Critical Care Nurse. 2005;25: 28-37
Copyright © 2005 by the American Association of Critical-Care Nurses.
Clinical Article
CE Article
Clinical Usefulness of the EASI 12-Lead Continuous Electrocardiographic Monitoring System
Mary Jahrsdoerfer, RN, MHA
Karen Giuliano, RN, PhD
Dean Stephens, RN, MS
Mary Jahrsdoerfer is a clinical consultant for Philips Medical Systems in the New York–New Jersey metropolitan area.
Karen Giuliano is a clinical research specialist for Philips Medical Systems in Andover, Mass.
Dean Stephens is the assistant director of nursing at North Shore University Hospital in Manhasset, NY.
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*This article has been designated for CE credit. A closed-book, multiple-choice examination follows this article, which tests your knowledge of the following objectives:- Identify the differences between standard and continuous 12-lead systems
- Identify 3 clinical conditions that are appropriate for continuous 12-lead monitoring
- Describe the benefits of examining 12-lead electrocardiograms in the clinical setting
The 12-lead electrocardiogram (ECG) is the diagnostic reference standard for evaluating cardiac rhythm and myocardial ischemia. The traditional 12-lead ECG system requires 10 electrodes that are strategically placed on the chest and the extremities (Figure 1
). This lead configuration, however, is not practical for ambulatory patients in the telemetry setting. Subsequently, over the years, we have used a tool that captures limited and partial ECG data with just 1 or 2 views of the heart. This tool consists of a portable monitor (commonly referred to as a telemetry box), and 3 or 5 leads (depending on the type of telemetry that the institution has installed). Typically, 4 leads are attached to the patient’s torso (modified extremity leads), and the last lead acts as the precordial or "V" lead (Figure 2). This method has been used successfully for continuous cardiac monitoring.1
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Figure 1 Placement of leads for standard 12-lead electrocardiography.
Abbreviations: LA, left arm; LL, left leg; RA, right arm; RL, right leg.
Used with permission from Philips Medical Systems, Andover, Mass.
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Figure 2 Typical placement of leads for 3-lead (left) or 5-lead (right) telemetry with the Mason-Likar format.
Used with permission from Philips Medical Systems, Andover, Mass.
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Telemetry monitoring was not designed, however, to obtain 12 views of the heart, but rather to derive continuous monitoring information from the 1 or 2 most commonly used ECG leads. Recent research and national guidelines on monitoring for myocardial ischemia2–6 recommended continuous 12-lead ST-segment monitoring for at-risk patients because silent ischemia (without chest pain) is common and may not always be detected with the same leads. The busy environment of areas devoted to patients’ care, coupled with the need for frequent 12-lead ECG analysis, makes using a conventional, diagnostic 12-lead ECG device an impractical approach for continuous interpretation of cardiac rhythms and myocardial ischemia. What is most needed in the clinical monitoring environment is a continuous 12-lead ECG monitoring system to replace the traditional 3- to 5-lead telemetry monitoring system now used in most hospitals for continuous ECG monitoring.
In this article, we describe the clinical outcomes that occurred after implementation of a new 12-lead derived ECG system designed for continuous, bedside monitoring as a replacement for a traditional 3- to 5-lead system.
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Derived 12-Lead ECG for Continuous Monitoring
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Any time that the limb leads are moved away from the extremities and placed on a patient’s torso and the precordial leads are modified down to a single lead that is calculated for the unipolar precordial V lead on the horizontal plane, the result is termed a derived ECG.7 The main difference between a derived ECG configuration and the standard 12-lead ECG configuration is that all 12 views of the heart are available on a continuous basis with the derived configuration, rather than the periodic basis available with the traditional 12-lead ECG system.
On the basis of the clinical need for continuous 12-lead ECG monitoring and the impracticality of using a traditional 12-lead configuration for continuous monitoring, Dower and colleagues7,8 developed in 1980 a 12-lead ECG based on the vector ECG principles described by Frank.9 Instead of the traditional 10 electrodes, ECG signals are derived from 4 chest electrodes and 1 reference electrode (Figure 3). The electrodes are placed on the upper part of the sternum (S), the lower part of the sternum (E) at the level of the fifth intercostal space, and on the right and left midaxillary lines (I and A) at the same level as the electrode on the lower part of the sternum. A fifth ground electrode can be placed anywhere. This 5-lead configuration gives us a 3-dimensional portrayal of the electrical activity through the heart’s conduction system, rather than independent channels of unipolar and bipolar energy. The result is 12 leads of information based on acquisition of simulatneous events in the frontal, horizontal, and sagittal cardiac planes. A mathematical calculation using standardized and fixed coefficients for each lead results in a linear transformation of the vectors, creating a derived 12-lead ECG.
