Continuous cardiac monitoring and the acquisition of the 12-lead electrocardiogram (ECG) have become common practice in sophisticated prehospital emergency medical services across the world. In the context of acute coronary syndrome, multiple studies show benefits in the use of the prehospital ECG and there is little doubt that when used appropriately, it can reduce the time to treatment and improve outcomes for patients.
Many articles have already been written about how to acquire an ECG, including within the Journal of Paramedic Practice (Crawford and Doherty, 2009; Simpson-Scott, 2018). The present article will adopt a different approach and try to highlight why we should all always follow best practice by identifying the problems associated with failing to do so.
In terms of best practice, the Society for Cardiac Science and Technology (SCST) provides a consensus guideline on the recording of a standard 12-lead ECG (Campbell et al, 2024), and it is highly recommended reading.
Indications for continuous cardiac monitoring
Continuous cardiac monitoring is carried out using a 3-lead ECG (although there will often be more than three electrodes). There are obvious cardiac indications that require monitoring but there are other non-cardiac conditions that should also be monitored. Indications include, but are not limited to the following:
- All patients who are at significant risk of an immediate, life-threatening arrhythmia
- All unconscious or recently unconscious patients
- Patients with a reduced level of consciousness
- Patients presenting with chest pain (even if the chest pain is atypical of a cardiac origin)
- Patients with endocrine disorders (endocrine disorders manifest as a disturbance in the milieu of multiple organ systems including cardiac)
- Patients with suspected poisoning
- Dyspnoeic patients
- Hypoxic patients
- Patients with abnormal vital signs
- Patients who appear poorly perfused or who are clinically shocked
- Patients in cardiac arrest or who have been recently resuscitated (may be through defibrillation pads)
- Major trauma.
Monitoring should also be used when administering drugs that may impact the cardiac electrical conduction system.
In situations where a patient refuses treatment or where a non-conveyance decision is being considered, the value of undertaking an ECG should be analysed and a recording made if there is any doubt.
12-lead ECG
The 12-lead ECG provides a graphical representation of the electrical activity within the heart and is used for diagnostic purposes. It provides 12 views of cardiac electrical activity, recording the impulses generated by the heart through electrodes placed on specific areas of the body. Six of the views are generated from the four limb electrodes that use a combination of three electrodes placed on three limbs (right arm, left arm and left leg) plus one neutral electrode placed on the right leg to reduce interference. The remaining six views are derived from the six chest electrodes, also known as the precordial leads (Garcia and Miller, 2019).
Indications for 12-lead ECG recording
A 12-lead ECG should be undertaken for any patient where detailed ECG analysis is required. This may be indicated after initial examination of a 3-lead ECG or it may be an essential requirement because of the nature of the presenting complaint.
A 12-lead ECG will always be indicated in the following circumstances:
- Suspected acute coronary syndromes (ACS)
- Cardiac dysrhythmias
- Conduction disturbances
- Electrolyte imbalances
- Drug toxicity
- Electrical injuries.
A 12-lead ECG may also be indicated following any of the indications for continuous monitoring outlined earlier.
Considerations when undertaking an ECG
ECG settings
A standard electrocardiograph is set to run at a paper speed of 25 mm/second, although some machines are set to run at different speeds. In the ECG paper, the vertical lines are set to show time; these show small boxes that equate to 0.04 seconds and larger boxes (made up of five small boxes) that equate to 0.2 seconds (hence there are five large boxes per second). The horizontal lines show amplitude; in a standard electrocardiograph, the paper is set to record 1 cm (10 mm) per millivolt (mV). A standard ECG recording will provide a calibration signal of 1 mV prior to the recording to ensure that the machine is correctly calibrated for interpretation.
Patient preparation
Recording of an ECG in the out-of-hospital environment is not without its challenges as it takes place in a multitude of different situations and states of urgency. However, it is important that best practice and correct placement of electrodes is followed whenever possible. Any deviation needs to be documented for those caring for the patient later, especially when a 12-lead ECG is undertaken. Dignity is of paramount importance, especially in the placement of the chest electrodes on women.
Patient position
When recording a three-lead ECG, patient positioning is probably less important than for a 12-lead ECG, as it is being used primarily for arrhythmia detection rather than as a diagnostic tool. For a 12-lead, a standardised approach should be used wherever possible as it allows for a like-for-like interpretation of the reading. Any significant variation should be documented on the ECG and on patient records.
The patient should be in a semi-recumbent position of approximately 45o. Evidence has shown significant variation between an ECG recorded from a patient in a supine position and one recorded with the patient in an upright position (Baevsky et al, 2007; Khare and Chawala, 2016) or inclined at 60o or greater to the horizontal (Bergman et al, 1989). There is no evidence of any significant effect on the ECG if the patient is placed between supine and 45o to the horizontal but many patients find it difficult to lie supine, hence the recommendation for a semi-recumbent position. The limbs should be supported on the stretcher/bed to minimise artefact owing to muscle tension.
