Arsenal recorded their 11th consecutive away win at West Brom following a penalty shootout triumph in the Capital One Cup.
Nine months on from their chastening quarter-final defeat to Bradford from 12 yards the Gunners' second-string feared a similar fate when Serge Gnabry had his penalty saved by Luke Daniels.
But misses from Craig Dawson and Albion substitute Morgan Amalfitano allowed Nacho Monreal to strike the decisive kick for a 4-3 shootout win.
Arsène Wenger fielded several young players inexperienced at this level, such as 20-year-old first-time starter Eisfeld, debutant Isaac Hayden, Kris Olsson - another making his senior bow - and Chuba Akpom, with the latter pair coming off the bench and scoring in the shoot-out.
Arsenal will host Chelsea in round four, and Wenger said: "I am proud. We played with many young players and they showed they are not only good footballers but that they can fight as well.
"I decided to go for young players with the penalties and they coped well with the pressure."