How Anaesthesia Works
General Anaesthesia
General anaesthesia (GA) works by inducing a controlled, reversible state of unconsciousness in which the patient has no awareness, sensation, or memory of the procedure. It is achieved through a combination of intravenous agents and/or inhaled gases.
Induction is typically done intravenously using agents like propofol or thiopentone, which act rapidly on the central nervous system by enhancing inhibitory GABA receptors and suppressing excitatory pathways in the brain.
Once unconscious, anaesthesia is maintained either through volatile inhaled agents such as sevoflurane or desflurane, or via a continuous intravenous infusion (total intravenous anaesthesia, TIVA) using propofol and an opioid like remifentanil. Muscle relaxants such as rocuronium or suxamethonium are often added to facilitate intubation and surgical access.
The patient's airway is secured with an endotracheal tube or laryngeal mask airway (LMA), and ventilation is mechanically controlled throughout.
Monitoring of depth of anaesthesia, haemodynamic stability, oxygen saturation, and end-tidal CO₂ is continuous.
At the end of the procedure, agents are withdrawn, reversal drugs may be given (e.g. neostigmine for neuromuscular blockade), and the patient is allowed to emerge from anaesthesia under close supervision.
Monitored Anaesthesia Care
Monitored anaesthesia care (MAC) is a service provided by an anaesthesiologist in which the patient remains conscious or in a state of light sedation, while still receiving careful monitoring, anxiolysis, and pain management. Unlike general anaesthesia, the patient retains their own airway reflexes and spontaneous breathing throughout.
It typically involves the administration of short-acting sedative and analgesic agents such as midazolam, fentanyl, ketamine, or low-dose propofol, titrated carefully to achieve comfort without rendering the patient fully unconscious.
MAC is commonly used for minimally invasive or superficial procedures such as endoscopies, cataract surgery, biopsies, or interventional radiology, often in combination with local anaesthesia administered by the surgeon at the operative site. The anaesthesiologist continuously monitors vital signs including heart rate, blood pressure, oxygen saturation, and respiratory rate, and is prepared to convert to deeper sedation or general anaesthesia if the clinical situation demands it.
The key advantage of MAC is its favourable safety profile — reduced physiological perturbation, faster recovery, and lower risk of airway complications — making it particularly suitable for elderly patients or those with significant comorbidities who may not tolerate a full general anaesthetic well.
Neuraxial Anaesthesia
Neuraxial anaesthesia refers to techniques that deliver local anaesthetic agents directly into the central neuraxial space — either the subarachnoid (spinal) space or the epidural space — to block nerve transmission along the spinal cord. In spinal anaesthesia, a small volume of local anaesthetic (e.g. hyperbaric bupivacaine) is injected into the cerebrospinal fluid (CSF) at the lumbar level, producing a rapid, dense, and predictable sensory and motor block below the level of injection. It is commonly used for lower limb, urological, and caesarean section surgeries.
Epidural anaesthesia involves placing a catheter into the epidural space, through which larger volumes of local anaesthetic (with or without opioids) are infused to block nerve roots as they exit the spinal cord. This technique offers more titratability and can be maintained for prolonged procedures or extended into the postoperative period for analgesia.
A combined spinal-epidural (CSE) technique combines the rapid onset of spinal with the flexibility of an epidural catheter. Neuraxial anaesthesia keeps the patient conscious (or lightly sedated) while providing profound regional anaesthesia, avoiding the systemic effects and airway manipulation associated with general anaesthesia.
Key physiological effects include sympathetic blockade, which can cause hypotension and bradycardia and must be carefully managed.