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Figure 3 Placement of leads for derived 12-lead electrocardiography. EASI lead placement: E, electrode placed on the lower part of the sternum, level with the fifth intercostal space; A, electrode placed at the level of the fifth intercostal space, on the left midaxillary line; S, electrode placed on the upper part of the sternum; I, electrode placed at the fifth intercostal space, on the right midaxillary line. The fifth ground electrode can be placed anywhere.
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Importance of Continuous Monitoring of Multiple ECG Leads
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One of the strongest arguments for continuous monitoring of all 12 leads of the ECG is the situation in patients with suspected or confirmed myocardial damage. ECG monitoring is more sensitive than a patient’s self-reporting of symptoms (eg, chest pain, chest discomfort) for detecting transient myocardial ischemia because 70% to 90% of the episodes of myocardial ischemia detected with ECG are clinically silent.2–5 A consensus group of experts in ST-segment monitoring6 have suggested a variety of clinical conditions and diagnoses other than myocardial ischemia that are appropriate for continuous 12-lead ECG monitoring (Table 1).
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Table 1 Recommendations for clinical conditions for which continuous 12-lead electrocardiographic monitoring may be beneficial6
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Implementation of a New 12-Lead Continuous Monitoring System
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In March 2002, we implemented a research study for a newly installed telemetry system at North Shore University Hospital, one of the leading medical centers on Long Island in New York. The study was implemented in 3 separate intensive care telemetry step-down units, a total of 128 telemetry beds. Before the new system was installed, ECG monitoring were accomplished by using a traditional 3-lead ECG system, with periodic standard 12-lead ECGs obtained as needed to detect various dysrhythmias or myocardial damage. Nonlicensed personnel (monitoring technicians) specially trained for ECG monitoring do the continuous observation of ECG rhythms in the unit.
The new telemetry system installed at North Shore University Hospital is currently being marketed as the EASI 12-lead system (Philips Medical Systems, Andover, Mass). The EASI 12-lead monitoring system is integrated into patients’ monitors, allowing display of up to 3 of the 12 continuously monitored derived leads. The EASI is the only system in the marketplace that does continuous 12-lead monitoring with only 5 electrodes. By using the stored data (referred to as the full-disclosure feature), data from all 12 leads of the ECG can be displayed and printed in either real time or retrospectively for any period in the preceding 24 to 96 hours. Automatic documentation of derived 12-lead ECGs can also be triggered by heart-rate alarms or ST-segment changes.7,8
All nursing and monitoring technical staff received extensive education and training on the new monitoring system before use of the system was implemented. The educational program consisted of information about the benefits of examining 12 leads of the ECG to improve diagnostic interpretation, how a 12-lead ECG is derived from the 5 leads of the 12-lead continuous monitoring system, and how and where to apply the 5 leads to the chest. Staff had ample time for hands-on experience with the new system, and each staff member completed a competency checklist before the new system was implemented.
Four months after implementation of the new monitoring system, data were collected for 4 months to evaluate the system’s performance. Members of the clinical staff were asked to complete a voluntary questionnaire whenever they were assigned to monitor ECG rhythms for the unit (Table 2).
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Table 2 Evaluation questionnaire completed by personnel monitoring electrocardiograms (ECGs) after implementation of a new continuous 12-lead ECG system
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Clinical Performance Outcomes
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During the 4 months of data collection (May through August 2002) to evaluate the new 12-lead system, a total of 64 surveys were completed by staff members. Survey responses were based on patients who were being monitored for a variety of reasons; the most common problems were actual or potential myocardial damage (myocardial infarction, chest pain, rule out myocardial infarction, 58%), atrial fibrillation (16%), and ventricular tachycardia (7%; Table 3).
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Table 3 Primary reasons for continuous electrocardiographic monitoring for 64 patients included in the evaluation
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In response to the questions about the use of the new 12-lead system, most surveys indicated use of the full-disclosure function of the system more than 10 times per shift, similar to the frequency of accessing the EASI 12-lead ECG (Table 4). Most of the time (75%), the decision to use the 12-lead components of the system was independently made by the monitoring technician, as opposed to deciding to use it because a physician requested it.
