Utility of current risk stratification tests for predicting major arrhythmic events after myocardial infarction.

OBJECTIVES: We surveyed the literature to estimate prediction values for five common tests for risk of major arrhythmic events (MAEs) after myocardial infarction. We then determined feasibility of a staged risk stratification using combinations of noninvasive tests, reserving an electrophysiologic study (EPS) as the final test. BACKGROUND: Improved approaches are needed for identifying those patients at highest risk for subsequent MAE and candidates for implantable cardioverter-defibrillators. METHODS: We located 44 reports for which values of MAE incidence and predictive accuracy could be inferred: signal-averaged electrocardiography; heart rate variability; severe ventricular arrhythmia on ambulatory electrocardiography; left ventricular ejection fraction; and EPS. A meta-analysis of reports used receiver-operating characteristic curves to estimate mean values for sensitivity and specificity for each test and 95% confidence limits. We then simulated a clinical situation in which risk was estimated by combining tests in three stages. RESULTS: Test sensitivities ranged from 42.8% to 62.4%; specificities from 77.4% to 85.8%. A three-stage stratification yielded a low-risk group (80.0% with a two-year MAE risk of 2.9%), a high-risk group (11.8% with a 41.4% risk) and an unstratified group (8.2% with an 8.9% risk equivalent to a two-year incidence of 7.9%). CONCLUSIONS: Sensitivities and specificities for the five tests were relatively similar. No one test was satisfactory alone for predicting risk. Combinations of tests in stages allowed us to stratify 91.8% of patients as either high-risk or low-risk. These data suggest that a large prospective study to develop a robust prediction model is feasible and desirable.

Dysrhythmia hazard after hospitalization for myocardial infarction: two ECG prognostic methods compared.

We retrieved reports of heart rate variability and signal-averaged electrocardiograms (SAECG) used to predict risk of a dysrhythmic event. From each report the number of cases with and without events was extracted to establish accurate values for true positive rate (tpr = sensitivity) and false positive rate (fpr = 1 minus specificity). For all the heart rate variability reports, these values were collected and tpr values were plotted versus fpr. The (fpr,tpr) data were summarized by a meta ROC graph using the method of Moses and Shapiro. A composite weighted mean value and 95% confidence interval were also derived. A summary meta-ROC curve for the SAECG reports was similarly obtained., Meta-ROC analysis of multiple reports better summarizes the performances of different prognostic methods and allows the effect of combining tests for a larger population to be simulated.

NHLBI workshop on the utilization of ECG databases: preservation and use of existing ECG databases and development of future resources.

Baseline examinations and periodic reexaminations in longitudinal population studies, together with ongoing surveillance for morbidity and mortality, provide unique opportunities for seeking ways to enhance the value of the electrocardiogram (ECG) recorded with digital technology as an inexpensive and noninvasive tool for prognosis and diagnosis. Clinicians, epidemiologists, and engineers from industry, government, and academic medical centers gathered at a workshop sponsored by the National Heart, Lung, and Blood Institute (NHLBI) on June 11-12, 1997, to discuss the research potential of ECG databases, their preservation and accession, and standards for recording and storage. Databases considered were those acquired in ongoing and future NHLBI-funded studies and in clinical settings in which the ECG continues to provide valuable information for evaluation and treatment. The accessibility of existing databases, the quality of their data, and the availability of ancillary demographic and clinical information were major themes. Also discussed were appropriate statistical methodologies to be used with these data for developing and testing ECG algorithms. The workshop participants affirmed the value of these databases and urged the establishment of an ECG advisory and review group to (1) resolve technical and proprietary issues for the utilization of currently existing databases; (2) develop standards for recording, storage, and utilization of ECGs in future NHLBI-supported studies; (3) oversee the creation of a national ECG database resource, consisting of an archive of ECG databases from past and ongoing NHLBI-supported studies, and a registry of ECG databases that would eventually include digital ECGs from populations currently underrepresented in the demographic spectrum of the NHLBI databases.

American National Standard for nonautomated sphygmomanometers. Summary report.

The Association for the Advancement of Medical Instrumentation develops voluntary standards for medical devices so that manufacturers might provide information on their product and basic safety and performance criteria that should be considered in qualifying the instrument for clinical use. American National Standards are generated through a consensus process by committees of experts in research, development, and design from user, industry, and government communities. Draft standards are made available for public review and become American National Standards after review by the American National Standards Institute. This report is a summary of the American National Standard that establishes both the safety and performance requirements of pneumatic and other nonautomated sphygmomanometers (especially mercury gravity and aneroid sphygmomanometers) used in the indirect measurement of blood pressure. This standard was developed by a consensus panel from academia, industry, and government; it updates the previous standard approved by the American National Standards Institute in 1986. This summary report does not cover all of the provisions of the revised 1994 American National Standard, thus users, researchers, and instrument designers should refer to the standard for detailed requirements.

Panoramic display of the orderly sequenced 12-lead ECG.

