What should the nurse include as the correct sequence of the transmission of electrical impulses?

A Beginners Guide to Normal Heart Function, Sinus Rhythm & Common Cardiac Arrhythmias

Cardiac Conduction System

Going back to the analogy of the central heating system, the pump, pipes and radiators are of no use unless connected to a power supply. The pump needs electricity to work. The human heart has a similar need for a power source and also uses electricity. Thankfully we don't need to plug ourselves in to the mains, the heart is able to create it's own electrical impulses and control the route the impulses take via a specialised conduction pathway.

This pathway is made up of 5 elements:

1. The sino-atrial (SA) node
2. The atrio-ventricular (AV) node
3. The bundle of His
4. The left and right bundle branches
5. The Purkinje fibres

Image: The cardiac conduction system

The SA node is the natural pacemaker of the heart. You may have heard of permanent pacemakers (PPMs) and temporary pacing wires (TPWs) which are used when the SA node has ceased to function properly.

The SA node releases electrical stimuli at a regular rate, the rate is dictated by the needs of the body. Each stimulus passes through the myocardial cells of the atria creating a wave of contraction which spreads rapidly through both atria.

As an analogy, imagine a picture made up of dominoes. One domino is pushed over causing a wave of collapsing dominoes spreading out across the picture until all dominoes are down.

The heart is made up of around half a billion cells, In the picture above you can see the difference in muscle mass of the various chambers. The majority of the cells make up the ventricular walls. The rapidity of atrial contraction is such that around 100 million myocardial cells contract in less than one third of a second. So fast that it appears instantaneous.

The electrical stimulus from the SA node eventually reaches the AV node and is delayed briefly so that the contracting atria have enough time to pump all the blood into the ventricles. Once the atria are empty of blood the valves between the atria and ventricles close. At this point the atria begin to refill and the electrical stimulus passes through the AV node and Bundle of His into the Bundle branches and Purkinje fibres.

Imagine the bundle branches as motorways, if you like, with the Purkinje fibres as A and B roads that spread widely across the ventricles . In this way all the cells in the ventricles receive an electrical stimulus causing them to contract.

Using the same domino analogy, around 400 million myocardial cells that make up the ventricles contract in less than one third of a second. As the ventricles contract, the right ventricle pumps blood to the lungs where carbon dioxide is released and oxygen is absorbed, whilst the left ventricle pumps blood into the aorta from where it passes into the coronary and arterial circulation.

At this point the ventricles are empty, the atria are full and the valves between them are closed. The SA node is about to release another electrical stimulus and the process is about to repeat itself. However, there is a 3rd section to this process. The SA node and AV node contain only one stimulus. Therefore every time the nodes release a stimulus they must recharge before they can do it again.

Imagine you are washing your car and have a bucket of water to rinse off the soap. You throw the bucket of water over the car but find you need another one. The bucket does not magically refill. You have to pause to fill it.

In the case of the heart, the SA node recharges whilst the atria are refilling, and the AV node recharges when the ventricles are refilling. In this way there is no need for a pause in heart function. Again, this process takes less than one third of a second.

The times given for the 3 different stages are based on a heart rate of 60 bpm , or 1 beat per second.

The term used for the release (discharge) of an electrical stimulus is "depolarisation", and the term for recharging is "repolarisation".

So, the 3 stages of a single heart beat are:

1. Atrial depolarisation
2. Ventricular depolarisation
3. Atrial and ventricular repolarisation.

As the atria repolarise during ventricular contraction, there is no wave representing atrial repolarisation as it is buried in the QRS.

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Have you ever wondered what causes your heart to beat? Your heart beats as a result of the generation and conduction of electrical impulses. Cardiac conduction is the rate at which the heart conducts electrical impulses. These impulses cause the heart to contract and then relax. The constant cycle of heart muscle contraction followed by relaxation causes blood to be pumped throughout the body. Cardiac conduction can be influenced by various factors including exercise, temperature, and endocrine system hormones.

