Anaesthesia: Preoperative assessment and principles of anaesthesia

• Before surgery: The Perioperative Pathway | Preoperative Assessment | ASA Grade and Risk Scores | Preoperative Investigations | Cardiac and Respiratory Risk
• Optimising the patient: Medicines Before Surgery | Anticoagulants and Antiplatelets | Diabetes and Steroids | Fasting and Consent
• Principles of anaesthesia: Types of Anaesthesia | The Stages of General Anaesthesia | Spinal and Epidural Anaesthesia | Monitoring and the WHO Checklist | Special Groups — Elderly, Obese, Pregnant and Children
• Revision: Preoperative Assessment at a Glance | Spot the Diagnosis | Exam-style Scenarios | Key Points | Guidelines and Further Reading

🩺 Preoperative assessment and principles of anaesthesia. Good outcomes from surgery start long before theatre: finding and optimising medical problems, managing medicines, sharing decisions about risk and planning the anaesthetic. Doctors on surgical and medical wards are often asked to prepare patients for theatre, so these principles matter to everyone — not just anaesthetists.
Your four questions for every patient before surgery: How fit is this patient — and what is their risk? Can anything be optimised before surgery (and is it worth the delay)? What do I do with each of their medicines? What type of anaesthetic suits the patient and the operation — and have they consented to it?

The Perioperative Pathway

• Perioperative medicine — care from the moment surgery is considered until full recovery ("decision to recovery") — led in the UK by the Centre for Perioperative Care (CPOC).
• Stages:
  • Contemplation of surgery — shared decision-making (BRAN — Benefits, Risks, Alternatives, doing Nothing); "is surgery the right choice?".
  • Preoperative assessment clinic — nurse-led with anaesthetic input; screening, investigations, risk assessment, optimisation.
  • "Prehabilitation" — exercise, nutrition, smoking and alcohol cessation, anaemia correction, psychological support — the "teachable moment".
  • Day of surgery — admission, WHO checklist, anaesthesia and surgery.
  • Postoperative care — recovery, ward, enhanced care or critical care; enhanced recovery programmes.
  • Discharge and follow-up.
• Enhanced Recovery After Surgery (ERAS) — evidence-based bundle: patient education, minimal fasting with carbohydrate drinks, avoiding bowel preparation where possible, goal-directed fluids, minimally invasive surgery, opioid-sparing analgesia, PONV prevention, early eating, drinking and mobilisation, early removal of drains and catheters — shorter stay and fewer complications.
• Emergency surgery — less time to optimise; higher risk (NELA — National Emergency Laparotomy Audit — risk scoring, consultant-led care, critical care for high-risk patients).

Preoperative Assessment

• History:
  • Cardiovascular — IHD, heart failure, valve disease (aortic stenosis), arrhythmias, pacemaker or ICD, hypertension, exercise tolerance (can they climb 2 flights of stairs — about 4 METs?).
  • Respiratory — COPD, asthma, obstructive sleep apnoea (STOP-BANG), recent chest infection, smoking.
  • Other systems — diabetes, CKD, liver disease, neurological disease, reflux (aspiration risk), rheumatoid arthritis (atlanto-axial instability), bleeding disorders, frailty and cognition.
  • Previous anaesthetics — problems (difficult airway, PONV, awareness), family history — malignant hyperthermia, suxamethonium (butyrylcholinesterase) deficiency.
  • Drugs and allergies — including latex, chlorhexidine, antibiotics, neuromuscular blockers.
  • Social — smoking, alcohol, recreational drugs, home support.
• Examination — cardiovascular and respiratory examination, BMI, airway assessment (mouth opening, Mallampati score, thyromental distance, neck movement, dentition, jaw protrusion — see the airway page), venous access, sites for regional anaesthesia.
• Functional capacity — METs (Duke Activity Status Index), cardiopulmonary exercise testing (CPET) for major surgery — anaerobic threshold under 11 mL/kg/min and low peak VO₂ predict complications.
• Frailty — Clinical Frailty Scale (CFS 5 or more — higher risk); comprehensive geriatric assessment (POPS — Proactive care of Older People undergoing Surgery).
• Anaemia — investigate and treat before elective surgery (IV iron for iron deficiency) — patient blood management.

