Anaesthesia – general


Definitions


Anaesthesia (αυαισθεσια = Without Perception): 


1. a partial or complete loss of all forms of sensation caused by pathology in the nervous system; 2. a technique using drugs (inhalational, intravenous or local) that renders the whole or part of the organism insensible for variable periods of time. Analgesia: the loss of pain sensation. Hypnotic agent: a sleep-inducing drug. Muscle relaxant: a drug that reduces muscle tension by affecting the nerves that supply the muscles or the myoneuronal junction (e.g curare, succinylcholine). Sedation: the production of a calm and restful state by the administration of a drug.


General Anaesthesia: 


relies upon generalized suppression of some functions of the cerebral cortex to induce a generalized state of insensibility.


Regional Anaesthesia: 


relies upon blockage of nerve impulses or spinal transmission of impulses to induce analgesia and immobility.







Key Points


  • Fasting – while food should be avoided for several hours preoperatively, water may be given freely to most patients up to 2 hours before operation.
  • Pre-operative assessment and risk is based on the ASA classification and the urgency and complexity of surgery.
  • General anaesthesia comprises safe induction, active maintenance of anaesthesia and safe recovery.
  • Regional anaesthesia is preferred for many procedures, e.g. obstetrics, eye surgery, orthopaedics.
  • Spinal/epidural anaesthesia is contraindicated in the anticoagulated patients.





Pre-Operative Assessment


Prior to an operation the anaesthetist will assess the patient and devise a plan for anaesthesia based on the following:



  • The condition of the patient (ASA classification) determined by:


history

physical examination

selective investigations.


  • The complexity of the surgery to be performed.
  • The urgency of the procedure (emergency or elective).




















Class ASA pre-operative physical status classification
Class I Fit and healthy
Class II Mild systemic disease
Class III Severe systemic disease that is not incapacitating
Class IV Incapacitating systemic disease that is constantly life-threatening
Class V Moribund – not expected to survive >24 hours without surgery

ASA = American Society of Anesthesiologists.


General Anaesthesia


Pre-Operative Fasting



  • Rationale:


GA reduces reflexes that protect against aspiration of stomach contents into lungs

fasting reduces volume and acidity of gastric contents.


  • Adults:


no food for 6 hours pre op

may drink clear fluids up to 2 hours pre op – this may include carbohydrate supplements (as in ERAS programmes)

caution in elderly, pregnant, obese and patients with stomach disorder.


  • Children:


no food for 6 hours pre op

no breast milk for 4 hours

may drink clear fluids (water, apple juice) for up to 2 hours pre op.


  • Emergency surgery:


cricoid pressure is applied as a part of ‘rapid sequence’ intubation – the cricoid cartilage is pushed against the body of the sixth cervical vertebra, compressing the oesophagus to prevent passive regurgitation.

Aims and Technique



  • To induce a loss of consciousness using hypnotic drugs which may be administered intravenously (e.g. propofol) or by inhalation (e.g. sevoflurane).
  • To provide adequate operating conditions for the duration of the surgical procedure using balanced anaesthesia, i.e. a combination of hypnotic drugs to maintain anaesthesia (e.g. propofol, sevoflurane), analgesics for pain (e.g. opiates, NSAIDs) and, if indicated, muscle relaxants (e.g. suxamethonium, tubocurarine) or regional anaesthesia.
  • To maintain essential physiological function by:


providing a clear airway (laryngeal mask airway or tracheal tube ± IPPV)

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Apr 19, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Anaesthesia – general

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