You and your EMT partner respond to a call for a 43-year-old male complaining of chest pain.Your 12-lead ECG shows Q waves and negative T waves in leads II and III, and aVF.You interpret these changes as being possibly associated with inferior wall myocardial ischemiaand transmit the ECG to the emergency department (ED). However, the cardiology teamat the hospital determines your patient is not having an AMI.
After a second look, you realize that, although Q waves and inverted T waves are present,there_s no ST-segment elevation of one or more millimeters in two or more contiguous leads.You were caught by a mimic. You re-check the leads and find a lead reversal.After you replace the patches, a repeat ECG shows normal sinus rhythm.
While following up on the case, you learn the patient_s lab studies at the hospitalrevealed no elevation of cardiac-specific enzymes and your patient was dischargedwithin two hours of presentation to the ED with musculoskeletal pain.Your medical director wants to meet with you.
The 12-lead ECG has become an exceptional prehospital tool in the detection and treatment of life-threatening arrhythmias and cardiac-associated complaints. When performed correctly, the 12-lead ECG provides a more in-depth view than the traditional three-lead ECG. Technology has advanced, and 12-lead ECG data can now be transmitted to receiving facilities to improve recognition and treatment ofcardiac-related emergencies in a timely manner consistent with evidence-based practice guidelines.
Lead misplacement, however, can cause misinterpretation of ST changes, electrical axis, location of bundle branch blocks and location of infarct. Although lead misplacement isn_t a common mistake among well-trained EMTs and paramedics, and no significant costs to an agency are associated with reapplying patches, it could cause the wrong diagnosis and delay proper treatment. Worse, it could cause a provider to initiate treatment from inaccurate ECG data that could be detrimental to the patient.
It_s estimated that 4% of all ECGs run are recorded with incorrect lead placements; estimates are even higher for ECGs recorded in areas that necessitate a higher level of care, such as the prehospital environment.1
The Recorded Cardiac Cycle
In 1924, a Nobel Prize was awarded to Willem Einthoven for his development of the first practical electrocardiogram (ECG). Einthoven initially used a magnet, which was connected to limb leads, to produce a scrolling record of the heart_s electrical activity. As his work advanced, he went on to describe the conduction of the cardiac electrical cycle as we know it today, with the naming of the various components of the ECG (P wave, QRS complex and T wave).2,3
The heart_s electrical conduction cycle begins with the sinoatrial (SA) node, which is often referred to as the central pacemaker. As the SA node discharges, an upward or positive deflection, known as the P wave, is created on the ECG. While this occurs, depolarization of the atria happens, producing contraction of the atria and forcing blood flow down into the ventricles.
Conduction of the cardiac cycle continues with stimulation of the atrioventricular (AV) node. At the AV node, conduction slows to allow blood to enter and fill the ventricles. Conduction continues rapidly down both the right and left bundle branches and terminates in the Purkinje fibers. The QRS complex is created bythe depolarization of the ventricles. As the cardiac cycle nears completion, the T wave is created during repolarization of the ventricles.
The 12-lead ECG displays the cardiac cycle in four-lead groups that describe the cardiac cycle in relation to lead placement and also provides 12 distinct views of the cardiac cycle. The four different lead groups are lateral (I, aVL, V5 and V6), inferior (II, III and aVF), septal (V1 and V2) and anterior (V3 and V4) (see Figure 1).
Correct Lead Placement
To obtain a 12-lead ECG, a total of 10 electrodes are used. Six precordial chest leads (V1ÏV6) are placed on the anterior chest at predetermined, anatomically referenced landmarks.
To place the precordial chest leads, it_s best to begin with V1, which is placed at the fourth intercostal space on the right sternal border. Given that V1 is the starting point for placing all the precordial leads, it_s important to ensure this lead is properly placed.
To find the fourth intercostal space, begin at the suprasternal notch and move down to the manubriosternal angle (angle of Louis), which will resemble a bony horizontal ridge. Once you locate the manubriosternal angle, move your fingers to the patient_s right, and you_ll be at the second intercostal space. Move two intercostal spaces down, and you can now place V1 in its proper location.
V2 is placed at the fourth intercostal space on the left sternal boarder. Skipping V3, move on and place V4 at the midclavicular line of the fifth intercostal space. V3 is then placed directly between V2 and V4. V5 is placed at the fifth intercostal space at the anterior axillary line. V6 is placed at the fifth intercostal space of the midaxillary line.
The four remaining leads to be placed are the limb leads (LA, LL, RA and RL). The limb leads require less precision in placement. The arm limb leads can be attached anywhere on the arms, as long as they_re not touching the thorax. It_s best to place the leg leads on the medial aspect of the calves or upper thighs. Avoid placement of leads over bony prominences as it may cause unnecessary artifact.
When ECG lead misplacement is suspected, limb lead reversals are commonly the culprit. Reversal of the RA and LA is the most frequent lead reversal; this error will result in negative P and QRS waves in lead I (see Figure 2). Negative P and QRS waves are uncommon, and even more uncommon in the presence of cardiac disease.