ASA Grade and Risk Scores

• ASA physical status classification (American Society of Anesthesiologists):

  ASA	Definition	Examples
  I	Normal healthy patient	Fit, non-smoker, minimal alcohol
  II	Mild systemic disease	Smoker, well-controlled hypertension or diabetes, BMI 30–40, pregnancy
  III	Severe systemic disease	Poorly controlled diabetes or hypertension, COPD, BMI 40 or more, dialysis, MI over 3 months ago, pacemaker
  IV	Severe systemic disease that is a constant threat to life	MI or stroke in the last 3 months, ongoing ischaemia, severe valve disease, sepsis, severely reduced ejection fraction
  V	Moribund — not expected to survive without the operation	Ruptured AAA, massive trauma
  VI	Brain-dead organ donor	—

• "E" is added for emergency surgery (e.g. ASA IIIE).
• Risk prediction tools:
  • Revised Cardiac Risk Index (Lee) — high-risk surgery, IHD, heart failure, cerebrovascular disease, insulin-treated diabetes, creatinine over 177 µmol/L.
  • SORT (Surgical Outcome Risk Tool) — 30-day mortality (UK).
  • NELA risk calculator — emergency laparotomy (over 5% predicted mortality = high risk — consultant surgeon and anaesthetist, critical care).
  • P-POSSUM — physiological and operative severity score.
  • Nottingham Hip Fracture Score.
• Surgical severity — minor, intermediate and major or complex (NICE NG45).

Preoperative Investigations

• NICE NG45 (routine preoperative tests for elective surgery) — tests depend on ASA grade and surgical grade — avoid unnecessary tests.
• Summary:

  Test	When (NICE NG45 — simplified)
  FBC	Major surgery; intermediate surgery in ASA III–IV with cardiovascular or renal disease or symptoms
  U&E	Major surgery in ASA II and above; intermediate surgery if at risk of AKI; ASA III–IV
  Clotting	Not routinely — only if liver disease, anticoagulants, bleeding history (ASA III–IV major surgery — consider)
  ECG	Major surgery over 65 years; ASA II–IV with cardiovascular, renal or diabetic disease
  HbA1c	People with diabetes (within the last 3 months)
  Pregnancy test	Offer to all women who could be pregnant (with consent)
  Sickle cell test	Not routine — ask about family history and status
  Echocardiogram	Murmur with symptoms, signs of heart failure
  Lung function / ABG	Only if needed (e.g. severe lung disease, lung resection)
  Chest X-ray	Not routine

• MRSA screening — per local policy (elective admissions, high-risk surgery); COVID testing per current policy.
• Group and save / cross-match — depending on expected blood loss (maximum surgical blood ordering schedule).
• BNP or NT-proBNP — used for cardiac risk stratification before non-cardiac surgery (ESC 2022).

Cardiac and Respiratory Risk

• Recent MI — delay elective surgery for at least 2 months (ESC 2022 — ideally longer for high-risk surgery).
• Coronary stents — delay elective surgery for 6 months after elective PCI with a drug-eluting stent (3 months may be acceptable if urgent) and 12 months after ACS, to complete dual antiplatelet therapy; never stop DAPT early without cardiology advice (stent thrombosis — often fatal).
• Heart failure — optimise before surgery; decompensated heart failure — postpone elective surgery.
• Aortic stenosis — severe or symptomatic — risk of cardiovascular collapse (fixed cardiac output — avoid vasodilatation and tachycardia; careful spinal anaesthesia); echo; consider valve intervention first (TAVI).
• Hypertension — postpone elective surgery if BP is 180/110 mmHg or more (AAGBI/BHS) — primary care readings under 160/100 in the last 12 months are acceptable.
• Pacemakers and ICDs — check device and indication; diathermy interference — bipolar diathermy; ICDs deactivated (magnet or reprogramming) with external defibrillation available; check afterwards.
• Postoperative pulmonary complications — atelectasis, pneumonia, respiratory failure — risk factors: upper abdominal or thoracic surgery, COPD, smoking, obesity, OSA, age, ASA III or above, prolonged surgery.
• Reducing respiratory risk — stop smoking (ideally 8 weeks, but any time helps), optimise COPD and asthma, treat infection (postpone elective surgery for 2–4 weeks after a URTI if possible — longer after severe chest infection), CPAP for OSA, regional anaesthesia, good analgesia, early mobilisation, physiotherapy.
• Obstructive sleep apnoea — STOP-BANG (Snoring, Tired, Observed apnoea, Pressure (hypertension), BMI over 35, Age over 50, Neck over 40 cm, male Gender) — 5 or more high risk; bring own CPAP; minimise opioids and sedatives; monitored bed.

