Veterinary Medicine, Shri Surat Panjarapole Prerit Nandini Veterinary Hospital, Surat, Gujarat, India
Canine heart is a four-chambered organ having two atria, two ventricles, mitral valve, tricuspid valve, aortic valve, pulmonary valve, arteries, arterioles, veins, and venules. The major function of the heart is to provide nutrients to all organs of the body and to remove the waste products of metabolism from the organs of the body through its pumping action. Malfunctioning of pumping action and/or electrical events in the heart adversely affects nutrient and oxygen supply to organs and removal of waste products from the body making the survival reasonably difficult. During recent years cardiac disorders in canines have assumed greater significance owing to its frequent occurrence and variable outcome. Cardiac functioning is affected not only in primary heart diseases but also in diseases of other organs. Without correct diagnosis of cardiac abnormalities, treatment may be futile with a fatal outcome. Nowadays diagnosis of cardiac abnormalities is facilitated with the aid of modern diagnostic technology. Though clinical examination plays a significant role in the diagnosis of heart diseases, clinical significance of murmurs and/or arrhythmias is baffling without further investigations. Sometimes non-cardiac ailments also manifest symptoms mimicking heart diseases. Clinical signs such as coughing, tiredness, weakness, dyspnea, and respiratory crackling are also evident in lung diseases and need differentiation whether these signs are due to cardiac or pulmonary origin. Before the advent of electrocardiography much reliance was paid to ancillary approach (analyzing history, clinical symptoms, and clinical examination) that lacks differentiating ability of various cardiac diseases. Nowadays it is possible to evaluate animals at risk of cardiac diseases even when clinical manifestations are not apparent. Recent advances in cardiology during last few decades, most notably in the areas of diagnostic imaging and biomarkers, have considerably improved our diagnostic skill in differentiating various heart diseases of which diagnosis was unimaginable during the early twentieth century. A bird eye view of different systematic approaches for evaluating heart is presented below.
1.1 History and Physical Examination
Despite the technical ability of electrocardiography and echocardiography, the history and physical examination remain the most important and desirable diagnostic steps in making a correct diagnosis of heart diseases. Findings from these examinations prompt the clinician to undertake further specific examinations on the basis of clinical suspicion so as to make specific diagnosis. History of coughing particularly during night in dogs with cardiac involvement indicates pulmonary venous congestion and edema owing to left atrial enlargement affecting the bronchi adversely. Coughing episodes worsening with exercise/excitement or at night in a lying animal are strongly suggestive of cardiac cough. Reduced cardiac output in heart diseases leads to exercise intolerance, cyanosis, syncope, marked weakness, inappetence, and weight loss. These manifestations are commonly narrated by the owners in the history.
Physical examination of cardiovascular system is comprised of inspection (looking for abducted elbow, jugular distension or pulse, edema of ventral abdomen or limbs, tachypnea/dyspnea), vital indices (temperature, pulse, respiration, mucus membrane color, capillary refill time), palpation (ascertain position of apex beat, presence or absence of precordial thrill, femoral pulse, ascites, hepatomegaly), and auscultation (rate, rhythm, intensity of heart sounds, murmurs, or abnormal respiratory sounds).
Auscultation is facilitated by the stethoscope. It was invented by a French physician Laennec Rene Theophile Hyacinthe in 1816 in France (Laennec 1819). Since its invention the stethoscope has become almost universal insignia of a physician or a veterinarian. Auscultation of chest/thorax (Figs. 1.1 and 1.2) is a very important part of cardiac examination to provide first-hand information on heart rate and its rhythm; presence of muffled heart sound; absence of heart sound; heart murmurs and gallop sounds (S3 or S4); systolic clicks and split heart sound (S1 or S2); as well as presence or absence of crackles and rales in the lungs. Normal heart sound is basically a three-character sound, viz., LUB-DUB-PAUSE. The first heart sound (S1-LUB) is associated with the closure of mitral and tricuspid valves. The second heart sound (S2-DUB) is associated with the closure of aortic and pulmonic valves. Heart is to be auscultated at all the valve areas, i.e., mitral valve at fifth intercostal space around costochondral junction on the left side (opposite to elbow point in standing dog), aortic valve at fourth intercostal space dorsal to mitral valve on the left side (at the level of the point of shoulder), pulmonic valve at third intercostal space at sternal border (axilla) on the left side, and tricuspid valve on third to fourth intercostal space at costochondral junction on the right side of the thorax. The intensity of normal heart sound increases in dogs with thin body, in young dogs, and in dogs with fever, anemia, or hyperthyroidism. In obese dogs the intensity of heart sound decreases. The intensity of heart sound also decreases in dogs with pleural or pericardial effusions. Though arrhythmias and murmurs are detected on heart auscultation, differentiation of arrhythmias or cardiomyopathy is not possible only on auscultation. Sometimes murmurs are physiological as observed in cases with anemia and/or hypoproteinemia. These physiological murmurs may be detected in young growing animals even in the absence of heart disease, and these are resolved as the animal grows. Heart murmurs are abnormal extra sound of relatively long duration and are generated due to turbulence within the heart owing to disturbance in blood flow. Detection of pathological murmurs may be indicative of valvular insufficiency, valvular stenosis, interatrial or interventricular septal defects, patent ductus arteriosus (PDA), or defect of great vessels. As per the timings of occurrence of the murmurs at the point of maximum intensity, these are divided into three classes as systolic (occurring during systole), diastolic (occurring during diastole), and continuous (occurring all the time). Systolic murmurs are generally soft and occur during early systole. Diastolic murmurs are of low frequency, and their occurrence is not very common. Continuous murmurs vary in intensity and are associated with patent ductus arteriosus (PDA). As per their intensity, murmurs have been graded into six grades as follows:
Grade 1. Murmurs are very soft and localized. Generally detected on prolonged auscultation.
