4.2.3 Practical use of echocardiography
Echocardiography is a skill which must be learned and for which there is no substitute other than practice. However, it is important that even the most experienced echocardiographer follows a standardised examination technique so that oversights are avoided. The examination is facilitated by effective restraint of the animal and thorough patient preparation.
Animals must be suitably restrained so that they do not damage expensive equipment, but most horses accept the feel of the transducer after a few seconds. It is important that animals are treated calmly so that they become used to the presence of the echocardiographer and equipment; excitement-induced tachywill not help assessment of the examination. In fact, in some horses the heart rate falls during the echocardiographic examination to a value below that recorded during the initial clinical examination. This true resting rate may itself be useful information. Stocks are useful in the majority of cases, although they may not be suitable for some horses. It may be possible to position a fractious animal next to a stable door so that the machine is kept at a safe distance (long transducer leads are a useful feature). Chemical restraint is best avoided if at all possible. Alpha2 agonist sedatives such as detomidine and xylazine result in a marked reduction in systolic function, although diastolic measurements may still be of some clinical value after sedation with low doses if examination is precluded without the use of sedatives. Acepromazine may be the best sedative from the point of view of effect on quantitative echocardiography, but even this should avoided if possible.
Because ultrasound will not pass through air, coupling gel is applied to produce good contact between skin and transducer. In most animals it is easier to obtain a good image if hair is clipped from the axillae. In thin, fine-haired animals such as racehorses in training this is not necessary so long as the coupling gel is worked well into the coat. In other animals, clipping is essential, even if this means that fractious horses have to be sedated some hours before the echo-cardiographic examination in order to allow the effects of the sedative to wear off. Cleaning of the skin with soapy water and/or spirit prior to the application of coupling gel removes grease and dirt and allows the gel to soak the hair and skin. It is helpful to use low viscosity gel for application to the skin and a thicker gel on the transducer as this stays on the transducer head longer. Only once good acoustic contact between the transducer and body is established can a good quality examination begin. Often a few minutes are required for the gel to soak into the skin before the best image is obtained.
Right parasternal views are most easily obtained using the left hand to hold the transducer, with the examiner facing in the same direction as the horse, so that the transducer can be angled into the axilla. Similarly, left parasternal views are most easily obtained using the right hand. Subdued lighting is required to see the screen clearly.