Medicines Before Surgery

Anticoagulants and Antiplatelets

• Balance — bleeding risk of the operation vs thrombotic risk of stopping.
• Warfarin:
  • Stop 5 days before surgery; check INR the day before (vitamin K 1–5 mg oral if INR 1.5 or more).
  • Bridging with LMWH only for high thrombotic risk — mechanical mitral valve, recent VTE (within 3 months), recent stroke, AF with very high CHA₂DS₂-VASc and previous stroke (BRIDGE trial — no bridging for most AF).
  • Emergency reversal — IV vitamin K plus prothrombin complex concentrate (PCC).
• DOACs (apixaban, rivaroxaban, edoxaban, dabigatran) — stop 24–48 hours before surgery (longer for dabigatran in renal impairment and high-bleeding-risk surgery or neuraxial anaesthesia — e.g. 72 hours or more); no bridging; reversal — idarucizumab (dabigatran), andexanet alfa (factor Xa inhibitors — limited use) or PCC.
• Aspirin — usually continued (stop 7 days before for some surgery — intracranial, spinal, prostate, posterior eye).
• Clopidogrel, prasugrel, ticagrelor — stop 5–7 days before (prasugrel 7) if safe — but not within the stent window without cardiology discussion.
• Restart — when haemostasis is secure (often 24–72 hours after surgery); thromboprophylaxis in the meantime.
• Neuraxial (spinal or epidural) anaesthesia — strict timing rules for LMWH and anticoagulants (e.g. at least 12 hours after a prophylactic LMWH dose and 24 hours after a treatment dose) — epidural haematoma risk.
• VTE prophylaxis (NICE NG89) — assess every surgical patient; LMWH and/or mechanical prophylaxis (anti-embolism stockings, intermittent pneumatic compression); extended prophylaxis after major cancer, hip and knee surgery.

Diabetes and Steroids

• Diabetes (CPOC 2022/2023 guideline):
  • HbA1c over 69 mmol/mol (8.5%) — consider optimising before elective surgery (higher infection and complication risk).
  • First on the list; minimise fasting.
  • Short starvation (one missed meal) — modify usual medication (reduced long-acting insulin dose — e.g. 80% of the evening or morning dose; omit short-acting insulin with missed meals; omit sulfonylureas on the day; SGLT2 inhibitors omitted earlier).
  • Longer starvation (more than one missed meal), poorly controlled diabetes or emergency surgery — variable rate IV insulin infusion (VRIII) with glucose-containing fluid (e.g. 5% glucose with 0.45% saline and potassium); continue long-acting basal insulin.
  • Target capillary glucose 6–10 mmol/L (4–12 acceptable); check hourly in theatre.
  • Metformin — usually continued unless contrast or AKI risk or more than one missed meal.
• Steroids and adrenal suppression (Association of Anaesthetists / Society for Endocrinology 2020):
  • Patients on prednisolone 5 mg or more daily for over 4 weeks (or equivalent) may have adrenal suppression — risk of adrenal crisis (hypotension).
  • Continue the usual steroid on the day; give IV hydrocortisone 100 mg at induction for major surgery, then 200 mg over 24 hours (infusion or 50 mg 6-hourly) until eating, then double oral dose for 48 hours (minor surgery — smaller doses).
  • Same approach for Addison's disease and hypopituitarism.

