Cardiac monitoring ( ECG) Circulation (Advanced)

Week 4

Context

The electrocardiogram (ECG) is a diagnostic tool that is routinely used to assess the electrical and muscular functions of the heart. While it is a relatively simple test to perform, the interpretation of the ECG tracing requires significant amounts of training. The heart is a two stage electrical pump and the heart's electrical activity can be measured by electrodes placed on the skin. The electrocardiogram can measure the rate and rhythm of the heartbeat, as well as provide indirect evidence of blood flow to the heart muscle.

A standardised system has been developed for the electrode placement for a routine ECG. Ten electrodes are needed to produce 12 electrical views of the heart. An electrode lead, or patch, is placed on each arm and leg and six are placed across the chest wall (see below). The signals received from each electrode are recorded. The printed view of these recordings is the electrocardiogram.

By comparison, a heart monitor requires only three electrode leads – one each on the right arm, left arm, and left chest. It only measures the rate and rhythm of the heartbeat. This kind of monitoring does not constitute a complete ECG.

To understand the electrocardiogram you need to have an understanding of the electrical and muscular activity of the heart itself.

With grateful thanks to Michael Sampson (Arrhythmia Nurse Specialist, St George's Hospital & Senior Lecturer London South Bank University) for his help with this week's resources.

Introduction

This section will enable you:

To understand how electrical impulses control the beating of the heart
To review the specialised system that conducts these impulses through the heart
To understand how the heart’s electrical activity can be recorded on an ECG
To develop some basic skills in ECG interpretation

Content

Cardiac conduction and contraction

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:

The sino-atrial (SA) node
The atrio-ventricular (AV) node
The bundle of His
The left and right bundle branches
The Purkinje fibres

The SA node is the natural pacemaker of the heart. Permanent pacemakers (PPMs) and temporary pacing wires (TPWs) can be 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, in a domino effect. 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 empty 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.

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 another 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. 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:

Atrial depolarisation
Ventricular depolarisation
Atrial and ventricular repolarisation.

As the atria repolarise during ventricular contraction, there is no wave representing this on the ECG.

Cardiac Monitoring

	The cardiac monitor detects voltage differences within the body surface and amplifies and displays these as a signal. The device can offer useful information to healthcare professionals, such as indicating myocardial ischaemia and cardiac arrhythmias.
The cardiac monitor uses three standard bipolar leads, I, II and III: red, yellow and green (or black). The monitor leads are connected to adhesive electrode pads placed on the patient’s chest.

Cardiac monitoring and the ECG can be extended to improve the views of the heart. In some instances you may see 5 lead cardiac monitoring being carried out. These leads allow additional views of the heart to be monitored, usually seen in use by paramedics and health care professionals in an acute clinical areas.

Once connected to a monitor, you may observe that the patient has a ‘normal’ heart rhythm (sinus rhythm), portraying that electrical impulses are travelling from the sino-atrial node to the atrio-ventricular node, down the septum of the heart, into the Bundle of His and then the left and right bundle branches and the Purkinje fibres. Remember: Observing a cardiac monitor is no substitute for observing your patient.

Cardiac monitoring during the A to E assessment is cruical to ascertain the patients heart rhythm. A 3-lead or 5-lead ECG monitor should be used. A reading in lead 11 is usually advised during A to E assessment situations, with a 12 or 15 lead ECG being performed in the investigations stage.

The electrocardiogram (ECG)

The ECG machine is designed to recognise and record any electrical activity within the heart. It prints out this information on ECG paper made up of small squares 1mm squared. Skin electrodes pick up the heart's electrical activity and send it back to the ECG machine or monitor. The result can be displayed on a screen or printed out on graph paper. For a standard '12 lead' ECH there are actually 10 electrodes: Four limb electrodes (Red, Yellow, Green, and Black), and sic chest electrodes (V1-V6). To ensure that the readings obtained can be interpreted clinically, there is a standard for the attachment of the leads. The limb leads can be attached in one of two ways, and the chest leads are set out in a 'string' across the chest, as in the picture below.

(from:http://www.biolog3000.com/electrode.htm Links to an external site.)

V1: Fourth intercostal space to the right of the sternum.
V2: Fourth intercostal space to the Left of the sternum.
V3: Directly between leads V2 and V4.
V4: Fifth intercostal space at midclavicular line.
V5: Level with V4 at left anterior axillary line.
V6: Level with V5 at left midaxillary line.
(Directly under the midpoint of the armpit)

Reading the ECG

The trace form a single lead used for monitoring patients in clinical areas might look like this:

One from a '12 lead' ECG might look like this:

In both examples you can see the standard normal waveform, sometimes called the PQRST complex.

The first part of the complex is called the P wave. The P wave represents the initiation of depolarisation in the sinus node and subsequent atrial contraction. It is relatively small due to the small muscle mass of the atrium. The normal P wave should be upright (positive) in lead II and less than 2.5mm tall.

The QRS complex represents the conductance and sequential depolarisation of the ventricles. It is a much larger wave form that the P wave due to the large muscle mass of the ventricles. An initial downwards (negative) deflection in the QRS is termed the Q wave. A Q wave may or may not be present depending on the lead and the pathology of the ECG. The R wave is large and upright (positive). The S wave follows the R wave and is a downwards (negative) deflection.

The ST segment follows the QRS complex. The ST segment represents the period of time in which the ventricles are isoelectric. The position of the ST segment relative to the baseline prior to the preceding P wave is considered, ie. is the ST segment elevated, depressed or normal relative to the previous baseline.

The T wave is the last portion of the PQRST complex. The T wave represents the repolarisation of the ventricles. The T wave is normally upright in all leads except V1 and aVR (isolated T wave inversion in III can also be normal).

The activity is measured by looking at the amplitude and frequency of the waveforms.

Amplitude

1 small square = 0.1 millivolt
1 large square = 0.5 millivolt

Frequency

1 small square = 0.04 sec (40 milliseconds)
1 large square = 0.2 sec (200 milliseconds)

Normal sinus rhythm

The heart rate is between 60-100 beats/min
The rhythm is regular
Normal P waves are present
P waves precede in QRS In a 1:1 ratio
The PR interval is 0.12-0.20 seconds (3-5 small squares)
The QRS complex is usually narrow (<3 small squares)

Pathological Variations

Becoming expert at reading ECGs takes time and practice. There is a publicly available book 'Cardiology Explained' which has a very good chapter on ECGs and pathological variations. You can download a pdf Download download a pdf

of the Chapter here, or visit the site itself Links to an external site..

References and Further Reading

Ashley, EA. and Niebauer, J. (2004) Cardiology Explained. London: Remedica; . Chapter 3, Conquering the ECG. Available from: http://www.ncbi.nlm.nih.gov/books/NBK2214 Links to an external site.

ECG Library http://www.ecglibrary.com/ecghome.html Links to an external site.

Hampton, J. (2008). ECG Made Easy 7th edn. London: Churchill Livingstone.

Bennett DH (2013) Bennett’s Cardiac Arrhythmias: Practical notes on interpretation and treatment, 8th edition, London, Hodder Arnold.

Hamilton R (2006) Treating a patient with a supraventricular tachycardia. British Journal of Cardiac Nursing 1(1), 34-38.

Lafuente-Lafuente C, Mahé I & Extramiana F (2009) Management of atrial fibrillation, British Medical Journal, 339, b5216