Of the surveys completed, 69% of the staff said that they changed their interpretation of ECG rhythms because of information obtained after examining the 12-lead stored information in the system (Table 4
, Figures 4–7). Survey results indicated that these changes in ECG interpretation led to a different therapeutic regimen 50% of the time.
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Figure 4 Change in interpretation of cardiac rhythm and patient’s treatment after stored 12-lead information is examined. A, monitor layout; B, traditional layout. Patient is monitored in leads I and V1 (A). Monitor technician, nurse, and physician all agree that rhythm is "V-Tach" (A, B). By using full-disclosure tools, EASI 12-lead electrocardiogram is pulled up to the screen (A). The rhythm shows torsades de pointes. Leads 1 and V1 are the only leads in which torsades de pointes is not evident on the electrocardiogram (A). The patient’s treatment was changed.
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Figure 5 Change in interpretation of cardiac rhythm with no change in patient’s treatment after stored 12-lead information is examined. Patient monitored in leads II and V1. V1 shows apparent 6-beat run of ventricular tachycardia. EASI 12-lead electrocardiogram is put on the screen. V6 shows downward beats of atrioventricular dissociation. EASI 12-lead electrocardiogram provided critical information that would have otherwise been missed with the 2-lead monitoring system. Interpretation of the rhythm was changed, but the treatment remained the same.
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Figure 6 Change in evaluation of pacemaker function. Patient was monitored in leads II and V1, with no evidence of pacemaker malfunction. EASI full-disclosure format was pulled up on the screen. The 12-lead electrocardiogram shows that the pacemaker was undersensing. The patient was sent to the electrophysiology laboratory for reprogramming of the pacemaker.
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Figure 7 Early warning of myocardial infarction. Patient complains of chest pain. Monitor technician pulls up EASI 12-lead electrocardiogram (ECG) immediately, which shows an evolving myocardial infarction. Minutes pass before staff can get an order for and obtain a standard 12-lead ECG. Physician compares findings of the 2 methods and finds identical ECG patterns of an evolving myocardial infarction. Patient receives treatment for acute myocardial infarction.
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Staff described 49 anecdotes of differences in rhythms observed on the continuous 2-lead display and when the full-disclosure 12-lead system was accessed (Table 5). Of the 49 anecdotes, 15 described situations involving ventricular tachycardias or wide complex tachycardias for which interpretations of rhythms were changed after review of the full-disclosure function of the 12-lead EASI.
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Implications of Clinical Performance Outcomes Monitoring of Cardiac Rhythm
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In previous surveys10 of critical care clinicians, researchers found that the most common ECG lead used for continuous ECG monitoring in a single-channel monitor was lead II (74%); the second most commonly used was lead MCL1 (18%). Half of the survey respondents used a single-channel monitor, 48% used a dual-channel monitor, and the remaining 2% used systems that allowed 3 or 4 leads to be displayed simultaneously. The clinical reality that most clinicians use lead II for continuous monitoring is in stark contrast to a growing body of ECG monitoring research10–24 that monitoring and diagnostic capability are better when multiple leads and ST-segment monitoring are used.
As indicated in several studies by Drew and colleagues,11–13 ventricular tachycardia and supraventricular tachycardia with aberrant conduction clearly cannot be correctly differentiated with lead II. Only a third of these tachycardias were correctly identified when lead II of the ECG was examined; the best leads for correct diagnosis were leads V1 and MCL1. The visibility of the P wave and the diagnostic morphology of the QRS complex are both better in lead V1 or lead MCL1 than in lead II; thus leads V1 and MCL1 are better than lead II for differentiating supraventricular tachycardia with aberrant conduction from ventricular tachycardia. Because treatment of supraventricular tachycardia with aberrancy differs dramatically from treatment of ventricular tachycardia, misdiagnosis of these cardiac rhythms could result in deleterious clinical outcomes.14,15 Correct identification of rhythm is critical and requires use of leads other than lead II.
Other arrhythmias that may not be easily identified in lead II, compared with other leads, include atrial flutter, bundle branch blocks, and heart blocks.1,16 In addition, many abnormal cardiac rhythms are identified correctly more often when more than a single lead is used for evaluation.1,11–13
Our experience with the implementation of a continuous, derived 12-lead ECG monitoring system emphasizes the results of prior research on the best leads for monitoring. The ability to check multiple leads when evaluating a dysrhythmia resulted in numerous anecdotal notations of changes in diagnoses from initial interpretations, and many of those changes resulted in different therapies. Many of these changes in interpretation were related to differentiation of ventricular tachycardia from supraventricular tachycardia with aberrant conduction.
