QT interval is time representing ventricular depolarization(QRS) and repolarisation (ST segment and T wave), it is calculated on the ECG from Q wave to the end of T wave. Normal range of QT interval: QTc: 470 – prolonged QTc Females: QTc 460 – 479 = borderline prolonged, QTc > 480 – prolonged QTc QTc >500msec – increases risk of Torsades de Pointes (2-3x) During childhood until puberty, the QT interval is similar in boys and girls. After puberty, females tend to have a slightly longer QTc than males due to the influence of sex hormones, such as testosterone, which accelerates ventricular repolarization. This age difference disappears after the age of 75. We correct the QT interval for heart rate using various formulas, such as Bazett, Fridericia, Framingham, Hodges, and Rautaharju. You can use online calculators, via this link. Corrected QT is abbreviated by QTc. Long QT syndrome (LQTS) is a disorder of myocardial repolarization. Prolonged QTc, if left untreated, can lead to Torsades de Pointes (a polymorphic ventricular tachycardia), syncope, cardiac arrest, and/or sudden cardiac death. Long QT syndrome can be either congenital (e.g., Jervell and Lange-Nielsen syndrome, Romano-Ward syndrome) or acquired, caused by metabolic disorders (e.g., hypokalemia, hypomagnesemia, hypocalcemia, hypothyroidism, starvation, anorexia nervosa, liquid protein diets) or more commonly by medications. It is crucial to be aware of medications that prolong the QT interval on an ECG. Medications that prolong QT: These are just examples, and the list is not exhaustive. Many of these drugs interfere with and block the delayed rectifier K+ channel (abbreviated as Ik), which prolongs repolarization. To understand this well let´s have a look on the action potential diagram of the cardiac myocyte. Myocardial action potential: 0 – Rapid depolarization (Opening voltage gated Na+ channels) 1 – Early repolarization (Inactivation of Na+ channels, initial opening of K+ channels) 2 – Plateau (Ca2+ influx) 3 – Repolarisation (K+ efflux via delayed-rectifier K+ channels (Ik)) 4 – Resting potential (Diastole occurs during this phase; maintained by Na+/K+ ATPase and inward rectifier K+ channels (IK1) You may recall this from medical school, but it serves to illustrate how blocking Ik during Phase 3 leads to prolongation of the T wave and the QT interval. The general approach to acquired long QT syndrome is to switch the patient to an alternative medication. Let's consider a clinical example. Imagine, you are treating the community acquired pneumonia. Typically, you would start empirical treatment to cover both "typical" (e.g., Streptococcus pneumoniae, Haemophilus influenzae) and atypical pathogens (e.g., Mycoplasma pneumoniae, Legionella pneumophila, Chlamydia pneumoniae). A common treatment approach includes administering a beta-lactam antibiotic (e.g., a Cephalosporin or Aminopenicillin like Amoxicillin/Clavulanate) to cover Streptococcus pneumoniae and Haemophilus influenzae, along with a Macrolide or Tetracycline (Doxycycline) to address atypical pathogens. While Fluoroquinolones are not the 1st-line option, they can cover both. To treat community-acquired pneumonia in a patient with a prolonged QT, it may be advisable to avoid Macrolides and Fluoroquinolones to minimize further prolongation of the QTc and reduce the risk of Torsades de Pointes. For such patients, Ceftriaxone + Doxycycline instead of a Macrolide (e.g., Azithromycin) or Amoxicillin/Clavulanate + Doxycycline would be a good choice. Author: Mamuka Asatiani, MD
