While interruptions are forbidden at any point throughout the Pesukeh De'zimra service, it is especially important to avoid interruptions while reciting the thirteen expressions of praise for Hashem in Yishtabah, the concluding blessing of Pesukeh De'zmira. These expressions begin with "Shir U'shhaba Hallel Ve'zimra" and continue through "Kedusha U'malchut," and they must be recited without any interruptions, as they correspond to G-d's thirteen attributes of mercy. These thirteen expressions should not be recited in one breath. To the contrary, they should be recited slowly and patiently, with concentration. Rav Yaakob Kassin (1900-1994), in his work Kesinei Eretz, writes a responsum (Siman 5) addressing the question of whether these words should be recited in one breath, and he convincingly demonstrates that such an idea is a mistake. The Zohar teaches that they should be recited without interruption, and some erroneously understood this to mean that they should be said in one breath. Rav Kassin laments the fact that some Siddurim instruct reciting these thirteen expressions in one breath, and he writes that it is a Misva to publicize the fact that this is wrong. These words should be recited slowly and with concentration, and thus one specifically should not recite them quickly in a single breath. If one hears Kaddish while reciting the thirteen expressions of praise, he should interrupt for the first five "Amen" responses, as these are considered a legitimate interruption. When responding, "Amen Yeheh Shemeh Rabba," one should continue until "Yitbarach." This applies to all "Debarim Bi'kdusha" – portions of the prayer service to which the congregation responds, such as Nakdishach and Barechu. One who hears these during Yishtabah may interrupt for the purpose of responding with the congregation. Preferably, however, if one hears in the beginning of Yishtabah that the congregation will soon reach Kaddish, Barechu or Nakdishach, then he should pause before he begins "Shir U'shbaha" to avoid having to interrupt during the thirteen expressions of praise. If one had already begun the final part of Yishtabah, starting from "Baruch Ata Hashem," then he may join the first five "Amen" responses of Kaddish, but not any other responses.