(a) Representative surface ECG traces of adult (8–12 wk) Fhf2WT/Y and Fhf2KO/Y mice subjected to hyperthermia-induction protocol. External heat lamp was used to raise core body temperature from 37 to 43 °C followed by cooling to 37 °C (1 °C min−1). Fhf2KO/Y ECG parameters were similar to Fhf2WT/Y animals at 37 °C (n=10) but were significantly prolonged in mutant mice at elevated temperatures (≥40 °C) (n=9 mutants and 7 controls). Coved-type ST elevations and high degree AV block with periods of ventricular asystole were noted in Fhf2KO/Y mice above 40 °C. Fhf2WT/Y mice did not exhibit altered ECG parameters at any temperature (37–43 °C). QRS duration is plotted against core body temperature. (b) Representative intracardiac electrogram traces during hyperthermia-induction protocol. Intracardiac conduction intervals (AH, HV, AVI) of Fhf2KO/Y were similar to Fhf2WT/Y animals at 37 °C but were significantly prolonged in mutant mice (n=3) at elevated temperatures (≥40 °C). HV interval duration is plotted against core body temperature. (c) Flecainide (15 mg kg−1) challenge was performed in adult (8–12 wk) Fhf2WT/Y and Fhf2KO/Y mice (n=5). Representative surface ECG traces five minutes after administration of flecainide demonstrates significantly prolonged ECG parameters in Fhf2KO/Y compared with Fhf2WT/Y mice at 37 °C (upper panels). Representative surface ECG traces 15 min after administration of flecainide 15 mg kg−1 in Fhf2KO/Y showing progressive heart block (lower panel). (d) Flecainide (30 mg kg−1) challenge was performed in adult (8–12 wk) Fhf2WT/Y mice in combination with the hyperthermia-induction protocol. Representative surface ECG traces of flecainide treated Fhf2WT/Y mice (n=5) at 37 °C and 43 °C. Fhf2WT/Y mice treated with flecainide did not exhibit any further prolongation of conduction parameters with elevated temperature (37–43 °C). (e) Langendorff-perfused Fhf2WT/Y and Fhf2KO/Y whole hearts (n=4) subjected to hyperthermia-induction protocol. Volume-conducted ECG recordings of isolated Fhf2KO/Y hearts recapitulated the in vivo findings. (f) Representative ventricular activation maps from Fhf2WT/Y and Fhf2KO/Y at 37 and 39 °C (n=3). Hearts were paced at a 200 ms basic cycle length. Isochrones are drawn 5 ms apart. AH, atrial-His; HV, His-ventricular; AVI, atrioventricular interval; CV, conduction velocity. Scale, 1 mm (f). Data represent mean±s.e.m. *significant P values, Student’s t-test.