Reducing Hypotension and Bradycardia after a Subarachnoid Block

Reducing Hypotension and Bradycardia after a Subarachnoid Block in the Obstetric Population: Is Zofran the Answer? Lauren Tennis, BSN, RN, SRNA York College of Pennsylvania

Objectives • • • Review of SAB and the physiologic effects Discuss the role of important reflexes Review serotonin and the 5 -HT 3 receptor Review pharmacology of Zofran Appraise literature on the use of Zofran to attenuate hypotension and bradycardia • Evaluate what the literature says on recommended dosing • Discuss areas of concern: Zofran and adverse fetal outcomes

Subarachnoid Blocks

Subarachnoid Block

Subarachnoid Blocks Class A-α A-β A-γ Function Motor Light Touch, Proprioception Pressure & Pain Muscle Spindles A-δ B C Temp & Pain Autonomic Pain & Pressure Myelin +++ ++ + - Diameter (μm) 12 -20 5 -12 1 -4 1 -3 0. 5 -1 30 - 70 12 - 30 15 1. 2 Conduction 70 -120 Speed (m/sec) Block Progression Last SNS blockade exceeds sensory dermatome by about 2 dermatomes. Motor blockade is about 2 dermatomes below the sensory blockade. First

Physiologic Effects of SAB Neural • Blocks C fiber A fibers • Sympathetic > Temperature > sensory > motor Gastrointestinal • Nausea and Vomiting • Decreased hepatic blood flow Respiratory • Paralysis of intercostal and abdominal muscles • Phrenic nerve usually preserved Cardiovascular • Block cardio-accelerator fibers • Dilation of arteries and venous capacitance vessels

Why do we have hypotension & bradycardia after SAB? • • • Sympathetic blockade Activation of the Bezold-Jarisch Reflex Activation of baroreceptors Blockade of cardio-accelerator fibers T 1 -T 4 Unopposed parasympathetic nervous system

Sympathetic Blockade • Sympathetic chain located in thoracolumbar region • Efferent stimulus from spinal cord in blocked and unable to transmit through sympathetic ganglion • Inability to vasoconstrict and increase HR

Bezold-Jarisch Reflex • Inhibitory reflex • Mechanoreceptors detect low volume and decreased stretch • Chemoreceptors sense increased levels of serotonin • Heart attempts to compensate by hypercontracting the LV • Afferent vagal nerve endings in LV send signal to NTS of brain • Brain sends out parasympathetic signals via vagus nerve and slows HR and causes vasodilation

Baroreceptor Activation Low Volume Heart Rate Decreased

Baroreceptor Activation

Blockade of Cardio-accelerator Fibers • Originate from T 1 -T 4 from the stellate sympathetic ganglion • SAB at T 4 level blocks sympathetic output to heart • Unopposed parasympathetic system causes bradycardia

Ways to Treat Hypotension and Bradycardia • • IV Fluids Vasoactive medications Head down position Zofran?

Ondansetron (Zofran) • 5 -HT 3 receptor antagonist • Works peripherally (GI tract) and centrally (chemoreceptor trigger zone) • Dosing: 2 -8 mg • Onset: 3 -5 min (IV) • Half-life: 3 -6 hours • Metabolized by the liver • Common routes of administration: PO, IV • FDA Category B

Serotonin • Excitatory and inhibitory neurotransmitter • Located in… – Enterochromaffin cells 90% peristalsis, N/V, diarrhea – Platelets 8 -10% local vasoconstriction – CNS 1 -2% mood, appetite, pain, sleep • Receptors are both metabotropic and ionotropic

5 -HT 3 Receptor • Ligand-gated ion channel • Structurally similar to n. Ach receptor • Located in CNS, blood vessels, GI tract, afferent vagal nerve endings

Zofran and the Bezold-Jarisch Reflex Sympathectomy from SAB Bradycardia, vasodilation and N/V Decreased blood volume and blood flow Reduced vasodilation, bradycardia, and N/V Parasympathetic outflow Less parasympathetic outflow Increased serotonin levels bind to the afferent vagal nerve endings stimulating the BJR and N/V No transmission of vagal afferents and block CTZ Release of serotonin from platelets and enterochromaffin cells Zofran: 5 -HT 3 Antagonist

Literature Review

Max Dose of Phenylephrine Use 240 mcg 209 mcg 106 mcg 125 mcg 195 mcg

Dosing Recommendations • 1/3 of Group 04 and Group 06 experienced hypotension • 40 -60% of Group S, Group 02, and Group 08 experienced hypotension • 4 mg and 6 mg of Zofran is adequate

Dosing Recommendations • 6 mg vs 12 mg vs control group (NSS) • Control MAPs < Zofran group MAPs • Control required Ephredrine for hypotension and Atropine for bradycardia • No significant difference between 6 mg and 12 mg Zofran

Zofran and Neonatal Outcomes • Category B • Clinical trials had no effect on birth malformations • APGAR scores 7 -10 • PO Zofran safe: no difference in spontaneous abortions, stillbirths, premature births, malformations, or low birth weight

Summary • Hypotension and bradycardia are common physiologic changes after a SAB • Zofran reduces hypotension by blocking the BJR and decreasing parasympathetic outflow • No significant change in bradycardia • 4 -8 mg is adequate • No evidence of adverse outcomes in neonates

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