<prompt> Which of the following is true about spinal anesthesia?Onset of analgesia takes about 15 minutesAnesthesia is injected in subarachnoid spaceMost common complication is PDPHMay lead to intra-operative tachycardia  <response>: Ans. is b i.e., Anesthesia is injected in subarachnoid space In spinal anesthesia LA is injected into subarachnoid space space between pia matter and arachnoid matter. Structure pierced during SA from outside in - Skin - subcutaneous tissue - Supraspinous intraspinous ligament - Ligamentum falvum - Durameter - Arachnoid matter. Site of spinal anesthesia is L2-3 or L3-4 intervertebral space in adult In adult spinal cord ends at lower border of Llvertebrae. In children it is L4-5 intervertebral space spinal cord ends at lower border L3 vertebrae in children. Spinal anesthesia leads to creation of a zone of differential blockade, i.e., motor fibres are blocked two levels lower and autonomic fibers are blocked two levels higher than the sensory blockade due to different sensitivity of different fibers. High spinal Total spinal anesthesia: - It occurs when the effect of spinal anesthesia ascends to higher cervical levels. It can also occur following attempted epiduralcaudal anesthesia if there is inadvertent intrathecal injection. It causes severe hypotension, bradycardia and respiratory insufficiency with loss of consciousness.About other options Hypotension is the most common complication of spinal anesthesia. The onset of analgesia is approximately 15-30 minutes in an epidural, while it is approximately 5 minutes in spinal anesthesia. 