If possible, time should be taken to ensure that the patient is relaxed and comfortable to reduce the risk that somatic muscle potentials will be recorded by the ECG as well as the cardiac activity (Campbell et al, 2024). Somatic potentials can make the ECG more difficult to interpret and could impact its clinical value. Some patients may find it difficult to relax; for example, those with painful conditions such as arthritis, or those who are anxious as a result of their acute medical condition. Other patients may have underlying conditions that cause a tremor (for example, Parkinson's disease, multiple sclerosis, alcoholism). In these situations, make the patient as comfortable as possible and then annotate the ECG if the tracing is sub-optimal (Campbell et al, 2024). It should also be noted that it is unlikely that a high-quality ECG will be recorded if the patient has clenched fists or stiff arms, or is moving her/his fingers.
Optimal patient position for recording an ECG is as follows:
- Patient in a semi-recumbent position of approximately 45o
- Limbs supported on the stretcher/bed.
Skin preparation
Skin preparation is often required to help produce an artefact-free accurate ECG (Campbell et al, 2024). It is not uncommon for paramedics to manage patients with excessive chest hair or patients who are sweating profusely, and both will impact the adhesion of electrodes as well as increasing impedance. Several recommendations that are designed to reduce the impedance between the skin and the electrode have been made. The SCST suggests the following methods be used (Campbell et al, 2024):
- The removal of chest hair using a razor to allow for greater contact between the electrode and the skin. This requires consent, a clean razor and sharps disposal box
- Exfoliation of the skin may be required and should be undertaken gently using a paper towel, gauze swab or specifically designed abrasive tape. This can further reduce impedance between the skin and the electrode
- It may be necessary to cleanse the skin to remove excess contaminants or sweat. A variety of methods can be used such as soap and water or alcohol wipes; however, care should be taken if there is broken or sensitive skin.
ECG electrode placement
3-lead ECG
The 3-lead ECG is non-diagnostic so there are several variations of lead placement used in clinical practice. In paramedic practice, it is usual to use the modified limb-lead placement where the electrodes are placed on the torso rather than on the limbs. This was first described by Mason and Likar (1966) and was developed for exercise testing. Moving the limb leads away from the extremities reduces artefact and allows for patient movement (Pahlm and Wagner, 2008), which is helpful for continuous monitoring on a moving vehicle.
Note: The modified limb-lead positions alter the amplitude of the QRS voltage in the inferior leads and show a rightward shift of the cardiac axis (Jowett et al, 2005). They are not to be used when undertaking diagnostic 12-lead ECGs except in exceptional circumstances. Where a modified lead position is used, it should be noted on the patient's records.
The modified lead positions are as follows:
- Right arm limb lead (RA/or red electrode) – below the right clavicle
- Left arm limb lead (LA/or yellow electrode) – below the left clavicle
- Left leg limb lead (LL/or green electrode) – lower part of the rib cage, on the left-hand side.
- Some machines may have a fourth lead (right leg lead (RLL)/or black electrode), this lead acts as an ‘earth’ and is neutral to prevent current interference. This electrode can be placed anywhere but is typically placed on the right side of the abdomen.
Please note that these colours refer to European equipment and differ in other countries including the United States (US).
The 12-lead ECG
The 12-lead ECG is a diagnostic tool and therefore the placement of the electrodes is paramount. Incorrect positioning of precordial electrodes presents a risk to patients as it can lead to morphological changes in the ECG (Bond et al, 2012; Kania et al, 2014; Walsh, 2018) with subsequent misinterpretation. In their study, Kania et al (2014) found more prominent morphology changes of ECG waves when the electrodes were displaced from the optimal position by 2 cm or more, so there is not a lot of room for error. The level of risk is difficult to quantify but where morphological changes occur because of electrode misplacement, it is conceivable that treatments may be administered or withheld based on an incorrect recording.
The accuracy with which electrodes are placed is concerning as most studies show suboptimal placement by various healthcare professionals. Rajaganeshan et al (2008) found that V1 was only identified correctly with any consistency by cardiac technicians (90%). Nurses (49%), physicians (excluding cardiologists) (31%) and cardiologists (31%) all had high levels of inaccuracy. In the same study, V5 and V6 were also frequently malpositioned. In another study, Medani et al (2018) found that only 10% of participants (doctors, nurses and cardiac technicians) applied all leads correctly with frequent misplacement of V1 and V2 (too superiorly) and V5 and V6 (too medially). Other studies (e.g. McCann et al, 2007; Wenger and Kligfield, 1996) also show poor levels of accuracy among healthcare professionals.
What about paramedics then – surely we're better than that? Well, a UK study using human volunteers as patients found a high level of variation in the placement of chest ECG electrodes by UK-registered paramedics (Gregory et al, 2021), while an Australian manikin study (Clopton and Hyrkäs, 2024) found similarly concerning results.
It is unclear why there is so much inaccuracy found in the placement of ECG electrodes, but we owe it to our patients to place them correctly.
Limb electrodes
The limb electrodes should be placed proximal to the wrists and ankles whenever possible. Moving the electrodes up the limbs may alter the appearance of the ECG and should be avoided unless there is a significant tremor, or a limb has been amputated (Campbell et al, 2024).
