Left and right-side echocardiographic measurement of Pulmonary Artery Stiffness in Thoroughbred racehorses

The Pulmonary Artery Stiffness (PAS) is a non-invasive echocardiographic index of pulmonary artery elasticity. In equine medicine, it has been demonstrated that PAS can be easily measured non-invasively by pulsed-wave Doppler echocardiography across the pulmonary artery valve from the right parasternal short axis view. In literature, to assess the diastolic function of the right ventricle, it has been described the acquisition of Doppler signals across the pulmonary artery from the left side of the thorax in horses. However, there are no studies that measured PAS from this side in the literature. Hence, the present study aimed to measure PAS by pulsed-wave Doppler echocardiography from both right and left sides of the thorax to evaluate if there was a difference between the two measurements in Thoroughbred racehorses. The Institutional Animal Care Committee approved the present prospective study (OPBA_14_2023). Twenty-five Thoroughbred racehorses were included and underwent an echocardiographic examination to measure PAS. Pulsed-wave Doppler of the pulmonary artery outflow was acquired from both sides of the thorax in each horse: from the right side, using the right parasternal short-axis view, as already described in the literature; from the left side, using the parasternal angled view of the right ventricular inflow/outflow, with the sample volume positioned just above the bifurcation of the pulmonary artery. Maximal Frequency Shift (MFS) and Acceleration Time (AT) were measured from at least three non-consecutive Doppler flow traces to obtain the mean values, and the mean PAS value was calculated as the ratio between MFS and AT. The Wilcoxon signed-rank test was used to compare right and left-side PAS measurements. The Intraclass Correlation Coefficient (ICC) and their 95% Confidence Intervals (95% CI) were used to assess the reliability of inter-observer, intra-observer, and day-to-day (measurements obtained from Doppler acquisition in two different echocardiographic sessions) left-side PAS measurements5. Finally, the Coefficient of Variance (CV) was calculated and expressed as a percentage to assess the inter-operator variability, intra-operator variability, and day-to-day variability. From the right side, the mean PAS value was 16.4 kHz/s; from the left side, it was 24.7 kHz/s. A statistically significant difference (p < 0.001) between the two sides was found. All inter-observer (ICC = 0.921, 95% CI: 0.808 – 0.968), intra-observer (ICC = 0.904, 95% CI: 0.786 – 0.957), and day-to-day (ICC = 0.984, 95% CI: 0.874 – 0.998) left-side PAS measurements showed an excellent reliability level. Finally, a low variability was found for inter-observer (CV = 8%), intra-observer (CV = 9%), and day-to-day (CV = 6%) left-side PAS measurement. The angle formed by the transmitted wave and the blood flow direction is important information to consider when performing a Doppler examination. In fact, the closer to parallel the transmitted wave is with the blood flow direction, the higher the Doppler measurement. In the present study, finding significantly higher left-side PAS measurements could be likely due to a better parallel alignment of the sample volume to the pulmonary vein when the pulsed-wave Doppler was acquired from the left side. However, as already described in the literature, the more cranial probe placement from the left side was poorly tolerated by some horses, making the acquisition of high-quality Doppler images challenging. In addition, in some cases, a high Body Condition Score contributed to increase the difficulty of obtaining good-quality images. Finally, the left-side PAS measurement showed high repeatability and reproducibility, demonstrating how PAS can be measured consistently from the left side of the thorax. The next research steps will focus on increasing the number of subjects assessed in an attempt to establish specific reference ranges for each side in horses.