types of anaesthesia

• Pre-op anaesthetic assessement 

  

  Used to establish rapport with patient, make anaesthetic plan, inform patient about the anaesthetic and its risks, and answer any questions.

  History

  Establish basic details:
  • Patient ID.
  • Type of surgery and side.
  • Type of anaesthetic to be used: general, regional, or local.
  Anaesthetic history:
  • Any previous anaesthesia, and any problems encountered.
  • Ask about post-op nausea and vomiting (PONV), airway problems, and post-op ICU admission.
  Family anaesthetic history:
  • Anybody have difficulty during an operation?
  • There are 2 important familial anaesthetic-related conditions. Suxamethonium apnoea, due to a pseudocholinesterase deficiency which prevents suxamethonium breakdown, and malignant hyperthermia, which is triggered by volatile anaesthetic agents or suxamethonium.
  Past medical history. Focus on:
  • Current and recent health: any infection, cold, fever.
  • Cardiovascular: HTN, IHD, HF, AF, surgery or pacemakers. Pacemakers need to be switched off as may be activated by diathermy.
  • Respiratory: asthma or COPD – including hospital admissions, steroids, and home O2 – and obstructive sleep apnoea.
  • Gastrointestinal: GORD, hiatus hernia. May need PPIs to prevent intra-operative aspiration, as anaesthesia relaxes the lower oesophageal sphincter.
  • Nervous system: previous CVA, epilepsy and seizure frequency.
  • MSK: RA and ankylosing spondylitis, which have implications for airway and positioning.
  • Other systems: CKD, liver disease, diabetes, thyroid disease, coagulopathy, anaemia.
  Dental:
  • Caps or crowns.
  • False teeth.
  Medications and allergies:
  • If on steroids, may need extra to prevent Addisonian crisis.
  Social history:
  • Smoking history.
  • Alcohol intake.
  • Functional status and exercise tolerance including walking distance, getting up stairs etc.
  Fasting times:
  • Ask when they last ate or drank. A sip for their tablets is fine.
  • Latest recommended times: food or unclear fluid (milk) 6 hours, breast milk 4 hours, clear fluids or chewing gum 2 hours.

  Examination

  Neck exam for airway assessment:
  • Open mouth for Mallampati score, which gives an indication of intubation difficulty based on how visible the uvula is (without saying 'ahh'). Less visible = harder to intubate.
  • Check flexion and extension, to assess ease of head tilt-chin lift.
  • Measure thyromental distance: <6.5 cm (4 fingers) suggests difficult intubation.
  • Check jaw protrusion: can they move bottom teeth in front of top. If not, jaw thrust may be difficult.
  Also consider cardio, respiratory, and neuro exams.

  Investigations

  • Pre-op bloods: FBC, U&E, LFT, G+S.
  • Pre-op imaging: ECG, echo, CXR.
  • Extent of testing depends on patient and procedure. Minimal basic bloods may be sufficient for young healthy patients.

• General anaesthesia

  Overview

  • The three components of general anaesthesia are hypnosis, analgesia, and paralysis. Different agents achieve each of these goals, but in reality each agent may produce aspects of the other e.g. high dose opioids cause hypnosis as well as analgesia. Combining them reduces toxicity from any one.
  • A general anaesthetic begins with a pre-op assessment, pre-medication (if needed), followed by the anaesthetic itself. This begins with induction into the anaesthetised state, followed by its maintenance, and finally waking up and extubation.

  Anaesthetic plan

  Plan should be formulated for each patient.
  Pre-operative care plan:
  • Optimisation for surgery and anaesthesia.
  • Pre-medication may involve anxiolytic, antiemetics, analgesics, antacids, and antisialogogues (e.g. kids with a cold). For most patients, nothing is required
  Intra-operative care plan:
  • Type of anaesthesia: general, regional, or local.
  • Airway plan: adjunct, LMA, or ET tube.
  • Breathing plan: spontaneous, ventilated, or assisted.
  • Circulation plan: IV cannula, arterial line, and/or central venous catheter.
  • Environment plan: positioning to prevent pressure damage, and temperature management. Heating device (e.g. Bair Hugger) often needed for longer procedures.
  Post-operative care plan:
  • Antiemetic and analgesia plan.
  • Post-recovery room destination: home (day case), ward, or critical care.

  Beginning

  Induction

  Can be gas or IV.
  IV induction:
  • IV injection of propofol, thiopentone, or (rarely) etomidate. Work through potentiating (propofol) or activating (thiopentone, etomidate) GABAA receptors.
  • Commonest approach.
  • Quicker and more controlled, as the exact amount given is known.
  • Propofol and thiopentone cause ↓BP, so avoid in hypovolaemic patients.
  Gas induction:
  • Inhalation of a hypnotic volatile agent such as sevoflurane or desflurane in oxygen. Volatile agents are liquid at room temperature but easily evaporate to form a vapour.
  • Slower, and generally used to avoid the distress of IV cannulation e.g. in kids or needle phobics.
  • Patient may be seen to pass through the planes of anaesthesia, from 1 to 4, while IV is too quick to see.
  Extras:
  • An analgesic such as IV morphine may be given just before.
  • Nitrous oxide may be used in combination with other agents to reduce the dose required.

  Intubation

  • Protects the airway while the patient is in a state of unconsciousness and muscle paralysis. Connected to mechanical ventilator as the patient's respiratory muscles are paralysed.
  • Patient is pre-oxygenated with bag-valve ventilation to maximize the O2 capacity in the lungs, giving a several minute window of opportunity to safely attempt intubation. An airway adjunct may also be used at this point.
  • Laryngoscope blade is passed – with the left hand – along the right side of the tongue, until the tip sits in the vallecula, and is manipulated such that the vocal cords are visualised. This usually involves applying ventral pressure on the mandible, while taking care to avoid levering the laryngoscope blade off the top teeth.
  • An endotracheal (ET) tube is then passed so that its tip sits a few centimetres below the vocal cords. The cuff is inflated to secure it in place, aid positive pressure ventilation, and prevent aspiration.

