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American Journal of Critical Care. 2002;11: 318-325
Copyright © 2002 by the American Association of Critical-Care Nurses.
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Association of Transient Myocardial Ischemia With Adverse In-Hospital Outcomes for Angina Patients Treated in a Telemetry Unit or a Coronary Care Unit

By Michele M. Pelter, RN, PhD, Mary G. Adams, RN, PhD and Barbara J. Drew, RN, PhD. From the Department of Physiological Nursing, University of California, San Francisco (MMP, BD) and the Department of Nursing, State University of New York, Buffalo (MGA).


   Abstract
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 Abstract
Methods
 Results
 Discussion
 Conclusions
 References
 
• Background Little is known about the frequency or consequences of transient myocardial ischemia in patients admitted to a telemetry unit for treatment of angina.

• Objectives To compare the rate of transient myocardial ischemia in a group of patients with angina treated in a telemetry unit with the rate in a similar group treated in a coronary care unit and to determine if transient myocardial ischemia is associated with adverse in-hospital outcomes.

• Methods Continuous 12-lead electrocardiography was used to monitor changes in the ST segment in 186 patients in the coronary care unit (1994–1996) and 186 patients in the telemetry unit (1997–2000). Transient myocardial ischemia was defined as a change from baseline of 100 µV or more in the ST segment in 1 or more leads lasting 60 seconds or longer.

• Results The rate of transient myocardial ischemia was 15% for patients in the telemetry unit and 19% for patients in the coronary care unit. Regardless of hospital unit, patients with transient myocardial ischemia were more likely than those without this complication to experience death or acute myocardial infarction after hospital admission. Most patients did not experience signs or symptoms during transient myocardial ischemia: 71% of patients in the telemetry unit versus 58% of patients in the coronary care unit (P = .28).

• Conclusions Transient myocardial ischemia is common among patients with angina treated in a telemetry unit. ST-segment monitoring may be useful for detecting patients with ischemia who may benefit from more aggressive therapies aimed at abolishing ongoing ischemia.

In patients with acute coronary syndromes, myocardial ischemia may develop spontaneously. The mechanisms responsible for acute ischemia include fissure and/or rupture of plaque, stimulation and aggregation of platelets, and eventual thrombus formation, with resultant coronary occlusion. Continuous ST-segment monitoring is a simple noninvasive method that can be used to reflect the status of myocardial perfusion. Importantly, changes in the ST segment are often the only indication of myocardial ischemia, because more than three quarters of ischemic episodes are clinically silent.4–7

ST-segment monitoring has been used primarily in the coronary care unit (CCU), because this unit has traditionally served as the initial hospital admission site for patients with acute coronary syndromes. However, many hospitals have replaced, or substantially reduced, the use of the CCU in favor of the more cost-efficient telemetry unit. For example, the telemetry unit, rather than the CCU, is now the initial admission site for patients who have angina8 and for patients who have had percutaneous coronary interventions.9 This change has resulted in a substantial increase in the number of patients with coronary artery disease (CAD) in the telemetry unit who are at risk for myocardial ischemia.

Despite the increased use of the telemetry unit, the rate and consequences of transient myocardial ischemia (TMI) diagnosed via ST-segment monitoring have not been determined. The purpose of our study was to compare the frequency of TMI in a group consisting of patients with unstable angina or patients with CAD admitted for cardiac catheterization and possible percutaneous coronary interventions treated in the telemetry unit from 1997 to 2000 with the frequency in a similar group of patients treated in the CCU from 1994 to 1996. A secondary aim was to determine if patients with TMI were more likely than patients without TMI to experience adverse in-hospital outcomes.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 Conclusions
 References
 
Research Design
The study reported here was a secondary analysis of data on more than 1100 patients with acute coronary syndromes who were enrolled in 2 prospective clinical trials: the ST Analysis Trial (STAT) study5 and the ST Analysis and Monitoring of Patients and Evaluation of a Derived Electrocardiogram (STAMPEDE) study.10 The purpose of the 2 trials was to determine the value of an experimental 12-lead configuration for electrocardiographic (ECG) detection of TMI. In the STAT study, CCU patients were enrolled from January 1994 through December 1996; in the STAMPEDE study, telemetry unit patients were enrolled from August 1997 through April 2000.

