Anesthesia for Surgeons

Anesthetic Agents

1. General Anesthesia
• elements of general anesthesia include unconsciousness with amnesia, analgesia, and muscle relaxation
• usually produced by a combination of IV and inhaled drugs
• IV induction agents are used to initiate anesthesia
• anesthesia is maintained by inhalational agents supplemented with IV opioids and muscle relaxants


1. IV Induction Agents
1. Propofol
• short half-life, lack of accumulation allows for rapid emergence from anesthesia with no nausea
• side effects include hypotension, respiratory depression, pain on injection, and rare anaphylactic reactions
• must be used in caution with patients who are hypovolemic or who have CAD


2. Etomidate
• rapid onset and offset of consciousness
• minimal cardiovascular effects, making it valuable in patients with CAD or hypovolemia
• side effects include a high incidence of post-op nausea, pain during injection, and transient adrenocortical suppression


3. Midazolam (Versed)
• reduces anxiety and produces amnesia
• usually administered in the immediate preoperative period
• may cause severe hypotension in hemodynamically unstable or hypovolemic patients
• effects of midazolam can be reversed by flumazenil


4. Ketamine
• only induction agent that increases blood pressure and heart rate, making it useful in hypotensive patients
• has potent analgesic properties
• is a bronchodilator, making it useful in patients with asthma
• may cause delirium or hallucinations
• increases myocardial oxygen demand
• causes copious oropharyngeal secretions


2. Inhalational Agents
• used as maintenance agents in adults
• at high doses, all agents can provide unconsciousness, analgesia, and muscle relaxation
• since at high doses the side effects are unacceptable, IV opioids and muscle relaxants are used as supplemental agents, allowing for a lower dose of the inhalational agent


1. Volatile Liquids
• isoflurane, sevoflurane, desflurane, and halothane are the most commonly used agents
• cause depression of cardiac contractility and stroke volume, resulting in dose-dependent hypotension
• triggering agents for malignant hyperthermia


2. Nitrous Oxide
• rapid onset and offset
• reliably produces amnesic and analgesic effects
• may be combined with a volatile agent to permit a lower dose of the volatile agent
• has minimal cardiovascular effects
• associated with increased intracranial pressure
• causes expansion of gas-filled cavities, and is contraindicated in patients with small bowel obstruction or pneumothorax


3. Analgesia
1. Opioids
• morphine, hydromorphone, fentanyl, sufentanil
• act on CNS μ-receptors
• not reliable hypnotic or amnesic agents
• cause minimal cardiac depression
• respiratory depression, sedation, itching, post-op nausea, and constipation are major disadvantages
• Naloxone is used to reverse the side effects of opioid overdose


2. Ketorolac (Toradol)
• IV NSAID
• COX-1 and COX-2 inhibitor
• reduces prostaglandin synthesis
• major side effects include gastric bleeding, platelet dysfunction, kidney and liver damage


3. IV Tylenol
• reduces the amount of opioids required


4. Neuromuscular Blocking Agents
1. Depolarizing Agents
• succinylcholine
• mimics acetylcholine and keeps the post-synaptic membrane depolarized, resulting first in contractions (fasciculations) and then decreased muscle excitability
• fast onset (< 60 seconds) and brief duration of action (5 – 8 minutes)
• used primary during rapid endotracheal intubation
• cannot be pharmacologically reversed
• side effects include bradycardia, hyperkalemia, and triggering of malignant hyperthermia
• should not be used in burn or trauma patients


2. Nondepolarizing Agents
• acetylcholine receptor antagonists – pancuronium, vecuronium, rocuronium
• peripheral nerve stimulator is required to gauge the depth and reversal of muscle relaxation
• can be reversed by anticholinesterase drugs (neostigmine, edrophonium)


2. Local Anesthetics
1. Agents
• amides: lidocaine, bupivacaine, mepivacaine
• esters: cocaine, procaine, tetracaine
• all agents reversibly block nerve conduction by stabilizing sodium channels in their closed states
• lidocaine has a more rapid onset and shorter duration of action than bupivacaine
• all agents are largely ineffective in acidic (inflamed) fields


2. Toxicities
• toxic dose of lidocaine is 5 mg/kg
• toxic dose of bupivacaine is 3 mg/kg
• 1% lidocaine solution = 10 mg/mL
• for a 75 kg person, the toxic dose of lidocaine equals (75 kg x 5 mg/kg) / 10 mg/mL = 37.5 mL



1. CNS Toxicity
• early symptoms include restlessness and tinnitus
• slurred speech, seizures, unconsciousness follow


2. Cardiovascular Toxicity
• hypotension, increased P-R intervals, bradycardia, cardiac arrest may occur
• bupivacaine is more cardiotoxic than lidocaine


3. Additives
• epinephrine increases the duration of action and slows absorption of the local anesthetic
• epinephrine-containing agents should not be injected into fingers or toes, because vasoconstriction can lead to loss of a digit


3. Regional Anesthesia
• avoids the complications of general anesthesia and airway manipulation
• sedation must be provided by IV administration of other drugs


1. Spinal Anesthesia
• local anesthetic is injected directly into the dural sac surrounding the spinal cord, usually as a single bolus injection
• the level of injection is below L1 – L2, where the spinal cord ends in most adults
• useful for inguinal hernia repair or lower extremity procedures
• the block causes loss of sensory, motor, and sympathetic function
• complications include hypotension from vasodilation, high spinal block leading to respiratory compromise, spinal headache, urinary retention


2. Epidural Anesthesia
• a catheter is inserted into the epidural space, such that repeated dosings are possible
• catheter may be left in place for several days to provide post-op analgesia
• particularly valuable in thoracic and vascular surgery
• may be associated with less post-op ileus and earlier ambulation in abdominal surgery patients
• to avoid spinal hematomas, strict protocols regarding the timing of placement and removal of catheters in patient receiving anticoagulants (LMWH) must be followed


3. Peripheral Nerve Blocks
• local anesthetic can be injected peripherally adjacent to a large nerve or plexus
• some common examples include ankle blocks, intercostal blocks for rib fractures, transversus abdominal plane (TAP) block for abdominal incisions


4. Conscious Sedation
• refers to sedation administered for procedures done outside the OR by non-anesthesia personnel
• drugs used include an opioid for pain, often combined with an anxiolytic such as Versed
• an induction agent such as propofol may also be used for this purpose
• there is a narrow margin of safety between minimal sedation, which may be inadequate for the procedure, and deep sedation, which may result in cardiovascular or respiratory depression
• major complications are hypoventilation and hypoxemia
• Joint Commission rules require that patients be managed and monitored as if they were in the OR with an anesthesiologist present
• a monitoring assistant must be present who has no other responsibility during the procedure except monitoring
• airway and resuscitation equipment must be present
• patients must be continuously monitored until the sedation has worn off
• physicians who provide conscious sedation must be appropriately credentialled
• office procedures need to follow the same precautions as practiced in the hospital