Equis ISSN 2398-2977

Heart: echocardiography

Contributor(s): Christopher Brown, Lesley Young

Introduction

Routine echocardiography
 
  • High-frequency sound waves are directed at tissues.
  • Tissues have different acoustic impedances.
  • Sound waves are reflected when they meet tissue interfaces.
  • Amplitude of the echo and the time taken for wave to return to source are computed to produce an image.
  • B Mode:
    • B for 'brightness' mode.
    • Produces a real-time two-dimensional image (2-D)   Heart: normal RHS long axis 01 - echocardiograph      Heart: normal RHS long axis 02 - echocardiograph            Heart: normal RHS short axis 01 - M-mode echocardiograph    Heart: normal RHS short axis 02 - echocardiograph     .
  • M Mode:
    • M for 'motion' mode.
    • A one-dimensional image is produced which plots time against the position of tissues relative to the transducer    Heart: mitral valve 01 - M-mode echocardiograph      Heart: mitral valve 02 - M-mode echocardiograph  .
    • Single imaging beam.
    • 2-D image is used to place the M mode cursor accurately.
    • Usually used with an ECG trace   Cardiovascular: ECG (electrocardiography)    Heart: mitral valve 01 - M-mode echocardiograph  to assess the timing of events in the cardiac cycle (also applies to B mode).
    • Provides better temporal resolution than B mode imaging, but newer digital machines are less frame rate limited at the depths needed for equine echocardiography, so the need for guided M mode is reducing.

Doppler echocardiography

  • To investigate blood flow through targeted areas of the heart and great vessels.
  • Useful to assess:
    • Valvular regurgitation   Heart: mitral valve 03 - color echocardiograph doppler    Heart: mitral valve 04 - color echocardiograph doppler        .
    • Septal defects    .
    • Complex congenital defects.
  • Ultrasound is directed at a moving stream of blood.
  • The frequency of the reflected ultrasound is altered dependent on direction and velocity of blood flow. This is derived from the Doppler principle.
  • The change in frequency is then used to calculate blood flow velocity.
  • Velocity is displayed against time, conventionally blood flowing toward the transducer is shown as positive while blood flowing away from the transducer is negative.
  • Low-frequency transducers produce better information in Doppler echocardiography.
  • Most accurate estimates of velocity are obtained when the transducer beam is parallel with the direction of blood flow.
  • If the direction of the insonation beam is >20° away from the direction of blood flow, the derived velocity will greatly underestimate true blood flow velocity.

Continuous-wave Doppler

  • Two ultrasound crystals are used, one to emit ultrasound and one to receive reflected signal.
  • Produces accurate velocity measurements from the whole length of the ultrasound beam. It is not therefore possible to localize the source of the velocity information   Heart: LAV valve regurgitation 01 - echocardiograph doppler  .

Pulsed-wave Doppler

  • Transducer first emits signal then switches to reception mode.
  • Using this method, blood flow at specific points in vessels or organs can be examined.
  • This time-limits the number of pulses that can be emitted and therefore the resolution in terms of velocity measured is also limited.
  • When measured velocity is higher than a limit determined by transducer frequency and depth (Nyquist limit) aliasing can occur.
  • This results in flow being plotted in the opposite direction to that in which it occurs. It is a limitation imposed by the sampling fequency.

Color-coded Doppler

  • A form of pulsed Doppler, where direction and velocity information is superimposed on a two-dimensional image   Heart: mitral valve 04 - color echocardiograph doppler  .
  • Red denotes flow towards the transducer   Heart: tricuspid valve - color echocardiograph doppler  .
  • Blue denotes flow away from the transducer   Heart: pulmonary artery - pulsed echocardiography doppler  .
  • Usually, increasing velocity is indicated by a lighter shade.
  • Some systems code disturbed flow as green   Heart: tricuspid valve - color echocardiograph doppler  .
  • Aliasing also occurs with color Doppler in which blood flowing above the Nyquist limit appears to be moving in the opposite direction.
  • Has abolished the need for cardiac catheterization.
Print off the Owner factsheet on Diagnostic imaging to give to your clients.

Uses

  • Evaluation of cardiac function and disease.

