Basic M-mode and two-dimensional echocardiography (2DE) transformed our understanding of equine cardiac disease in the late 1970s and early 1980s because, for the first time, we had a reliable way to look into the horse’s heart, evaluate its size and in particular identify changes within specific chambers which were the result of shifts in volume and pressure induced by individual cardiac disease. Close on their heels came Doppler echocardiography which allowed shunts and regurgitant flow to be mapped with colour flow and functional indices to be derived with spectral Doppler modes.
Although the horse’s size is a fundamental challenge in any form of imaging, as equipment improved and became more accessible through technological improvement, numerous institutes, clinics and individuals were able to offer equine cardiac consultation services using equipment which could very adequately display the equine heart and demonstrate intracardiac structures such as heart valve leaflets in reasonable detail. A key requirement for basic equine echocardiography is that the unit’s imaging depth is at least 30cm, necessary so that in all horses with normal heart size, all four chambers can be visualised from the right parasternal window. Good quality echocardiographic units which are small enough to take to a farm to perform a basic echo (M-mode, 2DE and Doppler flow mapping) are now affordable but advanced echocardiographic modalities have some important benefits over conventional modes.
What is advanced echocardiography?
Advanced echocardiography, in the current forms, consists of tissue Doppler imaging (TDI), speckle tracking echocardiography (STE) and three-dimensional echocardiography (3DE). Each of these modalities is now available in more than one equine referral centre in UK. While arguably not essential for every horse with heart disease, individually each of these modalities has specific advantages which allow clinicians to gain deeper insight of cardiac function. TDI and STE mainly provide insights on cardiac function, while 3DE has advantages in both functional and structural assessments. The specific clinical benefits of these modalities depend on the particular disease under investigation.
Tissue Doppler imaging
TDI does what the name implies: using the Doppler principle whereby the frequency shift induced by bouncing sound off a moving object can be used to derive an estimate of its velocity, TDI allows the movement of specific regions of the myocardium to be interrogated. There are two forms: spec-tral TDI is a pulsed wave technique where in real-time a sample volume is placed in the area of interest and velocity data are collected; and colour TDI data are collected simultaneously from all points on a sector with offline analysis to generate velocity curves from the area of interest. The derived measurements are similar but not identical and colour values are a mean of the velocities within the sample area and thus are slightly lower than spectral values. From these velocity data, displacement and deformation can be calculated. Strain, the magnitude of deformation, is defined by the deformation of an object relative to its original shape and expressed as a percentage while strain rate provides the closest estimate of contractility.
TDI can be used to examine function in both systole and diastole but in horses, due to anatomical limitations, it can only be used to derive velocities in the radial
plane limiting its usefulness in comparison to its role in canine and human cardiology. TDI is not, in fact, all that new in equine medicine and it was first explored for use in horses around 15 years ago (Sepulveda et al., 2010). Since then, most papers have explored its use in various breeds, age groups and examination conditions (Schefer et al., 2010; Decloedt et al., 2013b; Flethoj et al., 2016; Gehlen and Schlaga, 2019) with relatively little information about clinical applications. TDI has proved useful in documenting LV dysfunction in horses with myocardial disease (Decloedt et al., 2012a; Koenig et al., 2017) and can be used very effectively to assess atrial function: following treatment for atrial fibrillation (AF), mechanical function is not necessarily immediately restored (Schwarzwald et al., 2007; Decloedt et al., 2013c). Using TDI and 2D estimates to assess when atrial function has improved can help guide individual post-conversion exercise and rehabilitation programmes (Figure 1). TDI can also be used during pre-treatment assessment of AF cases. AF cycle length (Figure 2) is an indicator of atrial effective refractory period and can be estimated with colour TDI (Decloedt et al., 2013a). During AF, electrical remodelling decreases the effective refractory period which stabilises the fibrillation waves and, potentially, impacts likelihood of successful treatment and has been shown to be useful in identifying horses at increased risk of AF recurrence (De Clercq et al., 2014).
