Cardiac Reflexes - Notes for MD Anaesthesiology

OVERVIEW

1. BARORECEPTOR REFLEX (Carotid Sinus Reflex)

Physiology

• Located in the carotid sinus (bifurcation of common carotid artery) and aortic arch
• Mechanoreceptors sensitive to circumferential and longitudinal stretch
• Carotid sinus receptors respond to both increases AND decreases in pressure
• Aortic arch receptors respond primarily to increases in pressure
• Most sensitive to rapid changes in arterial pressure (rate of change > absolute level)

• Carotid sinus baroreceptors → Nerve of Hering → Glossopharyngeal nerve (CN IX)
• Aortic arch baroreceptors → Vagus nerve (CN X)
• Afferents synapse in the Nucleus Tractus Solitarius (NTS), located in the cardiovascular center of the medulla

• The cardiovascular centre in the medulla has two functional areas:
  • Depressor area (central/caudal): reduces BP; also integrates input from hypothalamus and limbic system
  • Pressor area (lateral/rostral): increases BP
• Typical activation of stretch receptors occurs when systemic BP exceeds 170 mmHg

Anatomic configuration of the baroreceptor reflex - Miller's Anesthesia 10e, Fig. 13.20

Anaesthetic Relevance

• Acute blood loss/shock: baroreceptor reflex is beneficial (maintains BP)
• Reflex arc becomes non-functional when BP drops below 50 mmHg
• Volatile anaesthetic agents blunt the baroreceptor reflex in a dose-dependent manner
• Chronic hypertension: baroreceptors reset to a higher set point and maintain rather than correct the elevated pressure
• Sex differences (hormonal status) alter baroreceptor sensitivity
• During spinal anaesthesia - extensive sympathectomy (T5-L2) causes venous pooling; compensatory baroreceptor response above the block level triggers vasoconstriction and tachycardia, but this may be overcome at higher block levels

2. CHEMORECEPTOR REFLEX

Physiology

• PaO2 < 50 mmHg (hypoxaemia)
• Acidosis (↓ pH)

• Sinus nerve of Hering → Glossopharyngeal nerve (CN IX) and Vagus nerve (CN X) → chemosensitive area of medulla

1. Stimulates respiratory centres → ↑ ventilatory drive
2. Activates parasympathetic system → ↓ HR, ↓ myocardial contractility
3. With persistent hypoxia - direct CNS stimulation → ↑ sympathetic activity (overrides initial parasympathetic response)

Anaesthetic Relevance

• Hypoxia during anaesthesia may initially trigger bradycardia (parasympathetic) before sympathetic override
• Important consideration during induction in patients with severe hypoxaemia

3. BAINBRIDGE REFLEX

Physiology

• ↑ Heart rate (via two mechanisms):
  1. Inhibition of parasympathetic (vagal) activity
  2. Direct effect on SA node via atrial stretch

Anaesthetic Relevance

• Explains tachycardia with rapid IV fluid administration
• Relevant during procedures with large fluid shifts
• Operates in opposition to the baroreceptor reflex (one tends to slow heart, the other to speed it)
• The interplay between Bainbridge and baroreceptor reflexes determines the net heart rate response during fluid loading

4. BEZOLD-JARISCH REFLEX

Physiology

1. Hypotension
2. Bradycardia
3. Coronary artery dilatation

• ANP and BNP receptors may modulate the reflex; therefore it may be less pronounced in patients with cardiac hypertrophy or atrial fibrillation
• Serotonin (5-HT3) can trigger the reflex (hence interest in ondansetron as a modulator)

Clinical Anaesthetic Relevance - HIGH YIELD

1. Spinal anaesthesia-induced cardiac arrest - particularly in young, healthy, conscious patients. Mechanism: hypovolemia + extensive spinal block → small end-systolic LV volume → mechanoreceptor-mediated bradycardia via Bezold-Jarisch reflex
2. Neuraxial anaesthesia (subarachnoid block):
   • Cardiac accelerator fibers (T1-T4) blocked → vagal predominance → bradycardia
   • Decrease in cardiac filling pressures → further stimulation via Bezold-Jarisch reflex
   • Cardiac arrest has been reported; early use of epinephrine is critical in resuscitation
3. Shoulder surgery (beach chair/sitting position):
   • Ventricular underfilling → Bezold-Jarisch reflex → sudden bradycardia and hypotension
   • Intraoperative use of epinephrine, fentanyl, hydralazine, propofol implicated
4. Serotonin modulation: Ondansetron (5-HT3 antagonist) has been studied to blunt spinal anaesthesia-induced hypotension via modulation of this reflex
5. Myocardial ischaemia/infarction, thrombolysis, reperfusion: Reflex activation causes vagal bradycardia post-MI

