I. Overview: Subarachnoid space block is an anesthetic method in which local anesthetic is injected into the cerebrospinal fluid in the subarachnoid space, and the local anesthetic can spread with the flow of cerebrospinal fluid, so that the spinal nerve roots and spinal cord surface are blocked. Subarachnoid space block is one of the commonly used anesthetic methods in clinical practice because of its rapid onset, satisfactory analgesia and good muscle relaxation. Second, subarachnoid space block anesthesia physiology: cerebrospinal fluid (adult): total volume: 120~150ml, spinal segment subarachnoid space only 23~25ml. appearance: clear, transparent, 37℃, pH=7.35 specific gravity: 1.003~1.009, individual differences exist. Blocking sequence: vegetative (sympathetic/parasympathetic) nerves → sensory nerves → motor nerve fibers (with myelinated proprioceptive fibers) are blocked last, and the fading sequence is exactly the opposite of the blocking sequence. Blocking plane: The sympathetic blocking plane is not parallel to the sensory blocking plane. The sympathetic block plane is 2~4 nerve segments higher than the sensory block plane, and the motor nerve block plane is often 1~4 segments lower than the sensory block plane. Therefore, when determining the level of block, the loss of sensation should prevail, while the fall in blood pressure that occurs when the sympathetic block is too wide should be taken into account. Ropivacaine hydrochloride is a new type of long-acting amide local anesthetic with low neurological and cardiac toxicity. The U.S. FDA has approved 0.5% ropivacaine hydrochloride for subarachnoid space block, and the recommended safe concentration should not exceed 0.5%.