What is the best description of a sinus arrhythmia in an anesthetized dog as seen on an ECG?

What is the best description of a sinus arrhythmia in an anesthetized dog as seen on an ECG?

• A. Normal if under light anesthesia: reflects vagal tone; Inspiration ↑ HR; Expiration ↓ HR
• B. Always indicates pathology
• C. No relationship with respiration
• D. Occurs only at deep anesthesia

Answer: (A)

Respiratory sinus arrhythmia is a normal, vagally mediated variation in heart rate that tracks the breathing cycle. In dogs, when anesthesia is light and autonomic tone is preserved, inspiration tends to increase the heart rate (shorter RR interval) and expiration tends to decrease it (longer RR interval). On the ECG this shows as a fluctuating rate that follows respiration, while the underlying sinus rhythm and P-QRS-T morphology stay normal. This pattern is not a sign of disease; it simply reflects the natural influence of breathing on vagal tone. It may be diminished or absent with deeper anesthesia, when vagal tone is suppressed.

Respiratory sinus arrhythmia is a normal, vagally mediated variation in heart rate that tracks the breathing cycle. In dogs, when anesthesia is light and autonomic tone is preserved, inspiration tends to increase the heart rate (shorter RR interval) and expiration tends to decrease it (longer RR interval). On the ECG this shows as a fluctuating rate that follows respiration, while the underlying sinus rhythm and P-QRS-T morphology stay normal. This pattern is not a sign of disease; it simply reflects the natural influence of breathing on vagal tone. It may be diminished or absent with deeper anesthesia, when vagal tone is suppressed.