INTRAOPERATIVE PRINCIPLES

LEARNING OBJECTIVES

  • To explain the basic principles of monitoring in anaesthesia
  • To outline the key drugs used in general anaesthesia
  • To describe the principles of fluid resuscitation and apply this to the management of a patient
  • To outline the principles in neuraxial anaesthesia/analgesia and to explain the peri-operative implications of this
  • To demonstrate the ability to perform basic airway manoeuvres
  • To demonstrate the ability to perform peripheral intravenous cannulation

GENERAL ANAESTHESIA

General anaesthesia is a drug-induced state of reversible unrousable unconsciousness with absence of recall.

When giving a general anaesthetic, a combination of different agents are used, including benzodiazepines, hypnotic agents, opioids, and muscle relaxants. Due to side effects of some of these drugs (e.g. nausea/vomiting, hypotension), it is common to use anti-emetics as well as emergency drugs such as metaraminol. See the figure below for a summary of agents used in a general anaesthetic:

Triad of Anaesthesia

  • Hypnosis – unconsciousness
  • Analgesia – pain relief
  • Muscle relaxation – apnoea and to aid surgical access

Conduct of Anaesthesia

Induction:

  • From consciousness to ‘surgical plane’ of anaesthesia (loss of protective reflexes)
  • Usually intravenous agent

Maintenance

  • Physiological stability for procedure
  • A compromise between cardiovascular/respiratory depressant and response to surgical stimulation
  • Usually inhalational agent, though use of intravenous agent (i.e. propofol) is also reasonable

Emergence

  • From anaesthesia to restoration of consciousness (with return of airway reflexes) and reversal of muscle relaxation
  • Often the most dangerous period of the anaesthetic

Depth of Anaesthesia

To determine the depth of anaesthesia, the anaesthetist relies on physical signs elicited from the patient. In 1937, Guedel described a detailed classification based on the use of depth of anaesthesia with ether (paralysing agents were not available) in patients premedicated with morphine and atropine. Despite EEG monitoring systems and changes to anaesthetic pharmacology, these stages are still useful to be aware of.

Guedel’s Stages of Anaesthesia

  1. Stage I (stage of analgesia or disorientation)
    • From beginning of induction of general anaesthesia to loss of consciousness
  2. Stage II (stage of excitement or delerium)
    • From loss of consciousness to onset of automatic breathing
    • Eye lash reflexes disappear but other reflexes remain intact and coughing and vomiting may occur; respiration can be irregular with breath holding
  3. Stage III (stage of surgical anaesthesia)
    • From onset of autonomic respiration to respiratory paralysis
    • It is divided into 4 planes:
    • Plane I – From onset of automatic respiration to cessation of eyeball movements. Eyelid reflex is lost, swallowing reflex disappears.
    • Plane II – From cessation of eyeball movements to beginning of paralysis of intercostal muscles. Laryngeal reflex is lost although inflammation of the upper respiratory tract increases reflex irritability, secretion of tears increases (useful sign of light anaesthesia), respiration is automatic and regular, movement and deep breathing as a response to skin stimulation disappears.
    • Plane III – From beginning to completion of intercostal muscle paralysis. Diaphragmatic respiration persists but there is a progressive intercostal paralysis, pupils dilated and light reflex is abolished. This was the desired plane for surgery when muscle relaxants were not used.
    • Plane IV – from complete intercostal paralysis to diaphragmatic paralysis (apnoea).
  4. Stage IV: (stage of anaesthetic overdose)
    • From halting of respiration till death
    • Anaesthetic overdose causes medullary paralysis with respiratory arrest and vasomotor collapse. Pupils are widely dilated and muscles are relaxed.

REGIONAL ANAESTHESIA

Neuraxial regional anaesthesia

  1. Spinal
  2. Epidural

Non-neuraxial anaesthesia

Here is a 15 minute lecture that explains regional anaesthesia including spinals, epidurals and limb blocks.

Of particular relevance in the pre- and post-operative periods is the use anticoagulants, antiplatelets, or patients with abnormal coagulation due to other reasons. At the end of this page is a link to a joint publication by the AAGBI, the Obstetrics Anaesthetists’ Association, and Regional Anaesthesia UK with clear guidelines on how to manage such patients.

AIRWAY MANAGEMENT

With the induction of anaesthesia it is important to be familiar with both basic and advanced airway management.

Here is a short clip on basic airway management, which includes chin lift/jaw thrust, bag-mask ventilation (+/- adjuncts, i.e. oropharyngeal/nasopharyngeal airways), and insertion of a laryngeal mask (LMA):

Here are two short clips on intubation:

MONITORING AND EQUIPMENT

The anaesthetist must be present and care for the patient throughout the conduct of an anaesthetic. Monitoring devices must be attached before induction and continued until the patient has recovered from the effects of anaesthesia. A summary of the information gathered should be recorded on an anaesthetic chart at five-minute intervals.

Clinical observations include: palpation of the pulse, movements of the chest wall or auscultation of breath sounds, mucosal colour, pupil size, autonomic response to surgical stimuli. Where appropriate, measurement of urine output and blood loss should also occur. Monitoring devices supplement clinical observation and so audible and visual alarms must be set to appropriate values.

Monitoring in theatre:

Essential

  • Non-invasive blood pressure
  • ECG
  • Inspired oxygen concentration (FiO2)
  • Oxygen saturations
  • Airway gas analysis
  • Airway pressures

Available:

  • Temperature
  • Nerve stimulator

If appropriate:

  • Invasive arterial blood pressure
  • Central venous pressure
  • Cardiac output monitoring
  • Depth of anaesthesia monitoring: BIS (Bispectral index), Entropy
  • Urine output
  • Blood loss

Patient safety

  • Eye protection
  • Positioning and pressure area care
  • DVT prophylaxis – TEDS, pneumatic calf-compressors
  • Warming – forced air warming blanket, fluid warmer

Regional Anaesthesia and Patients with Abnormalities in Coagulation

https://anaesthetists.org/Home/Resources-publications/Guidelines/Regional-anaesthesia-and-patients-with-abnormalities-of-coagulation

Anaesthesia Basics http://www.anaesthesia.med.usyd.edu.au/resources/lectures/anaesthesia_basics.html