Sample
Patients admitted to either the CCU or the telemetry unit for treatment of acute coronary syndromes were enrolled as soon as possible after admission. Verbal consent was obtained for the non-invasive ECG monitoring studies in all of the patients as approved by the appropriate institutional review board. Subjects enrolled in the parent studies were patients with acute myocardial infarction or unstable angina and patients with CAD admitted for cardiac catheterization and possible percutaneous coronary interventions. The diagnosis of CAD was based on (1) documentation of prior myocardial infarction, percutaneous coronary interventions, or coronary artery bypass surgery; (2) coronary angiogram showing a blockage of 70% or greater in one or more coronary vessels; or (3) occurrence of acute myocardial infarction during the current hospitalization. Patients with acute myocardial infarction were excluded from the analysis reported here because the CCU group would contain a disproportionately greater number of patients with acute myocardial infarction than the telemetry unit group would, a situation that would make the 2 groups different in terms of expected outcomes. Therefore, the sample for the current analysis included patients with CAD admitted to the hospital with unstable angina and/or for diagnostic cardiac catheterization and possible percutaneous coronary interventions. Also excluded from the analysis were patients with left bundle branch block or ventricular pacing rhythm, because these conditions distort the ST segment, making it difficult to determine the presence of TMI.

Because the major aim of the 2 parent studies was to compare various multilead ECG monitoring systems, specially trained research nurses were required to initiate ST-segment monitoring to ensure accurate lead placement. Thus, enrollment of patients occurred when the research nurse was available. During the STAT study, research nurses enrolled consecutive patients Monday through Sunday from 7 AM to 7 PM. During the STAMPEDE study, research nurses enrolled patients Monday through Friday from 7 AM to 7 PM.

Data Collection
All patients had routine ECG monitoring per the protocol of the respective units. In addition, 12-lead ST-segment monitoring was initiated by using a separate ECG device designed specifically for detection of ST-segment changes indicative of TMI. The 12-lead ECG device used for ST-segment monitoring was configured so that there were no audible alarms or alarm-triggered ECG tracings; hence, clinicians did not have access to this ECG data for clinical decision making. The ECG device for ST-segment monitoring stores ECG data, which are then downloaded to a personal computer for off-line analysis.

In order to maintain continuous 12-lead ECG monitoring in the parent studies, 4 part-time research nurses and 1 full-time project director were employed who provided clinical coverage as described. The research nurse responsible for clinical coverage provided in-house coverage from 7 AM to 7 PM. This same research nurse was then available to the nursing staff by pager from 7 PM to 7 AM to assist with troubleshooting problems via telephone.

Equipment
The Mortara ELI 100 ST Monitor (Mortara Instrument Inc, Milwaukee, Wis) was used for continuous 12-lead ST-segment monitoring. The ELI 100 monitor is a portable, programmable, microprocessor-based device that acquires, analyzes, and stores 12-lead ECGs at programmed intervals or when a change in the ST segment is detected. The monitor was programmed with filter settings of 0.05 to 109 Hz, as recommended by the American Heart Association for ischemia analysis,11 with a standard calibration setting of 100 µV = 1 mm and a paper speed of 25 mm/s, which are the standards used for clinical practice.12 Stored ECGs from the ELI 100 monitor were transmitted to a personal computer with additional analysis software (Mortara ST Review Station Computer) for in-depth analysis of the computerized ST-segment measurements.

