The principal goals of preoperative medication are
to relieve anxiety and provide sedation;
to induce amnesia;
to decrease secretion of saliva, gastric juices;
to increase the gastric pH;
to prevent allergic reactions to anesthetic drugs;
to prevent postoperative nausea.
Medication is usually given 0.5–2 hours before the induction of anesthesia. It is not necessary to give medication specifically to facilitate the induction of anesthesia. The selection of drugs is largely subjective. Sedation can be achieved by benzodiazepines and sometimes opioids (or both), if the patient has acute pain. To avoid an intramuscular injection, diazepam, 0.12 mg/kg, is especially effective for sedation when given orally 1–2 hours preoperatively. Midazolam is a benzodiazepine with powerful amnestic properties and can be given intravenously 10–30 minutes before going into the operating room, usually at a dose of 1.0–2.0 mg/70 kg. Patients should be observed for possible respiratory depression. Also, 2-adrenergic agonists (eg, clonidine and depomedetomidine) are being used for premedication. Gastric secretion can be decreased both in volume and pH by H2-receptor antagonists such as cimetidine or ranitidine. These drugs need to be given 2–3 hours preoperatively. Antacids, such as sodium citrate, can be given acutely to decrease gastric fluid pH. Sometimes, drugs that stimulate gastric emptying (eg, metoclopramide) are used. Anticholinergics such as atropine are rarely indicated. The traditional practice of prolonged fasting (eg, NPO after midnight or for 8 hours before induction of anesthesia) in patients without risk factors (eg, obesity, bowel obstruction, severe pain) is being reevaluated. The volume and pH of gastric contents are not affected by fluids ingested more than 2 hours previously. Although it is reasonable to allow clear fluids orally up to 2 hours before induction of anesthesia, this does not apply to patients with known risks for aspiration or those who have ingested solid food.
In the past few years, emphasis has been on pharmacologic approaches to decreasing the incidence of postoperative vomiting. Certain types of patients, surgical procedures, and anesthetics (eg, opioids) are known to enhance postoperative nausea. For moderate- to high-risk patients, ondansetron or dolasetron are effective. Droperidol was commonly used, but was given a “black box” because of QT prolongation.
The anesthesiologist’s explanation to the patient of what will occur can substantially alleviate fears about anesthesia and surgery. It has been shown that a thorough explanation has a calming effect comparable to that of medications given to relieve anxiety.
For some other conditions associated with surgery, it is better to give medication as the need arises. Cardiac vagal activity is best controlled with glycopyrrolate given just before anticipated vagal stimulation. Postoperative analgesia is better achieved by giving opioids intravenously just before they are needed.
Selection of Anesthesia
Many factors influence the choice of anesthesia for a given patient, and it is common practice to discuss the question with the patient preoperatively. The site of surgery and positioning of the patient on the operating table are obviously important factors. Epidural anesthesia, along with some peripheral nerve blocks, are increasingly being used not only for sometime superior intraoperative anesthesia but also as options for postoperative analgesia. Yet, a regional nerve block may be contraindicated in a patient with neuropathy due to diabetes mellitus. Spinal anesthesia is inappropriate for thyroidectomy. Different types of anesthesia may be given for elective or emergency surgery, particularly if the patient requiring emergency surgery has a full stomach. Coexisting diseases (eg, hypertension, cardiac disease) must be considered. The age and preferences of the patient must also be taken into account.
Preparation for Administration of Anesthesia
Anesthesia usually begins by starting an intravenous infusion and engaging standard monitors, which include noninvasively measured arterial blood pressure, electrocardiography, pulse oximetry, in some cases peripheral nerve stimulation and body temperature.
The machine for administering anesthesia must be checked for proper functioning, and drugs and other necessary supplies must be at hand (eg, the apparatus needed to suction the pharynx and ventilate the lungs with oxygen via a cuffed endotracheal tube).
National standards are evolving for patient monitoring during anesthesia. In general, minimum monitoring dictates measurement of arterial blood pressure, heart rate every 5 minutes, and the ECG should be displayed continuously.
In addition, standard practice during anesthesia is the use of pulse oximetry, capnography, an oxygen analyzer in the anesthetic circuit, a disconnect alarm, measurement of body temperature. Other monitors, such as transcutaneous PaO2, intra-arterial blood pressure, central venous pressure, transesophageal echocardiography, and Bispectral Index, are optimal depending on the anticipated extent of surgery, the anticipated duration and depth of anesthesia. The Bispectral Index is derived from continuous electroencephalographic monitoring and is thought to reflect the hypnotic component of anesthesia. Whether use of this type of monitoring will decrease the incidence of intraoperative “awareness” during anesthesia is controversial. Even the definition of “intraoperative awareness” during anesthesia is not well defined. The use of automated anesthetic records, in which all vital sign and other data are downloaded to a common record that can be connected to all activities in the perioperative period (eg, the postanesthetic care unit) is increasingly being recommended by organizations such as the Anesthesia Patient Safety Foundation.
Positioning on the Operating Table
The patient must be positioned properly on the operating table to avoid physical or physiologic complications. Immediate complications or long-term complications (eg, peripheral neuropathy) can result from improper positioning. In fact, nerve damage is the second most common type of anesthetic complication represented in the Society of Anesthesiologists closed claims database. Nerve damage can be caused by placing the patient in a position that stretches or applies pressure to a nerve. Pressure on a vulnerable area may lead to skin necrosis and ulceration, which in rare cases requires skin grafting. Damage to the toes or fingers may occur when positioning of equipment (eg, Mayo stand) is adjusted. Because anesthesia blunts the normal compensatory mechanisms, a sudden change in the patient’s position can cause cardiovascular changes.
The Society of Anesthesiologists has specific guidelines that may facilitate prevention of peripheral neuropathies; they include preoperative assessment, upper and lower extremity positioning, protective padding, equipment (eg, shoulder braces), postoperative assessment, and documentation.