5. OCULOCARDIAC REFLEX (OCR) - HIGH YIELD

Definition

Physiology

Pain/pressure/traction impulses → Short and long ciliary nerves → Ciliary ganglion → Ophthalmic division of trigeminal nerve (CN V1) → Gasserian (trigeminal) ganglion → Sensory nucleus of trigeminal nerve → Motor nucleus of vagus

Motor nucleus of vagus → Vagus nerve → Heart (↓ HR, ↓ contractility)

• Sinus bradycardia (most common)
• Junctional rhythm
• Ectopic atrial rhythm
• Atrioventricular block
• Ventricular bigeminy / multifocal PVCs
• Wandering pacemaker
• Idioventricular rhythm
• Ventricular tachycardia
• Asystole

Factors that Exacerbate OCR

• Hypoxaemia
• Hypercapnia
• Inadequate depth of anaesthesia
• Young age
• Use of opioids (fentanyl, alfentanil, remifentanil)
• Regional eye block (paradoxically can also elicit it)

Management

• IV atropine (20 mcg/kg) within 30 minutes of surgery - reduces incidence
• IV glycopyrrolate (0.01 mg/kg) - less tachycardia than atropine; useful alternative
• IM anticholinergics in premedication are ineffective for prophylaxis
• Note: prophylactic atropine in adults is generally NOT recommended routinely due to its own dysrhythmogenic potential (VF, VT, LBBB reported)
• Ketamine induction (vs. propofol) associated with lower OCR incidence in children (Choi et al.)
• Retrobulbar block: not uniformly effective; can itself trigger OCR

1. Ask surgeon to immediately stop surgical stimulation
2. Assess depth of anaesthesia
3. Check and correct ventilation status (oxygenation and CO2)
4. If arrhythmia persists after above:
   • IV Glycopyrrolate 0.1-0.2 mg (paediatric: 10 mcg/kg)
   • IV Atropine 10-20 mcg/kg
5. If severe bradycardia or asystole: IV Epinephrine

6. VALSALVA MANEUVER

Phases

• Phase IV overshoot + reflex bradycardia confirms intact baroreceptor reflex
• Absent phase IV response indicates autonomic dysfunction (e.g., diabetic neuropathy, spinal cord injury)
• Valsalva terminates paroxysmal supraventricular tachycardia via vagal stimulation

7. CUSHING REFLEX (Vasopressor Response)

Physiology

1. Raised ICP → cerebral ischaemia at medullary vasomotor center
2. Initial activation of sympathetic nervous system → ↑ HR, ↑ BP, ↑ myocardial contractility (to improve cerebral perfusion)
3. High vascular tone → detected by baroreceptors → reflex bradycardia via parasympathetic activation

• Hypertension (widened pulse pressure)
• Bradycardia
• Irregular respirations

Anaesthetic Relevance

• Sign of severely raised ICP and impending brainstem herniation
• Do not treat the hypertension in isolation - treat the underlying ICP
• Seen in neurosurgical emergencies and head injuries

SUMMARY TABLE

KEY EXAM POINTS

1. OCR afferent = CN V (trigeminal); efferent = CN X (vagus) - the most commonly asked fact
2. OCR is more common in children (higher vagal tone); tachyphylaxis occurs with repeated stimulation
3. First step in OCR management = ask surgeon to stop stimulation (not atropine)
4. Bezold-Jarisch reflex triad = hypotension + bradycardia + coronary dilatation; most relevant in spinal anaesthesia, beach chair position
5. Bainbridge reflex explains tachycardia with rapid IV fluid administration
6. Baroreceptor reflex set point is reset upward in chronic hypertension
7. Baroreceptor reflex loses function when BP < 50 mmHg
8. Cushing reflex = raised ICP triad: hypertension + bradycardia + irregular respirations
9. Ondansetron may mitigate spinal anaesthesia hypotension by blocking 5-HT3-mediated Bezold-Jarisch activation
10. Volatile agents blunt the baroreceptor reflex in a dose-dependent manner

