Zanpaktou's heal based on the vitality/reitsu and ability of the Shinigami.
The two best examples I think are:
#1, Ichigo versus Kenpachi: Zangetsu breaks, Ichigo's then powered up by Zangetsu/Shirosaki, Zangetsu heals immediately.
#2, Ikkaku versus Eduardo: Hoo-whatever breaks, Ikkaku powers...