Low temperatures slow the ion fluxes of the metabolic processes that determine electrophysiological properties of the heart. Figure 1 is an electrocardiogram from a 78-year-old man whose temperature was 90°F. Apart from a high voltage in leads V3–V5 (and nowhere else), the striking feature is the secondary wave following the QRS complex, which is usually about as wide as the QRS, pointed, and concordant with QR polarity. This is not bundle branch block and it disappeared with re-warming. It is visible, though reduced, in leads V5 and V6 as well as leads II, III, and aVF. It is the J or Osborne wave and probably belongs to the QRS because low temperatures lengthen action potential duration, which determines QRS as well as T duration. At least in humans, profound hypothermia affects both depolarization (QRS) and repolarization (ST–T). Lesser degrees of hypothermia can cause only T-wave abnormalities, seen here as a prolonged J–T interval and sagging of the early part of the T waves, which are clearly reduced out of proportion to the QRS amplitude in all leads except for V1 and V2. J waves are so named because they appear at the J-point, i.e. the junction between QRS and T. Their genesis has not been completely explained, and J waves may also be present during hypercalcemia and certain abnormalities of the central and peripheral nervous systems in patients who are normothermic. Their occurrence during accidental hypothermia permitted their discovery, and more recently they have been seen when therapeutic hypothermia has been induced in animals and patients. A similar wave occurs at the same time in some patients with Brugada syndrome. A common differential diagnosis is bundle branch block, either right or left; in this case the similarity to left bundle branch block is obvious.