Procedures
  Analysis of ST-Segment Monitoring Data.   TMI was defined as a change in the ST segment (elevation or depression), compared with baseline data, of 100 µV or greater, in 1 or more ECG leads, lasting 60 seconds or longer.12 A second event in a single patient was counted only if the ST segment returned to a baseline level for at least 60 seconds and then became deviated again. The process for determining the presence of TMI with the ST Review Station was as follows: At the end of the monitoring period, all stored ECGs with a noisy signal or with changes in the ST segment due to nonischemic conditions (ie, accelerated ventricular rhythm, intermittent ventricular pacing rhythm, or intermittent bundle branch block) were excluded from the analysis. In addition, in order to limit false-positive changes in the ST segment due to changes in body position,13,14 the research assistants obtained "template" body-position ECGs when patients were supine, lying on the left side, and lying on the right side. Any ECG obtained during a potential ischemic event was then compared with the template ECGs to ensure that the observed changes in the ST segment were not due to changes in body position.

The "cleaned" ECG data were then evaluated for changes in the ST segment (ie, elevation or depression) indicative of TMI, and computer-generated values were used to determine if the changes exceeded 100 µV of ST-segment deviation. When a potential ischemic event was observed, three 12-lead ECGs were printed out: (1) a "baseline" 12-lead ECG obtained before the changes in the ST segment, (2) an "event" ECG obtained during the changes, and (3) a "return to baseline" ECG obtained after the event.

  Comparison of Groups.   In order to determine whether the CCU group was comparable to the telemetry unit group in terms of prognosis, the Norris Coronary Prognostic Index (CPI) was used.15 Readily available clinical data, including age, history of heart disease, and evidence of heart failure, are used in this scoring system. The research nurses in the parent studies prospectively obtained information used to determine the Norris CPI, including demographic information, medical history, and a chest radiograph obtained at the time of admission to the hospital.

  Adverse In-Hospital Outcomes.   In-hospital complications were defined as acute myocardial infarction after hospital admission, as evidenced by elevation of serum levels of troponin I, or cardiovascular death.

  Symptomatic Ischemia.   For every ECG-detected ischemic event, the research nurses determined whether the patient experienced any signs or symptoms during the event. A detailed 24-hour diary was maintained by the research nurses, who communicated frequently with patients, nurses, and physicians about signs and symptoms and who abstracted each patient’s medical chart, including nurses’ and physicians’ notes. Symptoms of ischemia were defined as any documented complaint by the patient of chest pain, pressure, heaviness, tightness, squeezing, or dullness in the center of the chest. In addition, any anginal equivalents, such as diaphoresis, shortness of breath, nausea, or pain in the jaw, neck, or left arm, were counted as a sign or symptom of ischemia.

Statistical Analyses
Descriptive statistics were used to report demographic and clinical information. These values are expressed as mean and SD or percentages. Chi-square analysis was used to compare categorical variables. Chi-square analysis was also used to determine if the rate of in-hospital outcomes differed among patients with and without TMI. Two-tailed, unpaired t tests were used to compare continuous variables. A P value less than .05 was adopted as the critical value to determine if the differences between the 2 groups were significant.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 Conclusions
 References
 
Characteristics of the Sample
A total of 446 patients who had unstable angina or who had CAD and were admitted for cardiac catheterization and possible percutaneous coronary interventions were considered for the analysis reported here. Of the 446 patients, 236 (53%) were from the STAT database (1994–1996), and 210 (47%) were from the STAMPEDE database (1997–2000). Of the 236 patients from the STAT study, 50 (21%) were excluded because they were readmitted to the hospital during the study period. Of the 210 patients from the STAMPEDE study, 15 (7%) were excluded because they were readmitted to the hospital during the study period, and 9 (4%) were excluded because they had technically unanalyzable ECG data. Hence, a total of 372 patients were included in the final analysis. Coincidentally, 186 were monitored in the CCU from 1994 through 1996, and 186 were monitored in the telemetry unit from 1997 through 2000.

The clinical characteristics of the 372 patients in the study sample are given in Table 1Go. Even though patients with acute myocardial infarction were excluded, patients in the CCU group (1994–1996) had more advanced CAD (ie, prior myocardial infarction and higher Norris CPI score) and fewer prior percutaneous coronary interventions than did the telemetry group (1997–2000). The telemetry group was older than the CCU group and had fewer percutaneous coronary interventions procedures during hospitalization. Not surprisingly, the telemetry unit, rather than the CCU, was the initial admission site for 161 (87%) of the 186 patients from the most recent group of patients (1997–2000), whereas the CCU was the initial admission site for 161 (87%) of the 186 patients in the earlier period (1994–1996).


