Cardiovascular reflexes

Question 2

What is the Branham reflex? Name one example where this reflex is clinically relevant in our patient population.

Answer 2

= slowing of the HR when an abnormal connection between an artery and a vein (an AV fistula) is suddenly compressed or closed –> systemic vascular resistance increases –> BP rises –> baroreceptors –> vagus nerve –> Bradycardia to reduce the pressure

Ligation of a PDA:
PDA = persistent connection between the aorta and the PA –> blood from the high-pressure aorta flows into the low-pressure PA –> left-to-right shunt = low-resistance pathway allowing excess blood flow.

When the PDA is suddenly tied off –> shunt is eliminated, removing the low-resistance pathway –> sudden rise in systemic vascular resistance + aortic pressure –> Baroreceptors in the aortic arch + carotid sinus sense this pressure increase –> triggering of a vagal (parasympathetic) reflex –> Bradycardia (slow heart rate)

Question 3

What is the Bainbridge reflex (atrial reflex)? Give an example.

Answer 3

The Bainbridge reflex = reflex increase in HR due to an increase in venous return.

Examples: rapid IV bolus, inspiration, IV bolus in an already hypervolemic patient

–> walls of the RA stretch due to the extra blood –> activates stretch receptors in the atrium –> send signals via the vagus nerve to the brainstem –> increasing HR to help pump the extra blood more efficiently

Question 4

What is the Hering–Breuer reflex?

Answer 4

= protective breathing reflex that helps prevent over-inflation of the lungs

deep inhalation –> activation of stretch receptors in bronchi + alveoli –> sends signals via vagus nerve to respiratory centers in medulla + poms (brainstem) –> inhibition of further inhalation + triggering of exhalation

–> helps maintain normal breathing rhythm and protects the lungs from overdistension

important in neonates, when breathing control is still developing

adults: less prominent but contributes during forced or deep inspiration (like exercise or respiratory diseases).

Question 5

What is the Bezold-Garisch reflex?

Answer 5

= cardiovascular reflex characterized primarily by hypotension, bradycardia and peripheral vasodilation.

Origin: triggered by activation of chemosensitive and mechanosensitive receptors primarily in the ventricular walls of the heart (esp. LV) –> vagal nerve –> medulla (brainstem) –> activation of parasympathetic outflow –> bradycardia + peripheral vasodilation –> hypotension

Triggers:
* Chemical substances (e.g., serotonin, propofol in volume depleted patients, nicotine, chemotherapeutics)
* Myocardial ischemia
* Strong mechanical stimulation (like during sudden ventricular contraction with low filling)

Clinical relevance:

• can contribute to vasovagal syncope (fainting) in certain situations where there is sudden cardiac underfilling or irritation of the ventricular receptors