Electrical activity of the heart

An electrical signal known as Action Potential (AP) initiates and organizes the contractile sequence.

All vertebrate hearts are myogenic and will continue beating even if all nervous connections are removed.

ECG waveforms correspond with the electrical activity of the heart.

Blood flows through the left AV valve to the left ventricle.

Deoxygenated blood returns from systemic circulation to the right atrium via vena cavae.

ECG waveforms correspond with the electrical activity of the heart.

Sinus tachycardia is characterized by a normal ECG except that T waves overlap P waves, and it is probably initiated by SA node pacemaker cells. The heart rate is approximately 235 bpm, which is extremely rapid for a resting dog.

Cardiac muscle is myogenic, meaning the initiation of an action potential is spontaneous and does not require input from the nervous system.

The Bundle of His penetrates the annulus fibrosis and divides into left and right bundle branches, facilitating the conduction of action potentials to the ventricles.

It transmits depolarisation between the atria and ventricles, enabling a coordinated response.

It is the period during which cells cannot be depolarised again until they return to resting potential after repolarisation.

Blood flows through the right AV valve to the right ventricle.

Intercalated discs allow rapid conduction of the depolarization between cells.

The heart propels blood by alternately contracting and relaxing.

Spontaneous depolarization of autorhythmic cells occurs in the sinoatrial node (SAN), which contains 'pacemaker' cells.

2 or more electrodes are applied to the skin surface either side of the heart. Voltages recorded between a pair of electrodes (+ ve and - ve) are displayed on a video screen or printed on a strip of paper.

Depolarisation spreads rapidly through atrial muscle.

It allows time for filling of the cardiac chambers and coordinated muscle contraction.

If the SAN becomes dysfunctional, the Atrioventricular Node (AVN) will take over the role of pacemaker.

The delay ensures adequate time for ventricular filling.

The functional network is called a syncitium, where cells contract in synchrony as a unit.

A system of limb leads that record electrical activity reaching the body surface in the horizontal (frontal) plane, with the heart assumed to be located approximately in the center of an equilateral triangle formed by the two front limbs and the left hind limb.

Purkinje fibres form an extensive network that branches into the ventricular myocardium, allowing rapid transmission of action potentials to both ventricles, with larger animals having thicker fibres that conduct action potentials more rapidly.

T wave morphology is extremely variable due to changes in the pattern of repolarisation.

Atrial fibrillation is associated with large atria and high vagal tone, and may also involve mitral insufficiency.

A single lead is required.

Depolarisation occurs most rapidly in the Sinoatrial Node (SAN).

Depolarisation enters and traverses the A-V node relatively slowly.

HR = number of R - R intervals in 6 seconds x 10

The delay is important to ensure proper filling of the ventricles before they contract.

There is decreased parasympathetic activation during inspiration.

Alterations in conductivity between heart and skin.

The six standardised ECG leads are typically the limb leads (I, II, III) and the augmented leads (aVR, aVL, aVF).

Oxygenated blood enters the left atrium via pulmonary veins.

Atrial fibrillation is common in horses and results in poor performance due to reduced cardiac output.

Depolarisation begins in the S-A node.

Dr. Willem Einthoven.

The right ventricle pumps blood through the pulmonary valve to the lungs via the pulmonary trunk.

The annulus fibrosus separates the atria and ventricles.

Atrial muscle begins to repolarise nearly simultaneously as depolarisation spreads into the ventricles.

Lead

It leads to forceful ventricular contraction.

6 standardized leads: I, II, III, aVR, aVL, aVF

All deviations from the normal regular rhythm are termed arrhythmias.

Yes, this indicates a normal relationship between atrial and ventricular activity.

The A-V node is a collection of modified cells that conducts action potentials more slowly than ordinary muscle cells, ensuring a delay in impulse transmission between the atria and ventricles for adequate filling before contraction.

The sinoatrial node (SAN) initiates action potentials that are transmitted throughout the contractile cells of the heart, resulting in contraction.

Specialised conducting cells that conduct the electrical impulse from the atria to the ventricles.

Base - Apex configuration.

It ensures that the heart muscle cannot develop a tetanic spasm and allows for one-way conduction.

Depolarisation spreads down the conduction system faster than it could through ordinary ventricular muscle.

High vagal (parasympathetic) tone.

A naturally occurring variation in heart rate that occurs during the breathing cycle.

They can result in tachycardia.

The two types of cardiac cells are autorhythmic cells and contractile cells.

The ECG detects and amplifies the tiny electrical charges on the skin caused by the depolarisation of cardiac muscle during each heartbeat.

The left ventricle pumps blood through the aortic valve to systemic circulation via the aorta.

