CARDIAC TRANSMEMBRANE POTENTIALS— HOFFMAN 



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marizc'd as follows. The absolute refraclnry ])erin(l Ijes^iiis with the upstroke of the 

 action potential and lasts until repolarization is approximately two-thirds completed. 

 The relative refractor}- period, in turn, extends from this point until shortly after 

 the end of repolarization (fig. 6). Action potentials elicited by suprathreshold 

 stimuli applied during the relative refractory period are altered in shape (fig. 7) 



(f) 



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Fig. 6. — Tracing of transmembrane action potential of isolated ventricnlar fiber and curve of 

 recovery of excitability. ARP = Terminal boundary of absolute refractory i>eriod ; TRP ^ 

 Terminal boundary of total refractory periwl. Time and voltage calibrations shown in figure. 



Fig. 7. — .\ction potentials recorded from a single isolated ventricular fiber showing the efifect 

 of prematurity on induced extrasystoles. Stimulus artefacts indicated by downward marks on 

 baseline. Action potential upstrokes retouched. Note change in amplitude and duration of 

 extrasystole. 