Modified-limb electrode positions must not be used when recording a 12-lead ECG as it will cause significant changes to wave amplitudes and can invalidate the use of the recording for many diagnostic purposes (Campbell et al, 2024).
The correct anatomical positions for the limb leads have been defined and are shown in Table 1 (Campbell et al, 2024).
Table 1. Limb electrode positions
| Right leg limb lead (RL, black) | Right forearm, proximal to wrist |
| Left arm limb lead (LA, yellow) | Left forearm, proximal to wrist |
| Left leg limb lead (LL, green) | Left lower leg, proximal to ankle |
| Right arm limb lead (RA, red) | Right lower leg, proximal to ankle |
Precordial (chest) electrodes
The position of the precordial electrodes was defined in 2007 (Kligfield et al, 2007) and must always be used unless access is not possible. The centre of the active area of the electrode should be placed on the appropriate anatomical landmark (Table 2).
Table 2. Precordial electrode positions
| V1, red (C1) | Fourth intercostal space at the right sternal edge |
| V2, yellow (C2) | Fourth intercostal space at the left sternal edge |
| V3, green (C3) | Midway between V2 and V4 |
| V4, brown (C4) | Fifth intercostal space in the mid-clavicular line |
| V5, black (C5) | Left anterior axillary line at the same horizontal level as V4 |
| V6, purple (C6) | Left mid-axillary line at the same horizontal level as V4, V5 |
Techniques for locating the precordial electrode positions
It is imperative that the anatomical landmarks are measured, and that guesswork is eliminated. Guesswork is lazy and leads to errors that may render the recording invalid for later comparison.
V1 and V2
The first step to accurate identification of the appropriate intercostal spaces requires palpation to establish the location of the sternal angle, also known as the angle of Louis. To do this, locate the suprasternal notch and gently run the finger down until a bony horizontal ridge is found. From here, slide the finger down and to the right to identify the second intercostal space. It is possible to count down to the fourth intercostal space from here.
Repeat the procedure on the left side to correctly position the V2 electrode. The left- and right-sided rib spaces may be offset, so avoid placing V2 adjacent to V1 without counting the rib spaces.
V4
Next is placement of the V4 electrode. This should be placed in the 5th intercostal space in line with the mid-point of the clavicle on the left side of the chest. Identification of the 5th intercostal space is simple as it is just one down from the V2 lead; however, palpation of the clavicle should be undertaken to accurately identify the midpoint.
V3
The V3 electrode should then be placed mid-way between the V2 and V4 electrodes
V5
The V5 and V6 electrodes should then be positioned in horizontal alignment with the V4 electrode. The V5 electrode should be placed on the anterior axillary line; the V6 electrode should be placed on the mid-axillary line. It is important that these electrodes are on the same horizontal plane rather than following the curve of the ribs around.
Note: If electrode positioning varies from the recommended positions, it is essential that this is documented on the ECG recording, including electronically-stored ECGs.
Specific considerations
Female patients
When recording an ECG from female patients, it is convention to place the V4, V5 and V6 electrodes beneath the left breast when breast tissue overlies the correct anatomical positions (Campbell et al, 2024). There is limited evidence suggesting that positioning of these electrodes over the breast may not significantly attenuate the signal but the current recommendation remains to place beneath breast tissue where possible (Campbell et al, 2024).
Dignity should be preserved when moving breast tissue to place the electrodes and it is generally recommended that using the back of the hand to lift the breast can be helpful in minimising contact. It may also be possible to ask the patient to move their own breast but being careful that they do not move too much during this process.
Patients with obesity
Obesity can make landmarks more difficult to identify but the same processes should be used. Care needs to be taken not to press too firmly when identifying landmarks as this may add to patient discomfort.
Training
Appropriate training leads to fewer errors in ECG recordings and the guidelines recommend that any medical personnel responsible for the recording of ECGs should have periodic re-training in skin preparation, proper electrode positioning, and proper patient positioning (Kligfield et al, 2007). However, the guidelines are not specific on the frequency of training and it is not known if, or how often, paramedics receive refresher training in this area. Any person undertaking ECG recording needs to ensure that they are up to date with guidelines and best practice, and always deliver that best practice.
Removal of electrodes
Standard procedures recommend that the ECG electrodes be removed once the recording has been taken and this seems to be typical practice in ambulance services. If correct procedures have been followed and the electrodes have been correctly placed, this is a reasonable approach. However, it does remove the ability of those caring for the patient to see where the electrodes were placed in the case of an error. Although it would be good to believe that all prehospital clinicians measure and place electrodes correctly, this may not be the case (Gregory et al, 2021). It is not unreasonable to argue that precordial electrodes should be left in situ to allow those caring for the patient later to see how the previous ECG was recorded. There is the obvious caveat where there is a high risk of cardiac arrest and defibrillation pads have been placed.
If you take nothing else away from this article, please remember the inspiring words of Kirstie and Phil – location, location, location!