Grade 2. Murmurs are soft and localized. Detected easily.
Grade 3. Murmurs are moderately intense and detected at more than one place.
Grade 4. Murmurs are moderately intense and detected at many places in left and right chest.
Grade 5. Murmurs are loud over point of maximum intensity. Precordial thrill is also there.
Grade 6. Murmurs are very loud associated with precordial thrill.
1.2 Blood Pressure Monitoring
Another important tool for cardiac evaluation is blood pressure. Blood pressure monitoring is a routine clinical determinant in humans during clinical examination. On the contrary, blood pressure monitoring has been given a little casual attention in dogs. Blood pressure monitoring may serve as a valuable major determinant of ventricular wall stress and myocardial oxygen consumption. The advantage of measuring blood pressure in canines and felines is being increasingly recognized owing to deleterious effect of hypertension on ocular, renal, cardiovascular, and cerebrovascular systems. There is a growing assumption that hypertension is quite common in dogs with renal and endocrinological disorders. Though direct blood pressure monitoring in dogs dates back to 1800, its cumbersomeness restricts it’s clinical utility. For taking blood pressure in dogs, auscultative method (Fig. 1.3), ultrasonic Doppler sensing device (Fig. 1.4), oscillometric method, or electronic devices have been tried. Blood pressure has also been monitored in dogs using an aneroid sphygmomanometer with Velcro cuff of 5 cm × 22 cm size (Fig. 1.3). The dogs are restrained in right lateral recumbency, and Velcro cuff is applied over left hind limb on cranial tibial artery at the distal medial aspect of tibia (Fig. 1.3). Mean systolic (102.5–162.5 mm Hg), diastolic (44.5–76.0 mm Hg), and arterial blood pressure (70–102.5 mm Hg) is variable in healthy dogs. Hypertension (>160 mm Hg systolic, >100 mm Hg diastolic) is generally seen secondary to diseases like diabetes mellitus, left ventricular enlargement, and chronic renal failure. Hypotension (fall in blood pressure) is observed in severe gastroenteritis, hypothermia, and shock. It is in fitness of things that blood pressure measuring is adopted in routine clinical practice.
1.3 Thoracic Radiography
Chest/thorax radiography continues to play a significant role in the assessment of cardiovascular diseases or assessing congestive heart failure. For better results lateral (Fig. 1.5) and dorso-ventral (Fig. 1.6)/ventro-dorsal (Fig. 1.7) views are taken with high kilo voltage peak (kVp) and low milliamperes (mAs). When evaluating cardiac size and shape in dog chest, chest conformation should always be considered. It is always better to take radiograph at the peak of inspiration.
Ventricles occupy approximately three intercostal spaces in normal thorax in dogs. Location of the heart extends from third to sixth ribs and heart touches nearly diaphragm. Right atrium and right ventricle are on the cranial border; and left atrium and left ventricles are on caudal border of heart silhouette. Both atria, pulmonary arteries, aorta, and vena cava (cranial and caudal) are situated dorsally in the right lateral radiographs (Figs. 1.8, 1.9, and 1.10).
Vertebral heart score (VHS)—VHS is calculated on the lateral thoracic radiographs to assess cardiac size. Long axis of cardiac silhouette is measured from the carina of the main bronchus to the apex of the heart and short axis at the widest part of the heart. These axis (long and short) measurements are transferred to the vertebrae starting from cranial edge of T4 and the number of vertebrae fall under each axis are counted. The number of vertebrae falling under the both axes (Fig. 1.11) is counted. A VHS > 10.5 is generally denotes cardiomegaly. It is a rough estimate of the size of the heart and cannot be taken as sole criteria of heart enlargement.