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R D Rifkin

Publications and source records attributed to R D Rifkin.

16 recordsLinked to original sources
PubMed1995

Effects of correlated and uncorrelated measurement error on linear regression and correlation in medical method comparison studies.

It is well known that when uncorrelated measurement error affects both variables in linear regression, there is attenuation of the correlation coefficient and regression slope. The effect of correlated measurement error, however, has received little attention. In medical method comparison studies, such error correlation results from the presence of other, unknown explanatory variables that affect the results of the new test method and the reference test method to which it is being compared. The contribution of correlated measurement error to the observed correlation coefficient can be accounted for by the expression rho t1t2 = rho1 rho2 + rho E1E2 (1-rho2(1))1/2(1-rho 2(2))1/2 where rho t1t2 is the observed correlation between tests 1 and 2, rho 1 and rho 2 are the correlation with true values for tests 1 and 2, respectively, and rho E1E2 is the correlation between the test errors. The first term describes the attenuation due to uncorrelated error, the second term describes the effect of correlated error. A positive correlation between the measurement errors reduces the attenuation of observed correlation and slope, but, when the reference method is excellent, the effect is very small. For poorer reference tests whose correlations with true values are less than 0.9, however, error correlation may result in a slope and correlation coefficient that differ importantly from the values obtained with either uncorrelated error or with no reference test error. Negatively correlated measurement errors magnify the attenuation of slope and correlation. One might suspect the presence of correlated error when the observed regression slope is close to or exceeds 1 and the reference test is known to have suboptimal reliability. This paper provides several clinical examples of potentially correlated diagnostic methods.

Cardiac Output↗
PubMed1995

Comparison of proximal isovelocity surface area method with pressure half-time and planimetry in evaluation of mitral stenosis.

OBJECTIVES: This study sought to 1) compare the accuracy of the proximal isovelocity surface area (PISA) and Doppler pressure half-time methods and planimetry for echocardiographic estimation of mitral valve area; 2) evaluate the effect of atrial fibrillation on the accuracy of the PISA method; and 3) assess factors used to correct PISA area estimates for leaflet angulation. BACKGROUND: Despite recognized limitations of traditional echocardiographic methods for estimating mitral valve area, there has been no systematic comparison with the PISA method in a single cohort. METHODS: Area estimates were obtained in patients with mitral stenosis by the Gorlin hydraulic formula, PISA and pressure half-time method in 48 patients and by planimetry in 36. Two different factors were used to correct PISA estimates for leaflet angle (theta): 1) plane-angle factor (theta/180 [theta in degrees]); and 2) solid-angle factor [1-cos(theta/2)]. RESULTS: After exclusion of patients with significant mitral regurgitation, the correlation between Gorlin and PISA areas (0.88) was significantly greater (p < 0.04) than that between Gorlin and pressure half-time (0.78) or Gorlin and planimetry (0.72). The correlation between Gorlin and PISA area estimates was lower in atrial fibrillation than sinus rhythm (0.69 vs. 0.93), but the standard error of the estimate was only slightly greater (0.24 vs. 0.19 cm2). The average ratio of the solid- to the plane-angle correction factors was approximately equal to previously reported values of the orifice contraction coefficient for tapering stenosis. CONCLUSIONS: 1) The accuracy of PISA area estimates in mitral stenosis is at least comparable to those of planimetry and pressure half-time. 2) Reasonable accuracy of the PISA method is possible in irregular rhythms. 3) A simple leaflet angle correction factor, theta/180 (theta in degrees), yields the physical orifice area because it overestimates the vena contracta area by a factor approximately equal to the contraction coefficient for a tapering stenosis.

Adult↗
PubMed1991

Detection of coronary artery disease by vasodilator thallium imaging of the heart with amyl nitrite inhalation: a pilot study.

