Subarachnoid Block

• ​Theoretical definition: Subarachnoid block is a method of regional anesthesia in which local anesthetics are injected directly into the subarachnoid space of the spine, creating total autonomic, sensory, and motor blockade below the level of the injection.[1]

• Operational definition: Subarachnoid block is a common technique to create surgical conditions without inducing general anesthesia. Sympathectomy caused by SAB can result in side effects of hypotension, bradycardia, and vasodilation.[1]
   

Prophylaxis

• ​Theoretical definition: Prophylaxis is defined as measures taken to preserve health or prevent illness.[5]

• Operational definition: Prophylaxis is demonstrated by administering an intervention before a procedure, compared to no intervention, causing reduced rates of adverse outcomes.[5]
   

​Hypotension          

• ​Theoretical definition: Hypotension is defined as blood pressure that is below baseline values for a patient.[23]

• Operational definition: Hypotension is measured by a decrease in systolic blood pressure or mean arterial pressure by twenty percent or greater of a patient’s baseline blood pressure.[23]
   

​Bradycardia

• ​Theoretical definition: Bradycardia is a heart rate below a patient's baseline values.[23]

• Operational definition: Bradycardia is measured as a decrease in heart rate by twenty percent or less of a patient’s baseline heart rate.[23]
   

Bezold-Jarisch Reflex 

• ​Theoretical definition: The Bezold-Jarisch reflex is a triad of vasodilation, hypotension, and bradycardia that results from ventricular activation of vagal afferent pathways. Various chemical or mechanical stimuli can activate these vagal afferent fibers, including sudden decreases in preload, creating relative intraventricular hypovolemia.[30]

• Operational definition: The Bezold-Jarisch reflex can be identified by the presence of hypotension or bradycardia after the administration of SAB.[23]
   

​Ondansetron

• ​Theoretical definition: Ondansetron is a 5-HT3 receptor antagonist usually administered as an antiemetic.23,24It is theorized that ondansetron in ordinary therapeutic doses can attenuate the Bezold-Jarisch reflex and reduce the prevalence of hypotension and bradycardia associated with sympathectomy caused by spinal anesthesia.[23]

• Operational definition: In this proposal, ondansetron is operationally defined as an intravenous injection of ondansetron given before the placement of a subarachnoid block.[24]