, which represents the time between ventricular contraction and repolarisation.
The T wave, which represents the repolarisation of the ventricles and the end of the cardiac cycle.

Analysis of the timing, duration and sequence of events in the cardiac cycle, allows for the detection of any abnormal electrical activity or rhythm disturbances.

Cardiac Conduction Cycle

The cardiac conduction cycle refers to the sequence of electrical events that occur in the heart, triggering the contraction of the heart muscle and producing a heartbeat. The cardiac conduction cycle starts in the sinoatrial (SA) node, which is the natural pacemaker of the heart, and spreads throughout the atria and ventricles, causing them to contract in a co-ordinated manner.

The cardiac conduction cycle can be broken down into three main stages:

Depolarisation: The electrical signal from the SA node spreads throughout the atria, causing them to contract and pump blood into the ventricles.
Conduction: The electrical signal then travels to the atrioventricular (AV) node, which serves as a relay centre, and then to the Bundle of HIS and the Purkinje fibres, which conduct the electrical signal to the ventricles, causing them to contract and pump blood into the circulation.
Repolarisation: After contraction, the heart muscle returns to its resting state during repolarisation, which is the electrical event that signals the end of the contraction phase.

Depolarisation

Depolarisation refers to the phase of the cardiac cycle in which the heart muscle changes from a resting state to an excited state, resulting in contraction. Depolarisation is the electrical event that triggers the heart muscle to contract and pump blood.

During depolarisation, positively charged ions such as sodium (Na⁺) and calcium (Ca²⁺) enter the heart muscle cells, causing a rapid increase in the electrical potential of the cell. This change in electrical potential leads to the initiation of an action potential, which spreads rapidly throughout the heart muscle, triggering contraction.

Depolarisation is an essential component of the cardiac cycle, as it leads to the pumping of blood from the heart and the distribution of oxygen and nutrients to the body. Abnormalities in depolarisation can result in arrhythmias and other cardiac rhythm disturbances.

Repolarisation

Repolarisation refers to the phase of the cardiac cycle in which the heart muscle returns to its resting state after contraction. Repolarisation is the electrical event that follows depolarisation and signals the end of the contraction phase of the heart muscle.

During repolarisation, positively charged ions such as potassium (K⁺) leave the heart muscle cells, causing a decrease in the electrical potential of the cell and returning it to its resting state. This change in electrical potential signals the end of the action potential and the return of the heart muscle to its relaxed state.

Repolarisation is an essential component of the cardiac cycle, as it allows the heart to refill with blood and prepare for the next contraction.

Cardiac Action Potential

A cardiac action potential is the electrical event that occurs in the heart muscle cells that triggers the contraction and relaxation of the heart. The cardiac action potential is a change in the electrical potential of the heart muscle cells that results in depolarisation and repolarisation of the cells.

The cardiac action potential can be divided into five phases:

Phase 0 (Depolarisation): This is the rapid upstroke phase where positively charged ions, such as sodium (Na⁺), enter the heart muscle cells, causing a rapid increase in the electrical potential of the cell and triggering an action potential.
Phase 1 (Early Repolarisation): This is a rapid decline in the electrical potential of the cell, caused by the efflux of positively charged ions, such as potassium (K⁺).
Phase 2 (Plateau): This is a plateau phase where the electrical potential of the cell remains relatively stable for a brief period.
Phase 3 (Repolarisation): This is the rapid decline in the electrical potential of the cell, caused by the efflux of positively charged ions, such as potassium (K⁺).
Phase 4 (Resting): This is the resting phase where the electrical potential of the cell is at its normal resting state, preparing for the next depolarisation.

The cardiac action potential is an essential component of the cardiac cycle, as it triggers the contraction and pumping of blood from the heart. Abnormalities in the cardiac action potential can result in arrhythmias and other cardiac rhythm disturbances.