The first step of cardiac conduction is impulse generation. The sinoatrial (SA) node (also referred to as the pacemaker of the heart) contracts, generating nerve impulses that travel throughout the heart wall. This causes both atria to contract. The SA node is located in the upper wall of the right atrium. It is composed of nodal tissue that has characteristics of both muscle and nervous tissue.

The atrioventricular (AV) node lies on the right side of the partition that divides the atria, near the bottom of the right atrium. When the impulses from the SA node reach the AV node, they are delayed for about a tenth of a second. This delay allows atria to contract and empty their contents into the ventricles prior to ventricle contraction.

The impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.

At the base of the heart, the atrioventricular bundles start to divide further into Purkinje fibers. When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta.

Cardiac conduction is the driving force behind the cardiac cycle. This cycle is the sequence of events that occur when the heart beats. During the diastole phase of the cardiac cycle, the atria and ventricles are relaxed and blood flows into the atria and ventricles. In the systole phase, the ventricles contract sending blood to the rest of the body.

Disorders of the heart's conduction system can cause problems with the heart's ability to function effectively. These problems are typically the result of a blockage that diminishes the rate of speed at which impulses are conducted. Should this blockage occur in one of the two atrioventricular bundle branches that lead to the ventricles, one ventricle may contract more slowly than the other. Individuals with bundle branch block typically don't experience any symptoms, but this issue can be detected with an electrocardiogram (ECG). A more serious condition, known as heart block, involves the impairment or blockage of electrical signal transmissions between the heart's atria and ventricles. Heart block electrical disorders range from first to third degree and are accompanied by symptoms ranging from light-headedness and dizziness to palpitations and irregular heartbeats.

The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. The electrical system of your heart is the power source that makes this possible.

Your heartbeat is triggered by electrical impulses that travel down a special pathway through your heart:

1. SA node (sinoatrial node) – known as the heart’s natural pacemaker. The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles. The SA node sets the rate and rhythm of your heartbeat. Normal heart rhythm is often called normal sinus rhythm because the SA (sinus) node fires regularly.
2. AV node (atrioventricular node). The AV node is a cluster of cells in the center of the heart between the atria and ventricles, and acts like a gate that slows the electrical signal before it enters the ventricles. This delay gives the atria time to contract before the ventricles do.
3. His-Purkinje Network. This pathway of fibers sends the impulse to the muscular walls of the ventricles and causes them to contract. This forces blood out of the heart to the lungs and body.
4. The SA node fires another impulse and the cycle begins again.

At rest, a normal heart beats around 50 to 99 times a minute. Exercise, emotions, fever and some medications can cause your heart to beat faster, sometimes to well over 100 beats per minute.

How fast does the normal heart beat?

How fast the heart beats depends on the body's need for oxygen-rich blood. At rest, the SA node causes your heart to beat about 50 to 100 times each minute. During activity or excitement, your body needs more oxygen-rich blood; the heart rate rises to well over 100 beats per minute.

Medications and some medical conditions may affect how fast your heart-rate is at rest and with exercise.

How do you know how fast your heart is beating?

You can tell how fast your heart is beating (your heart rate) by feeling your pulse. Your heart-rate is the amount of times your heart beats in one minute.

You will need a watch with a second hand.

Place your index and middle finger of your hand on the inner wrist of the other arm, just below the base of the thumb.

You should feel a tapping or pulsing against your fingers.

Count the number of taps you feel in 10 seconds.

Multiply that number by 6 to find out your heart-rate for one minute:

Pulse in 10 seconds x 6 = \__ beats per minute (your heart-rate)

When feeling your pulse, you can also tell if your heart rhythm is regular or not.

Normal Heart Beat

1. The SA node sets the rate and rhythm of your heartbeat.

2. The SA node fires an impulse. The impulse spreads through the walls of the right and left atria, causing them to contract. This forces blood into the ventricles.

3. The impulse travels to the AV node. Here, the impulse slows for a moment before going on to the ventricles.

4. The impulse travels through a pathway of fibers called the His-Purkinje network. This network sends the impulse into the ventricles and causes them to contract. This forces blood out of the heart to the lungs and body.

5. The SA node fires another impulse. The cycle begins again.