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  		Table 1 Clinical characteristics of the sample*

 
Rate of Ischemia
The variables related to continuous 12-lead ST-segment monitoring and the frequency of TMI are presented in Table 2Go. The mean time from hospital admission to the initiation of continuous 12-lead ST-segment monitoring did not differ signif icantly between the CCU group and the telemetry group (15 hours vs 19 hours, respectively; P = .11). However, the CCU group was monitored a mean of 8 hours longer than the telemetry unit group was (P<.001), a situation that made it likely that more ST events would be detected in the CCU group. Despite more advanced CAD and longer monitoring time, the frequency of TMI in patients in the CCU did not differ significantly from that of patients in the telemetry unit (19% and 15%, respectively; P = .27). The percentage of patients with TMI who had silent ischemia was higher in the telemetry unit (71%) than in the CCU (58%), but this difference was not significant (P = .28).


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  		Table 2 Electrocardiographic variables and the frequency of transient myocardial ischemia in patients treated in the cardiac care unit or the telemetry unit*

 
Adverse In-Hospital Outcomes
The rate of death or acute myocardial infarction after hospital admission in the 2 groups of patients is given in Table 3Go. Regardless of hospital unit (CCU or telemetry), patients with TMI were more likely than patients without TMI to experience death or acute myocardial infarction after hospital admission. Although the percentage of patients with TMI who experienced acute myocardial infarction or death after hospital admission was higher in the telemetry unit (39%) than in the CCU (33%), the difference was not significant (P = .62). Additionally, we found that unplanned transfer (eg, because of unrelieved chest pain, major arrhythmia, or shock) from the telemetry unit to the CCU occurred significantly more often (P<.001) among patients with TMI (21%) than among patients without TMI (1%). The ECGs of a patient who was transferred from the telemetry unit to the CCU because of unrelieved chest pain are shown in the FigureGo.


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  		Table 3 Rate of death or acute myocardial infarction during hospital admission in patients with and without transient myocardial ischemia (TMI)*

 


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  		ST-segment monitoring (lead V3) in a 72-year-old man admitted to the telemetry unit for treatment of unstable angina who was transferred from the telemetry unit to the coronary care unit for more aggressive medical management because of unrelieved chest pain. Several hours before the transfer, he experienced 8 separate ischemic events while in the telemetry unit (top panel). The first 7 events were clinically silent, but he experienced chest pain during the final ischemic event. Importantly, these ST-segment data were not available to the clinicians. The panel at the bottom left of the figure shows electrocardiographic findings before the maximal ischemic event in the telemetry unit in the 6 precordial leads V1 through V6. The panel at the bottom right shows the maximal telemetry unit event (ST depression) in the same 6 precordial leads, which occurred just before transfer to the coronary care unit.

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
Conclusions
 References
 
To our knowledge, this study is the first one to describe the frequency and consequences of TMI detected with 12-lead ST-segment monitoring in a cardiac telemetry unit. Overall, the results indicated that 15% of patients admitted to the telemetry unit for treatment of unstable angina or CAD with cardiac catheterization and possible percutaneous coronary interventions experienced ischemia. A novel finding was that the rate of TMI in patients monitored in the telemetry unit (1997–2000) was no different from the rate of TMI in patients monitored in the CCU (1994–1996). Importantly, most ischemic events would have gone unrecognized because more than half of the events were clinically silent. In this investigation, patients who experienced TMI, whether monitored in the CCU or telemetry unit, were more likely than patients without TMI to experience serious in-hospital complications, including nonfatal myocardial infarction and death.

