Observed and expected reliability of echocardiographic volumetric methods and critical change values for quantification of mitral regurgitant fraction in dogs

Background: Reliability of echocardiographic calculations for stroke volume and mitral regurgitant fraction (RF_MR) are affected by observer variability and lack of a gold standard. Variability is used to calculate critical change values (CCVs) that are thresholds representing real change in a measure not associated with observer variability.

Hypothesis: Observed intra- and interobserver accuracy and variability in healthy dogs help model CCV for RF_MR.

Animals: Reliability cohort of 34 healthy dogs; allometric scaling cohort of 99 dogs with heart disease and 25 healthy dogs.

Methods: Accuracy, variability, and CCV of 2 observers using geometric and flow-based echocardiography were prospectively compared against a standard of RF_MR = 0% and extrapolated across a range of expected RF_MR values in the reliability cohort partly derived from cardiac dimensions predicted by the allometric cohort.

Results: Accuracy of methods to determine RF_MR in descending order was 4-chamber bullet (Bullet_4CH), mitral inflow, cube formula, and Simpson's method of disks. Intraobserver variability was relatively high. The CCV for RF_MR ranged from 28% to 88% and was inversely related to RF_MR when extrapolated for use in affected dogs. For both observers, the Bullet_4CH method had the lowest intraobserver CCV (Operator 1:28%, Operator 2:41%). Interobserver strength of agreement was low with intraclass correlation coefficients ranging from 0.210 to 0.413.

Conclusions and clinical importance: Echocardiographic volumetric methods used to calculate stroke volume and RF_MR have low accuracy and high variability in healthy dogs. Extrapolation of observed CCV to a range of expected RF_MR suggests observers and methods are not interchangeable and variability might hinder routine clinical usage. Individual observers should be aware of their own variability and CCV.