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Introduction to General Anaesthesia

Pre-operative assessment

  • Usually done at pre-assessment clinic, and the anaesthetist will do a quick check on the morning of theatre. Very important as it is the anaesthetist’s job to assess suitability for anaesthesia.
  • History: what operation having and why, thorough PMH including chronic lung disease and smoking, hypertension or heart disease, surgical history, anaesthetic complications and dental problems, FH of anaesthetic complications, any allergies.
  • Examination: cardiovascular and respiratory. Assess for difficulty of intubation: examine mouth and give Mallampati grading (a scale taking into account the visibility of structures in the oropharynx), measure thryomental distance (mental process to thyroid cartilage, less than 6cm equates to a difficult intubation), examine cervical spine for mobility.
  • Investigations: consider the patient before ordering. In a young healthy individual, FBC is sufficient [1], but for the older and more ill patient then U's and E's (diabetes or renal failure), LFT's (liver disease, alcoholic), blood glucose, (diabetic) blood crossmatch (for larger procedures), INR (if on warfarin, bleeding disorder), sickle cell screen (Afro-Caribbean or positive family history) are all reasonable. Order an ECG if over 50, do lung function tests in chronic lung disease, and do a CXR if known cardio-respiratory disease.
  • In some units U+Es are being done routinely on all surgical patients, to help guide prescribing of anti-coagulants post-op.
  • MSUs are compulsory for all orthopaedic patients.
  • Pregnancy tests are compulsory for certain females - your hospital will usually have a protocol to follow.
  • On the basis of this, give the patient an ASA grade and then decide if this patient is fit to be operated on. If patient is complicated, refer to a senior anaesthetist's pre-operative assessment clinic. If there are untreated medical conditions, e.g. asthma, hypertension, UTI, get these controlled.
  • Explain the procedure, tell them not to eat for 6h before, or drink clear fluids for 2h before, and inform patient of risks of anaesthetic: aspiration, MI, nerve injury, strokes, pneumonia, damage to teeth, sore throat, PONV. Give them some booklets to take home if worried.
  • Some medications will need to be stopped prior to theatre; for example, ACE inhibitors are often requested to be omitted the morning before surgery, anti-platelets are usually stopped 7 days before surgery, and herbal remedies such as garlic or St John's Wort need to be stopped 2 weeks before!

 

    Preparation for theatre

    • Go and see the patients, alleviate any last minute worries, check any points not covered in pre-op assessment.
    • Check the site and side of surgery.
    • Check if patient has signed consent form, and inform surgeon if they haven't.
    • A benzodiazepine could now be prescribed to be taken orally as pre-medication an hour pre-operatively. In an emergency patient, give some PO omeprazole to prevent damage caused by aspiration of gastric contents.
    • Check anaesthetic equipment is functioning normally.
    • Draw up the necessary medications: this means checking the medications for name and date with another health care professional, breaking the tops of the ampoules and filling up the syringes.
    • Patient brought into the anaesthetic room. Operating Department Practitioner (ODP) will run through a check-list with the patient for last minute safety, including checking the patient's identity and side and site of operation.

     

      Anaesthetic induction

      • Pre-oxygenate the patient with a face mask with 100% oxygen.
      • Obtain IV access (put a pink cannula in the dorsal metacarpal veins on the back of the hand) and give some diazepam IV to calm the patient, if not taken orally earlier. Also give any required prophylactic antibiotics.
      • Attatch pulse oximeter, ECG leads (three leads used, red on right shoulder, yellow on left shoulder and green on left lower chest) and blood pressure cuff with the assistance of the ODP.
      • Give the induction agent, usually propofol, (others include ketamine, thiopental sodium, and etomidate), a lipid soluble drug that swiftly affects the brain, but causes cardiovascular and respiratory depression. Induction agents have a short half life, which is why the maintenance gases are required. The patient has achieved “surgical anaesthesia” when their muscle tone is reduced, activity of the intercostal muscles is reduced and their pupils dilate.
      • If patient is a child or severely needle phobic, give a gas induction e.g sevoflurane in oxygen, then obtain IV access once anaesthetised.
      • Manage the airway initially by performing a head-tilt chin lift and jaw thrust, then by intubating the patient by opening the mouth and oropharynx with a laryngoscope and passing in an endotracheal tube and inflating the cuff, or by placing an LMA over the larynx and inflating the cuff. If intubated, watch the chest and auscultate the lungs to ensure the tube is in the trachea, not the oesophagus or a bronchus. In an emergency, put pressure over the cricoid before attempting intubation to prevent aspiration (rapid sequence induction).
      • The patient should now be inhaling oxygen and the maintenance anaesthetic gas, usually a flurane e.g isoflurane, desflurane, sevoflurane, and nitrous oxide may be used as a carrier gas. The amount required of these medications is dependant on their Minimum Alveolar Concentration (MAC), which is the concentration required to prevent movement in 50% of patients when a scalpel is used on them, and is lower in the elderly, opioid use and hypotension.
      • Positive pressure ventilation is used to force air into the lungs.
      • Administer a muscle relaxant only if the patient has an endotracheal tube and a secured airway. The choice is between a depolarizing agent e.g. suxamethonium, which depolarizes the post-synaptic membrane, or a non-depolarizing agent such as atracurium, which competes with acetylcholine at the neuromuscular junction.
      • Transport the patient through to theatre with the assistance of the ODP's and scrub nurses.
      • Re-attach the monitoring equipment and ventilatory equipment.
      • Put sandbags under areas vulnerable to nerve damage.
      • Tape the eyes closed.
      • Cover patient with a blanket, and use warm IV fluids to prevent hypothermia.
      • Start a litre bag of IV Hartmann's solution to maintain adequate hydration, decide on what time period based on predicted fluid losses.