Thallium imaging of the heart using dipyridamole-induced coronary arteriolar vasodilation has proven to be an effective means of detecting significant coronary stenosis. However, intravenous dipyridamole has not yet been made available for general use. We therefore examined the feasibility of substituting amyl nitrite inhalation as an arteriolar vasodilator prior to thallium imaging. Seventeen patients, all of whom had catheterization-proven coronary stenosis, inhaled amyl nitrite for 2-5 min. Thallium was injected after 45-60 s of inhalation. Completion of inhalation was followed immediately by planar imaging. Of 6 patients who inhaled amyl nitrite for at least 4 min, 5 had moderate or severe image defects on immediate scans which completely resolved on delayed scans. Only 3 of 11 who inhaled amyl nitrite for 2 min or less prior to scanning had similarly positive tests. Overall sensitivity for significant stenosis was 8 of 17 (47%). Inhalation was well tolerated with only one episode of angina and hypotension. We conclude that amyl nitrite inhalation for at least 4 min may offer an effective and readily available alternative to intravenous dipyridamole for vasodilator imaging of the heart.

Administration, Inhalation↗
PubMed1990

Comparison with radionuclide angiography of two new geometric and four nongeometric models for echocardiographic estimation of left ventricular ejection fraction using segmental wall motion scoring.

Two new echocardiographic methods for estimating left ventricular ejection fraction, both based on segmental wall motion scoring on a continuous scale, were developed and compared to radionuclide angiography in 50 patients. The first method used a geometric model in which the ventricle was represented by a tubular base section joined to a hemielliptical apex section, each containing 4 quadrants. Contraction was scored visually in 12 regions of the ventricle. The 12 scores were converted to dimensional shortenings, which were then used to calculate separate ejection fractions for each of the 8 quadrants. These 8 values were averaged to obtain the global ejection fraction. Assuming an elliptical end-systolic cross-section shape for each quadrant, the correlation between the echocardiographic and radionuclide ejection fractions was 0.917 with a regression slope of 0.93 and an intercept of 1.5. Results were identical assuming circular end-systolic quadrant shapes. The second approach used a linear combination of the mean wall motion score at the base, midleft ventricle and apex to predict ejection fraction. Coefficients for the 3 mean scores were optimized by least squares best fit with radionuclide ejection fraction in all 50 patients. The simplest linear model showed a correlation of 0.90 between the echocardiographic and radionuclide ejection fraction but underestimated low, and overestimated high, ejection fractions. However, these systematic errors could be removed by regression, which added a constant term of 8.1 and a slope factor of 0.87 to the linear model, improving the correlation with radionuclide ejection fraction to 0.91.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Effect of self-correlation on the relation between QT interval and cardiac cycle length.

The electrocardiographic QT interval, with a wide variety of practical and theoretic applications in medicine, pharmacology, and cardiology, is critically dependent on heart rate as expressed by cycle length (RR). To account for this and permit intraindividual and interindividual comparisons of changing QT intervals, a wide variety of formulas have been offered as single rate-correcting expressions over the range of heart rates. Yet as rate rises, the QT interval occupies more and more of the cycle length, until at the highest heart rates almost the entire cycle length is QT interval, and the QT-RR correlations become largely self-correlation. To elucidate this we investigated 150 consecutive patients divided into high, mid, and low rate terciles. For the entire group, and in each tercile, we determined the level of correlation between the QT interval and cycle length (RR interval). To remove the self-correlation, we repeated the calculations, correlating QT interval with RR interval minus QT interval. For the group, correlation (r) of QT with RR was 0.65. For the high, mid, and low rate terciles, correlation fell sharply, respectively: 0.58 to 0.36 to 0.25. When the self-correlation was removed by subtracting QT from cycle length (RR-QT), all correlations were even further sharply reduced for each tercile, respectively: 0.35, 0.16, and 0.03 (the latter being virtually no correlation). We conclude that at low heart rates, when it is not being correlated with itself, the correlation between QT and RR is low; at high heart rates the higher correlation is spurious because it is mainly self-correlation.