If the P wave has a positive deflection and the QRS wave remains positive, the problem isn_t a lead wire reversal, but an abnormal axis deviation resulting from underlying pathology or dextrocardia.4
Reversal of the RA and LL can produce a significant clinical finding of an inverted P-QRS complex in lead II. An inverted P-QRS could mimic an inferior myocardial infarction (see Figure 3).1,5,6
When precordial leads are misplaced, the cause is often specific lead placement on the chest wall rather than misplacement of the actual lead wires. One study that looked at accuracy of chest lead placement found V1 and V2 to be superiorly misplaced 50% of the time.4 When V1 and V2 are superiorly misplaced, poor R-wave progression can occur. R-wave progression can be characterized by the R wave becoming more prominent and the S wave becoming smaller or less prominent in leads V1ÏV4. Poor R-wave progression can result from the following conditions: left ventricular hypertrophy, right ventricular hypertrophy, obstructive pulmonary disease, anterior or anteroseptal infarction, conduction defects (such as a left bundle branch block) and cardiomyopathy.6
Precordial chest lead placement can also be hampered in obese patients and women with large, pendulous breasts. In these patients, special attention must be given to placing the leads at the correct anatomical landmarks.4
Catching & Preventing Mistakes
Being able to recognize lead misplacement errors can reduce erroneous decision-making and therapy initiations.1 Providers should be intimately familiar with the normal presentation of lead groups and the varying baseline morphology that each presents. Having this information will enable a provider to make comparisons to an ECG reading in which they suspect a lead misplacement has occurred.
In an ECG with proper lead placement, lead I will have a positive P wave and a positive R wave. AVR should have a negative P wave and a negative R wave. R waves will progress in size (become larger) across the chest leads of V1ÏV4, which is a normal variant found within the 12-lead ECG. The S wave will decrease in size between V4ÏV6.5
Learning baseline ECG morphology and the associated lead groups will allow you to quickly discover if a lead misplacement has occurred. The four lead groups and correlating 12-lead ECG morphology are presented in Figure 1.
A few suggestions on reducing ECG lead misplacements:
Keep a pocket reference card in your monitor case showing correct lead placement and 12-lead ECG morphology.
Require at least yearly training and review sessions that include 12-lead ECG placement and interpretation. Also, conduct training whenever your ECG manufacturer issues changes in technology or design.
Task one provider per call with applying leads and lead wires. This single assignment and responsibility reduces the chance of misplacement and lead wires connected to the wrong terminal.
Consider forming a utilization group to provide feedback on ECGs obtained in the field. If your service transmits 12-lead ECGs to facilities to activate cardiac teams for emergency cardiac treatment, this group should include experts from those facilities.
The utilization of 12-lead ECGs in the prehospital setting has been shown effective in detecting life-threatening arrhythmias, as well as identifying acute coronary conditions requiring immediate prehospital intervention or definitive care in a cardiac catheterization lab.7 Because many systems apply 12-lead ECGs early in most medical calls in order to rule out cardiac problems first, providers must be aware that improperly placed leads can misguide patient care and lead to inappropriate treatment and potentially unnecessary allocation of health-care resources.8
EMS personnel must always keep in mind that even with a normal ECG, a patient still may be having an MI. Therefore, signs and symptoms and medical history should support your treatment plan. Technology enables us to diagnose and treat a wide variety of illnesses and injuries, but it can only perform as well as the provider operating the equipment.
Wayne A. Riddle, RN, MSN, CFRN, PHRN, has been involved in EMS for more than 10 years, as both a flight nurse with PennSTAR flight, Hospital of theUniversity ofPennsylvania,Philadelphia, and as a firefighter/PHRN with the Berwyn Fire Company,Berwyn,Pa. Contact him firstname.lastname@example.org.
1. Batchvarov VN, Malik M, Camm AJ: ˙Incorrect electrode cable connection during electrocardiographic recording.Ó Europace. 9(11):1081Ï1090, 2007.
3. Dubin D: Rapid interpretation of EKGs, sixth edition. Cover Inc.:Fort Myers,Fla., 2000.
4. Drew BJ: ˙Pitfalls and artifacts in electrocardiography.Ó Cardiology Clinics. 24(3):309Ï315, 2006.
5. Coleman ME: ˙A technicians_ perspective: Tips and techniques for using disposable tab electrocardiogram electrodes; an easy to remember method for preventing electrocardiogram lead interchange.Ó Journal of Electrocardiology. 39(4):355Ï357, 2006.
6. Rudiger A, Sch·b L, Follath F: ˙Influence of electrode misplacement on the electrocardiographic signs of inferior myocardial ischemia.Ó American Journal of Emergency Medicine. 21(7):574Ï577, 2003.
7. Sejersten M, Sillesen M, Hansen PR, et al: ˙Effect on treatment delay of prehospital teletransmission of 12-lead electrocardiogram to a cardiologist for immediate triage and direct referral of patients with ST-segment elevation acute myocardial infarction to primary percutaneous coronary intervention.Ó American Journal of Cardiology. 101(7):941Ï946, 2008.
8. Hedôn B, Ohlsson M, Holst H, et al: ˙Detection of frequently overlooked electrocardiographic lead reversals using artificial neural networks.Ó American Journal of Cardiology. 78(5):600Ï604, 1996.