Routine echocardiography

  • Congenital diseases:
    • Ventricular septal defects   Heart: ventricular septal defect    Heart: ventricular septal defect 01 - echocardiograph  - usually just below the tricuspid valve; demonstrate with a right parasternal long-axis view of aorta and a short axis view below the level of the aortic valve; combine with Doppler echocardiography for prognosis        Heart: ventricular septal defect 05 - echocardiograph  .
    • Cardiac valve abnormalities (mitral, tricuspid, aortic, other) - use 2-D echocardiography, visualizing valve, chordae, papillary muscles, outflow and inflow tracts and great vessels in multiple planes.
    • See normals   Heart: normal LHS long axis 01 - echocardiograph    Heart: normal RHS long axis 01 - echocardiograph    Heart: normal LHS short axis 01 - echocardiograph    Heart: normal RHS short axis 01 - M-mode echocardiograph        .
  • Valvular disease:
    • Direct evidence of valvular abnormality is difficult to detect from echocardiography in the horse, due to lack of detail, and especially without the assistance of color flow Doppler to assess regurgitation.
    • Look for 'floppy' appearance or apparent prolapse into atrium during systole.
    • M mode can demonstrate vibration of the anterior leaflet of the left atrioventricular valve when there is aortic insufficiency and a regurgitant jet.
    • Ruptured chordae tendinae   Chordae tendineae: rupture  - look for a 'flail' leaflet in the left atrioventricular valve    moving into the atrium during systole (not a consistent sign), especially if minor chordae are involved.
    • Myocarditis and endocarditis   Heart: endocarditis  - may see vegetations on the valves, or abnormal echogenicity in the myocardium    .
  • Pericardial disease  Heart: pericarditis  :
    • Heart separated from pericardium by an anechoic space.
    • Hyperechoic bands of fibrin in effusion, maybe attached to the myocardium.
    • Echo can be used to guide pericardiocentesis to drain pericardial sac.
  • Cardiac function assessment:
    • To monitor response of heart to abnormal blood flow.
    • Volume and pressure overload due to valvular regurgitation - chamber enlargement.

    Alpha-2 agonists such as xylazine   Xylazine  , romifidine   Romifidine  decrease fractional shortening and heart rate.

Doppler echocardiography

  • To identify the source of murmurs    .
  • To assess valve and chamber function (cf normal)    .
  • Timing of regurgitant flow determined using size, color flow or continuous wave Doppler. This can be used to help assess the severity of regurgitant flow and valve dysfunction   Heart: LAV valve regurgitation 01 - echocardiograph doppler  .
  • Significance of abnormal flow can be assessed by a calculation of pressure difference between chambers based on the maximum velocity of abnormal flow; so that a large pressure difference indicates a small or restricted defect but relative equality in pressure suggests a large defect   Heart: ventricular septal defect 05 - echocardiograph  .
  • Can be used to calculate cardiac output and other measures of ventricular systolic and diastolic function.

Advantages

  • Non-invasive.
  • Widely available.
  • Dynamic.

Disadvantages

  • Given the size of the horse, there is an inevitable trade-off between depth of penetration and image quality.
  • Access to heart limited by position of triceps muscle and lung fields.
  • It is almost impossible to get images of diagnostic quality in some very fat, large horses.

Requirements

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Preparation

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Procedure

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Buhl R, Ersboll A K, Eriksen L & Koch J (2005) Use of color Doppler echocardiography to assess the development of valvular regurgitation in Standardbred trotters. JAVMA 227 (10), 1630-1635 PubMed.
  • Buhl R, Ersboll A K, Eriksen L & Koch J (2005) Changes over time in echocardiographic measurements in young Standardbred racehorses undergoing training and racing and association with racing performance. JAVMA 226 (11), 1881-1887 PubMed.
  • Fregin G F (1992) Medical evaluation of the cardiovascular system. Vet Clin North Am Equine Pract (2), 329-346 PubMed.
  • Long K J (1990) Doppler echocardiography in the horse. Equine Vet Educ 2 (1), 15-17 VetMedResource.
  • Reef V B et al (1990) Echocardiographic detection of an intact aneurysm in a horse. JAVMA 197 (6), 752-755 PubMed.
  • McGladdery A J & Marr C M (1990) Echocardiography for the practitioner. Equine Vet Educ 2 (1), 11-14 Wiley Online Library.
  • Bonagura J D, Herring D S & Welker F (1985) Echocardiography. Vet Clin North Am Equine Pract 1 (2), 331-333 PubMed.
  • Reef V B (1985) Evaluation of the equine cardiovascular system. Vet Clin North Am Equine Pract (2), 275-288 PubMed.


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