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ECGs: how to recognise an abnormal recording

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VOL: 98, ISSUE: 21, PAGE NO: 40

Amanda Roberts, RN, DipHE, is staff nurse, Intensive Care Unit, Royal Cornwall Hospital, Truro

This article focuses on the practical aspects of taking and interpreting an ECG. Guidelines for recording an accurate ECG, such as those described by Mallett and Dougherty (2001), are helpful. Before attempting any recordings, however, familiarise yourself with the machine you will be using by referring to the manufacturer's instructions. It is especially important to ensure that the paper speed is correct and that the machine has been calibrated to 10mm per mV (the calibration mark is the positive part of a square waveform that should be no wider than one large square and no taller than two large squares). In the UK, paper speed should be 25mm a second. This is often automatically recorded in writing on the ECG by newer machines.

This article focuses on the practical aspects of taking and interpreting an ECG. Guidelines for recording an accurate ECG, such as those described by Mallett and Dougherty (2001), are helpful. Before attempting any recordings, however, familiarise yourself with the machine you will be using by referring to the manufacturer's instructions. It is especially important to ensure that the paper speed is correct and that the machine has been calibrated to 10mm per mV (the calibration mark is the positive part of a square waveform that should be no wider than one large square and no taller than two large squares). In the UK, paper speed should be 25mm a second. This is often automatically recorded in writing on the ECG by newer machines.

Positioning the patient
To get an accurate tracing of the heart it is good practice to, where possible, explain the process to the patient, dispelling any anxiety that may affect the quality of the reading, and gain consent. Check the temperature of the room because a cold environment may cause shivering if patients need to remove any clothing for the electrodes to be applied. This type of movement can affect the accuracy of the recording. ECGs readings taken when a patient is restless or affected by rigors will also be affected.

Patients should be supine but if they have to sit in a chair to record an ECG, or if the patient is an amputee or has a thoracic deformity, it is good practice to document this on the tracing, along with the time, date, patient's name, hospital number, blood pressure and any symptoms the patient may be experiencing.

Skin preparation and electrode placement
The skin may be cleaned with soap and water, and dried thoroughly to facilitate contact with the gel-covered pads that are used to attach the electrodes. It is advisable to remove any hair that may interfere with electrode contact but, again, it is good practice to obtain consent for this where possible.

Three-lead recordings are usually used for continuous monitoring. The waveforms are displayed on a screen and most machines can produce printouts of the information displayed. The leads are usually coloured. In the UK, the red (RA) lead is placed on the right arm or right infraclavicular space, the yellow (LA) on the left arm or left infraclavicular space, and the black/green (LL) electrode on the left leg or lower left abdomen.

Twelve-lead ECGs give a more detailed recording of the electrical activity over the heart as the current sweeps across it. A 12-lead ECG uses 10 electrodes - six on the chest and one on each limb (Roberts, 2002). The four limb connections produce six viewpoints of the heart along a vertical plane by 'looking' at one another, while the six chest leads produce another six viewpoints of the heart on a horizontal plane, creating 12 images altogether. The placement of chest electrodes is shown in Fig 1.

Each patient's ECGs are compared regularly to assess changes over time, so it is good practice to be consistent and position the electrodes in the same places for each recording. A protocol will help to ensure this. For example, it is important to decide whether electrodes are to be placed at the wrists and ankles or nearer the torso.

Before printing the ECG, ensure that you have a steady baseline. This can be done by checking the screen before printing, or on older models you can print a short test strip. A 'wandering' baseline, where the tracing undulates or disappears off the screen or paper, may be caused by an electrode not being in good contact with the skin. Check each one to ensure that it is positioned in the right order. If necessary, clean the skin again and apply new electrodes.

Interpretation
As with any skill, it takes time and practice to be proficient in interpreting ECGs. The key is to memorise which view of the heart each electrode has and whether the flow of impulses should be towards it, away from it or moving straight past it. Following a guide, such as that described in Box 1, can help nurses to make an initial assessment of an electrocardiogram for any obvious abnormalities, but it is important to discuss the recording with the patient's doctor.

