CANINE CARDIOLOGY.
The Components of a Stethoscope
Most stethoscopes are designed with a bell and a diaphragm. The diaphragm is designed to pick up high frequency sounds and should be held firmly against the skin. The bell is designed to amplify lower frequency sounds when applied with light pressure. When using the bell, avoid applying firm pressure because the skin beneath the bell will act as a diaphragm and negate the amplification of low frequency sounds (Fox, 1988). The ear pieces should fit comfortably in your ear canals. If the stethoscope comes with different sized ear pieces, select the ear pieces that form the tightest seals in your ear canals. The tubing connecting the binaurals to the bell-diaphragm may be single or double layered. Double layered tubing eliminates more background noise.
How To Use a Stethoscope Correctly
For auscultation to be effective, it should be performed in a quiet room with little extraneous noise. The stethoscope is most effective if:
1. When placed in the ears, the binaurals follow the same direction as the ear canals (see diagram below).
2. The ear pieces fit snugly in the ears forming a tight seal and the hand holding the stethoscope is still, relaxed and placed against the animal with a constant amount of pressure.
An Introduction to Canine Cardiac Auscultation
The four basic principles required for successful auscultation are:
1. Correct use of the stethoscope.
2. Recognition of external landmarks that correspond to valve locations and that due to acoustical effects within the heart, the locations where sounds are heard loudest do not necessarily correspond to their anatomical source.
3. Correct differentiation, interpretation and understanding of normal versus abnormal sounds. Studies have shown that both students and veterinary practitioners can correctly describe the physical features of heart sounds, however, their ability to correctly interpret these sounds is often lacking (Naylor et al., In Press). This finding may be because traditional teaching methods rely on a verbal description of an audible anomaly rather than actual audio recordings. The authors of this website hope that with the multimedia technologies used herein, specifically the pairing of actual cardiac audio recordings with visual interpretation, individuals will improve their diagnostic and interpretive skills. For further information on cardiology, you may consult "Hearing Horse Hearts: An Interactive Guide to Equine Cardiac Auscultation" (Naylor, 2000).
4. The ability to convey your understanding and interpretation of what you heard to others using standard veterinary terminology.
Anatomy
The canine heart projects into both thoracic cavities, particularly the left, from the third to the sixth intercostal space. The long axis of the heart is rotated cranially so that it lies at an angle with the base more cranial than the apex. The base of the heart is fixed by the great veins and arteries while the apex can move freely within the pericardial sac. The so-called right and left sides of the heart are more correctly understood to be the dextro-cranial and levo-caudal sides because the left ventricle lies behind and slightly left of the right ventricle. The left ventricle is more conical and massive than the right ventricle which is more crescent shaped.
Landmarks
If the dog is standing square, much of the heart lies medial to the triceps mass. A horizontal line drawn through the point of the shoulder lies slightly above the level of the heart valves. As opposed to using features of the forelimbs (e.g. The point of the shoulder and position of the olecranon) to locate heart valves, palpation of the apex beat is more accurate because its position is independent of the dogs forelimbs.
Internal landmarks for the heart valves largely rely upon their positions relative to intercostal spaces and costochondral junctions. The following guidelines (Tilley and Goodwin, 2001) may be helpful for auscultation.
Cardiac Auscultation
Cardiac auscultation should be performed in a quiet room free of excessive noise. Cardiac auscultation should also be performed as soon as the animal enters the exam room or when the dog is stressed since this increases the probability that a transient or subtle murmur will be detected. The probability of detecting a murmur increases with stress because sympathetic activation increases heart rate, cardiac contractility and cardiac output. Turbulent flow, which gives rise to murmurs, is more likely at higher blood velocities.
Cardiac auscultation should proceed in a logical manner. The apex beat (mitral valve area) should be palpated and the heart rate measured either by cardiac auscultation or palpation of the femoral pulse. The femoral pulses should be palpated in each hindlimb and compared for fullness, sharpness and regularity. Next the femoral pulse should be palpated simultaneously with cardiac auscultation in order to detect pulse deficits due to arrhythmias. Each valve should be ausculted in the order M itral, A ortic, P ulmonic (acronym MAP). Some palpate the apex beat (mitral valve area) and move cranially from there. However, if you wish to auscult in a particular intercostal space it is easier if you start counting spaces from the last rib (13 th) cranially.
Arrhythmias
An arrhythmia or dysrhythmia is a deviation from the regular rhythm. In dogs this may be normal or abnormal and may result from abnormal cardiac impulse formation, conduction, rate or regularity.
Regularity
Regularity refers to the predictability of an arrhythmia. Some arrhythmias occur in a predictable fashion and are said to be regularly irregular. These rhythms may be normal (e.g. sinus arrhythmia) or pathological. In others the onset of the next beat is completely unpredictable and the rhythm is said to be irregularly irregular (e.g. atrial fibrillation). Irregularly irregular rhythms are pathological in origin.
Classification of Arrhythmias
Tilley and Goodwin (2001) classify arrhythmias according to:
Origin
Supraventricular arrhythmias arise from the atria or AV node whereas ventricular arrhythmias arise from the ventricles.
Rate
Arrhythmias with slow rates are bradyarrhythmias while those with fast rates are tachyarrhythmias.
Regularity
Fibrillation is a rapid, irregular, chaotic rhythm while tachycardia is a rapid but regular rhythm.