Speckle tracking echocardiography
STE can be applied to both 2D and 3D imaging. Currently in equine practice, it is primarily used with 2D images. The mode documents myocardial motion using the speckle pattern in the myocardium to identify specific regions. The speckle pattern created as ultrasound interacts with tissue is a mixture of interference patterns and acoustic reflections – the imaging technique relies on identifying clusters of echoes and not individual speckles and these clusters are tracked to determine velocity. As with TDI, velocity can be used determine strain, strain rate and displacement. Because 2D imaging (unlike Doppler-based technologies) is less impacted by angle, STE can determine not only radial (Figure 3) and circumferential movement (Figure 4) but also longitudinal movement (Figure 5) which makes the modality very appealing for equine use. However, because frame rates used to generate the images are much lower than those used with TDI, it is inherently less accurate in determining velocity.
Early work applying STE on horses focused on methods and normal values for radial (Figure 3) and circumferential (Figure 4) imaging (Schefer et al., 2010; Decloedt et al., 2012b; Decloedt et al., 2013b; Flethoj et al., 2016; Gehlen and Bildheim, 2018; Gehlen and Schlaga, 2019). The pay-off between frame rate and imaging width needed for the large equine heart means it is difficult to image the entire ventricle and depending on preference, different groups have chosen to focus on the apex or the base – getting both at once is difficult in many larger horses. In humans, an important indication for STE is identification of regional pathology, specifically myocardial infarction, a pathology which is very rare in horses. Equine clinical studies have highlighted its use in myocardial disease (Decloedt et al., 2012a) and to document changes in LV function associated with primary valvular disease (Figure 5) (Ven et al., 2018; Decloedt et al., 2020), while a recent study has looked at how it can be applied to examine left atrial function (Eberhardt et al., 2020).
3DE is the most major recent advance in echocardiography enabled by improvements in both computer and transducer technologies. Instead of the “slice” produced with 2DE, with 3DE a pyramid of data is collected and subsequently this can be manipulated – sliced, rotated, flipped and reversed – so that the echocardiographer can interrogate internal cardiac structures in any plane. There is always a pay off between resolution, image size and frame rate; by collecting a series of small pyramids over multiple beats, the sub-images can be stitched together to create a larger pyramid – known as multi-beat acquisition. A regular heart rhythm and a stationary patient are key to successful multi-beat acquisition, which can be a problem if horses are sedated for examination as this often induces second degree block.
A fact which was much overlooked in the past is that the heart operates by moving volumes of blood and therefore understanding how disease modifies the heart’s ability to move volume is the ideal way to assess any patient with heart disease. Conventional echocardiography is based on functional measures such as fractional shortening (the difference between LV diameter in diastole and systole as a function of diastolic diameter) derived from a single dimension image), or M-mode and 2DE estimates of ejection fraction which rely on various geometric assumptions which may or may not apply to horses, and may or may not apply to the diseased heart of an individual horse. Being able to actually measure volume is set to revolutionise our ability to determine function of the cardiac chambers.
3DE also offers new views of heart valves and the structural changes associated with congenital disease (Redpath et al., 2020) (Figure 6). Rather than slicing through structures, images can be processed to reveal anatomy as though it was being examined from the inside – or en face (Figure 7). Greater detail can be examined but processing is time-consuming, and for many “routine” equine cardiac cases of mild to moderate valvular regurgitation, this extra effort offers little reward. However, in horses with valvular lesions such as dysplasia (Figure 7) and infective endocarditis (Figure 8) and with subtle or hard-to-find lesions such as small vegetations or rupture of a minor chorda tendinea, 3DE provides greater insight on the extent of pathology. 3DE technology can also be combined with colour flow mapping for a greater understanding of the complex anatomy of regurgitant jets. In horses the required imaging depths can limit the usefulness of this approach, and the use of multiplane modes, which present two perpendicular image planes simultaneously, is often more achievable.
The foundations of an echocardiographic examination remain M-mode, 2DE and Doppler flow mapping; however, advanced imaging modalities have specific advantages depending on the clinical condition being evaluated. Together, these modalities look set to help individualise and improve management of horses with heart disease.