Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (61 page)

   Who Should Be Suspected?

   Risk factors for sudden cardiac arrest (SCA) are driven largely by risk factors for coronary artery disease and structural heart disease, the most frequent cause of SCA in patients over 35 years of age (80%). The presence of known cardiac structural disease raises the risk of SCA six- to eightfold and is the index presentation of coronary disease in 15% of patients. Valvular disease and hypertrophic cardiomyopathy each account for 5% of cases in adults.

   In contrast, hypertrophic cardiomyopathy accounts for 48% of SCA in patients ≤35 years.

   In structurally normal hearts (5–10% of all SCA), the most common inherited arrhythmogenic diseases contributing to SCA include long- and short-QT syndromes, Brugada syndrome, Wolf-Parkinson-White syndrome, ARVC (see above), and catecholaminergic polymorphic VT. These conditions may account for 10–12% of SCA in young patients, and 5% in adult populations.

   Laboratory and Diagnostic Testing

   Potentially reversible causes of SCA should be immediately assessed in survivors and include assessment of electrolytes (particularly hypokalemia and hypomagnesemia, hypocalcemia), ischemia (ECG and troponins), arterial blood gas, recreational drugs, and medication lists scrutinized for proarrythmogenic effects (reference
www.qtdrugs.org
).

   Electrolyte abnormalities may be the result of hemodynamic derangement and resuscitative efforts. Attribution of primary etiology of SCA to electrolyte disturbance is appropriate only if other etiologies are excluded.

   It is essential that survivors of SCA undergo a full evaluation of structural heart disease including, but not limited to, ECG, cardiac catheterization and echocardiography.

   Cardiac MRI is indicated if a structural abnormality is uncertain after initial assessment and is particularly useful for the diagnosis of myocarditis, cardiac infiltrative disease (amyloid and sarcoid) and arrhythmogenic right ventricular cardiomyopathy.

   Electrophysiology testing is not routinely performed in survivors of SCA, but may be useful in patients whose evaluation reveals no clear etiology of SCA. Inducible arrhythmias are a nonspecific finding and lack of inducible rhythm does not confer low risk for recurrence.

   First- and second-degree relatives of SCA patients should be screened for cardiovascular disease and considered for genetic testing. Risk of SCA increases 1.57-fold in family members, and up to 9.4-fold if maternal and paternal history of SCA is present.

   OTC ≥ 440 ms in males and ≥460 ms in females should be screened for inherited long QT syndrome by family history and potentially genetic testing. Given the number of genes identified in long QT syndromes and uncertainly over the functional significance of some reported variants, referral to an experienced testing center is recommended. Genetic testing in long QT syndrome has a yield of 40% positive genotype at a cost of $13,000 per diagnosis (in contrast, EP study >$50,000).