▁layers ▁of ▁the ▁heart ▁wall ▁will ▁have ▁fully ▁rep olar ised , ▁and ▁are ▁therefore ▁ready ▁to ▁contract ▁again , ▁while ▁other ▁regions ▁are ▁only ▁partially ▁rep olar ised ▁and ▁therefore ▁are ▁still ▁within ▁their ▁ref ract ory ▁period ▁and ▁not ▁yet ▁able ▁to ▁be ▁re - ex c ited . ▁ ▁If ▁a ▁trigger ing ▁imp ulse ▁arriv es ▁at ▁this ▁critical ▁point ▁in ▁the ▁card iac ▁cycle , ▁the ▁wave front ▁of ▁elect rical ▁activation ▁will ▁conduct ▁in ▁some ▁regions ▁but ▁block ▁in ▁others , ▁potentially ▁leading ▁to ▁wave break ▁and ▁re - entr ant ▁arr h yth mi as . ▁ ▁The ▁second ▁mechanism ▁rel ates ▁to ▁the ▁increased ▁number ▁of ▁f ibr ill atory ▁wave lets ▁that ▁can ▁simultaneously ▁exist ▁if ▁the ▁action ▁potential ▁decre ases , ▁in ▁a ▁concept ▁known ▁as ▁the ▁arr h ythm ia ▁w avel ength . ▁During ▁f ibr ill ation , ▁the ▁cha otic ▁wave lets ▁rotate , ▁or ▁re - enter , ▁within ▁the ▁mus cle ▁of ▁the ▁heart , ▁contin ually ▁ex ting u ishing ▁and ▁reform ing . ▁The ▁volume ▁of ▁t issue ▁in ▁which ▁each ▁wave let ▁can ▁complete ▁a ▁re - entr ant ▁circuit ▁is ▁dependent ▁on ▁the ▁ref ract ory ▁period ▁of ▁the ▁t issue ▁and ▁the ▁speed ▁at ▁which ▁the ▁waves ▁of ▁dep olar isation ▁traverse ▁move   – ▁the ▁condu ction ▁velocity . ▁The ▁product ▁of ▁the ▁condu ction ▁velocity ▁and ▁ref ract ory ▁period ▁is ▁known ▁as ▁the ▁w avel ength . ▁In ▁t