The standard 12-lead electrocardiogram (ECG) has been developed over many years. The ECG has had a long and successful history of providing diagnostic information in clinical medicine. Cardiac arrhythmias have been elucidated by deductive reasoning from continuous ECG recordings with confirmation from electrophysiologic studies. Recently, there has been renewed interest in the morphology of the QRS complex, ST-segment, and T wave, which raises the important question of considering whether the usual method of display provides maximal diagnostic capabilities. The conventional display provides a logical visualization of precordial lead recordings representing the horizontal plane, but does not provide a logical visualization of the limb lead recordings representing the frontal plane. Many clinical problems require the consideration of serial ECGs necessitating the comparison of separate pages. An alternate format presenting serial recordings on a single page would be advantageous. Some automated ECG analysis systems already include the capability for multiple display formats, but these have not yet been widely used in clinical practice. This point of view paper introduces a new display format for the standard 12-lead ECG that includes: (1) a presentation of an orderly sequence of leads to facilitate scanning through different points in space and (2) a presentation of recordings of 12-lead sequences to facilitate scanning through different points in time. This display format could either replace or supplement the conventional ECG format.

National standard for measurement of resting and ambulatory blood pressures with automated sphygmomanometers.

The Association for the Advancement of Medical Instrumentation develops voluntary standards for medical devices so that manufacturers might provide information on their product and basic safety and performance criteria that should be considered in qualifying the instrument for clinical use. American national standards are generated through a consensus process by committees consisting of experts in research, development, and design from user, industry, and government communities. Draft standards are made available for public review and may become American national standards after review by the American National Standards Institute. The first American national standard for electronic and automated sphygmomanometers was published in monograph form in 1987. The objective of the revised 1992 standard for electronic and automated sphygmomanometers is to provide updated labeling, safety, and performance requirements that help ensure that consumers and health care professionals are supplied with safe, accurate devices for the indirect measurement of blood pressure, including ambulatory blood pressure recorders. This standard permits validation of the automatic or electronic device by comparison with either direct, intra-arterial blood pressure measurements or the noninvasive cuff/stethoscope technique, based on Korotkoff sounds identified by individuals trained in auscultation. This summary report of the 1992 American national standard for automatic sphygmomanometers provides recommendations for the methods of comparison, statistical analysis of the data, presentation of the results, and criteria for acceptability. Users, researchers, and instrument designers should refer to the American national standard monograph for detailed requirements.

Instrumentation and practice standards for electrocardiographic monitoring in special care units. A report for health professionals by a Task Force of the Council on Clinical Cardiology, American Heart Association.

The proposed recommendations for continuous electrocardiographic monitoring systems represent goals for future development. Description of a technique in the report does not constitute an endorsement of its clinical use. Lead systems for ECG monitoring must adequately sense the cardiac electrical field and the leads should be standardized. Future monitors should be capable of simultaneously displaying and analyzing multiple leads. Recommendations for electrode placement and position of patient are made. Important parameters in each category of standards for instrumentation published in 1983 in the American National Standard for Cardiac Monitors, Heart Rate Meters, and Alarms are listed. Selected procedures proposed by the Association for the Advancement of Medical Instrumentation to inform users of minimally acceptable accuracy of computerized systems in a standardized manner are presented. Emphasis is placed on the importance of nursing and medical staff capabilities. Personnel qualifications and training as well as systems to assure and maintain quality of immediate ECG diagnosis are highlighted.

Optimal lead subsets for reconstruction of QRS and ST-T in 35-lead precordial maps.

Precordial maps have been used for some 15 years to estimate the extent of myocardial injury in patients with acute anterior or lateral wall infarction. Estimates have been based on various QRS- and ST-T-derived parameters, including amplitude sum of ST elevations. Application of the electrodes, commonly 35, is cumbersome and time-consuming with the critically ill. A subset of 5 or 7 selected leads can be applied instead, and the remaining leads calculated from that subset with minimal loss of QRS and ST-T information. Maps were recorded from 100 patients within 72 hours of onset of anterior or lateral infarct. Optimal lead subsets for QRS and ST-T feature extraction were found by the sequential selection method of Lux. Subsets numbering between 2 and 15 leads were derived, with their lead-transform coefficients. Measures to estimate goodness of fit for reconstructed leads included correlations, error-to-signal ratios and root-mean-square errors. These measures were calculated separately over the QRS and ST-T complexes. Reconstructions from a 7-lead subset had a mean 0.92 correlation with ST-T in the original leads and root-mean-square error of only 0.04 mV. Sum of ST elevation differed by only 2% between original leads and reconstructions based on 5 or more leads. To confirm repeatability, lead-transform coefficients were also calculated from a training population of 50 patients and applied to the maps of the other 50.

Multicenter validation study of real-time ultrasonography, arteriography, and pathology: pathologic evaluation of carotid endarterectomy specimens.