8. ANO-CARDIAC REFLEX (Recto-Vagal / Perineo-Cardiac Reflex)

Definition

Anatomy and Innervation of the Anal Canal

• Above dentate line: Autonomic innervation - sympathetic (L1-L2) and parasympathetic (S2-S4 via pelvic splanchnic nerves)
• Below dentate line: Somatic innervation via the inferior rectal branch of the pudendal nerve (S2-S4) - carries pain, pressure, and stretch sensations
• The perianal skin and lower anal canal have dense sensory nerve endings that are exquisitely sensitive to stretch, traction, and pressure

Pathway

Stretch/pain/traction in anal canal or perianal region → Inferior rectal nerves (branch of pudendal nerve, S2-S4) → Pelvic splanchnic nerves → Sacral spinal cord → Ascending pathways → Medullary cardiovascular centre (nucleus tractus solitarius / dorsal motor nucleus of vagus)

Dorsal motor nucleus of vagus → Vagus nerve (CN X) → SA node → ↓ HR, ↓ BP

Response

• Bradycardia (sinus bradycardia, sometimes severe; junctional rhythm; asystole in extreme cases)
• Hypotension (due to vasodilation + reduced cardiac output)
• Pallor, sweating, nausea (in awake/sedated patients)

Clinical Settings - HIGH YIELD

Predisposing Factors (Same as General Vasovagal)

• High vagal tone (young, athletic patients)
• Anxiety or pain prior to/during procedure
• Inadequate depth of anaesthesia or inadequate regional block
• Hypovolemia / inadequate preload
• Use of propofol or opioids (fentanyl, remifentanil) - enhance vagal tone
• Performing under sedation/local anaesthesia (greater sensory awareness)
• Hypoxia and hypercapnia (exacerbate vagal response)

Interaction with Neuraxial Anaesthesia

• Sympathectomy from the spinal block reduces compensatory vasoconstriction
• Bezold-Jarisch reflex may be co-triggered by reduced venous return
• Cardiac arrest under spinal anaesthesia for perianal surgery has been reported in the literature (vasovagal + Bezold-Jarisch combination)

Prevention and Management

1. Adequate depth of anaesthesia or effective regional block - ensures the afferent limb is suppressed
2. Saddle block (hypobaric or hyperbaric) - appropriate level for perineal surgery while limiting hemodynamic instability
3. IV atropine (10-20 mcg/kg) or glycopyrrolate (0.01 mg/kg) - prophylactic anticholinergic, especially in high-risk patients
4. Optimal preloading with IV fluids before neuraxial block
5. Gentle surgical technique - avoid forceful traction, rapid distension
6. Avoid unnecessary stretch of anal canal
7. Patient positioning - avoid excessive lithotomy with legs elevated (reduces venous return further)

1. Ask surgeon to immediately stop surgical stimulation (first and most important step)
2. Lay patient flat / Trendelenburg position (↑ venous return)
3. Ensure adequate oxygenation (100% O2)
4. Check depth of anaesthesia / block adequacy
5. IV Atropine 0.6 mg (or glycopyrrolate 0.2 mg) - anticholinergic
6. IV Ephedrine (mixed alpha + beta agonist) - if hypotension is significant
7. If cardiac arrest: commence CPR + ALS algorithm; IV Epinephrine early

UPDATED SUMMARY TABLE (All 8 Reflexes)

Key Exam Points for Ano-Cardiac Reflex

1. Also known as the recto-vagal reflex or perineo-cardiac reflex - a form of vasovagal syncope specific to perianal surgery
2. Afferent = pudendal nerve (S2-S4); efferent = vagus nerve (CN X)
3. Most dangerous when anal surgery is performed under spinal anaesthesia - compound risk with Bezold-Jarisch reflex
4. First step in management = stop surgical stimulation (identical to OCR management principle)
5. Unlike OCR, tachyphylaxis does NOT reliably occur with this reflex
6. Rubber band ligation of hemorrhoids can elicit this reflex even under local anaesthesia - monitor patients for at least 20-30 minutes post-procedure