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12-lead ECG – posterior-lead placement:
When interpreting a 12-lead ECG it is important to use a systematic approach. This involves looking at the rhythm strip on the 12-lead ECG to determine heart rate and rhythm, using the previously recommended 6-step approach. The complex configuration and intervals of the complexes, for example, PR, QRS and QT should be examined and measured to identify any delays or abnormalities in conduction (for example, AV, Bundle Branch block or pre-excitation).
The ECG should be examined to look for evidence of the following:
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A 12-lead ECG is an important tool in clinical decision making and can provide a wealth of information, for example, diagnosis of evolving MI; identify potentially life threatening arrhythmias; long term effects of sustained hypertension; or acute effects of a pulmonary embolus. In contrast to cardiac monitoring, the 12-lead ECG shows the 3-dimensional electrical activity of the heart recorded from 12 different leads or viewpoints. It provides the whole picture. This is achieved by using Bipolar (I, II & III) and Unipolar leads (augmented leads & precordial chest leads).
ECG electrodes are placed on specific areas of the body in predetermined positions so that different pictures of the heart’s electrical activity can be recorded accurately and replicated between recordings.
The bipolar limb leads show the electrical differences between the positive and negative electrodes placed on the limbs as far away from the heart as possible. These 3 leads are equidistant from the heart and create an equilateral triangle (Einthovens triangle) which the heart sits in the centre of.
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When looking at an ECG its important to know what is normal.
Within the first 6 leads (I,II,III, avR, avL and avF) all, with the exception of aVR, should be upright.
In the precordial or chest leads the r wave progresses from small to tall from V1 to V6
The polarity or direction of the ST segment and T wave are generally the same as the preceding QRS complex, for example, if the QRS complex is positive or upright then the T wave will also be positive.
All ST segments should remain on the isoelectric line or baseline.
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See Common Cardiac Investigations: The Electrocardiogram (ECG) for a video from Glasgow University on indications, equipment, placement, preparation and procedure and recording an ECG.
The following section will allow you to apply this approach to analysing common arrhythmias.
This will include:
If you want to remind yourself of the 6-step approach you may want to revise the following rhythms.
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Having now completed the rhythm interpretation, we will now proceed to the 12-lead ECG.
Please view the following video which illustrates a systematic approach to rhythm interpretation.
Three or five lead monitoring can be applied for use with both static and telemetry cardiac systems. The 3 or 5 lead systems are attached to electrodes which are applied to specific locations on the patient’s chest. It is important to ensure proper skin preparation prior to application of the skin electrodes.
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See Additional information for common problems associated with cardiac monitoring.
Having set up your specific monitoring system, we will now go on to consider how to interpret the cardiac rhythm using a systematic 6-step approach.
Continuous cardiac monitoring is an important tool in the clinical assessment of patients with a variety of conditions. It allows the detection of changes in heart rate, rhythm and conduction, and is essential in the detection of life threatening arrhythmias. This is achieved using a cardiac monitor, connected to a cable lead and skin electrodes, which captures the electrical activity predominantly through a single view (commonly lead II).
The monitor function includes:
Alarms should never be ignored or turned off.
Patients who may require cardiac monitoring include those who are haemodynamically compromised and/or at clinical risk of adverse events, for example, patients with:
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The cardiac monitor or telemetry system will be connected to the patient via a 3- or 5-lead cable. This will now be explained.
Before commencing this section of the module, we would suggest that you review the electrical activity of heart (see Healthy Heart module: Electrical activity of the heart). You might also find it useful to review your knowledge and understanding of Cardiac Investigations: Common Cardiac Investigations.
Take a closer look at the PQRST waves below:
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This is the conversation that happens between a patient and their healthcare professional to reach a healthcare choice together. This conversation needs patients and professionals to understand what is important to the other person when choosing a test or treatment.
It is the role of the healthcare professional to collaborate with the patient and actively encourage him/her to be involved in the decision making, enabling them to make an informed choice about whether or not they agree to having the particular test done or which test they have done, if there are a few possibilities.
The conversation between the doctor/nurse and patient is core to shared decision-making based on the individual’s priorities and preferences, fears and concerns. The BMJ: Time to deliver patient-centred care: BMJ 2015;350:h530
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The overarching aim of patient-centred care is to ensure the patient is an active participant in making decisions about their investigations, treatment and care.
After explaining the risks and benefits of a particular investigation, such as, an angiogram, to the patient, they may feel that they are not willing to accept any risk of stroke and decline to have the angiogram done, even though you believe that they should go ahead with the test because their symptoms warrant it. Although we can explore the patient’s fears further, in the end, we have to accept the patient’s wishes.
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