Makindo Medical Notes

Related Subjects: | General Anaesthetics | Introduction to Anaesthetics | Spinal and Epidural Anaesthesia | General Anaesthetics | Local Anaesthetics for Suturing or other Procedures | Post-operative surgical care and complications | Rapid Sequence Intubation (RSI)

Overview of General Anesthesia

General anesthesia is a medically induced state of unconsciousness that provides amnesia (lack of memory), analgesia (pain relief), muscle relaxation, and reflex suppression. It enables patients to undergo surgical and other invasive procedures without pain, anxiety, or awareness. Anesthesia providers (anesthesiologists or nurse anesthetists) carefully select drugs and doses to balance efficacy, safety, and rapid recovery.

Pharmacological Principles

General anesthetics typically produce their effects by modulating neurotransmitter activity in the central nervous system (CNS). Common mechanisms include enhancing inhibitory pathways (e.g., GABA-A receptor agonism) or inhibiting excitatory pathways (e.g., NMDA receptor antagonism). The goal is to reduce or eliminate consciousness, pain perception, and responses to surgical stimulation.

• GABA-A Receptor Modulation: Many intravenous anesthetics, such as propofol and barbiturates, enhance the function of GABA, the primary inhibitory neurotransmitter in the brain. This leads to decreased neuronal firing and sedation.
• NMDA Receptor Antagonism: Drugs like ketamine block the NMDA receptor, preventing excitatory neurotransmission and producing profound analgesia and sedation.
• Voltage-Gated Ion Channel Effects: Inhalational anesthetics and some intravenous agents may also influence sodium, potassium, and calcium channels, altering neuronal excitability.

Common Drugs Used in General Anesthesia

• Intravenous (IV) Induction Agents:
  • Propofol: A popular induction agent due to its rapid onset and short duration of action. It enhances GABA-A receptor activity, leading to sedation and hypnosis. Side effects include hypotension (due to vasodilation) and respiratory depression.
  • Ketamine: Primarily an NMDA receptor antagonist. It provides excellent analgesia, preserves airway reflexes, and often stimulates the cardiovascular system (increasing heart rate and blood pressure). However, it may cause dissociative anesthesia and vivid dreams or hallucinations.
  • Etomidate: Known for cardiovascular stability (minimal effects on blood pressure), making it useful in patients with compromised heart function. It can, however, suppress adrenal steroid synthesis with repeated doses or infusions.
  • Thiopental (Thiopentone): A barbiturate historically used for induction; it enhances GABA-mediated inhibitory signals. It’s less common now due to the popularity of propofol.
• Inhalational Agents (Volatile Anesthetics):
  • Isoflurane: A widely used volatile anesthetic with moderate potency and solubility. It may cause dose-dependent decreases in blood pressure and respiratory function.
  • Sevoflurane: Known for its low airway irritation and relatively rapid induction/emergence profile. Often favored in pediatric anesthesia.
  • Desflurane: Characterized by low blood solubility, allowing very quick adjustments of anesthetic depth and rapid patient wake-up. It can irritate the airways, making it less ideal for induction in some patients.
  • Nitrous Oxide (N₂O): A weak anesthetic agent often used as an adjunct to reduce the dose requirements of more potent agents. It provides some analgesia but is insufficient as a sole agent for most surgeries.
• Adjunctive Drugs and Other Agents:
  • Opioids (Fentanyl, Morphine, Remifentanil): Provide analgesia and reduce the stress response to surgery. They can cause respiratory depression and, in higher doses, hypotension.
  • Benzodiazepines (Midazolam): Provide anxiolysis and amnesia. They potentiate GABA-A receptors but can cause respiratory depression and sedation.
  • Muscle Relaxants (Rocuronium, Vecuronium, Succinylcholine): Facilitate endotracheal intubation and optimize surgical conditions. They do not provide analgesia or sedation but paralyze skeletal muscles temporarily.
  • Reversal Agents (Neostigmine, Sugammadex): Help reverse the effects of non-depolarizing muscle relaxants, improving patient recovery and reducing residual paralysis.

Side Effects and Complications

• Hypotension: Many agents reduce systemic vascular resistance, causing low blood pressure.
• Respiratory Depression: General anesthetics can depress the respiratory drive and diminish protective airway reflexes.
• Cardiac Arrhythmias: Some drugs (e.g., volatile anesthetics) can predispose patients to arrhythmias.
• Nausea and Vomiting: Postoperative nausea and vomiting (PONV) are relatively common; antiemetics (like ondansetron) are often used prophylactically.
• Malignant Hyperthermia: A rare but life-threatening reaction triggered by certain inhalational agents (e.g., halothane) and the muscle relaxant succinylcholine. Characterized by a rapid increase in body temperature and severe muscle rigidity. Dantrolene is the antidote.
• Allergic Reactions: Though rare, serious anaphylactic reactions can occur to any anesthetic drug.
• Postoperative Confusion/Delirium: Especially in older adults, certain drugs and anesthesia depth can contribute to cognitive changes after surgery.

Monitoring and Safety Measures

Patient safety during general anesthesia is paramount. Standard monitoring includes continuous ECG (electrocardiogram), non-invasive blood pressure measurement, pulse oximetry (SpO₂), end-tidal CO₂, and temperature. In more complex cases, invasive monitoring such as arterial lines or central venous catheters may be used. Anesthesia providers also track anesthetic depth using clinical signs and, in some settings, brain monitoring devices (e.g., BIS monitor).

Other Considerations

• Preoperative Assessment: Identifying comorbidities (heart disease, diabetes, lung problems) and any previous issues with anesthesia helps tailor drug choices and reduces complications.
• Patient Positioning: Ensuring proper positioning is crucial to prevent nerve compression, pressure sores, and other musculoskeletal issues.
• Fluid Management: Balancing intravenous fluids and blood loss to maintain hemodynamic stability.
• Emergence and Recovery: As the anesthetic drugs wear off or are discontinued, patients regain consciousness in the post-anesthesia care unit (PACU). Monitoring continues to identify any immediate postoperative complications (e.g., airway obstruction, pain, or bleeding).

Through careful drug selection, vigilant monitoring, and expert management, general anesthesia remains a cornerstone of modern surgical practice, offering patients a safe and comfortable environment for undergoing even the most complex operations.