Dynamicity of hypothermia-induced J waves and the mechanism involved

Background: J waves develop during hypothermia, but the dynamicity of hypothermia-induced J waves is poorly understood. Objective: The purpose of this study was to investigate the mechanism of the rate-dependent change in the amplitude of hypothermia-induced J waves. Methods: Nineteen patients with severe hypothermia were included (mean age 70 ± 12 years; 16 men [84.2%]). The rectal temperature at the time of admission was 27.8° ± 2.5°C. In addition to prolonged PR, QRS complex, and corrected QT intervals, the distribution of prominent J waves was widespread in all 19 patients. Results: Nine patients showed changes in RR intervals. When the RR interval shortened from 1353 ± 472 to 740 ± 391 ms (P =.0002), the J-wave amplitude increased from 0.50 ± 0.29 to 0.61 ±0.27 mV (P =.0075). The J-wave amplitude increased in 7 patients (77.8%) and decreased in 2 patients (22.2%) after short RR intervals. The augmentation of J waves at short RR intervals was associated with a significant prolongation of ventricular activation time (35 ± 5 ms vs 46 ± 5 ms; P =.0020), suggesting accentuated conduction delay. Increased conduction delay at short RR intervals was suggested to accentuate the phase 1 notch of the action potential and J waves in hypothermia. None developed ventricular fibrillation, and in 2 of 9 patients with atrial fibrillation, atrial fibrillation persisted after rewarming to normothermia. Conclusion: J waves in severe hypothermia were augmented after short RR intervals in 7 patients as expected for depolarization abnormality, whereas 2 patients showed a bradycardia-dependent augmentation as expected for transient outward current–mediated J waves. Increased conduction delay at short RR intervals can be responsible for the accentuation of the transient outward current and J waves during severe hypothermia.