Abstract
Echocardiography is increasingly used for the diagnosis and management of congenital and acquired cardiac diseases. While a number of different echocardiographic formats are used in clinical practice, each involves reflection of ultrasound from cardiovascular tissues, specialised processing of returned (echoed) signals, and the ultimate display of this information in some recognisable visual or auditory format. Echocardiography has become increasingly sophisticated, and the combined modalities have largely replaced cardiac catheterisation and angiocardiography for diagnosis and assessment of cardiac lesions. Although the newest technologies are expensive and limited to referral hospitals and clinics, many practising veterinarians use, or will soon acquire, echocardiographs. Furthermore, veterinarians who are not yet performing echocardiographic studies, often find referral for echocardiography helpful or even essential for establishing a cardiac diagnosis, assessing ventricular function, determining a prognosis, and guiding medical or surgical therapy.
The echocardiographic examination must be placed within a proper clinical perspective (Table 1). Most importantly, echocardiography is not a substitute for a careful clinical examination and routine diagnostic studies such as measurement of arterial blood pressure, determination of serum thyroxine, measurement of the PCV, and thoracic radiography. Cardiac auscultation is still a useful and expedient method for identifying serious heart diseases; however, it should be appreciated that a significant percentage of cats with cardiomyopathy do not have an auscultable murmur or gallop. Furthermore, in up to 25% of cats examined with heart murmurs in our practice, we are unable to demonstrate significant morphologic lesions by 2D ECHO or unambiguous flow disturbances by Doppler studies.
Echocardiographical modalities
Haemodynamic load refers to the increase in volume pumped or pressure generated by the ventricle during each cardiac cycle. In general, the more severe the cardiac lesion, the greater the haemodynamic burden.
For M-mode indices of ventricular function, left ventricular shortening fraction = (ventricular diastolic dimension—ventricular systolic dimension)/diastolic dimension; velocity of circumferential fibre shortening = shortening fraction/ejection time; systolic time intervals include pre-ejection period (onset of QRS to aortic opening) and ejection time; mitral valve E-point to septal separation = distance in cm from from mitral valve opening to the ventricular septum.
Ejection fraction = ventricular stroke volume/ventricular end-diastolic volume. A special form of pulsed-wave Doppler—high pulsed repetition frequency Doppler—can measure high velocity flow signals; this is a hybrid of pulsed and continuous wave Doppler.
Echocardiography should be performed and echocardiograms are best interpreted by clinicians who are knowledgeable of feline heart disease. One must understand the pertinent issues and questions during the examination. Moreover, the clinical assessment and treatment plan prescribed should be directed by an individual capable of integrating information from all sources, including the history, physical examination, radiography and laboratory tests. Therapeutic decisions should not be abdicated to a consultant unless that individual has a complete understanding of the clinical situation and has examined the patient and medical records. The echocardiographer must appreciate ultrasound physics and instrumentation so that artifacts are not overinterpreted and important information is not suppressed. Echocardiographic studies are very repeatable in experienced hands, but results can be highly operator and equipment dependent.
The veterinarian should appreciate the advantages and limitations of an echocardiographic study. A complete echocardiographic study should: (1) reveal the pertinent congenital or acquired anatomic lesions (morphologic diagnosis); (2) estimate haemodynamic burden through quantitation of cardiac chamber size (dilatation and hypertrophy); (3) quantify ventricular systolic function: (4) estimate ventricular diastolic function; (5) evaluate valvular function; and (6) refine haemodynamics with pressures estimates. Properly gathered and interpreted, this information should lead to a definitive cardiac diagnosis and illustrate the haemodynamic consequences of structural and functional cardiac lesions. Such detailed information can be obtained through complementary echocardiographic modalities; however, limited echo studies can also be useful in selected situations, as with rapid screening for pericardial effusion or when an expedient estimate of left ventricular ejection fraction is needed.
The routine feline echocardiographic study should include images obtained from the right and often the left hemithorax. Short axis images from the papillary muscle, mitral, and aortic/left atrial levels are obtained. The papillary muscles and left ventricular walls should be assessed for hypertrophy and the left auricle examined for thrombus. The right ventricular outlet can be examined and interrogated by Doppler studies. Long axis images optimised for the left ventricular inlet (left atrium, mitral valve) and left ventricular outlet (outflow tract, aortic valve, aorta) are essential. Diastolic measurements of at least four wall segments should be taken using these 2D ECHO imaging planes. Diastolic wall measures of 6 mm or greater are considered indicative of hypertrophy. The routine M-mode echo is also recorded to measure left ventricular wall and chamber dimensions, but placing the cursor across the papillary muscles will lead to significant measurement error. Furthermore, the M-mode may fail to identify regional hypertrophy. The maximal left atrial systolic diameter should be measured by 2D studies. In most normal cats, this measure will be <16 mm. Volume depletion (including zealous diuresis) can lead to misleading results inasmuch as chamber dimensions are reduced and ventricular ‘pseudohypertrophy’ may be induced. Global ventricular function should be estimated. Dynamic obstruction should be sought using 2D imaging, M-mode interrogation of the valve and Doppler studies. Mitral regurgitation is common in cats with cardiomyopathy, and this systolic event can be identified by Doppler studies. The left hemithorax is examined when Doppler methods are available. Mitral regurgitation and dynamic outlet obstruction can be identified from an apical window. Transmitral flow patterns, isovolumetric relaxation time, and pulmonary venous flow patterns are recorded and measured to identify diastolic dysfunction. Tissue Doppler imaging may be employed to evaluate wall motion. The interior of the left auricle is scrutinised for thrombus using 2D ECHO from a craniodorsal transducer position whenever that chamber is enlarged.
Echocardiography is clearly useful in the assessment of cats with heart diseases, especially with acquired disorder of the myocardium. There are also indications for performing echocardiography in cats with congenital heart diseases. Salient features of these clinical conditions are summarised in Tables 2 and 3.
Usual echocardiographical findings in feline congenital heart diseases
Major points are emphasised; this table is not comprehensive. LA, left atrium; RA, right atrium; LV, left ventricle; RV, right ventricle; Ao, aorta; PA, pulmonary artery; PDA, patent ductus arteriosus; VSD, ventricular septal defect; ASD, atrial septal defect.
Mild increases in diastolic mitral (PDA, VSD) and systolic aortic velocities (PDA) will usually be recorded owing to increased volume flow.
The velocity depends on the relative pressure between the systemic and pulmonary circulations; generally the velocity is directly related to pressure differences and inversely related to the diameter of the defect.
Mild increases in diastolic tricuspid and systolic pulmonary artery velocities will usually be recorded owing to increased transvalvular flow.
Congenital atrioventricular valve stenosis is relatively rare, but would be associated with other findings including a narrowed valve orifice or tethered valve leaflets, dilated atrium, increased transvalvular diastolic velocities (E and A waves), prolonged pressure half-time, and often concurrent valvular regurgitation.
Usual echocardiographical findings in feline acquired heart diseases
For abbreviations, see Table 2.