13.6: Stages of General Anesthesia

Various sedation levels offer significant advantages in facilitating procedural interventions for patients undergoing medical or invasive surgical procedures. These levels span from anxiolysis to general anesthesia, providing a spectrum of sedative effects to cater to specific patient needs. Anxiolysis reduces anxiety and is achieved through minimal sedation, enabling patients to remain awake and responsive while feeling more at ease during the procedure. This level can benefit minor interventions or diagnostic tests where patient cooperation is essential. Moderate or conscious sedation induces higher relaxation and drowsiness, often achieved with benzodiazepines or opioids. While patients may experience decreased awareness and responsiveness, they can still maintain certain protective reflexes and respond to verbal commands. Deep sedation represents a more profound level of sedation, bordering on general anesthesia, in which patients are rendered unconscious and unresponsive, requiring assistance with maintaining airways and carefully monitoring vital signs.

The first stage is induction, where potent anesthetics are administered to produce unconsciousness. Opioids are also given for pain relief during this stage. The goal is to achieve a state of reversible CNS depression. Once induction is complete, maintenance begins. This stage involves the sustained use of anesthetics through intravenous administration or inhalation. Monitoring is crucial to ensure the proper balance of the drugs used. The final stage is recovery, which starts with the cessation of the anesthetic and continues until the patient's protective reflexes and physiological functions ( such as spontaneous respiration, blood pressure, and heart rate) fully return. Throughout the process, anesthesia produces a descending CNS depression, with higher functions being lost first and progressively lower areas affected.

Guedel (1920) described the four stages of anesthesia with ether, namely analgesia, delirium, surgical anesthesia, and medullary paralysis, dividing the third stage (surgical anesthesia) into four planes based on physiological responses. The present-day anesthesiologist assesses the depth of anesthesia by observing responses to painful stimuli and monitoring cardiac and respiratory functions. Additionally, the physiological responses to general anesthetics depend on the minimal alveolar concentration —used to indicate the potency of inhalational gases, and it decreases with age beyond 50 years old, correlating with the oil/gas partition coefficient.

Patients benefit from carefully managed sedation levels —characterized by distinct physiological changes— during medical procedures.

General anesthesia induces descending CNS depression, initially affecting higher functions, then progressively targeting lower brain areas.

The first stage is induction, where anesthetics are administered to induce unconsciousness and achieve reversible CNS depression. Opioids are usually co-administered to aid analgesia.

Before complete unconsciousness, patients might experience a brief delirium stage characterized by movement and incoherent speech.

Once the patient is unconscious, the maintenance of the surgical anesthesia stage begins. It involves sustained anesthetic administration through intravenous or inhalation routes with vital sign monitoring to ensure optimal drug balance.

Post-anesthetic cessation, the patient's recovery is monitored until his protective reflexes and physiological functions fully return

In sporadic cases of anesthesia overdose, patients might enter the medullary paralysis stage—causing respiratory arrest and, ultimately, death.

So, anesthesiologists diligently assess anesthesia stages and depth by observing reflexes and monitoring vital signs.