  Paralysis (neuromuscular blockade)

  Achieved with a depolarising or non-depolarising muscle relaxant.
  Depolarising agents:
  • In practice, suxamethonium (aka succinylcholine) is the only agent now used.
  • Causes depolarisation of the motor end-plate at the neuromuscular junction (NMJ), leading to an initial contraction – twitching spreading from eyes to arms then toes – then paralysis as the membrane cannot repolarise.
  • Short-acting.
  Non-depolarising agents:
  • Drugs: rocuronium, atracurium.
  • Most are based on the naturally occurring curare, hence have 'cur' somewhere in the name, which block the nicotinic ACh receptors at the NMJ.
  • Longer-acting.
  Typically given before intubation to aid the passage of the laryngoscope and ET tube.

  During the operation

  Maintenance

  Again, this can be gas or IV.
  Gas maintenance:
  • Commonest approach.
  • Minimal risk of awareness during surgery, as the anaesthetic machine can measure how much is inhaled and exhaled. Aim is the minimum alveolar concentration, the concentration of vapour in the lungs that prevents movement in 50% of people. In practice, the use of other agents – analgesics and muscle relaxants – ensure nearly all patients are anaesthetised at this level.
  Total intravenous anaesthesia (TIVA):
  • Requires a continuous infusion, as propofol is relatively short-acting (5-10 minutes).
  • There is a risk of IV access failing – e.g. cannula tissues, kink in the line – which could lead to awareness during surgery. The paralysed patient would be unable to alert anyone. In reality, the risk is extremely low.
  • Less post-op hangover, nausea, and vomiting than volatile agents, and a quicker waking up.
  • Does not require an anaesthetic machine, so particularly useful outside of theatre e.g. ECT, MRI.

  Ventilation

  • Intubated patients are mechanically ventilated.
  • The cuffed ET tube allows a closed loop system, where O2 comes from the anaesthetic machine and no ambient air enters. Subsequently, only very small amounts of O2 are needed e.g. 300 ml/min, reflecting the small amount actually used during normal breathing. CO2 is removed from the exhaled air by reactive granules (e.g. soda lime).

  Other considerations

  • Paralysis is maintained with regular injections of muscle relaxant, and analgesia intermittently given.
  • Fluids, vasoconstrictors, inotropes, and chronotropes may need to be given due to the bradycardia and hypotension often caused by anaesthetic drugs. These effects of anaesthesia are one reason why patients with HTN, IHD, or the elderly with stiffer arteries are high risk. A sudden drop in BP may lead to dangerous hypoperfusion of areas which are dependant on high blood pressure.
  • Antibiotics and antiemetics may be given to reduce post-op complications.

  Ending

  Anaesthetic agent is withdrawn:
  • For gas maintenance, sevoflurane (quick) or desflurane (quickest) supply is turned off, and they wear off rapidly.
  • For TIVA, last propofol dose wears off after 5-10 mins.
  Ending paralysis:
  • If the operation was on time and doses given at the right point, the muscle relaxants will wear off around this time. Otherwise, they may need to be reversed by a combination of a cholinesterase inhibitor and an antimuscarinic. This combination ensures the increased ACh works only at the NMJ (nicotinic receptor), countering the muscle relaxant, and not in the parasympathetic nervous system (muscarinic receptor).
  • Ways to check that paralysis has ended: checking for spontaneous breathing by checking end tidal CO2; using a nerve stimulator; and/or clinically, by response to voice and observation of chest movements.
  Extubation:
  • Paralysis must have ended before the ET tube is removed and ventilation stopped.
  • Ideally done while the patient is still unconscious. However, may require the patient to be (partially) awake if state of paralysis and integrity of airway unclear.

• Post-operative nausea and vomiting (PONV)

  Pathophysiology

  Complex, but likely multifactorial, involving drug side effects, fasting, and abnormal positioning and motion.

  Risk factors

  • Surgical: gynae surgery, ENT surgery, neurosurgery, GI surgery, squint correction, long operations.
  • Patient: age, female sex, motion sickness, non-smokers.
  • Anaesthetic: opioids, volatiles, nitrous oxide.
  • Previous PONV.

  Management

  Aim to prevent rather than treat.
  Non-pharmacological:
  • Fluids
  • TIVA
  • Minimize pre-op fasting period.
  • Wound site local anaesthesia to minimize opioid use.
  Anti-emetics:
  • Can be given pre, intra, and/or post-operatively.
  • Commonly used drugs: ondansetron, cyclizine, dexamethasone, prochlorperazine, metoclopramide, droperidol.
  • High risk patients may receive combination therapy.

• Management of post-operative pain

  Mild and moderate pain

  • Pain ladder e.g. regular simple analgesia plus PRN weak opioid.

  Severe pain

  Patient-controlled analgesia (PCA):
  • Allows dose of IV analgesic every 5 minutes.
  • Usually morphine or fentanyl, the latter better in renal impairment.
  • Pros: patient autonomy, appropriate dosing, and reduces work of nurses.
  • Cons: expensive equipment, requires patient understanding, risk of fatal technical errors.
  Eventually wean off onto regular weak opioids and simple analgesia, and PRN Oramorph.
  Alternatives if there are severe opioid side effects:
  • Epidural
  • Nerve catheter infusion.
  • Wound infusion device.

  Discharge

  • Prescribe regular analgesia.
  • Provide written info on dose – including daily maximum – and side effects.
  • Inform GP about intended duration of analgesia.