Equine echocardiography.

This review explains how veterinarians use ultrasound from the outside of a horse’s chest to visualize the heart and blood flow. It outlines when to use echocardiography, how to perform a complete exam, what to measure, and how both standard and newer techniques help diagnose heart disease in horses.

What the article covers

• Summarizes principles of transthoracic echocardiography (TTE) in horses, including image acquisition, measurements, and interpretation.
• Details indications, equipment choices, and technical considerations unique to equine patients.
• Describes a systematic, view-by-view approach using 2D, M-mode, and Doppler techniques.
• Explains methods to assess chamber size, systolic and diastolic function, atrial performance, hemodynamics, and valve regurgitation.
• Introduces selected applications of tissue Doppler imaging (TDI) and 2D speckle-tracking echocardiography (STE).

Indications for equine transthoracic echocardiography

• Characterization of cardiac murmurs and assessment of suspected valvular disease (e.g., mitral, tricuspid, aortic regurgitation).
• Evaluation of arrhythmias (e.g., atrial fibrillation), syncope, or exercise intolerance/poor performance.
• Screening and monitoring of congenital defects (e.g., ventricular septal defect) or acquired disease (e.g., endocarditis, myocardial disease).
• Investigation of signs of heart failure (e.g., jugular distension, edema, respiratory signs) or suspected pulmonary hypertension.
• Pre-purchase assessments in selected cases and follow-up of incidental findings from auscultation.

Equipment and technical setup

• Transducers: low-frequency phased-array or curvilinear probes (approximately 1.5–3.5 MHz) for adequate depth penetration in large thoraxes.
• Modalities: 2D imaging, M-mode, color Doppler, pulsed-wave (PW) Doppler, continuous-wave (CW) Doppler; ECG for timing events.
• Optimization: set appropriate depth and focus (often 20–35 cm), adjust gain to avoid dropout or noise, and tailor color scale/PRF to flow velocities.
• Patient preparation: minimal clipping when possible; generous acoustic coupling (alcohol/gel); calm standing restraint; avoid or minimize alpha-2 sedation as it alters heart rate, conduction, and loading conditions.
• Positioning: horse standing square; right and left parasternal windows typically at the 3rd–5th intercostal spaces beneath the triceps mass.

Standard acoustic windows and views

• Right parasternal long-axis views:
  • Four-chamber view: left and right atria and ventricles, AV valves, septa.
  • Left ventricular outflow tract (LVOT) view: LV, aortic root, mitral valve, proximal ascending aorta.
• Right parasternal short-axis views:
  • At papillary muscle/chordal level: LV cavity shape and wall motion; M-mode for dimensions.
  • At mitral valve level: leaflet motion and coaptation.
  • At aortic valve level: aortic root, right ventricular outflow tract (RVOT), left atrium.
• Left parasternal views:
  • Long-axis of aorta and LVOT; pulmonary artery bifurcation; interrogation of aortic and pulmonary flows.
• Ancillary windows:
  • High right/left parasternal for great vessels; subcostal or suprasternal as anatomy allows.

Systematic examination workflow

• Begin with 2D survey to identify anatomy, global function, and obvious lesions.
• Apply color Doppler systematically across all valves and septa to localize regurgitant or shunt flows.
• Acquire M-mode at standardized short-axis levels for reproducible chamber measurements.
• Use PW/CW Doppler to quantify inflow/outflow velocities (mitral, tricuspid, aortic, pulmonary) and any high-velocity jets.
• Record cine loops and representative stills with ECG; perform measurements on cardiac cycles free of ectopy and representative of steady rhythm.

Chamber measurements and quantification

• Left ventricle (LV):
  • M-mode at papillary muscle level for LV internal diameter in diastole/systole (LVIDd/LVIDs), interventricular septum (IVS), and LV free wall (LVFW) thickness.
  • 2D linear dimensions and cross-sectional area for fractional area change (FAC) when geometry is noncircular.
• Left atrium (LA):
  • 2D area or diameter measurements (often at aortic valve short-axis level); LA-to-aortic root ratio can contextualize enlargement.
• Right heart:
  • Right ventricular (RV) dimensions are more challenging; use qualitative assessment supplemented by linear/area indices when feasible.
  • Right atrial (RA) area estimation where indicated (e.g., TR, atrial fibrillation).
• Septal defects:
  • Localize and size ventricular septal defects (VSD) in multiple planes; relate size to aortic diameter when possible.

Allometric scaling and interpretation of reference values

• Rationale:
  • Equine body size varies widely; cardiac dimensions scale with body weight in a nonlinear fashion.
• Approach:
  • Apply allometric equations of the form Measure = a × (Body weight)^b to compare an individual horse to population norms.
  • Exponents (b) are typically near 0.3 for linear dimensions, near 0.6 for areas, and near 0.9 for volumes (reflecting geometric scaling).
• Contextual factors:
  • Breed, age, sex, and athletic conditioning influence cardiac size and wall thickness (“athlete’s heart”).
  • Interpret enlargement relative to scaled references rather than absolute cutoffs.