A non-invasive measurement of the electrical activity of the heart.

Autorhythmic cells have the ability to spontaneously depolarise.

Premature beats.

The negative electrode should be above and cranial to the heart.

SAN, atrial myocardium, AVN (junctional), and ventricular myocardium.

1st degree AV block and 2nd degree AV block.

A condition where different parts of the ventricular myocardium contract and relax at random, leading to the failure of the ventricles to pump blood.

There is increased parasympathetic activation during expiration.

Different leads examine the heart from different angles and help determine which area of the heart is affected.

2 major divisions = 1mV

The Nobel Prize in Physiology/Medicine in 1924.

Sinus bradycardia is characterized by a normal ECG but with a heart rate of only approximately 55 bpm, indicating that the SA node pacemaker is abnormally slow.

Cardiac cell depolarisation and repolarisation.

A difference in electrical potential is created between parts of the muscle that are depolarised and those that are not.

Count the number of R-R intervals in 6 seconds and multiply by 10.

The voltage difference created during depolarization sets up ionic currents in the tissues and body fluids surrounding the heart.

No, there are no electrical connections (gap junctions) that cross the fibrous layer.

It results in two functional syncitia.

Arrhythmias occur due to disturbances in impulse generation or impulse conduction.

Yes, consistent and reasonable relation indicates a stable rhythm.

Yes, uniformity in P-waves and QRS complexes suggests a normal rhythm.

Determining if the rhythm is regular or irregular helps in diagnosing the type of arrhythmia.

The RR interval measures the time between ventricular depolarisations and is used to calculate the ventricular rate.

Normal sinus rhythm is the heart rhythm originating from the sinoatrial node, characterized by a regular rate of 60 to 100 beats per minute and a consistent pattern of P waves preceding each QRS complex.

A ventricular ectopic beat is a premature ventricular depolarisation.

The positive electrode should be below and caudal to the heart.

A supraventricular (atrial) ectopic beat is a premature atrial depolarisation.

If transmission of action potentials is blocked, action potentials will be generated in the bundle of His or Purkinje network, resulting in an escape rhythm.

It rapidly results in death (sudden death) if not treated.

An ECG measures the voltage differences over time that are induced by the depolarization of cardiac muscle.

The ventricular myocardium lying nearest the ventricular chambers depolarises first due to its proximity to the cardiac conduction system.

Yes, this suggests that every atrial contraction is followed by a ventricular contraction.

The QRS duration measures the time required for ventricular depolarisation.

They measure voltages from one limb compared to the average of the other two.

The PP interval measures the time between atrial depolarisations and is used to calculate the atrial rate.

Atrioventricular node (AV node).

Rate, rhythm, and nature of cardiac depolarisation and repolarisation.

Depolarisation does not pass through the fibrous connective tissue layer.

Changes in myocardial mass.

Bradycardia (slow) and Tachycardia (fast).

The main components of the ECG include the P wave, QRS complex, and T wave.

Sinoatrial node (SA node).

Atrial depolarization.

It represents the time taken for the electrical impulse to travel from the SA node to the AV node.

Ventricular depolarization.

Abnormal ECG voltages are indicative of structural or electrical abnormalities.

The long refractory period prevents the reactivation of the atria.

It can only be reversed by defibrillation.

Additional chest leads are used to detect areas of abnormal conduction caused by infarcts.

25mm/s and 50mm/s

5 major divisions = 1 sec.

10 major divisions = 1 sec.

It helps to spread out ECG events in animals with rapid heart rates.

In a normal heart, the PP interval and RR interval should be equal.

Left and Right

The conducting system acts as an 'electrical window' through the fibrous layer.

The further away from the SAN, the slower the heart rate will be.

Metabolic abnormalities affecting the myocardium.

The electrical activity of the heart.

Augmented limb leads, also known as Goldberger’s leads, include aVR, aVL, and aVF.

The formula is 1500 divided by the R-R interval.

The instantaneous heart rate is 30 bpm.

The formula is 3000 divided by the R-R interval.

The PR interval is the time between the start of atrial depolarisation and the start of ventricular depolarisation, reflecting A-V node conduction velocity.

They are derived from the same 3 electrodes as leads I, II, and III.

The QT interval is the time from the start of ventricular depolarisation to the end of ventricular repolarisation.

They provide views of the heart from different angles (vectors).

The Q-T interval represents the time from the beginning of the Q wave to the end of the T wave.

The QRS complex represents the depolarization of the ventricles.

The P wave represents the depolarization of the atria.

The T wave indicates the repolarization of the ventricles.

The P-R interval is the time from the beginning of the P wave to the beginning of the QRS complex.