Setting a New Gold Standard

Abstract

The diagnostic term acute coronary syndrome (ACS) is more commonly used to describe myocardial ischemic events such as unstable angina, non-Q-wave myocardial infarction (MI), and Q-wave MI, and encompasses the cyclic and dynamic processes of atherosclerotic plaque rupture, platelet stimulation, coronary spasm, and thrombus formation related to a coronary occlusive event. Traditionally, patient history, physical examinations, and static 12-lead electrocardiogram (ECG) represented the “gold standard” for detection and diagnosis of acute myocardial infarction (AMI) and unstable angina. However, they did not detect the atypical symptoms such as heartburn or clinically silent symptoms of ongoing ischemia with which patients with ACS often present. In the recent past however, ST-segment monitoring studies have shown that patients with myocardial ischemia don’t often present with clinical symptoms such as chest pain, nausea, and shortness of breath. This validates the limitations of static ECG in the early detection, monitoring, and diagnosis of ACS. 1,2 Under normal conditions, the ST-segment is isoelectric because of an electric standstill between the end of ventricular depolarization and the beginning of ventricular repolarization. The isoelectric line is represented by the TP-segment of the ECG waveform, which does not deflect positively or negatively from the baseline. During episodes of tachycardia the PR-segment is considered to be the isoelectric line. During an acute coronary event, the ischemic myocardium is not adequately oxygenated. The electric current is disturbed, resulting in ST-segment abnormalities. ST-segment deviation is represented by the number of millimeters the ST-segment is displaced from the isoelectric line. This is measured at 60 or 80 ms after the J point (see ST-Segment Deviation, page 6). ST-segment elevations of more than 1 mm often indicate abnormalities with repolarization and are suggestive of myocardial ischemia.FIGUREFigure. ST-Segment Deviation: (A) Normal electrocardiogram complex. Measurement points used in ST-segment analysis are indicated. The TP or PR segment is used to define the isoelectric line. The ST-segment measurement point is measured at 60 or 80 ms past the J point. (B) ST-segment shown measures +4 mm. The measurement point used is 80 ms past the J point. (C) ST-segment depression. The ST-segment shown measures −4 mm. The measures point used is 80 ms past the J point. Source: Tisdale LA Drew BJ: ST-segment monitoring for myocardial ischemia. AACN Clin issues 1993, 4(T) 34–43. With permission.Mitchell Krucoff and colleagues introduced the concept of “ST fingerprint,” which is defined as the patient’s unique 12-lead ECG pattern of elevation and depression in the ST-segment, based on the anatomical site of the coronary artery that is occluded and receiving inadequate blood flow to perfuse the myocardium. 3 This unique ischemic pattern can be recorded on the 12-lead ECG during the first few hours of ACS, or during coronary angioplasty when the balloon is inflated and temporarily occludes the affected artery. On a 12-lead ECG, ST-segment elevation and T-wave inversion occur in the leads facing the ischemic myocardium, with ST-segment depression appearing in the reciprocal leads. In current clinical practice, continuous ST-segment monitoring is underused for patients with ACS even though the software is widely available in bedside cardiac monitors. This underuse appears to be attributed to technical problems—including false positive alarms and lack of hardware or software for accurate ST-segment analysis—that can interfere with transmitting an accurate ECG signal to the ST monitor. Other obstacles include absence of practice standards and guidelines and a lack of consensus by physicians about the necessity of continuous ST-segment monitoring. These issues need to be addressed during the process of developing and planning a successful ST-segment monitoring program. PREANALYSIS In planning a program, it’s important to first assess who would most likely benefit from ST-segment monitoring. ST-segment monitoring is not limited to patients with diagnosis of AMI, unstable angina, or postcoronary intervention (such as angioplasty, atherectomy, and stent placement). Others who may benefit are patients admitted to the emergency department with chest pain, as well as postoperative cardiac and noncardiac surgery patients. 2,4 Patients who are not candidates for ST-segment monitoring include those with left bundle branch block, intermittent left or right bundle branch block, and ventricular paced rhythm. Prospective or retrospective analyses of the number and length of hospital stays of patients with the aforementioned diagnoses can provide volume and frequency data to support program development. The analysis also needs to include a review of the nursing units or departments responsible for patients, identifying and prioritizing those departments to be ST-segment monitoring sites. Such sites include the emergency department, cardiac catheterization lab, and critical care, transitional care, and telemetry units. The operating and recovery rooms represent newly identified areas where patients at high risk for cardiac complications can also be monitored. Finally, in planning resource allocations, it’s important to consider the costs of hiring registered nurses compared to telemetry technicians for monitoring, initial training time, and ongoing education and clinical support. THE TEAM Members of this group should include a nurse and physician champion, advanced practice nurses, administrative nurses, clinical engineers, and a quality improvement (QI) analyst. The role of this interdisciplinary team will be to evaluate monitoring systems and develop policies, procedures, and implementation plans for the program. After ST-segment monitoring is implemented, this group will continually evaluate QI data related to process outcomes and standards, and will also function as the advisory group for the program. CHOOSING