Electrocardiogram: What It is and How It Should be Done
This is for the benefit of nursing students in their second and third years of training. It should not be used for diagnostic purposes. Only a qualified doctor can diagnose and treat patients. The author is a Lecturer in Health Sciences at a major university.
Definition:
An electrocardiogram is defined as a recording of the electrical activity of the myocardium of the heart. An ECG is recorded on ECG paper and it is used to investigate heart diseases. It does not provide information about the actual mechanical functioning of the heart and cannot be used to assess cardiac output, blood pressure or tissue perfusion.
Standard ECG paper is usually divided into 1mm squares. Vertically, ten blocks correspond to 1 mV, and horizontally each square represents 0.04 of one second (or 40 milli-seconds. During the recording of an ECG the paper speed is usually set at 25mm per second. Five horizontal and five vertical blocks make the “big blocks”. Each big block is equal to 0.20 of a second horizontally and 0.5mV vertically.
Key points to remember:
Horizontally: Each small square is 1mm by 1mm = 0.04 of a second.
Vertically: Ten small squares = 1mV (milli-volt).
Electrode placement: Ordinarily the word electrode means a conductor through which electric current is passed. However, in applied nursing and medicine an electrode is a sensor which is attached at the end of a wire coming from a cardiac monitor. These electrodes sense the amount of electricity which is passing through them. The electrical voltage is then conducted to the cardiac monitor (oscilloscope). An electrode may be made of a metal like copper, silver or lead. However, cardiac electrodes used for monitoring patients are made of non metals. Look at the picture below and note that the green center is the electrode portion. It is made of a gel. This gel is a good conductor of electricity. Also note the “silver button” on the other side of the electrode. This “button” connects to the cardiac monitor. Last but not least, the white or sticky portion of the electrode holds the electrode in place for continuous monitoring.
Patient preparation (general): As is usual in all nursing procedures follow the eight standard steps of nursing. If you need to revise the eight standard steps in nursing please see Tabbners page 952 or visit: triond.com and read the article entitled NURSING: Eight Standard Tasks in caring for all patients.
Placing the electrodes for continuous monitoring: Identify and locate the three spots as shown below where the electrodes are to be placed. The commonest monitoring methods use either three or four electrodes. The three electrode method uses three electrodes placed in the following areas. First electrode is placed on the right shoulder. It is usually the white electrode wire marked RA for right arm. The second electrode is placed on the left shoulder and is generally marked LA for Left arm. It is generally black in color. The third electrode is usually red and is generally placed on the LL (left leg). The fourth electrode is usually green and is often placed on the right leg, it may be labeled RL.
Dry skin is a poor conductor of electricity. Therefore, before you place the electrode make sure the skin is clean and free of oils secreted by the skin. Hairy skin surfaces must be shaved. Be alert to the fact that some religious groups never shave and may object to it. Others will want the shaved hair to be kept safely. You may need to give it to the patient or a senior family member.
Rubbing the clean skin with the rough part of the electrode’s protective plastic will insure proper conduction and a smooth tracing.
Now look at pictures one and two below and note the different parts of the electrodes and how they are placed on the patient. Also pay particular attention to the button part of the electrode which fits on the wire from the oscilloscope.
Note: The green gel is a good conductor. The electrodes will not work if they are old and the gel has dried up.
Cardiac electrodes: how to make them stick
New electrodes are of a very good quality and generally stick well. However, sticking electrodes on patients who have very dry skins for example Indigenous Australians can become a challenge. Application of moisturizing agents will make the problem worse. So clean the patients skin of dead, dried & hardened skin. Once this is done place the electrode firmly on the skin. Apply a gentle pressure. Wait briefly for the gel to diffuse into the dry skin.
Patients who are oily or sweaty can pose a similar challenge. The solution is similar, clean the skin before placing the electrodes. Sometimes a little extra micropore tape will do the trick.
It is good practice to cleanse the patient’s skin prior to application of electrodes as a recording that clear and free of interference is more useful for diagnostic purposes. Frequent false alarms result in reducing the alertness of Coronary Care Unit staff.
