  It has been shown recently [see, for example, S.-Y. Zhu and M. O. Scully,
Phys. Rev. Lett. {\bf 76}, 388 (1996)] that a dynamical suppression of
spontaneous emission can occur in a three-level system when an external field
drives transitions between a metastable state and {\em two} decaying states.
What is unusual in the decay scheme is that the decaying states are coupled
directly by the vacuum radiation field. It is shown that decay dynamics
required for total suppression of spontaneous emission necessarily implies that
the level scheme is isomorphic to a three-level lambda system, in which the
lower two levels are {\em both} metastable, and each is coupled to the decaying
state. As such, the total suppression of spontaneous emission can be explained
in terms of conventional dark states and coherent population trapping.