MONITORING SYSTEMS Ideally, a monitoring system needs to display and print the 12-lead ECG, perform ST-segment analysis of all 12 leads, and review full retrospective disclosure of all 12 ST-monitoring leads and alarms. Additional factors to consider when evaluating monitoring systems include the service reputation of the vendors, the support they provide (education and training as well as on-site support), the manufacturer’s research and development priorities with regard to advancing technologies and timelines for release of upgrades, and finally, accessibility to software upgrades that enter the market. In selecting a monitoring system, evaluate whether it’s capable of bedside monitoring with hard wires or wireless telemetry monitoring. Next, evaluate lead configuration choices as well as the associated number of electrodes (see Three 12-lead ECG Lead Configurations for ST-Segment Monitoring, page 8). There are three primary lead configurations available—Mason-Likar, Frank, and EASI—and all three have demonstrated effective ST-segment monitoring. The modified and derived lead configurations can produce a different ECG waveform from the traditional 12-lead ECG configuration. Mason-Likar, Frank, and EASI result in associated alterations in the ST-T wave morphology. It’s important that the traditional 12-lead ECG and the derived or modified 12-lead ECG not be compared for diagnostic purposes.FIGUREFigure. Three 12-lead ECG Lead Configurations for ST-Segment Monitoring: Left panel: Modified standard ECG (Mason-Likar) lead configuration. Middle panel. Frank vectorcardiographic lead configuration from which a 12-lead ECG is derived. Right panel: EAST lead configuration from which a 12-lead ECG is derived. Source: Drew BA, Krucoff MW, Multilead ST-segment monitoring in patients with acute coronary syndromes. A consensus statement for healthcare professionals. Am J Crit Care 1999; 8(6): 372–385. With Permission.Mason-Likar. The modified standard 12-lead ECG utilizes the Mason-Likar lead configuration in which the limb leads are placed on the torso. 5 Ten electrodes are required to record eight channels of ECG information (lead I, lead II, and the six precordial leads). The four limb leads—lead III, aVR, aVL, and aVF—are derived from leads I and II. This configuration is preferable to the standard 12-lead electrode positioning. The torso-positioned configuration reduces false alarms related to movement of the patient’s arms and legs, which cause artifact in the ECG tracing. Frank. The Frank vectorcardiographic lead configuration utilizes a second derived 12-lead method. This configuration requires eight electrodes to record three channels of ECG information (X, Y, and Z leads). Through vectorcardiography, the 12-lead ECG is derived from these three leads. The Frank lead configuration requires placing electrodes on the patient’s back as well as under the right axilla. This may be why the Frank lead system is more sensitive than the standard 12-lead ECG for detecting posterior wall and right ventricular myocardial ischemia. EASI. The EASI 12-lead ECG method utilizes a modified Frank lead configuration and requires five electrodes to record three channels of ECG information. Dower and colleagues utilized this five-electrode configuration to derive the 12-lead ECG referred to as the EASI 12-lead ECG. 6 It’s available as a bedside hard wire and telemetry monitoring system. THE CRITERIA Establish criteria to determine which patients require ST-segment monitoring based on risk stratifications, determination of the time frames for ST-segment monitoring, clinical goals, and recommendations for monitoring lead selection. It’s also important that practice guidelines outline what nurses need to do when a ST-monitoring alarm occurs. In 1999, Barbara Drew and Mitchell Krucoff were instrumental in developing a multilead ST-segment monitoring consensus statement for health care professionals, which provides recommendations for the initiation and duration of ST-segment monitoring. 2 According to the consensus statement, patients with unstable angina or AMI need to receive ST-segment monitoring for 24 to 48 hours after the onset of signs and symptoms of ischemia or diagnosis of ACS, or until they remain free of signs and symptoms for 12 to 24 hours. For acute ST-elevation MI, there are two high-risk periods in which monitoring is recommended. The early period is the first six hours after thrombolysis or coronary intervention. Then monitor for recurrent ischemia six to 48 hours after therapy (late period). The goals of monitoring in these situations are to assess blood flow and oxygen delivery through the affected coronary artery. In the emergency department, eight to 12 hours of ST-segment monitoring in combination with monitoring serum troponin levels is an efficient way to triage patients with chest pain. Initiating ST-segment monitoring in the cardiac catheterization lab with radiolucent electrodes and lead wires is recommended practice for patients undergoing coronary interventions. This provides practitioners with the opportunity to document the patient’s unique “ST fingerprint” associated with transient occlusion of the coronary artery during catheter balloon inflation. Monitoring for six to 12 hours postprocedure is recommended for patients with other ongoing medical problems, for hemodynamic instability, and for patients who undergo more complex multivessel catheter-based interventions. Postprocedure monitoring has two goals: to detect reocclusion at the anatomical site of the coronary artery that was blocked because of vessel dissection or thrombosis, and to distinguish ischemic from nonischemic chest pain. In contrast to angioplasty, abrupt, postprocedure reocclusion occurs less often with intracoronary stenting. Approximately 50% of patients who have a stent implanted and 12% of patients who undergo angioplasty experience chest pain after the procedure. 