Administration of General Anesthesia - Preoperative Preparation
The anesthetist assumes responsibility for the patient when the preoperative medication is administered. An anesthetist or other responsible clinician should accompany an unstable patient during transport to the OR.
Preoperative evaluations may be performed minutes to weeks before the administration of the anesthetic and sometimes not by the anesthetist of record. A detailed history and physical exam should be performed, and preoperative optimization should be completed. The administering anesthetist performs an airway examination and checks for interim changes in the patient's condition, medications, laboratory data, and consultant notes. Time of last oral intake is confirmed (see Table 15.1). Tube feeds in critically ill intubated patients may be continued before and during procedures outside the abdomen and thorax. Allergies and the anesthetic plan are reviewed with the patient, and proper informed consent for the administration of anesthesia is obtained from either the patient or his or her legal proxy.
Intravascular Volume. Patients may arrive in the OR with intravascular or total body hypovolemia due to prolonged NPO status, severe inflammatory illness, hemorrhage, fever, vomiting, or diuretic use. Currently available isotonic bowel preparations may not directly induce water loss but can decrease absorption of fluids ingested before surgery. The patient's volume status is evaluated either clinically or with appropriate monitors. If a fluid deficit is present, the patient should be adequately hydrated before the induction of anesthesia. The fluid deficit for fasting adults is estimated at 60 mL/h + 1 mL/kg/h for each kilogram greater than 20 kg (maintenance fluids). In general, at least half of this deficit is corrected before induction; the remainder may be corrected intraoperatively. The type and amount of fluids given may be modified in the presence of systemic diseases (see Chapters 2 to 6) or for specific types of surgery (Chapters 22 and 25).
IV Access. The size and number of IV catheters placed varies with the procedure, anticipated blood loss, and the need for continuous drug infusions. At least one large bore IV (>16-gauge) catheter should be placed when rapid fluid or blood infusion is anticipated. When continuous drug infusions are to be delivered concurrently with rapid fluid infusion, an additional IV catheter often is dedicated for this purpose. Some medications used for cardiovascular support (e.g., norepinephrine) are best delivered via a central venous catheter placed either before induction, if indicated, or after (see below under Monitoring).
- Anxiety. The preoperative period is one of high anxiety, especially for patients who have not had a prior interview with an anesthetist. Anxiety may be managed effectively with calm reassurance and expression of interest in the patient's well-being. When deemed appropriate, a benzodiazepine (e.g., diazepam and midazolam) with or without a small dose of an opioid (e.g., fentanyl and morphine) may be administered. Oral diazepam or lorazepam may be given with a small amount of water 30 to 60 minutes before the procedure. Patients complaining of pain on arrival in the OR may be given analgesics in incremental amounts to alleviate symptoms. Dosages are based on the patient's age, medical condition, and anticipated time of discharge (see Chapters 1 and 12). Appropriate monitoring should be used and resuscitative equipment available.
- Drugs to neutralize gastric acid (H2-antagonists, PPIs, nonparticulate antacids) and decrease gastric volume (metoclopramide, H2-antagonists, PPIs) should be used only when the patient is at increased risk of aspiration of gastric contents (i.e., recent meal, trauma, bowel obstruction, pregnancy, history of gastric surgery, increased intra-abdominal pressure, difficult airway, or history of active reflux; see Chapter 1). When bowel obstruction is present, promotility drugs may increase retrograde peristalsis and are contraindicated.
Monitoring. Standard ASA monitoring (see Chapter 10) is established before the induction of anesthesia. Invasive hemodynamic monitors (e.g., arterial catheter, central venous line, and pulmonary artery catheter) should be placed before induction of anesthesia when indicated by the patient's medical condition and potential anesthetic effects (e.g., an arterial line for a patient at risk for cerebral ischemia). Invasive monitors may be placed after induction of anesthesia when indicated primarily by the surgical procedure (e.g., a central line for a patient undergoing elective aortic surgery). Surgical-specific monitoring (e.g., evoked potentials) should be discussed with the surgical team to ensure compatibility with anesthetic plans.
ASA Practice Guidelines for Preoperative Fasting (Table 15.1)
|Ingested Material||Minimum Fasting Perioda|
|Clear liquids||2 h|
|Breast milk||4 h|
|Infant formula, nonhuman milk, light meals||6 h|
|Full meal||8 h|
a Healthy patients, elective cases.