Tachycardia

Tachycardia means “fast heart” (from Greek tachys, “fast” + kardia, “heart”). It describes a heart rate above the normal resting range for the individual dog. Normal resting heart rates in dogs vary by size:

• Giant breeds: 60-100 bpm
• Large breeds: 70-120 bpm
• Medium breeds: 80-140 bpm
• Small and toy breeds: 100-160 bpm

Heart rates above these ranges at rest constitute tachycardia.

Physiologic (Normal) Tachycardia

Not all tachycardia is pathologic. Heart rate increases normally in response to:

• Exercise: increased metabolic demand requires increased cardiac output
• Stress, fear, or excitement: sympathetic nervous system activation
• Pain: a consistent cause of elevated heart rate in veterinary patients
• Fever: heart rate increases approximately 10 bpm per degree Fahrenheit of temperature elevation
• Anemia: reduced oxygen-carrying capacity is compensated by increased heart rate

Physiologic tachycardia resolves when the stimulus is removed.

Pathologic Tachycardia

Sustained or recurrent tachycardia without an appropriate physiologic stimulus warrants investigation. Pathologic tachycardia is classified by its origin:

Sinus Tachycardia

The heart’s normal pacemaker (sinoatrial node) fires at an elevated rate. Causes include pain, anxiety, fever, anemia, heart failure, hyperthyroidism, and shock. Treatment targets the underlying cause.

Supraventricular Tachycardia (SVT)

Rapid heart rates originating above the ventricles (in the atria or AV node). Includes:

• Atrial fibrillation: the most common sustained cardiac arrhythmia in large-breed dogs with dilated cardiomyopathy or advanced mitral valve disease
• Atrial flutter and other supraventricular tachycardias: less common but clinically significant

Ventricular Tachycardia (VT)

Rapid heart rates originating in the ventricles. VT is the most dangerous form of tachycardia because it can degenerate into ventricular fibrillation and sudden death. Causes include:

• Dilated cardiomyopathy
• Arrhythmogenic right ventricular cardiomyopathy (common in Boxers)
• Myocarditis
• GDV (bloat)
• Splenic disease
• Trauma

Clinical Significance

Sustained tachycardia reduces cardiac efficiency because the heart does not fill adequately between beats (reduced diastolic filling time). This leads to decreased cardiac output despite the fast rate — the heart is beating fast but not pumping effectively. Prolonged tachycardia can cause tachycardia-induced cardiomyopathy (the heart weakens from chronic overwork), hypotension, and clinical decompensation.

Detection and Monitoring

Tachycardia is detected through cardiac auscultation (listening with a stethoscope), pulse palpation, and confirmed with electrocardiography (ECG). For intermittent tachyarrhythmias, a Holter monitor (24-hour ambulatory ECG) captures events that may be missed during a clinic visit.