4 Differentiating between abrupt occlusion requiring revascularization and benign pain is essential to prevent unnecessary recatheterization or transfer back into the intensive care unit. Since fluid shifts and hypercoagulable conditions occur during the first 24 to 48 hours after surgery, ST-segment monitoring is recommended during this period to monitor for myocardial ischemia. Monitoring ST-segment changes in all 12 ECG leads is recommended for the accurate detection of myocardial ischemia for patients with ACS. Myocardial ischemia may be the result of several contributing physiologic process that cause changes in different leads at different times. 1 The lead configuration recommended as most valuable for detecting occlusion in one of the three coronary arteries uses nine out of the 12 standard ECG leads. The classic ECG pattern produced by coronary occlusions is ST-segment elevation detected by the leads that lie directly over the ischemic myocardium. For a right coronary artery (RCA), ST-segment leads II, III, and aVF are recommended; the left anterior descending artery (LAD) requires leads V2, V3, and V4. For the left circumflex artery, a variety of leads might be monitored depending on which myocardial zone (lateral, inferior, or posterior) is affected. For myocardial zones, suggested monitoring leads for the lateral left circumflex branch are leads V5 and V6; the inferior left circumflex branch requires leads II, III, and aVF; and for the posterior region of the heart, leads V1, V2, and V3. In the absence of 12-lead ECG ST-segment monitoring analysis software, the use of leads III and V3 is recommended for patients with ACS if a two-lead bedside cardiac monitoring system is available. The desired three-lead combination is leads III, V3, and V5. The three-lead combination has limitations because most bedside cardiac monitors are only capable of monitoring a single precordial (V) lead with a single chest electrode. 2 Both the two-and three-lead systems exclude V1, which is considered the best lead to monitor for detection of arrhythmias. EDUCATION IS KEY Competency-based educational programs are critical if clinicians are to provide safe and effective ST-segment monitoring of patients. Initial educational offerings need to include a combination of didactic presentations and hands-on skills demonstrations for nurses, physicians, and telemetry technicians. Vendors usually have clinical systems specialists who can assist with initial education and hands-on training. Educational objectives need to achieve the following minimum clinical performance expectations recommended by the 1999 ST-segment monitoring consensus for health care professionals:2 Accurately place and consistently maintain ECG leads. Identify the lead that shows peak ST-elevation and recognize rapid ST-recovery versus sustained ST-elevation in AMI. Recognize abrupt reocclusion during monitoring of patients postcoronary intervention. Recognize false alarms that are related to a noisy signal or to transient arrhythmia. Assess the patient’s cardiac symptoms and hemodynamic status to determine the clinical importance of changes in the ST-segment. TROUBLESHOOTING Accurate ST-monitoring requires expertise in interpreting the 12-lead ECG, an understanding of the patient’s clinical situation, and knowledge of both the functions and limitations of the monitoring system. The presence of artifact in the ECG waveform can interfere with accurate diagnosis of a clinically significant ST-segment deviation. In order to ensure that the ST-segment monitoring is accurate and clinically relevant, careful preparation of the patient’s skin includes shaving areas where the electrodes will be placed and prepping the area with alcohol to remove skin oils. Marking the locations of the electrodes with indelible ink ensures accurate replacement of the electrodes if they need to be removed for any reason. ECG waveform changes resulting in misdiagnosis can occur, especially when the electrodes are misplaced as little as 1 cm away from the original location. A baseline ST level with the patient in a supine position needs to be established upon initiating ST-segment monitoring. Experts suggest programming the monitor to measure the ST-segment at J point plus 60 msec to decrease the false alarms that may occur if the patient develops tachycardia. 2 The ST alarm parameters need to be set 1 to 2 mm above and below the patient’s baseline ST level (rather than at the isoelectric level). It’s rare that the baseline ST-segment is isoelectric due to chronic repolarization abnormalities seen in patients with left ventricular hypertrophy or digitalis therapy. 7 The ST analysis monitor is configured to detect a change in the ST-segment and to record the degree, extent, and timing of the change. The bedside health care professional interprets whether or not the ST value is clinically significant. The ST findings in question need to be printed out on a graphic recording to determine whether the changes are related to ischemia, a transient arrhythmia, or tachycardia. Body positioning can produce false ST alarms from changes in polarity of the electric signal that occurs when turning from side to side. When an alarm condition occurs and the patient’s body position is unknown, a 12-lead ECG needs to be recorded from the ST monitor with the patient supine, and compared to the baseline ST measurement. ST-segment monitoring can provide a method for early detection of myocardial ischemia in patients with ACS. It’s a cost-effective way to rule out myocardial ischemia rather than admitting patients to the hospital for additional tests and monitoring. Early diagnosis and treatment of ischemia directly improves patient outcomes by minimizing damage to the heart muscle. However, further research is necessary to evaluate whether ST-segment monitoring does in fact contribute to decreased length of stay for patients and improved outcomes as a result of accurate, timely diagnosis, and treatment of ACS. Therefore, for ST-segment monitoring programs to significantly effect patient outcomes, commitment from health care institutions for a planning and implementation process will be essential.▾