Rate of Ischemia
Our results indicate that a shift occurred in the care of patients with angina at our facility during the 6-year study period. For example, the majority of the patients during the earlier period (1994–1996) were admitted directly to the CCU rather than to the telemetry unit. In contrast, the majority of patients from the more recent period (1997–2000) were admitted directly to the telemetry unit. The rate of TMI (15%) among the patients in the telemetry unit was similar to that reported in studies,5,16–20 conducted exclusively in the CCU, that included similar patients. Thus, despite an obvious shift in the care of patients at our facility, TMI, which is most often silent, remains a common problem.

Adverse In-Hospital Outcomes
In agreement with the findings of previous studies conducted exclusively in the CCU,5,21–25 our results indicate that ECG-detected ST events that occur in the telemetry unit are also associated with in-hospital complications. Specifically, 39% of the patients with TMI in the telemetry unit experienced death or acute myocardial infarction after admission, whereas only 5% of the telemetry unit patients without TMI experienced such events. Hence, continuous ECG monitoring for changes in the ST segment indicative of TMI, which is largely silent, can be used to detect patients at risk for serious in-hospital events.

Similar to other investigators, we found that TMI among patients with acute coronary syndromes was associated with unplanned transfer from the telemetry unit to the CCU for more aggressive therapy.26,27 In the earlier studies,26,27 chest pain was used as the only indicator of TMI. In contrast, we found that patients experienced changes in the ST segment, which were largely asymptomatic, before unplanned transfer from the telemetry unit to the CCU. In our study, clinicians did not have access to the ECG data obtained via ST-segment monitoring; thus, the decision to transfer a patient to the CCU was based on acute changes in the patient’s condition (eg, shock or unrelieved chest pain). This finding indicates that ST-segment monitoring may be useful in detecting patients with TMI who might benefit from currently available anti-ischemic therapies before the onset of acute complications that might necessitate unplanned transfer to the CCU.

Limitations
One limitation of this study was the interval between hospital admission and the start of ST-segment monitoring. Because TMI is most likely to occur shortly after admission before anti-ischemic therapies may have taken effect, we may not have detected all of the patients who had TMI. Thus, most likely TMI was underreported in our study. However, the time between admission and the start of ST-segment monitoring did not differ between the groups being compared (ie, CCU vs telemetry).

In addition, because patients in the CCU were monitored a mean of 8 hours longer than were patients in the telemetry group, we may have detected a higher rate of TMI in the CCU group. Had the 2 groups been monitored for the same length of time, the telemetry unit group might have had more TMI than the CCU group did, a finding that would make our findings even more significant.

Finally, although a detailed 24-hour diary was maintained prospectively by the research nurses, who communicated frequently with patients, nurses, and physicians about signs and symptoms of ischemia, some signs and symptoms could have been missed. However, the frequency of asymptomatic ischemia in our study is similar to that of other studies,23,28–30 a finding that supports the conclusion that TMI is mostly silent and thus will likely go unrecognized by patients and clinicians.


   Conclusions
Top
 Abstract
 Methods
 Results
 Discussion
 Conclusions
References
 
Before this investigation, clinicians had to extrapolate the rate and potential consequences of TMI in the telemetry unit from the results of studies conducted exclusively in the CCU. Our findings indicate that TMI is not uncommon among hospitalized patients with angina or patients with CAD who are undergoing cardiac catheterization who are treated in a telemetry unit. Moreover, our results indicate that compared with patients without TMI, patients in the telemetry unit who have TMI, which is largely asymptomatic, are at increased risk for acute myocardial infarction or death after admission to the hospital.

With the current emphasis on cost-efficiency and decreased use of high-level hospital and nursing care (ie, the CCU), our findings support the need for a non-invasive method in the lower cost telemetry unit that can be used to detect patients with TMI who may benefit from the multitude of currently available nonischemic therapies. Our results provide support for using continuous ST-segment monitoring to detect changes in the ST segment that indicate TMI. Patients with TMI are a high-risk group who may benefit from aggressive anti-ischemic therapies.


   ACKNOWLEDGMENTS
 
This study was supported by grant RO1NR03436 from the National Institute of Nursing Research, National Institutes of Health.

To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.


   REFERENCES
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 Methods
 Results
 Discussion
 Conclusions
 References
 

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