Induction and Maintenance of General Anaesthesia

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Introduction

An anaesthetic is a crucial part of most surgical operations. The β€˜rule of threes’ is a useful framework to approach anaesthetics:

  • Three main types of anaesthesia: general, regional and local
  • Three key components of an anaesthetic: analgesia, amnesia/unconsciousness and paralysis

A general anaesthetic (GA) is administered to facilitate a pain-free, reversible state of unconsciousness with or without paralysis.

Before a general anaesthetic, a comprehensive pre-operative assessment should be performed to determine anaesthetic drug choice and technique. This assessment should include an assessment of co-morbidities, fasting status and airway assessment.

There are three stages of a general anaesthetic:

  • Induction
  • Maintenance
  • Emergence

This article discusses the induction and maintenance of a general anaesthetic.

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Induction

Induction involves securing the airway and administering drugs to facilitate a rapid and smooth loss of consciousness before the surgical procedure starts.

Whilst this may sound relatively simple, there are numerous aspects to consider, and it is helpful to follow a basic ABCDE approach:

  • A: Airway adjuncts and considerations
  • B: Breathing
  • C: Cardiovascular system and emergency drugs, including vasopressors
  • D: Hypnosis, muscle relaxation
  • E: Monitoring, type and duration of surgery
  • F: Fluids, intravenous access

Several essential steps should occur before the patient is brought to the operating theatre:

  • Pre-operative visit and airway assessment by the anaesthetist
  • Theatre team brief
  • Preparation of airway tray, emergency and induction drugs
  • Ventilator check

Upon entering the anaesthetic room/theatre, the patient will undergo safety checks and have essential monitoring attached. This can vary according to the procedure, but at a minimum includes: ECG, blood pressure (NIBP), capnography, anaesthetic depth monitoring and SpO2.1

Intravenous access is secured, and the patient is ideally pre-oxygenated with 100% oxygen for 3-5 minutes.

Airway

As a general rule, any risk of airway soiling (regurgitation, unfasted status) or anticipated difficulty with ventilation (obesity, lung pathology, laparoscopic surgery) will warrant an airway being secured via tracheal intubation (using an endotracheal tube – ETT) over a supraglottic device (laryngeal mask airway – LMA).

If the patient is considered at high risk of airway soiling, a rapid sequence induction (RSI) technique can be performed. This technique involves a rapid, successive administration of induction and neuromuscular blocking (paralytic/muscle relaxant agents) drugs to achieve a state of unconsciousness and paralysis in the shortest time possible (usually within less than a minute) to secure the airway.

The pre-operative airway assessment and anticipated difficulty will inform the choice of laryngoscopy (video/direct/blade type) or if any advanced equipment is needed (fibreoptic scopes, tracheostomy kits etc.).

Drugs

Drugs used to induce anaesthesia can be simplified into three main categories and must be administered sequentially.

Pre-medication

These are usually opioids or benzodiazepines. They work synergistically with induction agents, reducing the dose required for surgical anaesthesia and minimising sympathetic stimulation (e.g. tachycardia and hypertension to a stimulus such as laryngoscopy).

Anaesthetic (induction) agents

Choice of agents include:

  • Intravenous: propofol, ketamine, thiopental, etomidate
  • Gas: sevoflurane, desflurane, nitrous oxide

Some agents (e.g. propofol, sevoflurane and nitrous oxide) are used to both induce and maintain an anaesthetic.Β 

Paralytic/muscle relaxant agents

These agents are not required for all general anaesthetics. They facilitate intubation and improve surgical access (particularly abdominal and laparoscopic procedures).

Suxamethonium and rocuronium in appropriate doses have the fastest onset (60 seconds and 90 seconds, respectively) and are commonly used in rapid sequence inductions.Β 

General anaesthetic induction procedure

A simple general anaesthetic induction β€˜recipe’ for tracheal intubation in a fit and well patient usually incorporates a quick-acting opioid (e.g. fentanyl) and propofol.

Loss of consciousness is confirmed by loss of vocal response/jaw thrust/eyelash reflex. The time taken for this to occur is within one arm-brain cycle. This is the time the drug takes to travel from the point of administration (e.g. cannula in hand) to the effect site (brain). This can vary between individuals, notably amongst the critically ill population and extremes of physiological age. Following the loss of consciousness, the paralytic agent is administered.