Three standard limb leads
On the 12 lead ECG these are shown as L1, L2 and L3. As the name implies, the limb leads are ECG tracings which are obtained from leads attached to the limbs. The exact points are not important but placement of electrodes on limbs must be bilaterally symmetrical. The fourth lead is considered to be neutral, if applied, for monitoring purposes it is attached to the right lower quadrant of the abdomen. Limb lead electrodes are usually labelled RA=Right Arm, LA=Left Arm, RL=Right Leg and LL=Left Leg. One of the commonest mistakes when doing a 12-lead ECG is the connection of the right side leads to the left side and vice versa. These leads are said to be unipolar because they measure electrical activity of the heart from one direction only.
When the three augmented unipolar leads are coupled with the three bipolar leads as is done on a 12-lead ECG they form the six limb leads of the ECG. On the ECG paper these will be marked as L1, L2, L3 and AVr, AVl and AVf. These six leads record electrical activity along the frontal plane of the heart.
Bipolar limb leads reflect condition of the frontal plane of the myocardium.
Lead 1: Right arm to Left arm. Information pertains from to right to left and lateral view.
Lead II: Right arm to Left inferior. Information pertains to superior inferior view.
Lead III Left arm to LF: Same as lead II above.
Augmented unipolar limb leads (frontal plane): (Also called standard limb leads)
aVR lead: RA to LA & LF. Right sided view.
aVL lead: LA to RA & LF. Left sided view.
aVF lead: LF to RA & LA. Inferior view.
Unipolar chest leads provide information along the anterior horizontal plane.
These six leads are also called augmented limb leads.
Leads V1, V2, V3: Provide information along posterior anterior plane.
Leads V4, V5, V6: Provide information along posterior anterior plane.
Where to Place Electrodes for a 12-lead ECG?
The four limb leads are placed on the limbs, bilaterally symmetrically. The Chest electrodes are placed as follows:
Chest lead 1, called V1: Fourth inter-costal space in line with outer aspect of neck on the right side of the chest.
Chest lead 2, called V2: Fourth inter-costal space in line with outer aspect of neck on the left side of the chest.
Chest lead 3, called V3: Fourth inter-costal space, directly between L2 and L4.
Chest lead 4, called V4: Fifth inter-costal space, mid-clavicular line.
Chest lead 5, called V5: In line with V4 at left anterior axillary line.
Chest lead 6, called V6: In line with V5 at left mid-axillary line, midpoint of armpit.
Placement of electrodes is a sticky issue. However, the “stickier, the better” for continuous monitoring. The placement of electrodes for continuous monitoring has been described above. The exact points are less important for continuous monitoring but are critical when doing a 12-lead ECG.
Picture 1. Electrode placement for cardiac monitoring. Three and four lead methods
Diagnosis: In many institutions Registered Nurses are trained to be able to accurately interpret 12-lead ECG. In Australia this is not a standard expectation, however, it is in the best interest of the Nurse, the Institution and the Patient to know how to do and read a 12-lead ECG. If you cannot read an ECG you must get it read by the Doctor as soon as possible otherwise it is not much good doing the ECG in the first place.
Last but not least, in Emergency departments when a patient is undergoing an acute event it is usually necessary to do serial 12-lead ECG for comparison purposes. Do not compare rhythm strips with 12-lead ECG.
Look at the 12 lead ECG below. Note that specific areas where changes will be apparent on a 12-lead ECG.
Picture 2: 12 lead ECG Changes: Where to look for changes?
In some situations you will see a poor signal and you may have to trouble shoot. Go through the process systematically.
- First, make sure that the cables have all been connected properly
- Make sure that the monitor is functioning properly and that it has been switched on
- Make sure that there is no interference from other high voltage cables nearby
- Check to make sure that the skin was prepared adequately
- Make sure that the electrodes are not dry
- Make sure that electrodes are not being pulled/tagged
Disclaimer: The information contained herein is for basic education. It is not to be a diagnostic tool to aid treatment and management of any cardiac condition. Only your doctor can diagnose and treat.
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