Carbon Monoxide Poisoning — SAQ

Stem

A candidate is asked to manage a classic presentation of Carbon Monoxide Poisoning in an exam setting. Use precise definitions, scores, doses, and decision thresholds.

Core knowledge (model answer backbone)

Carbon monoxide (CO) poisoning is a leading cause of unintentional poisoning deaths worldwide — a colourless, odourless, tasteless gas produced by incomplete combustion of carbonaceous fuels. The clinical syndrome arises because CO binds haemoglobin with an affinity approximately 240 times that of oxygen, forming carboxyhaemoglobin (COHb) that both reduces oxygen-carrying capacity and shifts the oxyhaemoglobin dissociation curve leftward (impairing oxygen unloading at tissue). Additional injury occurs through direct mitochondrial poisoning (cytochrome c oxidase / cytochrome aa3 inhibition), oxidative stress from neutrophil priming, and lipid peroxidation of myelin-rich white matter that produces the classic delayed neuropsychiatric syndrome. Acute clinical features range from headache, nausea and dizziness in mild exposure to syncope, myocardial ischaemia, seizures, coma and death in severe exposure; fetal demise may follow even moderately severe maternal exposure because fetal haemoglobin binds CO even more avidly. The diagnostic cornerstone is arterial or venous blood gas with co-oximetry — standard pulse oximetry FALSELY reads normal because it cannot distinguish COHb from oxyha

Red flags

• Altered consciousness, seizures, or coma — severe CO poisoning; immediate 100% oxygen via NRB and consider hyperbaric oxygen
• COHb over 25% in adults, over 15% in pregnancy, over 20% in children — threshold for hyperbaric oxygen referral
• Fire or smoke-inhalation victim with any neurological or cardiovascular sign — consider concomitant cyanide poisoning; give hydroxocobalamin 5 g IV empirically
• Myocardial ischaemia or arrhythmia with CO exposure — cardiac CO toxicity is a marker of severe poisoning; HBO indicated regardless of COHb

High-yield structure examiners expect

Cover: Overview & Definition, Classification, Epidemiology & Risk Factors, Pathophysiology, Clinical Presentation, Differential Diagnosis.

Key doses / thresholds (from topic teaching)

• hydroxocobalamin 5 g IV** empirically
• over 10 mmol/L), normal or near-normal COHb at the t
• over 10 mmol/L in fire victims is highly suggestive
• lactate over 10 mmol/L** in a fire victim is highly suggesti
• crystalloid 10 to 20 mL/kg), continuous ECG, **SpO2 + ETCO2 mon
• treated empirically with hydroxocobalamin 5 g IV** even before the cyanide level retu

Questions

a) Define the condition and give the most important classification or severity framework used in exams. (3 marks)

• Clear one-line definition matching standard teaching.
• Named classification / stages / types with discriminating features.
• One sentence on why classification changes management.

b) Outline pathophysiology in a mechanism chain that explains the main clinical features. (3 marks)

• Initiating insult → intermediate pathway → end-organ effect.
• Link at least two symptoms/signs to mechanism.
• Mention one complication pathway (e.g. shock, perforation, herniation, arrhythmia).

c) List discriminating clinical features and bedside assessment. (3 marks)

• Classic presentation plus one atypical group (elderly, pregnancy, child, immunocompromised).
• Named signs/manoeuvres if relevant.
• What must never be missed on exam/bedside (pregnancy test, airway, glucose, etc.).

d) Investigations with thresholds and one named score if applicable. (3 marks)

• First-line tests and what positive findings mean.
• Gold-standard or definitive investigation when needed.
• Score components reproduced exactly if a named score is standard for this topic.

e) Immediate resuscitation and definitive management with doses where standard. (3 marks)

• ABC / time-critical steps first.
• First-line drug(s) with agent + dose + route (or procedure steps).
• Escalation triggers (theatre, ICU, thrombolysis window, antidote, etc.).
• Disposition and safety-netting.

Marking tips

Full marks require specificity (numbers, names, doses) not generic "give antibiotics/fluids." Regional practice (ICMR / NICE / AHA) may be cited as alternative where relevant.