DIAGNOSIS

MH is a disorder of increased metabolism, and early signs may be subtle (see "Clinical Syndromes"). These must be distinguished from other disorders with similar signs (see Table 29-2 ). Postoperative fever alone seldom represents MH.

When the diagnosis is obvious (i.e., fulminant MH or succinylcholine-induced rigidity with rapid metabolic changes), there is marked hypermetabolism and heat production, and there may be little time left for specific therapy to prevent death or irreversible sequelae. If end-tidal carbon dioxide increases and ventilation is then increased to maintain normal end-tidal values, diagnosis of MH may be delayed. ^[114] A clinical grading scale aids in establishing the likelihood of MH in specific problem cases. It is based on weighted scores for muscle tone, muscle breakdown, acid-base parameters, temperature, tachycardia or other arrhythmias, and response to dantrolene. This scale is hampered if clinical laboratory evaluation has been minimal.^[115]

In general, MH is not expected to occur when nontriggers are administered (see "Anesthesia for Susceptible Patients"). When volatile anesthetics or succinylcholine is used, MH should be suspected if there is increased end-expired carbon dioxide, undue tachycardia, tachypnea, arrhythmias, mottling of the skin, cyanosis, increased temperature, muscle rigidity, sweating, or unstable blood pressure. If any of these occur, signs of increased metabolism, acidosis, or hyperkalemia must be sought. Analysis of arterial blood gases demonstrates metabolic acidosis and may show respiratory acidosis if the patient is unable to increase ventilation as metabolism increases. Central venous oxygen and carbon dioxide levels change more markedly than do those in arterial blood; therefore, end-expired carbon dioxide or venous carbon dioxide levels more accurately reflect whole-body stores. Venous carbon dioxide, unless the blood drains an area of increased metabolic activity, should have PCO₂ levels only about 5 mm Hg greater than that of expected or measured PaCO₂ . In small children, particularly without oral food or fluid for a prolonged period, base deficit may be 5 mEq/L because of their smaller energy stores. Any patient who is suspected of having an MH episode should be reported to the North American Registry using the rare disease protocol (AMRA) available from the MHAUS web site.