Basic Principles of General Anesthesia

General anesthesia (GA) is a controlled, reversible state characterized by loss of consciousness, amnesia, analgesia, muscle relaxation or immobility, and attenuation of adverse autonomic responses to painful stimuli. This state serves to maintain physiological homeostasis during surgical and other interventional procedures.

Preanesthetic Evaluation

Before administering anesthesia, all patients undergo an assessment of their health status, potential perioperative risks, and readiness for the planned procedure.

Evaluating perioperative risks helps identify vulnerable patients in advance, optimize comorbidities, determine the anesthesia plan (required monitoring, infusion strategy, postoperative observation), and conduct patient counseling, including discussion of expected risks and obtaining informed consent.

In clinical practice, the ASA Physical Status Classification is widely used as a basic indicator of patient severity, along with other scales such as the ACS NSQIP Surgical Risk Calculator for non‑cardiac surgical patients and the RCRI for assessing cardiovascular complications.

Main Stages of General Anesthesia

Modern GA includes three stages:

• Induction;
• Maintenance;
• Emergence.

1. Induction

Induction is the process of bringing the patient into an unconscious state by administering an adequate concentration of anesthetic, followed by continued maintenance of anesthesia. Induction is the most vulnerable stage, during which the patient transitions within minutes from preserved protective reflexes to apnea and potential hypotension.

Safety is ensured through prior preparation:

• Verification of patient identity, procedure, and consent;
• Checks of the anesthesia machine, equipment, and medications;
• Readiness for anticipated blood loss and availability of blood products;
• Airway evaluation and assessment of procedure‑related risks.

Primary Induction Agents

Induction may be achieved intravenously or via inhalation, depending on the clinical situation or patient age. Among adults, intravenous induction is preferred and typically includes a hypnotic agent (propofol, etomidate, ketamine), an adjuvant (an opioid and/or benzodiazepine), and a neuromuscular blocking drug if intubation is planned.

Inhalational anesthesia is often added as a supplemental component after initial loss of consciousness with IV hypnotics, or used as the sole method.

Airway Protection

Airway management is an integral part of GA, ensuring adequate ventilation and oxygenation, as well as delivery of the anesthetic gas mixture.

Airway Accessories

• Face mask: Used during induction before other devices, and during anesthesia for short procedures that do not require muscle relaxation.
• Supraglottic devices (laryngeal mask airway): Inserted into the oropharynx and positioned above the larynx. The device may be used as the primary airway during spontaneous or controlled ventilation but does not provide complete protection against aspiration. It can also serve as a conduit for intubation or be used in patients with difficult airways.
• Endotracheal tube: Placed via direct laryngoscopy with the distal end positioned in the trachea. The tube is used for patients at risk of aspiration, during prolonged procedures, or when neuromuscular blockade is required. Postoperative sore throat is the most common side effect of endotracheal intubation.

Special situations: Include high aspiration risk (requiring rapid sequence induction, RSI) and difficult airways.

2. Maintenance

The primary goal of this stage is to maintain Stage 3 surgical anesthesia at a safe depth, particularly in elderly patients and those with significant comorbidities. This includes:

1. Maintaining physiological homeostasis: hemodynamic stability, adequate ventilation and oxygenation, temperature control.
2. Preventing intraoperative awareness.
3. Providing adequate analgesia.
4. Neuromuscular relaxation: to facilitate surgery or prevent movement when required.

Maintenance may rely primarily on inhalational or intravenous techniques. An ideal anesthetic should exhibit:

1. Rapid onset;
2. Minimal cardiopulmonary effects;
3. Rapid clearance from the bloodstream.

No currently available anesthetic is ideal for all patients; all anesthetic agents have potential side effects.

Thus, maintenance may involve inhalational anesthetics alone, total intravenous anesthesia (TIVA), or one or more inhalational agents in combination with several intravenous sedatives and analgesics (opioid and nonopioid), with or without additional neuromuscular blockers.

Inhalational Anesthetics

These agents produce loss of consciousness and may also provide additional effects, including analgesia (nitrous oxide) and muscle relaxation (isoflurane). All inhalational anesthetics are either pressurized liquefied gases or volatile liquids. Potency is measured by the minimum alveolar concentration (MAC), defined as the concentration that prevents movement in response to a surgical stimulus in 50 % of patients at 1 atm.

Inhalational anesthetics include:

• Volatile agents (sevoflurane, desflurane, isoflurane, and halothane in some countries): may be used as the sole agent for maintaining GA;
• Nitrous oxide: used as an adjunct during maintenance.

Advantages of inhalational anesthesia:

• Easy and rapid control of anesthetic depth (via end‑tidal concentration).
• Dose‑dependent reduction in muscle tone, allowing lower doses of neuromuscular blockers.
• Predictable and rapid emergence.
• Cost‑effectiveness under certain conditions.

Disadvantages:

• Dose‑dependent respiratory and hemodynamic depression.
• Higher risk of postoperative nausea and vomiting (PONV).
• Potential trigger for malignant hyperthermia.

Intravenous Anesthetics

These agents produce rapid loss of consciousness when administered in appropriate doses (typically achieved after one arm‑brain circulation time). Maintenance of GA exclusively through intravenous (IV) administration is called total intravenous anesthesia (TIVA).

