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.

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.

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.

Clinical application of a second generation electrocardiographic computer program.

An electrocardiographic computer program based on multivariate analysis of orthogonal leads (Frank) was applied to records transmitted daily by telephone from the Veterans Administration Hospital, West Roxbury, Mass., to the Veterans Administration Hospital, Washington, D. C. A Bayesian classification procedure was used to compute probabilities for all diagnostic categories that might be encountered in a given record. Computer results were compared with interpretations of conventional 12 lead tracings. Of 1,663 records transmitted, 1,192 were selected for the study because the clinical diagnosis in these cases could be firmly established on the basis of independent, nonelectrocardiographic information. Twenty-one percent of the records were obtained from patients without evidence of cardiac disease and 79 percent from patients with various cardiovascular illnesses. Diagnostic electrocardiographic classifications were considered correct when in agreement with documented clinical diagnoses. Of the total sample of 1,192 recordings, 86 percent were classified correctly by computer as compared with 68 percent by conventional 12 lead electrocardiographic analysis. Improvement in diagnostic recognition by computer was most striking in patients with hypertensive cardiovascular disease or chronic obstructive lung disease. The multivariate classification scheme functioned most efficiently when a problem-oriented approach to diagnosis was simulated. This was accomplished by a simple method of adjusting prior probabilities according to the diagnostic problem under consideration.

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.

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.

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.

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.

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.

Differences between supine and sitting Frank-lead electrocardiograms.

Frank-lead electrocardiograms (ECGs) were recorded from 59 adult males with suspected coronary artery disease. Three records were recorded consecutively on frequency modulated tape for each patient in (1) supine position, (2) sitting position with arms relaxed, and (3) sitting position on bicycle with arms on bicycle handles. Electrodes were applied at the level of the fifth intercostal space with patients in the sitting position. Computer measurements of electrocardiographic amplitudes were averaged over ten seconds of each record with these results: (1) QRS spatial amplitudes and R amplitudes in lead z were significantly higher and R amplitudes in lead y lower for sitting than for supine positions. (2) Except for slightly higher R and S amplitudes in lead x for sitting with arms on bicycle, no significant differences were observed between the two sitting positions. (3) These postural differences are significantly greater than those resulting from day-to-day variability of electrode locations. It is hypothesized that electrode level shifts with postural changes are responsible for the observed x- and z-lead changes. For the y-lead changes, it is hypothesized that shifting blood volumes with postural changes are the cause. It is concluded that reference electrocardiographic measurements for stress testing should be obtained from resting ECGs with the patient in the same postural position as that maintained during exercise.

Technical factors affecting an immunoelectrophoretic reference system for analysis of mycobacterial antigens.

The length of immunoelectrophoretic separation patterns obtained with a reference mycobacterial antigen was reduced significantly when a continuous buffer system was used in place of the recommended discontinuous system. Varying the supporting medium by using 1% concentrations of different agar preparations in the discontinuous system also affected the separation length and resolution of the reference pattern. Measurement of the electrical resistivity of the agar medium provided an explanation of these differences on the basis of current density or field intensity applied per slide. Variation in the reference pattern obtained in differently designed electrophoresis chambers also was attributed to differences in field intensity. To avoid these obvious alterations in the separation pattern of the reference antigen, it was suggested that separation procedures described originally for the reference system be used without modification.

Distortions in infant electrocardiograms caused by inadequate high-frequency response.

Frank lead ECG's from infants were studied for frequency content by introducing low-pass filters of 50, 75, 100, and 150 150 Hz bandwidths before obtaining computer measurements. Results indicated that a minimum bandwidth of 100 Hz is required to avoid amplitude error of 10 per cent or greater. This bandwidth requierement is essentially the same as that required for adult ECG's despite the fact that infant QRS durations are usually about half those of adults. Although the average infant ECG spectrum is likely to contain higher frequencies than the average adult ECG spectrum, duration values for Q, R, and S waves overlap in these populations to such an extent that bandwidth requirements are practically identical.