Monitoring of Myocardial Ischemia
The strongest case for the use of continuous 12-lead ECG monitoring comes from clinical studies of patients at risk for myocardial ischemia.17–25 The value of continuous monitoring of ST segments in multiple leads of the ECG has been shown in a variety of patients. Continuous 12-lead ST-segment monitoring was highly sensitive and specific for identifying reperfusion and vessel patency after reperfusion therapy in patients with acute coronary syndromes17–20 and in patients being treated with anti-ischemic drugs.22
Continuous ST-segment monitoring is particularly beneficial in predicting vessel patency in patients with extensive initial elevation of the ST segment, patients who are at highest risk for adverse cardiac events. Studies of patients with acute coronary syndrome and no persistent elevation of the ST segment have shown that ST-segment monitoring in all 12 leads significantly increases the detection of ischemic episodes compared with ST-segment monitoring in 3 leads.21,23 Patients in the emergency department with suspected acute coronary syndrome who have transient ischemic episodes also benefit from continuous 12-lead ECG monitoring.24,25
Recent studies5,26,27 of patients at high risk for myocardial ischemia after elective surgery indicated a 6% to 11% incidence of myocardial ischemia after vascular surgery. Of interest in these studies is the inability to detect these episodes of ischemia with traditional single- or dual-lead monitoring. Even 2-lead ST-segment monitoring had a low sensitivity (<5%) for detecting myocardial ischemia postoperatively, because the leads selected for standard monitoring (II and V5) rarely were involved in the ischemia patterns (V2 to V4).5
An interesting finding of ST-segment research is that monitoring the ECG leads that have the greatest ST-segment deviation during acute myocardial infarction or balloon inflation during cardiac catheterization, also called the "fingerprint" leads, is not sufficient to detect future episodes of ischemia.10,21,22 On the basis of those studies, it is recommended that monitoring of patients with the potential for ischemia include all 12 leads rather than just the fingerprint leads.6
Conventional 12-Lead ECG Versus Derived 12-Lead Monitoring Systems
One aspect of the new monitoring system that initially concerned clinicians was how well the EASI derived 12-lead monitoring system approximated the standard 12-lead ECG. Although research has shown that use of the EASI 12-lead system for continuous monitoring results in the clinical detection of more ischemic episodes and the capture of more arrhythmias (because data are collected continuously rather than as a snapshot in time),12,28–36 the conventional 12-lead ECG remains the reference standard as a "diagnostic" tool.
Having a continuous ECG system with all 12 leads stored in the computer memory was also better than standard 12-lead ECGs for verifying changes in cardiac rhythm or myocardial oxygenation. Cardiac rhythms or ischemic episodes that were of short duration could always be recalled by using the full-disclosure system of the EASI system. Such recall not only allowed accurate documentation of the changes in rhythm or myocardial oxygenation but also allowed the staff to treat the rhythm or perfusion problem more quickly because the clinical information was readily available.
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Summary
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Although the conventional 12-lead ECG is routinely used for diagnostic purposes, it is not practical for continuous monitoring. However, the standard telemetry placement produces only some of the ECG information that is needed, highlighting only 1 or 2 views of the heart. Technological advancements now enable us to obtain all 12 views of the heart on a continuous basis by using just 5 electrodes. By shifting the axis in which the 5-lead ECG is obtained, we can have important 12-lead information at our fingertips on a continuous basis. The clinical advantages of knowing that valuable 12-lead information can be obtained at any given time or at a designated time during any shift is important for correct diagnosis and treatment of ECG abnormalities.
In this study, the ability to generate both real-time and retrospective 12-lead ECGs resulted in important differences in clinical management. These results suggest that use of continuous 12-lead monitoring of the ST segment in all patients in the cardiac step-down area would have a clinical benefit.
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Acknowledgments
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We acknowledge the staff in the cardiac telemetry units at North Shore University Hospital, Manhasset, NY. Without their assistance and support, this project would not have been completed. Philips Medical Systems (Andover, Mass) provided partial funding for this project.
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References
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- Gottlieb SD, Wisfeldt ML, Ouyang P, Mellits ED, Gerstenblith G. Silent ischemia as a marker for early unfavorable outcomes in patients with unstable angina. N Engl J Med. 1986;314:1214–1219.[Abstract]
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