How to Identify Malignant Hyperthermia
The key to identifying malignant hyperthermia is recognizing an unexplained, unexpected increase in end-tidal CO2 (ETCO2) that cannot be controlled by increasing minute ventilation, accompanied by an upward trend in heart rate, in any patient exposed to volatile anesthetics or succinylcholine 1.
Cardinal Clinical Features
The diagnosis centers on recognizing excessive carbon dioxide production as the earliest and most reliable indicator 1:
Primary Diagnostic Signs (Early Recognition)
- Increased ETCO2 despite attempts to control it by increasing minute ventilation - this is the most critical early sign
- Rising heart rate (the upward trend matters more than absolute values) - unless masked by beta-blockers or remifentanil
- Increased oxygen consumption (if your anesthetic machine calculates this)
- These three findings together, with exclusion of alternative causes, are sufficient to diagnose suspected MH and begin treatment 1
Secondary Clinical Features (May Appear Later)
- Masseter muscle spasm or generalized muscle rigidity (may occur even with non-depolarizing neuromuscular blockade present) 2
- Rising core temperature (>0.5°C per 15 minutes, often exceeding 40°C) - but temperature may still be normal when you first make the diagnosis 1, 3
- Metabolic and respiratory acidosis
- Hyperkalemia with peaked T-waves on ECG
- Arrhythmias (tachycardia progressing to more serious dysrhythmias)
- Cola-colored urine indicating myoglobinuria from rhabdomyolysis 3
- Skin changes: hot, flushed, sweaty exposed areas; mottling (more common in children) 1
Critical Timing Considerations
MH can present at highly variable times - this is a common pitfall 1:
- As early as 2-10 minutes after exposure to triggering agents 4
- May be delayed for several hours, especially with desflurane 1
- Can occur after discontinuation of volatile anesthetics (up to 40 minutes post-discontinuation in documented cases) 1
- More rapid and florid reactions typically occur when both succinylcholine and volatile anesthetics are used together 1
Diagnostic Algorithm
When volatile anesthetics or succinylcholine are in use:
- Monitor continuously: ETCO2, heart rate, temperature, oxygen consumption
- Suspect MH immediately if you observe:
- Rising ETCO2 that you cannot control with increased ventilation
- Plus unexplained rising heart rate
- Exclude other causes (equipment malfunction, inadequate anesthesia depth, other hypermetabolic states)
- Do NOT wait for fever - temperature increase often lags behind metabolic signs 1
- Call the diagnosis and treat when you have rising CO2 + rising heart rate without alternative explanation
Common Pitfalls to Avoid
- Waiting for hyperthermia: Temperature may be normal initially; by the time significant fever develops, the crisis is already advanced 1
- Dismissing early signs: An unexplained increase in ETCO2 deserves immediate attention, not rationalization
- Missing delayed presentations: Remain vigilant for hours after discontinuing triggering agents, particularly after cardiac surgery 1
- Confusing with other causes: Temperature increase that begins before evidence of increased CO2 production is not MH 1
- Assuming prior uneventful anesthetics rule out MH: First reactions can occur at any age after multiple previous exposures
Supporting Diagnostic Tools
Continuous monitoring recommended 2:
- End-tidal CO2 monitoring (essential)
- Core temperature monitoring
- Arterial blood gas analysis (will show mixed acidosis, hyperkalemia)
- Creatine kinase and myoglobin levels (for rhabdomyolysis)
- ECG monitoring (for arrhythmias and hyperkalemia changes)
The diagnosis is clinical - do not delay treatment waiting for laboratory confirmation. Recognition of the pattern of hypermetabolism in the context of triggering agent exposure is sufficient to initiate life-saving therapy 1, 3.