Tube placement within the trachea is confirmed with three essential checks whilst delivering 5 test breaths:

  • Symmetrical chest wall movement
  • Misting within the tube
  • More than five waveforms on capnography

Once tube placement is confirmed, the tube length at the lip or teeth is noted. It is secured with a cloth tie or tape to prevent tube migration which may lead to endobronchial intubation or accidental extubation. The patient is then connected to the ventilator.


Maintenance

After induction, the aim is to maintain a pain-free, unconscious state throughout surgery while ensuring physiological stability.

The choice of maintenance technique depends on the type and duration of surgery and clinician preference.

Maintaining a state of unconsciousness

This is achieved via intravenous accessΒ or inhalation, commonly using a volatile (gas) agent such as sevoflurane delivered into the breathing circuit via a vaporiser attached to the ventilator.

More recently, there has been a move towards total intravenous anaesthesia (TIVA) involving continuous, rapidly titratable IV infusions. This is commonly an anaesthetic agent (e.g. propofol) plus a rapid-acting opioid such as remifentanil. TIVA has numerous advantages, including improved recovery profiles and reduced greenhouse gas emissions.

Maintaining analgesia

It is important to distinguish between the unconscious state and being pain-free – the patient can be unconscious but still experience pain, and you will see the effects of this reflected in physiological parameters such as tachycardia and hypertension.

Anaesthetists aim to provide a β€˜multimodal analgesia’ strategy for managing pain. This involves administering medications from multiple pharmacological classes to provide effective analgesia and reduce opioid requirements.

For example, a commonly used combination for minor procedures in a healthy patient with no contraindications may include paracetamol, a non-steroidal, local anaesthetic to the surgical site and breakthrough opioids if required.

Other techniques for maintaining a pain-free intraoperative state include:

  • Bolus or continuous infusion of opioids
  • Regional techniques (e.g. via peripheral nerve blockade or a central neuraxial block – spinal or epidural).

Intraoperative monitoring

Continuous intraoperative monitoring of vital signs is mandatory, including blood pressure, heart rate, oxygen saturation, and end-tidal CO2 (capnography).

Other monitoring essential to safe anaesthetic practice includes depth of anaesthesia monitoring and neuromuscular blockade assessment.

Depth of anaesthesia is typically measured using a bispectral index (BIS) monitor, which analyses the brain’s electrical activity (EEG). Alternatively, mean alveolar concentration (MAC) can be used if volatile agents are used to maintain anaesthesia.

The degree of the neuromuscular blockade can be assessed using a peripheral nerve stimulator with leads typically over the facial or ulnar nerve.

The most common physiological change you can expect with a standard general anaesthetic is hypotension due to the potent vasodilator effects of most induction agents (except ketamine and etomidate). This must be identified and treated promptly to ensure adequate organ perfusion.

To deliver a safe anaesthetic, the anaesthetist must be familiar with commonly used drugs in anaesthesia, including doses, presentation and reconstitution, anticipated and adverse effects, as well as pharmacokinetic and pharmacodynamic properties of these drugs.

Emergency drugs include suxamethonium to quickly achieve muscle relaxation, antimuscarinics (atropine, glycopyrronium) to increase heart rate, vasopressors (ephedrine, metaraminol) to maintain blood pressure and adrenaline if the above agents fail, or cardiac arrest occurs.

For more information, see our guide to anaesthetic emergencies.


Summary

There are many considerations to the induction and maintenance of general anaesthesia. Β This includes patient, surgical and equipment factors, drug administration, and potential complications.

It is important to be aware of these critical steps in the induction and maintenance of a general anaesthetic:

  • Pre-operative and airway assessment
  • Preparation for the procedure, including emergency airway and drugs
  • Endotracheal tube placement checks
  • Intraoperative care goals, including physiological stability and monitoring required to achieve this

Editor

Dr Chris Jefferies


References

  1. Association of Anaesthetists. Recommendations for standards of monitoring during anaesthesia and recovery (2021) Available from: [LINK]

 

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