       

        Maintenance of anaesthesia

        • Write in the notes the medications used, the starting observations, and how cannulation was completed.
        • Monitor the patients observations and record regularly. The monitor should show lead 2 of a 12 lead ECG, heart rate, the blood pressure and mean systolic pressure, pulse oximeter waveform and saturation of oxygen, capnometry and the temperature.
        • If an arterial or central venous line is in place, then the direct blood pressure and central venous pressure can also be observed respectively.
        • If an alarm goes off, check the monitor, check attachment of equipment to patient and then start considering various causes of the problem.
        • Remember, the patient should be monitored by an anaesthetist throughout the operation.
        • Keep spare medications in a kidney dish with you.
        • Towards the end of the operation, administer IV fentanyl for pain relief. Give an anti-emetic, e.g. IV cyclizine to combat post op nausea and vomiting (PONV).
        • Temperature rising? Could be malignant hyperpyrexia induced by suxamethonium. Call senior and get ice packs on the patient, phone ITU.
        • Suspected anaphylaxis? Suspect if severely hypotensive or severe bronchospasm, flushing, hypoxaemia. Discontinue drugs likely to cause this, maintain a patent airway and oxygenise, call for senior help, give IV adrenaline, start IV colloids.
        • Aspiration of gastric contents? Suspect if the patient has gastric contents in their endotracheal tube, or if they are coughing on induction or recovery. Call for senior help, aspirate visible material and maintain airway.

         

          Recovery

          • Turn off the inhaled anaesthetics, increase the oxygen levels to flush the anaesthetic out, and allow the patient to begin to regain conciousness.
          • When the patient is regaining conciousness, reverse atracurium with neostigmine, an anticholinesterase, and also give glycopyrrolate, a muscarinic anticholinergic, to prevent increases in acetylcholine affecting these receptors. Suxamethonium is spontaneously cleared.
          • When the patient starts to cough or choke on the endotracheal tube or LMA, remove these.
          • Transfer the patient onto a bed and prescribe medication for the recovery nurses to administer, usually IV morphine, and more fluids if needed. Remove LMA or endotracheal tube, remembering to suction up saliva to prevent aspiration. Place an oropharyngeal airway in the mouth to maintain airway until fully concious.
          • Give oxygen via a facemask (either simple or non-rebreather, depending on requirements) and keep the patient warm with blankets. Keep on Hartmann's fluid.
          • PONV? try another anti-emetic.
          • Pain? try another analgesic, e.g. IV paracetemol, a PCA or a nerve block.
          • Slow to recover? Consider suxamethonium apnoea, where the enzyme to remove suxamthonium is lacking and therefore it takes a long time to metabolise. Keep them anaesthetised and ventilated.
          • Hypertensive? Call senior and give a vasodilator.
          • Hypotensive? Are they on sufficient fluids? Check for other signs of shock.
          • Atelactasis and pneumonia? Oxygenate well, start antibiotics and inform chest physiotherapists. Good analgesia allows the patient to breathe deeply and cough up any chest phlegm, unimpeded by pain [2].

           

            References

            1. NICE. The use of routine preoperative tests for elective surgery. http://www.nice.org.uk/Guidance/CG3 (accessed 9th March 2011).
            2. Gwinnutt C. Lecture notes: Clinical Anaesthesia. 3rd edition. Sussex: Wiley-Blackwell; 2008.
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