Assessment of the Height of Block
Assessment of the Height of Block
The first 5 to 10 minutes after administration of the spinal anesthetic are the most critical time in adjusting the level of anesthesia when hyperbaric or hypobaric solutions are used. In 15-20 minutes after injection of a local anesthetic into the subarachnoid space, the level of anesthesia is fixed to a certain degree. However, it is reported that it takes 40-50 minutes to fix finally. So the level of anesthesia should be examined frequently. It is strongly recommended to examine the level of anesthesia at 5, 10, 15 and, if possible, 30 and 60 minutes after injection (Table 8.1).
It is also strongly recommended to check it at the end of operation and at an emergency such as serious hypotension or respiratory depression. Even if it is difficult to check the level of anesthesia during the operation, it is recommended to examine frequently whether the highest level of anesthesia goes up to T4. It is strongly recommended to check the level of anesthesia within 3 minutes after injecting a local anesthetic, when the following conditions occur: the sudden blood pressure decrease, the level of anesthesia rises considerably high (above T4) in a short duration, and occurrence of respiratory depression. These may occur, for example, in a caesarean section, in the case of a giant tumor of the abdomen, morbid obesity, or very young.
Ways to examine the level of anesthesia: One is to rub the skin with cotton soaked with ethyl alcohol, then check whether the patient feels cold sensation or not. The area at which the patient does not feel cold is nearly the same as the sympathetic blockade area, and is two to several segments wider than the area of analgesia. The other way is to prick the skin slightly with a blunt needle, and check whether the patient feels tenderness or not (the pin prick test) sensory level. It is important, of course, to check precisely where the highest level of anesthesia is. So it is strongly recommended to wait for at least 15 minutes.
Table 8.1 Levels of Spinal Anesthesia Required for Common Surgical Procedures
|T4-5 (nipple)||Upper abdominal surgery including C/S|
|T6-8 (xiphoid)||Intestinal surgery including appendectomy, gynecologic pelvic surgery, and ureters and renal pelvic surgery|
|T10 (umbilicus)||Transurethral resection, obstetric vaginal delivery, and hip surgery|
|L1 (inguinal ligament)||Transurethral resection, if no bladder distension; thigh surgery; lower limb amputations, and so forth.|
|L2-3 (knee and below)||Foot surgery|
|S2-5 (perineal)||Perineal surgery, hemorrhoidectomy, anal dilation...|
The next 10 to 20 minutes are also the most critical in assessing the cardiovascular responses to spinal anesthesia. Frequent measurements of blood pressure and heart rate will allow early recognition of any degree of hypotension. After surgical anesthesia levels and cardiovascular stability have been achieved, the anesthetist may evaluate whether supplemental drugs should be administered to make the patient comfortable. Sedation may be achieved with repeated small doses of an intravenous hypnotic. It is generally considered good practice for all patients undergoing surgery under spinal anesthesia to be given supplemental oxygen by facemask at a rate of 2-4 liters/minute, especially if sedation has also been given.