Letter to the Editor.

Dear Editor,

We thank Drs. Kellihan and Chesler for their kind words and comments regarding our article “Diagnostic value of the right pulmonary artery distensibility index in dogs with pulmonary hypertension: Comparison with Doppler echocardiographic estimates of pulmonary arterial pressure” recently published in JVIM.1 In their letter, Drs. Kellihan and Chesler expressed concern regarding our use of the term “right pulmonary artery distensibility (RPAD) index” primarily because this index, as performed in our echocardiographic study, does not truly represent distensibility of the right pulmonary artery. As discussed, true pulmonary artery (PA) distensibility is determined by measuring the relative percent change in luminal area (cross‐sectional area) of the artery (usually obtained via magnetic resonance imaging) for a given change in PA pressure (or pulse pressure – usually obtained invasively via right heart catheterization) during a cardiac cycle and is expressed as %/mmHg.2, 3, 4

We appreciate the desire of Drs. Kellihan and Chesler to standardize terms used in defining pulmonary hypertension (PH) and agree that terminology can be misleading. By definition, the RPAD index is dissimilar to and does not quantify true distensibility of the right PA. Instead, the RPAD index quantifies relative (or percent) diameter change in the PA throughout the cardiac cycle and can be conceptually thought of as a shortening fraction of the right PA derived via echocardiography. Our rationale for using the term RPAD index was primarily based on it previously being used in the clinical veterinary literature to describe relative changes in pulmonary artery diameter throughout the cardiac cycle and its associated changes with varying degrees of PH5, 6 and because it represents a noninvasive, readily obtainable, and easily understood echocardiographic measure.6

Drs. Kellihan and Chesler advocate use of the term “right pulmonary artery relative area change (RPA RAC)” rather than “RPAD index” and suggest this alternate term will accomplish the goal of more accurately describing the right PA measurement utilized in our study. We respectfully disagree and also find this proposed terminology to be misleading. In our study, we did not quantify the right PA cross‐sectional area (or its relative change) but instead quantified the relative change in luminal diameter of the right PA throughout the cardiac cycle. Further, we recommend not deriving the cross‐sectional area of an artery from a diameter measurement, as small errors in diameter (radius) measurement will be greatly magnified based on the formula: π × radius2. Lastly, we and others1, 6 found relative diameter change in the right PA (RPAD index) to be a practical, repeatable, and accurate predictor of invasive PA pressure and echocardiographically derived tricuspid regurgitation systolic pressure gradient in dogs with naturally occurring PH.

In conclusion, we agree that the current terminology describing PA properties in dogs with PH might be misleading, particularly when considering measurements derived from advanced imaging techniques (eg, magnetic resonance imaging). However, we do not believe that RPA RAC is more appropriate than RPAD index. A more accurate term might be “right PA relative diameter change” rather than the currently used nomenclature. Despite disagreement in nomenclature, we maintain the clinical value of the RPAD index when echocardiographically screening dogs for PH and suggest continued use of this terminology due to it being consistently reported in this manner within the clinical veterinary literature.