Fasting and Consent

• Fasting rules (elective surgery — "6-4-1" or "6-2"):
  • Food (including milk drinks) — 6 hours.
  • Breast milk — 4 hours (3 hours in some newer paediatric guidance); formula — 6 hours.
  • Clear fluids (water, squash, black tea or coffee) — 2 hours in adults; 1 hour in children (APA 2018 consensus) — and increasingly adults (some guidance allows sips until sent for).
  • Chewing gum — don't delay surgery; essential medicines with a sip of water.
• Aspiration risk factors — emergency surgery, bowel obstruction, pregnancy (second half), hiatus hernia and reflux, obesity, diabetes (gastroparesis), GLP-1 agonists, opioids, reduced consciousness — rapid sequence induction.
• Avoid prolonged fasting — dehydration, hypoglycaemia, discomfort; preoperative carbohydrate drinks (ERAS).
• Consent (GMC; Montgomery 2015):
  • Discuss material risks — what a reasonable person in the patient's position would want to know, and what this patient would want to know.
  • Anaesthetic consent — anaesthesia type, sore throat, PONV, dental damage, awareness (about 1 in 19,000 — NAP5), nerve damage with blocks, serious complications (death about 1 in 100,000 for anaesthesia in healthy patients), blood transfusion.
  • Capacity (Mental Capacity Act 2005) — best interests decision if lacking capacity (consent form 4); LPA for health and welfare; IMCA if no one to consult.
  • Children — Gillick competence; parents with parental responsibility.
  • Jehovah's Witnesses — usually refuse blood (discuss acceptable products — cell salvage, albumin, clotting factors — and document; advance decision).

Types of Anaesthesia

• Regional vs general — regional anaesthesia avoids airway instrumentation, reduces opioid use, PONV and possibly delirium (hip fracture — no clear mortality difference — REGAIN trial).
• The "triad of anaesthesia" — hypnosis (unconsciousness), analgesia and muscle relaxation — plus reflex suppression.

The Stages of General Anaesthesia

• Preparation — check machine and drugs, monitoring, IV access, preoxygenation (100% oxygen for 3 minutes or until end-tidal O₂ over 85–90% — fills the functional residual capacity with oxygen and extends safe apnoea time).
• Induction:
  • IV induction (most adults) — e.g. propofol with an opioid (fentanyl or remifentanil).
  • Inhalational (gas) induction — sevoflurane — children, needle-phobic patients, some upper airway obstruction.
  • Rapid sequence induction (RSI) — when aspiration risk is high — preoxygenation, fixed doses of a fast-acting hypnotic and a fast-acting neuromuscular blocker (rocuronium or suxamethonium), no bag-mask ventilation traditionally (gentle ventilation is now often used), (cricoid pressure — now optional), intubation with a cuffed tube.
• Airway management — face mask, supraglottic airway device (e.g. i-gel, LMA) or tracheal tube.
• Maintenance — volatile agent (sevoflurane, desflurane — environmental impact; isoflurane) in oxygen and air, or TIVA (total IV anaesthesia — propofol and remifentanil infusions — less PONV); analgesia; neuromuscular blockers if needed; fluids, temperature management (warming), positioning, antibiotic prophylaxis.
• Depth of anaesthesia — end-tidal agent concentration (MAC — minimum alveolar concentration), processed EEG (BIS) monitoring — especially with TIVA and neuromuscular blockade (awareness risk — NAP5).
• Emergence — stop anaesthetic agents, reverse neuromuscular blockade (neostigmine with glycopyrronium, or sugammadex for rocuronium) — check with a nerve stimulator (train-of-four ratio over 0.9) — extubate when awake with airway reflexes (or deep in selected cases).
• Recovery (PACU) — airway, breathing, observations, pain, PONV; discharge to ward using criteria (Aldrete score).

Spinal and Epidural Anaesthesia

• Contraindications — patient refusal, coagulopathy or anticoagulants (timing rules), local or systemic infection, raised ICP, hypovolaemia, severe fixed cardiac output states (aortic stenosis — relative), allergy.
• Complications:
  • Hypotension (sympathetic block — vasodilatation) — fluids and vasopressors (phenylephrine, metaraminol, ephedrine).
  • Bradycardia — block above T4 (cardioaccelerator fibres).
  • High or total spinal — arm weakness, breathlessness, hypotension, loss of consciousness — airway and circulatory support.
  • Urinary retention, itch (intrathecal opioids), nausea.
  • Post-dural puncture headache — postural — blood patch if persistent.
  • Rare but serious — epidural haematoma or abscess (back pain, leg weakness, bladder dysfunction — urgent MRI and neurosurgery), nerve damage, meningitis.

