Learning Objective
Understand how changes in arterial pressure alter heart rate through the baroreceptor reflex, and how these changes are represented on BP/HR tracings.
Key Concepts: Baroreceptor Reflex
- ↑ BP → ↑ baroreceptor firing → ↑ PANS, ↓ SANS → ↓ HR
- ↓ BP → ↓ baroreceptor firing → ↓ PANS, ↑ SANS → ↑ HR
Blood pressure and heart rate change in opposite directions on tracings because of this reflex.
Classic Tracing Patterns
A. Sudden Increase in BP (e.g., phenylephrine, carotid sinus massage)
- BP tracing: sharp rise
- HR tracing: reflex bradycardia (sharp fall)
B. Sudden Decrease in BP (e.g., standing up quickly, hemorrhage, nitroprusside)
- BP tracing: sharp drop
- HR tracing: reflex tachycardia (sharp rise)
Drug and Lesion Effects on Tracings
- β₁-blockers: blunt tachycardic response
- Muscarinic antagonists: block reflex bradycardia
- Ganglionic blockers (NN antagonists): abolish both reflexes
- Carotid sinus hypersensitivity: exaggerated HR drop with neck pressure
Interpretation Principle
When reading tracings:
Always look at the BP change first.
The HR tracing will show the reflex response in the opposite direction.
