1.2 The heart (electrical activity) --> Part 2

Electrical activity of the heart

A part of the cardiovascular system that controls when the heart beats and pumps blood.

Another name for the electrical activity of the heart

The conduction system

Key components of the conduction system

• Sino-atrial (SA) node

• Atrioventricular (AV) node

• The bundle of His

• Purkinje fibres

Another spelling of ‘Purkinje’

Purkyne

Sinoatrial node

Often referred to as the heart’s peacemaker, located in the upper wall of the right atrium.

Functions of the sinoatrial node

• Regulates the heart rate in line with the body’s demand by sending out electrical impulses

• The impulses which travel across the muscle cells in the atria, cause them to contract

Atrioventricular node

A cluster of cells located at the bottom of the right atrium, near the ventricles.

Functions of the atrioventricular node

• Receives impulses from the SA node and delays it

• The delay ensures the atria have completely emptied their blood into the ventricles, before the ventricles contract

Bundle of His

A specialised pathway of fibres extending from the AV node down into the interventricular septum

Functions of bundle of his

• It is the only electrical bridge between the upper and lower heart chambers

• The impulses travel down the bundle of his from the AV node

• The bundle of his splits into right and left branches to carry the impulses down to the ventricles

Purkinje fibres

A complex, branching network of fibres that fan upward and outward from the bundle branches, into the muscular walls of both ventricles

Functions of the Purkinje fibres

• They rapidly transmit the impulses throughout the entire ventricular muscle mass

• Allows the heart’s conduction system to create synchronised contractions of its ventricles

• Therefore, they are essential for maintaining a consistent heart rhythm

Atrial depolarisation

The effect the SA node has on the atria, causing them to contract by providing an electrical stimulus across them

Ventricular depolarisation

The effect the AV node has on the ventricles, causing them to contract by providing an electrical stimulus

Atrial and ventricular repolarisation

Occurs during a brief time period following depolarisation and is the return of the heart’s electrical impulse to a baseline value

ECG

Electrocardiogram

Electrocardiogram

A test that that checks how well the heart is working by measuring the electric signals that travel through the heart.

How electrocardiogram works

• During a test, small sticky patches called electrodes are placed on the arm, chest and legs

• These electrodes pick up the electrical signals and send them to a machine that makes a pattern of the heart’s activity

• The pattern helps the doctor see if the heart is healthy or not

What ECG measures

Heart rate, rhythm and timing of electrical impulses

What ECG shows electrical impulses as

Waves

The waves ECG shows

P, Q, R, S and T waves

P wave

Represents the electrical impulse that triggers atrial contraction

QRS complex

Represents the electrical activity of the ventricles contracting

T wave

Represents the heart’s full electrical reset, preparing it for the next cardiac cycle

ST segment

Marks the end of ventricular contraction and the beginning of the heart’s recovery phase

Q-T interval

It measures the time it takes for the ventricles to contract and then start to recover

T-P interval

It measures the time when the ventricles are relaxed and filling up with blood. It is the period between the recovery phase (T wave) and the next electrical signal starting (P wave).

If the wave is irregular, absent or abnormal, it may suggest issues with the atrial contraction.

P wave abnormalities

If the wave is long or distorted, it can suggest issues with ventricular conduction or a blockage within the bundle of His.

QRS complex abnormalities

If raised or reduced, it can indicate ischemia or heart attack.

ST segment abnormalities

If the wave is inverted or unusually tall, it can signal electrolyte imbalances or ischemia.

T wave abnormalities

How heart conditions like; arrhythmias and fibrillation, are diagnosed using ECG

Patterns are studied in different conditions and compared to the standard ECG.

How the heart condition fibrillation is stopped

• Passing a strong electric current through the chest wall.

• Then the heart stops for up to 5 seconds after which it begins to beat in a controlled way.

How the heart rate can be modified

Heart rate can be modified by the nervous system and hormones.

How the nervous system modifies heart rate

• Sympathetic Nervous System: Fight or flight increases heart rate

• Parasympathetic Nervous System: Rest and digest decreases heart rate

How hormones modifies heart rate

Adrenaline and thyroxine both increases heart rate