Electrocardiography↗
PubMed1988

Value of fluoroscopy in the detection of coronary stenosis: influence of age, sex, and number of vessels calcified on diagnostic efficacy.

Although fluoroscopically detected coronary artery calcification is known to correlate with the presence of coronary artery stenosis, age, sex, and extent of calcification influence the strength of this association. To clarify its diagnostic potential, we performed fluoroscopy before coronary angiography in 600 patients and analyzed the results according to all three factors simultaneously. The sensitivity of fluoroscopy for significant stenosis exceeded 65% in all groups except women less than 45 years of age. Specificity exceeded 90% in patients less than 45 years and 85% in patients less than 55 years of age, and declined significantly with age. The number of vessels calcified was an important determinant of predictive value, except in those less than 45 years of age in whom even a single mild calcification markedly increased the chance of stenosis. In patients aged 45 to 64 years, calcification of two or three vessels substantially increased the chances of stenosis, but single-vessel calcification increased the risk only slightly. In patients more than 65 years of age, fluoroscopy was not helpful in detecting stenosis, regardless of the number of vessels calcified. Our findings were similar in men and women. We conclude that if both age and the number of vessels calcified are considered, fluoroscopy can provide useful information regarding the presence of stenosis in young and middle-aged patients.

Age Factors↗
PubMed1985

Maximum Shannon information content of diagnostic medical testing. Including application to multiple non-independent tests.

The increase in Shannon information available from a diagnostic test associated with grading of the test results into many outcomes, rather than simply positive or negative, was examined to determine its upper limit as the number of test outcomes is increased indefinitely. Numerical methods were employed to find the optimal locations of outcome boundaries when a single normally distributed test variable is classified into 2, 3, 4, 5, 6, 8, 14, or 20 outcome categories. In each case Shannon information was computed for values of prior probability between 0.01 and 0.99 and for distances between the means in diseased and nondiseased populations ranging from 0.5 to 5.0 standard deviations. There is an important improvement in Shannon information as the number of outcomes defined is increased, but the increment in information diminishes rapidly with each additional category. A 20%-30% increment in information may be achieved with three outcomes instead of two. A further important increase in information occurs with four to seven outcomes, but beyond this the increment in inforation is negligible. The findings were similar over a wide range of prior probabilities and distances between the means. The analysis was extended to the case of multiple nonindependent tests by demonstrating their application to a Fisher discriminant function incorporating such tests. It was concluded that for normally distributed test variables: grading of test results significantly improves the information content of both single and multiple tests; the value of information content for 8-20 outcomes represents very nearly the maximum information content of a test; there is little value in using more than five to seven test outcomes; multiple grading should not be neglected for discriminant functions.

Bayes Theorem↗
PubMed1984

Combined posteroanterior subepicardial fat simulating the echocardiographic diagnosis of pericardial effusion.

The location and relative size of echo-free spaces observed by cardiac ultrasound have been considered reliable signs for distinguishing pericardial fat from fluid; spaces that are exclusively anterior have been considered to represent fat, while spaces that are exclusively or predominantly posterior have been considered to represent fluid. In the present study, the location and relative size of echo-free spaces in eight patients suggested the diagnosis of pericardial effusion; evaluation by computed tomography or thoracotomy, or both, in six and necropsy in two, however, disclosed that these echo-free spaces--posterior as well as anterior--were exclusively due to fat. Age appeared to be as important a predisposing factor as obesity in the accumulation of excess subepicardial fat. No M-mode or two-dimensional features were found to be reliable in differentiating fat from fluid, although excessive amplitude of the posterior pericardial echo on the M-mode study favored the diagnosis of fat. Thus, the finding of echo-free spaces by cardiac ultrasound, even when the posterior space is isolated or larger than an accompanying anterior space, is not necessarily indicative of pericardial fluid. In elderly patients, in particular, posterior echo-free spaces due to fat may invite an incorrect diagnosis of pericardial effusion or pericarditis. In patients in whom echo-free spaces represent an unexpected finding of cardiac ultrasound examination, computed tomography of the chest may be helpful in establishing whether they are due to fat or fluid.

