The induction of general anesthesia involves administering anesthetic agents to induce unconsciousness, managing the airway, and patient positioning prior to the start of an operation.
Positioning and Preoxygenation
Following the preoperative evaluation, a patient is brought to the operating room and positioned on the operating room table. Typically, the patient is placed supine so that the anesthesiologist has adequate access to the airway. ASA standard monitoring is initiated including continuous oxygen saturation, electrocardiography, and serial blood pressure measurements. Preoxygenation is performed, where 100% oxygen is delivered to the patient, typically via a face mask that forms a good seal with the patient’s face. Preoxygenation aims to increase the safe apnea time by replacing the nitrogen in the alveoli with oxygen. The functional residual capacity (FRC) of the lungs then provides a reservoir of oxygen to prevent desaturation during periods of apnea. For patients with reduced FRC, for example, obese or pregnant patients, a ramp or reverse Trendelenburg positioning may be used to elevate the thorax above the abdomen, therefore increasing the FRC and thus increasing safe apnea time. The goal of preoxygenation is to achieve an end tidal oxygen concentration of at least 80%, although 90% is achievable in most patients with tidal volume breathing over 3 minutes or eight vital capacity breaths in 1 minute. High-flow nasal oxygen can also be used for preoxygenation and to allow for apneic oxygenation in select patients who may desaturate quickly or are predicted to potentially have difficult airways.
Intravenous Induction
Once a patient is adequately preoxygenated, anesthesia can be induced. This typically involves intravenous administration of a hypnotic agent to
induce unconsciousness. Typical hypnotic agents include propofol, etomidate, and ketamine, although other agents may be chosen to facilitate the specific procedure being performed, such as methohexital for induction of anesthesia for electroconvulsive therapy. Often lidocaine is administered intravenously prior to reduce pain related to propofol or etomidate injection. Laryngoscopy and intubation are associated with tachycardia and hypertension so opioids like fentanyl and remifentanil or beta blockers can be administered to attenuate these sympathetic responses.Inhalational Induction
In some circumstances, most commonly pediatric patients without intravenous access, anesthesia can be induced through inhalation. This involves administration of volatile anesthetic agents via mask. The concentration of these agents is increased until an adequate depth of anesthesia is reached to facilitate IV placement and/or airway manipulation. Inhalational inductions can also be used to maintain spontaneous ventilation in select patients.
Patient Positioning
Once the airway is secured, the patient can be positioned for the operation. Care should be made to pad pressure points to minimize the risk of peripheral nerve and pressure injuries. Special attention should be paid to covering the patient’s eyes to prevent corneal abrasions (typically done before airway manipulation), keeping the patient’s neck in a neutral position, and adequate padding of the upper extremities to avoid ulnar nerve compression or stretch injuries of the brachial plexus. Some positions pose unique challenges and risks to the patient, for example prone positioning requires careful positioning of the head and face to avoid pressure on the eyes. In addition, lithotomy can lead to lower extremity nerve injuries including femoral and common peroneal neuropathies. Safe positioning of the patient is a responsibility shared by the surgeons, anesthesiologists, and operating room staff.