The limit of plausibility for predictors of response: application to biventricular pacing.

OBJECTIVES: We sought a method for any reader to quantify the limit, imposed by variability, to sustainably observable R(2) between any baseline predictor and response marker. We then apply this to echocardiographic measurements of mechanical dyssynchrony and response.
BACKGROUND: Can mechanical dyssynchrony markers strongly predict ventricular remodeling by biventricular pacing (cardiac resynchronization therapy)?
METHODS: First, we established the mathematical depression of observable R(2) arising from: 1) spontaneous variability of response markers; and 2) test-retest variability of dyssynchrony measurements. Second, we contrasted published R(2) values between externally monitored randomized controlled trials and highly skilled single-center studies (HSSCSs).
RESULTS: Inherent variability of response markers causes a contraction factor in R(2) of 0.48 (change in left ventricular ejection fraction [ΔLVEF]), 0.50 (change in end-systolic volume [ΔESV]), and 0.40 (change in end-diastolic volume [ΔEDV]). Simultaneously, inherent variability of mechanical dyssynchrony markers causes a contraction factor of between 0.16 and 0.92 (average, 0.6). Therefore the combined contraction factor, that is, limit on sustainably observable R(2) between mechanical dyssynchrony markers and response, is ~0.29 (ΔLVEF), ~0.24 (ΔESV), and ~0.30 (ΔEDV). Many R(2) values published in HSSCSs exceeded these mathematical limits; none in externally monitored trials did so. Overall, HSSCSs overestimate R(2) by 5- to 20-fold (p = 0.002). Absence of bias-resistance features in study design (formal enrollment and blinded measurements) was associated with more overstatement of R(2).
CONCLUSIONS: Reports of R(2) > 0.2 in response prediction arose exclusively from studies without formally documented enrollment and blinding. The HSSCS approach overestimates R(2) values, frequently breaching the mathematical ceiling on sustainably observable R(2), which is far below 1.0, and can easily be calculated by readers using formulas presented here. Community awareness of this low ceiling may help resist future claims. Reliable individualized response prediction, using methods originally designed for group-mean effects, may never be possible because it has 2 currently unavailable and perhaps impossible prerequisites: 1) excellent blinded test-retest reproducibility of dyssynchrony; and 2) response markers reproducible over time within nonintervened individuals. Dispassionate evaluation, and improvement, of test-retest reproducibility is required before any further claims of strong prediction. Prediction studies should be designed to resist bias.