Sinus Arrhythmia
Sinus arrhythmia is a regularly irregular sinus rhythm which is a normal finding in most dogs (especially brachycephalic breeds). Sinus arrhythmia is characterized by slight variations in the S1-S1 interval. These variations are related to changes in vagal tone to the heart and are often associated with inspiration (negative pressure created in the thorax) or use of sedative or anesthetic drugs. You can demonstrate sinus arrhythmia by palpating the radial artery on your wrist. Once you feel your pulse take a big deep breath and you should feel your pulse quicken and then slow down as you exhale.
Altered Sinus Impulse Formation
Sinus Bradycardia (Slow Heart Rate)
Sinus bradycardia has a regular rhythm and may result from systemic disease (renal failure), toxicities, increased vagal tone, elevated intracranial pressure or compression of the eyeball, hypothermia, hypothyroidism or drugs (tranquilizers, propranolol, morphine, various anesthetics) (Fox, 1988). Sinus bradycardia is diagnosed when the heart rate is less than 65 beats / minute and an ECG shows sinus rhythm.
Sinus Tachycardia (Fast Heart Rate)
Sinus tachycardia; often caused by stress; is the most common arrhythmia observed in dogs and has a regular rhythm. Sinus tachycardia may result if there is increased metabolism and oxygen demand or increased requirement for cardiac output (pain, fright, excitement), pathology (fever, shock, anemia, hypoxia, hyperthyroidism) or pharmacological agents (atropine, epinephrine, ketamine) (Fox, 1988). Sinus tachycardia is diagnosed when the heart rate is more than 160 beats / minute for most dogs (>180 bpm for small / toy breeds or >220 bpm in puppies) and an ECG shows sinus rhythm (Fox, 1988).
Altered Supraventricular Impulse Formation
Atrial Fibrillation
Atrial fibrillation is a common pathological arrhythmia in dogs. Auscultable characteristics of atrial fibrillation include a completely unpredictable rhythm, sometimes called a "jungle-drums" rhythm. Listen for long diastolic pauses between some beats and very short intervals between others. Sometimes the beats are so close together that S2 is not generated and two S1 sounds follow each other. The other hallmark of atrial fibrillation is a pulse deficit. Sometimes this can be detected because there is a large disparity between the heart rate and the pulse rate. If the heart beat is slow it is more reliably detected by simultaneous auscultation and palpation of the pulse. Normally every S1 heart sound is followed by a pulse wave. Abscence of a wave is called a pulse deficit.
The most common causes of atrial fibrillation are chronic atrioventricular valvular insufficiency in small breeds, dilated cardiomyopathy in large breeds, and congenital heart defects. Less common causes include heartworm disease, cardiac trauma, digitalis toxicity and severe metabolic disorders (Fox, 1988). Auscultable or palpable characteristics of atrial fibrillation include inconsistently filled femoral pulses, detection of an S1 without an S2 and a pulse deficit.
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Disrupted Impulse Conduction
Second Degree Atrioventricular (AV) Block
Second degree AV block may be of two types: Mobitz I, usually type A or Mobitz II, usually type B. The two types of second degree AV block are best distinguished by ECG. Mobitz I is a normal finding in dogs, especially in young animals and disappears with exercise. Mobitz II is pathological in origin and will not disappear with exercise. Both types of second degree AV block are manifested by a dropped beat detectable during auscultation. By exercising and immediately ausculting the dog, you can determine if the AV block is a Mobitz I (the dropped beats have disappeared) or Mobitz II (the dropped beats are still auscultable). Second degree AV blocks can be associated with sinus arrhythmia, increased vagal tone, supraventricular tachycardia, electrolyte imbalances or drugs (digitalis, intravenous atropine, xylazine) (Fox, 1988).
Murmurs
Murmurs are sounds produced by turbulent blood flow. Rapid flow, a wide vessel, low blood viscosity and an uneven or constricted vessel wall all predispose to cardiac murmurs. They can be physiological, for example high blood flow though the aortic outflow tract. Pathological murmurs reflect heart disease, for example degeneration and roughening of a valve surface. Veterinarians require a uniform method of describing murmurs to facilitate communication between each other via a common understanding. Five parameters have been developed that serve to describe all of the important aspects of a murmur. Of the five parameters, the most important ones are position in the cardiac cycle, intensity, duration and pattern of intensity. The point of maximal intensity (PMI) identifies the location where the murmur is heard loudest and is often described using the valve location nearest (e.g. Mitral valve area). On the following page is a table summarizing the parameters and their descriptions (Naylor, 2000). In dogs, systolic or continuous murmurs are more common than diastolic murmurs.
In describing the duration of murmurs, pan- refers to a murmur that obliterates both heart sounds either through systole or diastole. Holo- refers to a murmur that lasts throughout systole or diastole but does not obliterate any heart sounds. A continuous or machinery murmur lasts throughout most or all of systole and diastole and may or may not obliterate heart sounds. Early- and late- describe murmurs that are positioned closer to one heart sound than to another. Crescendo, decrescendo or diamond are terms that describe the intensity profiles of murmurs as increasing, decreasing or increasing and then decreasing in loudness. Musical and blowing are terms used to describe the frequency profile of a murmur. Grade refers to the absolute intensity of murmurs determined on a 6 point scale where the higher the grade the more severe the murmur (Example: Grade 2 versus a grade 5 regurgitant murmur).
Research shows that most clinicians correctly describe the grade of a murmur. Localization of the murmur to systole or diastole is less consistent. A clue is the timing of the heart sounds (systolic murmurs occur in the short pause), however loud murmurs can be perceived as being of longer duration than they really are (Naylor et al., In Press). Another useful method is to palpate the pulse during auscultation. Pan- or holo-systolic murmurs should be heard coincident with the pulse wave.