⚠️ Safety Pearl: New or worsening leg weakness after an epidural or spinal is an epidural haematoma or abscess until proven otherwise. Stop the infusion, assess hourly, and get an urgent MRI — decompression within hours preserves function.

Monitoring and the WHO Checklist

• Minimum monitoring (Association of Anaesthetists 2021) — for every anaesthetic and sedation:
  • A trained anaesthetist present throughout.
  • Pulse oximetry, non-invasive BP, ECG.
  • Capnography (waveform) — for GA and for sedation; and in recovery when an airway device is in place.
  • Inspired oxygen and anaesthetic agent concentrations, airway pressure.
  • Neuromuscular monitoring (quantitative) when neuromuscular blockers are used.
  • Temperature (if anaesthesia over 30 minutes); depth of anaesthesia monitoring with TIVA and paralysis.
• "No trace = wrong place" — absent capnography trace after intubation means oesophageal intubation (even in cardiac arrest a trace is usually seen with CPR) — NAP4.
• Invasive monitoring — arterial line, central venous pressure, cardiac output monitoring (oesophageal Doppler) in major surgery.
• WHO Surgical Safety Checklist:
  • Sign in (before induction) — identity, site marked, consent, anaesthesia safety check, allergies, airway risk, blood loss risk.
  • Time out (before incision) — team introductions, confirm patient, procedure and site, antibiotic prophylaxis, critical events, imaging, VTE prophylaxis.
  • Sign out (before leaving theatre) — procedure recorded, instrument, swab and needle counts correct, specimens labelled, equipment problems, recovery concerns.
• Never events — wrong site surgery, retained foreign object, wrong route (e.g. IV drugs given intrathecally — vincristine), wrong implant, misplaced NG tube, unrecognised oesophageal intubation (added 2021).
• NatSSIPs — National Safety Standards for Invasive Procedures (version 2, 2023) — extend these principles to all procedures.

Special Groups — Elderly, Obese, Pregnant and Children

• Older people — frailty, multimorbidity, polypharmacy, reduced physiological reserve; postoperative delirium (prevention — avoid benzodiazepines and anticholinergics, good analgesia, glasses and hearing aids, orientation, sleep, early mobilisation); postoperative cognitive dysfunction; hypothermia; reduced drug doses; shared decisions and treatment escalation plans.
• Obesity (BMI 40 or more) — difficult airway and mask ventilation, rapid desaturation (reduced FRC — preoxygenate in a head-up "ramped" position), OSA, GORD, difficult venous access, drug dosing (ideal vs total body weight), VTE risk, pressure injuries; bariatric equipment.
• Pregnancy (non-obstetric surgery or obstetric anaesthesia):
  • Aspiration risk from the second trimester (RSI, antacid prophylaxis — ranitidine no longer available; omeprazole or famotidine plus sodium citrate).
  • Aortocaval compression after 20 weeks — left lateral tilt (15°) or uterine displacement.
  • Difficult airway (mucosal oedema), rapid desaturation, increased sensitivity to anaesthetic agents.
  • Avoid elective surgery in pregnancy; second trimester is safest for urgent surgery; fetal monitoring per obstetric advice.
  • Caesarean section — usually spinal (GA if very urgent — category 1 — or contraindications).
• Children — anxiety (parents present at induction, premedication — oral midazolam), gas induction common, weight-based dosing, airway differences, fasting (clear fluids up to 1 hour), URTI increases laryngospasm and bronchospasm risk, day-case surgery where possible, postoperative pain (codeine not under 12 years).
• Day surgery — now the default for many procedures (British Association of Day Surgery) — suitable patients selected on fitness and social support (responsible adult for 24 hours), not age or BMI alone.

Preoperative Assessment at a Glance

Spot the Diagnosis