Conduction abnormalities or dysrhythmias
Conduction abnormalities or dysrhythmias occur when damage to the structures of the heart, illness (chronic or acute) or external stressors (such as ambient temperature or emotions) affect the heart's ability to circulate blood efficiently and effectively to its own structures and the rest of the body. Areas of the normal conduction pathway may become irritated, disrupted, blocked or partly blocked, causing atypical waveforms on the ECG tracing.

Myocardial infarction
After myocardial infarction, for example, damaged areas will result in abnormal ECG tracings. Dead (unrecoverable) tissue will produce abnormal Q-waves, injured tissue that might be recoverable will cause elevation of the S-T area of the QRS complex, and ischaemic tissue that is likely to recover will produce T-wave inversion (T-waves will appear U-shaped when the QRS complexes are positive).

Cardiac axis
The impulses generated by the heart flow (roughly) from 11 o'clock to 5 o'clock, that is, from the top right of the heart down towards the bottom left of the heart.

This is known as the cardiac axis and deviation of the impulses too far to the left or right on the trace can signify problems that may require immediate attention. One example of such deviation is hemiblock, which occurs when one of the two main branches (or fascicles) of the left bundle branch (a branch of the bundle of His) fails to conduct impulses properly.

Escape rhythms and ectopics
When another area of the heart takes over as pacemaker, the resulting impulses are known as escape rhythms and appear after a P-wave fails to materialise where expected. These late waveforms act as a method of circumventing or escaping the normal (failing) conduction pathways of the heart to keep it beating. The atrioventricular node or the ventricles may generate escape rhythms.

Ectopics are waveforms that appear prematurely and can arise from the atria, the AV node or the ventricles. Ectopics are a common feature after myocardial infarction and may be aggravated by hypokalaemia and digoxin toxicity.

Ventricular ectopics that are asymptomatic and do not occur more regularly than five a minute are usually benign and do not require treatment (Whiteley et al, 1998). Atrial ectopics will have a P-wave and will appear earlier than anticipated. Ectopics generated by the AV node will probably be narrower than normal and will also appear before they are expected. Ectopics produced by the ventricles are wide, bizarre-looking waveforms. A pattern in which ventricular ectopics appear after every normal QRS complex is termed bigeminy.

Failure of the sinoatrial node
If the sinoatrial node fails to function, the AV node may begin to compensate by acting as an auxiliary pacemaker, but the rate is often slower at between 40 and 60 beats a minute. This may be expected to continue until the SA node recovers and begins to override the AV node, as it would in good health.

Waveforms generated by the AV node are often known as junctional rhythms and can be identified by an inverted P-wave in lead II because the flow of impulses generated by the AV node will flow backwards into the atria as well as forwards towards the ventricles. The P-R interval will also be too short because the impulse has originated from the AV node rather than having been conducted through it.

Failure of the AV node
If the AV pacemaker then deteriorates, a ventricular pacemaker will step in. This will be only between 15 and 40 beats a minute, the complexes generated will be wider than normal and P-waves will be absent. The slower heart rates can compromise homeostasis.

Dysrhythmias
The ECG tracings pictured (Fig 2) show normal recordings together with those that might be recorded in a patient with hyperkalaemia, which can occur with arrhythmias.

Conclusion
The interpretation of ECG recordings is a valuable skill. It takes time and practice to become proficient, but ultimately enhances patient care by allowing nurses to recognise potential problems. It is strongly recommended that the reader embark on further study, particularly practical workshops, to gain a greater understanding of the subject.

A nurse's ability to explain or clarify treatments can alleviate patients' anxieties, aid the process of recovery and promote effective teamwork among health care professionals. An understanding of the subject will enable nurses to explain the treatment options to patients, their relatives or carers, who may have questions about aspects of care. This can only help to meet the government's targets on preventing, detecting and treating heart disease.

- See Practical Procedures, Detecting cardiac arrhythmias in cardiac arrest, p45

- A range of ECG traces can be seen at: www.ecglibrary.com

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