Anesthesia · 5 min read
Anesthetic Agents
Intravenous Induction Agents
Propofol is the most commonly used induction agent in ambulatory anesthesia. It produces rapid onset of unconsciousness within one arm-brain circulation time, typically 30 to 45 seconds. Recovery is rapid due to redistribution and metabolism. Propofol has antiemetic properties, making it favorable for ambulatory surgery. Side effects include hypotension from decreased systemic vascular resistance and myocardial depression, respiratory depression, and pain on injection. Propofol is formulated in a lipid emulsion, and strict aseptic technique is required due to potential for bacterial contamination.
Etomidate provides hemodynamic stability, making it useful for patients with cardiovascular compromise. It has minimal effects on blood pressure and heart rate. Etomidate causes less respiratory depression than propofol. Side effects include myoclonus, pain on injection, nausea and vomiting, and adrenocortical suppression with repeated doses.
Ketamine is a dissociative anesthetic that provides analgesia, amnesia, and sedation while maintaining airway reflexes and spontaneous ventilation. It causes sympathetic stimulation, increasing heart rate and blood pressure. Ketamine is useful for patients with asthma due to bronchodilatory effects. Side effects include emergence delirium, increased secretions, and increased intracranial pressure. Subanesthetic doses are used for multimodal analgesia.
Inhalation Agents
Volatile anesthetics include sevoflurane, desflurane, and isoflurane. These agents provide maintenance of general anesthesia through continuous inhalation. Minimum alveolar concentration (MAC) describes potency; it is the concentration at which 50 percent of patients do not move in response to surgical incision.
Sevoflurane is the most commonly used volatile agent for induction and maintenance in ambulatory surgery. It has a pleasant odor, low blood-gas solubility for rapid onset and emergence, and minimal airway irritation. Sevoflurane is suitable for mask induction in pediatric patients.
Desflurane has the lowest blood-gas solubility among volatile agents, providing the most rapid emergence. However, it is irritating to airways and is not suitable for mask induction. Desflurane requires a heated vaporizer due to its low boiling point. It may cause sympathetic stimulation with rapid increases in concentration.
Nitrous oxide is an adjunct agent that provides analgesia and reduces requirements for other anesthetics. It has low potency and cannot produce surgical anesthesia alone. Nitrous oxide diffuses into air-filled spaces, making it contraindicated in patients with pneumothorax, bowel obstruction, or middle ear surgery. It may contribute to postoperative nausea and vomiting.
Neuromuscular Blocking Agents
Neuromuscular blocking agents (NMBAs) produce skeletal muscle relaxation for intubation and surgical procedures. They are classified as depolarizing or nondepolarizing agents.
Succinylcholine is the only depolarizing NMBA in clinical use. It produces rapid onset paralysis within 60 seconds and short duration of 5 to 10 minutes. Succinylcholine is metabolized by plasma cholinesterase. Side effects include fasciculations, hyperkalemia, increased intraocular and intragastric pressure, masseter muscle rigidity, and triggering of malignant hyperthermia in susceptible individuals. It is contraindicated in patients with burns, crush injuries, denervation injuries, and certain neuromuscular diseases due to risk of severe hyperkalemia.
Nondepolarizing NMBAs include rocuronium, vecuronium, cisatracurium, and atracurium. These agents competitively block acetylcholine at the neuromuscular junction. Onset ranges from 2 to 5 minutes depending on the agent and dose. Duration varies from intermediate (30 to 60 minutes) to long-acting. Rocuronium is most commonly used in ambulatory surgery due to intermediate duration and availability of sugammadex for reversal.
Reversal of neuromuscular blockade is essential before extubation. Neostigmine is an anticholinesterase that increases acetylcholine at the neuromuscular junction. It must be administered with an anticholinergic (glycopyrrolate or atropine) to prevent muscarinic side effects including bradycardia and increased secretions. Sugammadex is a selective relaxant binding agent that encapsulates rocuronium and vecuronium, providing rapid and complete reversal regardless of depth of blockade.
Opioid Analgesics
Opioids provide analgesia during and after surgery. Commonly used intraoperative opioids include fentanyl, sufentanil, alfentanil, and remifentanil.
Fentanyl is a synthetic opioid approximately 100 times more potent than morphine. It has rapid onset (2 to 3 minutes) and intermediate duration (30 to 60 minutes). Fentanyl is commonly used for intraoperative analgesia and as a component of balanced anesthesia.
Remifentanil is an ultra-short-acting opioid metabolized by plasma esterases. Its elimination is independent of hepatic and renal function. Remifentanil provides intense intraoperative analgesia but no postoperative pain control, requiring transition to longer-acting analgesics.
Morphine and hydromorphone are longer-acting opioids used for postoperative pain management. They have slower onset and longer duration than fentanyl. These agents may delay discharge due to sedation and increased incidence of nausea and vomiting.
Local Anesthetics
Local anesthetics block sodium channels, preventing nerve impulse conduction. They are classified as esters or amides based on chemical structure.
Ester local anesthetics include procaine, chloroprocaine, and tetracaine. They are metabolized by plasma cholinesterase. Allergic reactions are more common with esters due to metabolite para-aminobenzoic acid (PABA).
Amide local anesthetics include lidocaine, bupivacaine, ropivacaine, and mepivacaine. They are metabolized in the liver. Allergic reactions are rare with amides.
Lidocaine has rapid onset and intermediate duration. It is used for local infiltration, peripheral nerve blocks, and intravenous regional anesthesia. Maximum dose without epinephrine is 4.5 mg/kg; with epinephrine, 7 mg/kg.
Bupivacaine has slow onset and long duration, making it suitable for peripheral nerve blocks when prolonged analgesia is desired. It has significant cardiotoxicity if injected intravascularly. Maximum dose is 2.5 mg/kg.
Ropivacaine is similar to bupivacaine but with less cardiotoxicity and motor blockade. It is preferred for peripheral nerve blocks in ambulatory surgery.
Local anesthetic systemic toxicity (LAST) occurs from intravascular injection or excessive absorption. Early symptoms include perioral numbness, tinnitus, metallic taste, and dizziness. Progression leads to seizures, cardiac arrhythmias, and cardiovascular collapse. Treatment includes stopping injection, airway management, seizure control with benzodiazepines, and lipid emulsion therapy (Intralipid 20%).
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