Assessment of systolic ventricular function

• Conventional indices:
  • Fractional shortening (FS) from M-mode ([(LVIDd − LVIDs)/LVIDd] × 100%).
  • Fractional area change (FAC) from 2D short-axis as a geometry-agnostic alternative.
  • Qualitative global and regional wall motion assessment on 2D cine loops.
• Flow-derived metrics:
  • Stroke volume and cardiac output via aortic PW Doppler: cross-sectional area × velocity–time integral (VTI) × heart rate.
• Caveats:
  • Loading conditions, sedation, and heart rate strongly affect systolic indices.
  • Geometric assumptions limit translation of human small-animal EF methods; prioritizing consistent, breed/weight-adjusted serial trends is valuable.

Assessment of diastolic ventricular function

• Transmitral inflow (PW Doppler):
  • Early (E) and atrial (A) filling waves, E/A ratio, E-wave deceleration time, and isovolumic relaxation time (IVRT).
  • High vagal tone in resting horses can accentuate A; rhythm disturbances (e.g., physiologic 2nd-degree AV block) complicate measurement—choose representative sinus beats.
• Tissue Doppler annular velocities (with TDI):
  • Early diastolic (E′) and late diastolic (A′) velocities at the mitral annulus; E/E′ provides an estimate of filling pressures (interpret cautiously across vendors and breeds).
• 2D and color M-mode:
  • Qualitative LV relaxation and suction; propagation velocity methods are less standardized in equine practice.

Atrial function assessment

• Size:
  • LA and RA areas or diameters to detect enlargement associated with MR/TR or chronic arrhythmias.
• Functional phases:
  • Reservoir (conduit and booster) function inferred from phasic volume/area changes; LA fractional area change can be derived when end-systolic and end-diastolic areas are available.
• Rhythm influence:
  • Atrial fibrillation abolishes active atrial contraction, reducing late diastolic filling and altering transmitral patterns.

Hemodynamic and pressure assessment with Doppler

• Bernoulli relationship:
  • Use simplified ΔP ≈ 4 × (velocity)^2 to estimate pressure gradients across valves or defects.
• Pulmonary pressures:
  • Estimate RV systolic pressure from tricuspid regurgitation (TR) peak velocity plus right atrial pressure estimate; interpret cautiously in horses due to RA pressure uncertainty.
• Outflow obstruction or shunts:
  • Quantify aortic or pulmonary outflow velocities and gradients; for VSDs, the left-to-right gradient informs relative pressures and shunt severity.
• Flow volumes:
  • Aortic and pulmonary VTIs enable stroke volume and cardiac output estimation; comparison can inform shunt fraction when appropriate.

Evaluation of valvular regurgitation

• Detection and localization:
  • Color Doppler to identify jets at mitral, tricuspid, and aortic valves; sweep in multiple planes to define origin and extent.
• Severity assessment (semiquantitative):
  • Jet area relative to receiving chamber (context-dependent; affected by loading and gain).
  • Vena contracta width (narrow aliasing neck) as a more robust index when feasible.
  • CW Doppler jet density and contour; for AR, shorter pressure half-time suggests more severe regurgitation.
  • Pulmonary vein systolic flow blunting/reversal (severe MR) or hepatic vein systolic reversal (severe TR), when obtainable.
• Equine-specific considerations:
  • Trivial to mild MR/TR is common in fit horses and may be physiologic; AR prevalence increases with age due to valve fibrosis.
  • Serial examinations, clinical signs, and exercise tolerance guide management more than a single Doppler metric.

Newer methods: tissue Doppler imaging (TDI) and 2D speckle tracking (STE)

• Tissue Doppler imaging:
  • Measures myocardial velocities (S′ systolic, E′ and A′ diastolic) at annular or segmental sites.
  • Useful for quantifying longitudinal function and relaxation; angle-dependent and sensitive to tethering.
• 2D speckle tracking echocardiography:
  • Angle-independent assessment of myocardial deformation (strain and strain rate) in longitudinal and circumferential directions.
  • Can detect subclinical dysfunction and regional abnormalities; requires good image quality and adequate frame rates, with vendor-specific algorithms.
• Clinical role:
  • Adjuncts to conventional echo for nuanced assessment of systolic/diastolic performance and training effects; reference ranges and standardization are evolving in equine medicine.

Common pitfalls and limitations

• Acoustic challenges:
  • Ribs, large thoracic depth, and aerated lung obscure windows; patient conformation affects access.
• Physiologic variables:
  • Low resting heart rate, respiratory variation, and vagal tone alter Doppler profiles; transient AV block is common at rest.
• Doppler artifacts:
  • Aliasing, blooming, and angle dependence can mislead severity grading; use CW for high-velocity jets and optimize color scales.
• Measurement variability:
  • Interobserver differences and loading-state dependence necessitate standardized protocols and, when possible, serial comparisons.

Clinical applications and decision-making

• Risk stratification:
  • Integrate valve lesion severity, chamber remodeling, and function with clinical signs to advise on training, competition, or breeding.
• Monitoring:
  • Schedule follow-up echocardiography for progressive valve disease (especially AR and MR), significant chamber enlargement, or new arrhythmias.
• Interdisciplinary care:
  • Correlate echo findings with ECG/Holter, exercise testing, and hematologic/biochemical data; consider infectious workup for endocarditis.

Reporting, quality control, and follow-up

• Structured reporting:
  • Document acquisition windows, measurements (with body-weight scaling where applicable), Doppler velocities/gradients, and qualitative impressions.
• Image archiving:
  • Save cine loops and stills with ECG for re-review and serial comparison; note machine settings to enhance reproducibility.
• Follow-up planning:
  • Define surveillance intervals based on lesion type and severity, performance demands, and any clinical progression.