Infusion doses are adjusted based on age, BMI, frailty, and comorbidities. Synergism is achieved when combining IV anesthetics from different classes, allowing dose reduction of each agent.

Key components of IV anesthesia:

• Sedative‑hypnotic anesthetic: typically propofol.
• Analgesic component: usually opioids such as remifentanil, fentanyl, sufentanil, hydromorphone, or morphine.
• Adjuvants: used to reduce or replace propofol as the hypnotic or opioids as the analgesic component. These include benzodiazepines (midazolam, remimazolam), α2‑agonists (dexmedetomidine), local anesthetics (lidocaine), and others.

Advantages of TIVA:

• TIVA is useful when inhalational techniques are undesirable or not feasible (airway procedures, transportation).
• It does not trigger malignant hyperthermia.
• The technique shows lower risk of PONV.
• Environmental benefit (reduced atmospheric impact compared with inhalational agents).

Disadvantages of TIVA:

• Inability to directly measure target blood concentration and therefore depth of anesthesia.
• Risk of unintended awareness if drug delivery is interrupted, requiring high safety standards for infusion systems and EEG‑based monitoring.
• Dose‑dependent respiratory and hemodynamic depression, similar to inhalational agents.

Muscle Relaxants

Muscle relaxants, or neuromuscular blocking drugs (NMBDs), are divided into two classes based on their mechanism of action:

1. Depolarizing (succinylcholine).
2. Non‑depolarizing: characterized by a slower onset and longer duration of action. Non‑depolarizing muscle relaxants are further classified by duration of action into long‑acting (pancuronium), intermediate‑acting (vecuronium, rocuronium, atracurium, cisatracurium), and short‑acting (mivacurium).

Selection of a specific NMBD is based on the planned duration of the surgical procedure and the presence of severe renal or hepatic dysfunction.

Advantages of NMBDs:

• Optimized surgical conditions (muscle relaxation in the surgical field, which, for example, reduces insufflation pressure requirements during laparoscopy).
• Reduction in the total amount of anesthetic drugs and facilitation of synchrony with mechanical ventilation.

Disadvantages and Risks of NMBDs:

• Residual neuromuscular block is a significant risk factor for anesthesia‑related morbidity and mortality; therefore, quantitative TOF (train-of-four) monitoring is recommended.
• Anaphylaxis (NMBDs are among the most common causes of perioperative anaphylaxis).
• Succinylcholine, a depolarizing agent, is a known trigger of malignant hyperthermia.

3. Emergence

This stage signifies a gradual return of consciousness after anesthetics and adjuvant medications are discontinued at the end of the procedure, allowing the patient to transition smoothly from surgical anesthesia to wakefulness.

Key objectives of this stage:

1. Discontinue or reduce anesthetic delivery while maintaining adequate analgesia.
2. Restore spontaneous ventilation and protective airway reflexes.
3. Eliminate residual neuromuscular block before extubation; perform reversal of neuromuscular block (with neostigmine, sugammadex).
4. Prevent and treat PONV.
5. Manage potential complications during emergence from general anesthesia, such as hypertension, tachycardia, shivering, airway issues (laryngospasm, bronchospasm), or severe agitation.
6. Transport the patient from the operating room to the post‑anesthesia care unit (PACU) for further monitoring.

Monitoring in General Anesthesia

Monitoring is a fundamental and mandatory component of GA. The anesthesiologist must continuously assess the patient’s physiological variables as well as anesthesia equipment parameters, since anesthesia and surgery can cause rapid changes in vital signs.

Basic Monitoring

1. Pulse oximetry.
2. Electrocardiogram (ECG).
3. Noninvasive blood pressure measurement.
4. Thermometry.
5. Capnography (ETCO₂).
6. Low oxygen concentration and apnea/ventilation‑disconnection alarms.
7. Quantitative monitoring of exhaled gas volume.

Advanced Monitoring

1. Invasive arterial blood pressure (arterial line): indicated in patients at high risk of hemodynamic instability, during major complex surgeries, when rapid blood pressure fluctuations are expected, when arterial blood gas sampling is required, or for precise titration of vasoactive medications.
2. Quantitative neuromuscular monitoring (TOF/TOFR, electromyography, acceleromyography): indicated in patients receiving NMBDs for dosing and confirmation of recovery; the target before extubation is TOF≥ 0.9.
3. Depth of anesthesia monitoring using processed EEG ( bispectral index monitor, BIS): preferred when using TIVA.
4. Central venous line and central venous pressure (CVP) monitoring: indicated in cases of poor peripheral access, need for rapid infusion/transfusion, or as part of volume status assessment.
5. Advanced hemodynamic monitoring (transesophageal echocardiography, pulmonary artery catheterization — Swan — Ganz catheter, etc.): required in high‑risk patients, during cardiac surgery, and in other high‑risk procedures.
6. Continuous temperature monitoring: indicated for prolonged surgeries, when active warming is required, or when there is a risk of hypothermia.

Postoperative Care

Most patients are transported to the PACU after anesthesia to ensure timely management of postoperative side effects. Critically ill patients — whether conscious or intubated — are transported to the ICU for further treatment.