Adipose Tissue↗
PubMed1983

Magnetic measurement of S-T and T-Q segment shifts in humans. Part II: Exercise-induced S-T segment depression.

The direct-current magnetocardiogram, which shows the T-Q (baseline) shift, is used to clarify the cause of S-T depression induced by stress testing in the human heart. Measurements are made of the amount of baseline shift associated with the S-T depression. Results are presented of a well-documented patient, with typical coronary artery disease, undergoing a two-step exercise test. Before exercise, there was no S-T or baseline shift. During exercise, the S-T segment became depressed and the baseline segment was simultaneously elevated, at about 70% of the S-T amplitude. After termination of exercise, the baseline elevation disappeared somewhat more rapidly than the S-T depression. These results were consistent in repeated tests of this patient. Because the baseline shift is a reflection of an injury current, these results confirm the belief that exercise-induced S-T depression is mostly due to an injury current which is interrupted during the S-T interval. The baseline shift seen here is the first non-invasive measurement of an injury current in the human heart, and its presence and time-course generally agree with measurements in the animal heart. This work also confirms that the direct-current magnetocardiogram, although not practical for clinical purposes, is useful as a research tool.

Heart↗
PubMed1983

Classical statistical considerations in medical decision models.

The influence of sampling error on decision-analytic models was investigated to determine how these errors affect model reliability. Formulas were developed to relate statistical error in the probability decision threshold and gain in expected utility to the error in the data samples upon which such models are based. The formulas were validated in a simulation experiment and then applied to a hypothetical decision model and to the clinical problem of immediate surgery versus continued observation in suspected acute appendicitis. The results of this analysis show that modest statistical error affecting any variable in a decision model may be amplified into a substantially larger error in both the probability decision threshold and the gain in utility predicted by the model. In addition, when errors are present simultaneously in several variables, they may compound to unexpectedly large magnitudes, rendering the model unreliable over a wide range of disease probability. The interpretation of the results of a decision analysis should be viewed along a continuum that takes into account both the magnitude of the gain or loss in expected utility predicted by the model and a quantitative measure of the reliability of this prediction. Whenever possible, a determination of statistical error should be an integral part of any formal decision analysis.

Acute Disease↗
PubMed1979

Coronary calcification in the diagnosis of coronary artery disease.

Clinical, postmortem and angiographic studies of coronary calcification are reviewed to define the value of fluoroscopy in the diagnosis and management of coronary artery disease. Autopsy studies consistently show a unique association between calcification of the coronary arteries and atherosclerosis. The relation of coronary calcification to the presence of major stenosis is more variable but is strong enough to be of clinical value, particularly in the younger subject. The diagnostic value of fluoroscopy can be improved by attention to the detailed features of calcification observed with the technique. Combined use of fluoroscopy and exercise testing appears to be a valid and as yet unexploited approach to the noninvasive diagnosis of coronary stenosis. Fluoroscopy has been a neglected method of noninvasive diagnosis and is sufficiently promising to warrant greater clinical use.

Calcinosis↗
PubMed1977

Bayesian analysis of electrocardiographic exercise stress testing.

We evaluated the diagnostic accuracy of exercise-induced ST-segment depression in detecting coronary-artery disease by applying the likelihood-ratio formulation of Bayes's theorem to stress-test data, which were partitioned into half-millimeter ranges of depression. The graphic relation between the predictive value of a given test result and the pretest risk of disease in the test subjects was obtained for each of these half-millimeter intervals. This method reveals that the predictive value of testing depends on the degree of ST-segment depression, and that the pretest risk of coronary-artery disease is an important determinant of the predictive value of any test result in the individual patient. These findings suggest that the use of the terms "positive" and "negative" are inappropriate to describe most stress-test results. Instead, the results should be interpreted in terms of a continuum of risk based on the extent of ST-segment depression.

Adult↗