Thoracic auscultation in dogs and cats

Thoracic auscultation in dogs and cats is used to identify normal heart sounds, determine the presence or absence of abnormal sounds, assess heart rhythm and rate, and evaluate pulmonary sounds. Heart sounds are created by turbulent blood flow and associated vibrations in adjacent tissue during the cardiac cycle. Although many of these sounds are too low in frequency and/or intensity to be audible, others can be heard with the stethoscope or even palpated. Heart sounds are classified as transient sounds (those of short duration) and cardiac murmurs (longer sounds occurring during a normally silent part of the cardiac cycle). Cardiac murmurs and transient sounds are described using general characteristics of sound: frequency (pitch), amplitude of vibrations (intensity/loudness), duration, and quality (timbre); the timbre is affected by the physical characteristics of the vibrating structures. Because many heart sounds are difficult to hear, cooperation of the animal and a quiet room are important during auscultation.
If possible, the animal should be standing so that the heart is in its normal position. Panting in dogs is discouraged by holding the animal’s mouth shut. Respiratory noise can be decreased further by placing a finger over one or both nostrils for a short time. Purring in cats may be stopped by holding a finger over one or both nostrils, waving an alcohol-soaked cotton ball near the cat’s nose, or turning on a water faucet near the animal. Various other artifacts can interfere with auscultation, including respiratory clicks, air movement sounds, shivering, muscle twitching, hair rubbing against the stethoscope (crackling sounds), gastrointestinal sounds and extraneous room noises.

The traditional stethoscope has both a stiff, flat diaphragm and a bell on the chest piece. The diaphragm, when applied firmly to the chest wall, allows better auscultation of higher frequency heart sounds than those of low frequency. The bell, applied lightly to the chest wall, facilitates auscultation of lower frequency sounds. Some stethoscopes have a single-sided chest piece that is designed to function as a diaphragm when used with firm pressure and as a bell when used with light pressure. Ideally the stethoscope should have short double tubing and comfortable eartips. The binaural eartubes should be angled rostrally to align with the examine’s ear canals.

Both sides of the chest should be carefully auscultated, with special attention paid to the valve areas. The stethoscope is moved gradually to all areas of the chest. The examiner should concentrate on the various heart sounds, correlating them to the events of the cardiac cycle, and listen for any abnormal sounds in systole and diastole successively. The normal heart sounds are used as a framework for timing abnormal sounds. The point of maximal intensity (PMI) of any abnormal sounds should be located. The examiner should focus on cardiac auscultation separately from pulmonary auscultation because full assimilation of sounds from both systems simultaneously is unlikely.

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