The morphologic description and measurements of endarterectomy specimens are usually believed to be accurate and are used as the gold standard against which the findings of diagnostic procedures are judged. Pathology data on 289 endarterectomy specimens from five participating centers and the corresponding angiography and B-mode ultrasonography data provided a basis for scrutinizing the validity of using the morphologic measurements as a standard. Discrepancies of greater than 1 mm between pathology and angiography measurements of minimum residual lumen occurred in 35% of the cases and between pathology and B-mode ultrasonography measurements in 64% of the cases. Discrepancies of greater than 1 mm between pathology- and angiography-measured lesion width occurred in 81% of the cases and between pathology and B-mode ultrasonography measurements in 64% of the cases. The cases representing mismatches of greater than 1 mm at one participating center were subjected to a rigorous review, with remeasurement of all morphologic features, in an attempt to explain the discrepancies. Various types of artifactual distortion of the specimens, the presence of slit-like and occluded lumens that were likely related to loss of perfusion pressure, and an inability to match planes of interrogation used in angiography and B-mode ultrasonography with pathology planes contributed significantly to the existence of mismatches. On the other hand, fixation and decalcification produced minimal and insignificant distortional changes. We conclude that the acquisition of quantitative data from endarterectomy specimens and the acceptance of morphologic data as a standard are limited by a number of problems that can be defined but have been difficult to resolve.

Ventricular assist systems for temporary life support: device readiness testing.

A description is given of a quality control and reliability program for establishing short-term ventricular-assist-device (VAD) safety and efficiency prior to clinical use. Experience in the clinical use of temporary VADs indicates that the pre-clinical reliability testing program works, since no device related failures occurred. The procedures have become much stricter, and a goal of 80% reliability for in vitro testing currently exists for the permanent, ventricular-assist system with strict guidelines on how to determine device-related failure. Animal testing requires total animal-months of continued usage consistent with the 80% in vitro reliability. A critical feature of the program has been freezing of device and system design throughout the quality control/assurance program, reliability testing, and clinical use.

Multicenter validation study of real-time (B-mode) ultrasound, arteriography, and pathologic examination.

The ability of high-resolution ultrasound, angiography, and pathologic examination of endarterectomy specimens to identify and quantitate atherosclerosis was compared in a five-center study. The carotid bifurcation in 900 patients was evaluated by angiography and ultrasound. In 216 cases, high-quality endarterectomy specimens were available for comparison with the preoperative images. All comparisons were made in a blinded fashion. Results indicate that ultrasound is able to differentiate angiographically normal from abnormal arteries with a sensitivity of 88% (1077 of 1233 arteries) and accuracy of 79% (1251 of 1578 arteries). Angiographic stenoses equal to or greater than 50% diameter were accurately identified by ultrasound imaging in 72% (1133 of 1578 arteries) of cases, and this was improved by the addition of other functional data (i.e., Doppler spectral analysis and oculoplethysmography). There was only modest correlation of absolute measurements of lesion width, minimal lumen, and standard lumen by the two imaging techniques (r = 0.28 to 0.55). Ultrasound measurements of lesion width were on the average 2 mm greater than those of angiography. The lumen averaged 1.5 mm larger when measured by ultrasound techniques. In the subset in which data were available from endarterectomy specimens, ultrasound showed the best correlation with lesion width (mean difference -1.1 mm) and angiography correlated best with minimal lumen (mean difference -0.1 mm). Neither examination consistently identified ulcerated plaques. Although ultrasound imaging alone has limited usefulness in quantitating luminal stenosis, this can be improved by the use of Doppler spectral analysis and oculoplethysmography. Ultrasound is superior to angiography for quantifying atherosclerotic plaque (lesion width) and will be an important tool for further study of atherosclerotic lesions.

Effects of electrode area on electrocardiographic voltages.

Effects of two different sizes of chest electrodes--100 and 750 mm2 area--on x and z Frank leads were determined using electrocardiographic data from 25 subjects. In most cases, differences in Rx and Rz were below 50 uV, but in nine cases (36%) differences exceeded this value for either Rx or Rz or both. In six cases, differences exceeded 100 uV. For an additional 20 subjects, standard precordial leads were recorded using the same two electrode sizes. QRS amplitudes were significantly affected for V4 but not for V1 or V6. Variability caused by electrode size is greater than that caused by beat-to-beat variation and is comparable to that found in day-to-day variation. Interchangeability of data among ECG recording laboratories can be significantly improved by standardizing electrode size for precordial electrodes.

Electrode position effects on Frank lead electrocardiograms.

Frank lead electrocardiograms were recorded from 149 normal and abnormal adult males using four different electrode placements. All chest electrodes were placed at: (1) the fourth intercostal space level, (2) the fifth intercostal space level, (3) the fourth intercostal space level with V4 substituted for C, and (4) the fifth intercostal space level with V4 substituted for C. Differences in mean values of many commonly used amplitudes and orientations were not statistically significant among the four recording methods, but amplitude differences for individual subjects were often large and difficult to predict. When V4 is substituted for C, as commonly done in some laboratories, Rx decreased and Rz increased by more than 10 per cent in about 40 per cent of the cases. In about 70 per cent of the cases, Rx and Rz changed significantly when electrode level was shifted from the fifth to the fourth intercostal space. For these 70 per cent, it does not appear possible to accurately predict increase or decrease of Rx, Rz, or QRSm. Analysis programs which depend on individual amplitude measurements are likely to be significantly affected by electrode placement. It is suggested that criteria for analysis programs developed using a specified version of the Frank system should ideally be applied only to electrocardiograms recorded in the same manner.