QT/QTc- Interval

• Start of Q-wave to end of the T-wave (time of ventricular depolarization + repolarization). 
• Life threatening risk of prolonged QTc >500ms = Torsades de pointes (TdP).
• Prolonged QT/QTc interval may be a clue to electrolyte disturbances (hypocalcemia or hypokalemia), drug effects (quinidine, procainamide, amiodarone, or sotalol), or myocardial ischemia (usually with prominent T wave inversions). 
• Shortened QT intervals are seen with hypercalcemia and digitalis effect. 
• Each 10-millisecond increase in QTc contributes approx a 5% to 7% additional increase in risk for TdP.
• QTc of 540 milliseconds has a 63% to 97% higher risk of developing TdP than a patient with QTc of 440 milliseconds.

How do you measure it:

• Find a lead with the tallest T wave and count the little boxes from the start of the QRS complex to the point where the T wave comes back down to the isoelectric line. 
• Multiply the number of little boxes by 0.04 seconds. 
• Example if you counted 8 boxes then QT interval is 8 x 0.04 = 0.32 seconds (320 milliseconds).
• QT interval should be less than half the preceding R-R interval (Works for regular rates between 65-90).

Methods of calculating the QTc:

• Bazett formula, QTc = QT / √RR.  
• Fridericia formula (QTc = QT / RR1/3)
• Hodges [QTc = QT + 0.00175 x (HR - 60)]
• Framingham linear regression analysis {QTc = QT + 0.154 x (1 - RR)}
• Karjalainen et al. [QT nomogram] 
• Rautaharju formula, QTc = QT x (120 + HR) / 180

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Rhabdomyolysis

• Rhabdomyolysis is a clinical syndrome that comprises destruction of skeletal muscle with outflow of intracellular muscle content into the bloodstream. 
• The systemic complications associated with rhabdomyolysis result from the leakage of muscle intracellular components into the bloodstream. 
• Elevated Creatine kinase (CK) hallmark of rhabdomyolysis. 
• Defined based on CK values five times above the upper limit of normal. 
• Half-life of CK is 1.5 days; elevated<12hrs, peaks in 3 days, & normalizes in 5 days. 
• Myoglobin half-life of 2-3 hrs & rapidly excreted by kidneys. 
• Rapid & unpredictable metabolism makes myoglobin less useful marker of muscle injury.
• Antibiotics associated with rhabdomyolysis: Daptomycin, macrolides, trimethoprim-sulfamethoxazole, linezolid, fluoroquinolones, and cefdinir. 
• Rhabdomyolysis is associated with hyperkalemia, hypocalcemia, hyperuricemia, and hyperphosphatemia.

Management includes appropriate hydration to improve end-organ perfusion, close monitoring of urine output, correction of electrolyte abnormalities, identification of complications like compartment syndrome, and disseminated intravascular coagulation.

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