6.2 General Anesthesia and Adjunct Medications

Principles of General Anesthesia

General anesthesia is a complex, medically induced state that provides a reversible loss of consciousness, amnesia (loss of memory), analgesia (loss of pain), and akinesia (muscle relaxation). It allows surgeons to perform extensive, invasive procedures that would be intolerable or impossible in a conscious patient.

Phases of General Anesthesia

1. Induction: The rapid transition from an awake state to an unconscious state, typically achieved with fast-acting intravenous medications. Airway control (intubation) is established during this phase.
2. Maintenance: Keeping the patient safely unconscious and relaxed for the duration of the surgery. This is usually achieved using inhalation agents (gases) delivered via the anesthesia machine, combined with intravenous drugs.
3. Emergence: The highly controlled awakening of the patient at the end of the procedure, involving the cessation of anesthetic agents, administration of reversal drugs, and extubation.
4. Recovery: The immediate post-anesthetic period in the Post-Anesthesia Care Unit (PACU) where the patient is closely monitored until their vital signs are stable and protective reflexes have returned.

Intravenous Induction Agents

Induction agents are pushed intravenously to quickly render the patient unconscious.

Inhalation Agents (Volatile Anesthetics)

Inhalation agents are primarily used to maintain the anesthetic state. They are delivered via a calibrated vaporizer on the anesthesia machine.

• Sevoflurane (Ultane): Sweet-smelling and completely non-irritating to the airway. Because it does not cause coughing or laryngospasm, it is the agent of choice for inhalation induction (breathing the patient to sleep), which is highly utilized in pediatric anesthesia.
• Desflurane (Suprane): Has an extremely rapid onset and offset, allowing for very quick emergence at the end of surgery. However, it is highly pungent and irritating to the airway, meaning it cannot be used for mask inductions.
• Isoflurane (Forane): An older, highly effective, and cheaper agent with a pungent odor. It takes longer for the patient to emerge compared to the newer agents sevoflurane and desflurane.
• Nitrous Oxide (N2O): A true gas (not a volatile liquid vapor). It provides excellent analgesia but very weak anesthesia. It is often used as an adjunct to other gases to reduce their required concentration.
  • Clinical Trap: Nitrous oxide diffuses into air-filled spaces in the body faster than nitrogen can diffuse out. This expands those spaces, making it strictly contraindicated in patients with a bowel obstruction, pneumothorax, or undergoing middle ear surgery.

Neuromuscular Blocking Agents (Muscle Relaxants)

Muscle relaxants paralyze skeletal muscles. This is absolutely necessary to facilitate endotracheal intubation and to provide a relaxed surgical field (e.g., preventing the abdominal wall from contracting during a laparotomy).

Depolarizing Agents

• Succinylcholine (Anectine): The only depolarizing agent currently in clinical use. It mimics acetylcholine, binding to receptors and causing sustained depolarization of the muscle endplate.
  • Characteristics: Ultra-short acting, incredibly rapid onset (making it ideal for rapid sequence intubation).
  • Side Effects: Causes visible muscle twitching (fasciculations) before total paralysis sets in.
  • Important Trap: There is NO pharmacological reversal agent for succinylcholine; it must wear off naturally via pseudocholinesterase in the blood. It is also a known primary trigger for malignant hyperthermia.

Non-Depolarizing Agents

These agents competitively block acetylcholine at the neuromuscular junction without causing any initial depolarization (no fasciculations).

• Rocuronium (Zemuron): Intermediate-acting with a rapid onset.
• Vecuronium (Norcuron): Intermediate-acting.
• Pancuronium (Pavulon): Long-acting.

Reversal Agents

At the conclusion of the surgery, non-depolarizing muscle relaxants must be deliberately reversed to restore the patient's ability to breathe spontaneously before the endotracheal tube is removed.

• Neostigmine (Prostigmin): An acetylcholinesterase inhibitor. It increases the amount of acetylcholine at the neuromuscular junction to outcompete and displace the muscle relaxant.
  • Critical Co-administration: Neostigmine causes massive systemic parasympathetic stimulation (dangerous bradycardia, intense salivation, and bronchospasm). Therefore, it MUST be co-administered with an anticholinergic drug like Glycopyrrolate (Robinul) or Atropine to block these dangerous, unwanted muscarinic side effects.
• Sugammadex (Bridion): A revolutionary, highly specific reversal agent that acts by physically encapsulating and inactivating rocuronium and vecuronium molecules directly in the bloodstream. It completely reverses deep paralysis in minutes without the need for an anticholinergic drug.

Malignant Hyperthermia (MH)

Malignant hyperthermia is a rare, life-threatening, inherited pharmacogenetic disorder affecting skeletal muscle calcium metabolism.

• Triggers: It is exclusively triggered by all volatile inhalation anesthetics (sevoflurane, desflurane, isoflurane) and the depolarizing muscle relaxant succinylcholine.
• Signs: The very first and most sensitive sign is a sudden, unexplained rise in end-tidal CO2 (hypercarbia). This is rapidly followed by generalized muscle rigidity (especially masseter spasm of the jaw), unexplained tachycardia, and profound metabolic acidosis. A dramatic, rapid rise in body temperature is a late, yet devastating, sign.
• Treatment: The surgical and anesthesia teams must act instantly. Immediate discontinuation of the triggering agents, calling for the MH cart, hyperventilating the patient with 100% oxygen, and the rapid administration of the specific, life-saving antidote, Dantrolene Sodium (Dantrium / Ryanodex). Active cooling measures (ice packs to the groin and axillae, cold IV fluids) are simultaneously initiated.