Echocardiography

All echocardiographic studies¹ were performed by a board‐certified veterinary cardiologist (L.C.V or J.A.S.) or a cardiology resident under the direct supervision of a board‐certified veterinary cardiologist and all raw data were captured digitally for offline analysis at a digital workstation.² Dogs were manually restrained in right and left lateral recumbency. Use of sedation and supplemental oxygen was permitted if deemed necessary by the attending clinician. Conventional imaging planes21 were utilized with continuous ECG monitoring. Care was taken to align the TR jet as parallel as possible to the plane of the ultrasound interrogation cursor. Care was also taken to optimize visualization of the main and right pulmonary arteries via the 2D echocardiographic view from a right parasternal short axis basilar position. Quantification of a pulmonary valve insufficiency jet to estimate mean and diastolic pulmonary artery pressures was not performed for the purposes of this study.

All echocardiographic assessments, measurements and calculations were performed by the same individual (L.C.V.) and in the same order for each dog, with measurement of TR jet velocity obtained last. Therefore, the individual performing echocardiographic measurements was unaware of the TRPG when measuring all other echocardiographic indices. Peak systolic TR jet velocity obtained by 2D/color Doppler‐guided continuous‐wave Doppler was measured from the view that allowed the clearest jet profile and best alignment with direction of the jet. Tricuspid regurgitation jet velocity was only measured when a complete flow profile of the jet was present and peak velocity was clearly visualized. The value for each echocardiographic index consisted of an average of 3 representative but not necessarily consecutive measurements and was always measured while the dog was in sinus rhythm. Heart rate recorded represented the average heart rate of each of the 3 cardiac cycles used to determine each echocardiographic index value.

For determination of the RPAD index, the minimum diastolic (RPA_D; usually at the Q wave) and maximum systolic (RPA_S; usually coinciding with the largest T wave deflection or early‐to‐midsystole) internal diameter of the right PA was quantified at the same location of the right PA. This was performed using a trailing edge to leading edge technique. Care was taken to clearly visualize the internal borders of right PA throughout the cardiac cycle, and internal diameters of the right PA were measured as perpendicular as possible to the internal borders of the right PA. The RPAD index represents the percent change in diameter of the right PA throughout a single cardiac cycle according to the following formula: RPAD index = ([RPA_S−RPA_D]/RPA_S) × 100 (Fig ​(Fig1).1). For each individual dog's measurement, RPA_D and RPA_S were measured at the same location along the right PA, although this location could have varied slightly from dog to dog. The RPAD index was also determined in 12 randomly selected studies (3 from each of the PH groups) from a short axis 2D imaging view of the right PA (viewed from a right parasternal long axis view optimized for the heart base/right PA in short axis).6

Representative measurement and calculation of the right pulmonary artery distensibility (RPAD) index in a dog with a peak tricuspid regurgitation systolic pressure gradient (TRPG) <36 mmHg. RPA, right pulmonary artery; RPA_D, RPA at its minimum diameter in diastole; RPA_S, RPA at its maximum diameter in systole; RA, right atrium; RV, right ventricle; Ao, aorta; PA, pulmonary artery.

From the same view used to quantify the RPAD index, diastolic measurements of the internal dimensions of the main pulmonary artery (MPA) in long axis and the ascending aorta (Ao) in short axis were obtained, and the MPA:Ao ratio was calculated.10 The MPA measurement was obtained approximately midway between the pulmonary valve and origin of the right and left PAs.22

Pulmonary artery flow was measured and recorded with pulsed‐wave Doppler imaging from the standard right parasternal short axis view only, using color Doppler imaging to guide placement of the sample volume (1–3 mm) centrally between the opened pulmonary valve leaflets. AT was measured from the onset of the pulsed‐wave Doppler flow to peak flow velocity. ET was measured from the onset to the end of the pulsed‐wave Doppler PA flow signal, and an AT:ET ratio was calculated.11 The presence of systolic “notching” during deceleration of the PA flow profile was noted.4

Right ventricular (RV) size and function was assessed from a standard left apical four‐chamber view. Right ventricular function was quantified by FAC, where measurements of RV area were obtained by tracing the RV endocardial border at end‐diastole (RVA_D) and end‐systole (RVA_S).20 Right ventricular percent FAC was calculated using the formula: FAC = ([RVA_D−RVA_S]/RVA_D) × 100. Fractional area change values were compared to body weight‐specific reference intervals.20 Right ventricular size was assessed from the same view as FAC and, for the purposes of this study, the presence of (severe) RV enlargement was deemed present if the RV chamber and/or wall thickness was subjectively greater than that of the left ventricle.4

The presence of interventricular septal flattening was subjectively assessed throughout the cardiac cycle from standard short‐ and long‐axis views.

Left atrial (LA) size was assessed from a standard right parasternal long axis four‐chamber view and was determined by indexing the maximum systolic dimension23 to the aortic valve annulus, which was measured at the hinge points of opened leaflets in an early systolic frame from a separate long axis view optimized for the aortic valve annulus. A ratio greater than 2.6 indicated enlargement.24