Anaesthesiology
Preoperative Evaluation and Premedication
Preoperative anaesthetic assessment for MBBS: ASA physical status, airway assessment, fasting rules, medication management and premedication, mapped to NMC codes AS3.
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Preoperative Evaluation and Premedication
Preoperative anaesthetic assessment for MBBS: ASA physical status, airway assessment, fasting rules, medication management and premedication, mapped to NMC codes AS3.
This chapter covers how a patient is assessed and prepared before anaesthesia, from history and examination to targeted investigations and risk grading. It sets out the ASA physical status, airway assessment, fasting rules, the management of long-term medicines and the aims and drugs of premedication.
High-yield: Preoperative Evaluation and Premedication
- The American Society of Anesthesiologists physical status is graded from I (healthy) to VI (brain-dead organ donor), with the suffix E added for an emergency.
- ASA III is a patient with severe systemic disease that limits activity but is not incapacitating; ASA IV is disease that is a constant threat to life.
- The airway is assessed by the Mallampati class, mouth opening, thyromental distance, neck movement and the presence of loose or prosthetic teeth.
- The Mallampati class is scored with the patient sitting, mouth open and tongue out without phonation; a high class predicts a difficult laryngoscopy.
- A thyromental distance of less than about 6 centimetres and mouth opening of less than 3 centimetres suggest a potentially difficult airway.
- Standard adult fasting guidance is 6 hours for solids, 6 hours for milk-containing drinks and 2 hours for clear fluids before elective surgery.
- Investigations are guided by the patient and the surgery, not ordered routinely, so a fit young adult for minor surgery needs few or no tests.
- Metformin is usually withheld on the day of surgery, and the risk of perioperative hypoglycaemia guides adjustment of insulin and sulfonylureas.
- Aspirin is often continued for cardiac indications, while clopidogrel and warfarin are managed case by case with the surgical bleeding risk in mind.
- Angiotensin-converting enzyme inhibitors are frequently omitted on the morning of surgery because of the risk of refractory intraoperative hypotension.
- Premedication aims to relieve anxiety, provide analgesia, reduce secretions, blunt reflexes and lower the risk of aspiration.
- Benzodiazepines such as midazolam are used for anxiolysis and amnesia, and are a common component of premedication.
- Aspiration prophylaxis in the at-risk patient uses an H2 blocker or proton pump inhibitor, a prokinetic and sometimes sodium citrate.
- A functional capacity of 4 metabolic equivalents, such as climbing two flights of stairs, is a useful marker of adequate cardiac reserve for surgery.
ASA physical status
- **ASA I and II:** Healthy patient; mild systemic disease with no functional limitation.
- **ASA III:** Severe systemic disease that limits activity but is not incapacitating.
- **ASA IV and V:** Disease that is a constant threat to life; a moribund patient not expected to survive without the operation.
- **Fasting:** 6 hours for solids and milk, 2 hours for clear fluids before elective surgery.
NMC competencies in this chapter
- **AS3.1:** Principles of preoperative evaluation
- **AS3.2:** History taking in preoperative anaesthetic evaluation
- **AS3.3:** Clinical examination in preoperative evaluation
- **AS3.4:** Choosing and interpreting preoperative investigations
- **AS3.5:** Determining readiness for surgery
- **AS3.6:** Premedication for surgery
Frequently Asked Questions
What is the ASA physical status classification?
It grades a patient's fitness from I, a healthy person, to VI, a brain-dead organ donor, with the suffix E for an emergency. It is a global description of health, not a direct predictor of a specific risk.
What are the standard preoperative fasting times?
For elective surgery in adults, 6 hours for solids and milk-containing drinks and 2 hours for clear fluids. These times reduce the volume of gastric contents and the risk of aspiration.
How is the airway assessed before anaesthesia?
By the Mallampati class, mouth opening, thyromental distance, neck movement and dentition. A high Mallampati class with limited mouth opening and neck movement warns of a difficult laryngoscopy.
Which routine medicines are altered before surgery?
Metformin is usually withheld on the day, ACE inhibitors are often omitted to avoid hypotension, and antiplatelet and anticoagulant drugs are managed against the bleeding risk of the operation.
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