Left and right-side echocardiographic measurement of pulmonary artery stiffness in thoroughbred racehorses.

This study tested whether a left-side ultrasound view of the horse’s heart can reliably measure pulmonary artery stiffness and found it produces consistent results that differ systematically from the traditional right-side view. The left view yielded higher stiffness estimates, likely because the ultrasound beam lines up better with blood flow.

What the researchers measured and why it matters

• Pulmonary artery stiffness (PAS) is a non-invasive echocardiographic index reflecting how elastic (or stiff) the pulmonary artery is; stiffer arteries can indicate pulmonary vascular disease or higher pulmonary pressures.
• The PAS index used here is calculated as Maximal Frequency Shift (MFS) divided by Acceleration Time (AT), where:
  • AT = time from flow onset to peak velocity in the pulmonary artery (shorter AT often reflects higher pressure or stiffer vessels).
  • MFS = the highest Doppler frequency shift of blood flow (proportional to peak velocity along the ultrasound beam).
  • PAS = MFS/AT, yielding a stiffness-related parameter in kHz/s.
• In horses, PAS had previously been measured mainly from a right parasternal short-axis view; the study assessed whether a left parasternal angled view of the right ventricular inflow/outflow can provide repeatable and reproducible measurements and how those compare with the right view.

Study design and population

• Population: 40 Thoroughbred racehorses underwent pulsed-wave Doppler echocardiography from both right and left parasternal windows.
• Primary measurements from each view: AT, MFS, and PAS (MFS/AT).
• Repeatability and reproducibility plan:
  • Intra-operator repeatability (same operator repeating measures) in 40 horses.
  • Inter-operator reproducibility (three operators performing measurements) in 40 horses.
  • Day-to-day repeatability (two different days) in 5 horses.
  • Inter-operator reproducibility for image acquisition (two operators acquiring images, then measured) in 10 horses.
• Comparative analysis: Left vs right parasternal measurements compared in the same 40 horses.

Statistical methods and thresholds used

• Agreement and reliability were quantified using the intraclass correlation coefficient (ICC) and coefficients of variation (CV).
• Conventional ICC interpretation applied:
  • Poor: <0.5; Moderate: 0.5–0.75; Good: 0.75–0.9; Excellent: >0.9.
• Left–right comparisons used the Wilcoxon signed-rank test for paired nonparametric data.

Key results

• Intra-operator repeatability (same operator):
  • AT: ICC = 0.94 (excellent).
  • MFS: ICC = 0.95 (excellent).
  • PAS: ICC = 0.97 (excellent).
• Inter-operator reproducibility (three operators, same images/measurements):
  • AT: ICC = 0.63–0.87 (moderate to good, sometimes excellent).
  • MFS: ICC = 0.96–0.99 (excellent).
  • PAS: ICC = 0.83–0.96 (good to excellent).
• Day-to-day repeatability (two days, n = 5):
  • AT: ICC = 0.80 (good).
  • MFS: ICC = 0.71 (moderate to good).
  • PAS: ICC = 0.98 (excellent).
• Inter-observer agreement for image acquisition (two operators acquiring images, n = 10):
  • AT: ICC = 0.97 (excellent).
  • MFS: ICC = 0.86 (good).
  • PAS: ICC = 0.94 (excellent).
• Left vs right parasternal view differences (n = 40), all statistically significant (p-values not specified in the excerpt but reported as significant):
  • AT: lower from the left view (110 ms) vs right (151 ms).
  • MFS: higher from the left view (2.60 kHz) vs right (2.27 kHz).
  • PAS: higher from the left view (23.8 kHz/s) vs right (15.6 kHz/s).

Interpretation of the measurement differences

• The left parasternal angled view likely aligns the Doppler beam more closely with true pulmonary artery flow direction than the right short-axis view.
• Better alignment increases measured velocity (higher MFS) and sharpens the early systolic upstroke (shorter AT), which together elevate PAS (MFS/AT).
• Therefore, left-view PAS appears systematically higher than right-view PAS due to technical (angle-of-insonation) rather than biological differences.

Clinical and research implications

• Feasibility: PAS can be consistently obtained from a left parasternal angled view in Thoroughbred racehorses.
• Reliability: Excellent within-operator and generally strong between-operator agreement, especially for the composite PAS metric, supports use in longitudinal monitoring.
• Standardization: Because absolute values differ by imaging window, clinicians and researchers should:
  • Use the same window (left or right) consistently within a horse and across serial exams.
  • Avoid directly comparing left-derived PAS to right-derived reference ranges.
• Potential applications:
  • Screening and monitoring for pulmonary vascular stiffness and early pulmonary hypertension indicators in athletic horses.
  • Assessing training effects or recovery where pulmonary hemodynamics may change over time.

Strengths of the study

• Within-subject, paired comparison of left and right views in the same 40 horses controls for biological variability.
• Comprehensive reliability testing: intra-operator, inter-operator, day-to-day, and image acquisition reproducibility.
• Use of ICCs with clear performance bands allows practical interpretation of measurement robustness.

Limitations to consider

• Day-to-day repeatability was evaluated in a small subset (n = 5), which may limit precision of those estimates.
• The sample consisted solely of Thoroughbred racehorses; generalizability to other breeds, ages, or clinical populations is uncertain.
• Measurements were obtained at rest; exercise or recovery-state dynamics were not assessed.
• No invasive gold standard (e.g., right heart catheterization) was used to validate PAS against direct pulmonary pressure or stiffness measurements.
• Exact p-values and measures of dispersion for some comparisons were not provided in the excerpt.

Practical takeaways

• Both left and right parasternal windows can yield repeatable PAS measures, but absolute values differ systematically.
• Prefer the left parasternal angled view when feasible for stronger beam–flow alignment and robust PAS estimates, or adhere strictly to one window for consistency across serial exams.
• Focus on trends within the same horse using the same imaging window and operator whenever possible.

Bottom line

• PAS measurement from the left parasternal angled view in Thoroughbred racehorses is feasible, highly repeatable, and tends to produce higher values than the right view, likely due to superior Doppler alignment with pulmonary artery flow.
• Standardizing the imaging window is essential for meaningful comparisons and longitudinal monitoring.