Because Pesukei d’Zimrah are surrounded by berachot – Baruch She’amar before and Yishtabach after them – they are considered one segment and interruptions in the middle of their recital are forbidden.
However, in cases of a great need, such as to prevent loss, it is permissible to interrupt by speaking (see Mishnah Berurah 51:7, where he writes that before and after the interruption one should say the verses “Baruch Hashem” that appear before Vayevarech David). Similarly, in order to prevent insulting someone, one is permitted to greet him (see further in this book 16:6; the law concerning an interruption between Yishtabach and Barchu is explained further in this book 16:2).
There are those who say that when one must relieve himself in the middle of Pesukei d’Zimrah, he should delay the recital of Asher Yatzar until after the prayer service so as not to interrupt Pesukei d’Zimrah. However, it is best to recite Asher Yatzar immediately after relieving oneself, for if he delays its recital until after praying, he might forget to say it altogether.
If a person hears berachot or Kaddish in the middle of Pesukei d’Zimrah, he may not answer “Baruch Hu u’varuch Shemo,” although he is permitted to answer Amen. Saying Amen is allowed, but is not an obligation. Since he is engaged in the mitzvah of reciting Pesukei d’Zimrah, he is exempt from the mitzvah of responding to other matters of sanctity. If a person’s concentration during Pesukei d’Zimrah will be disturbed if he interrupts to answer Amen, it is best that he continues to recite Pesukei d’Zimrah with kavanah and refrain from answering Amen. However, if he hears the congregation saying Kedushah, he must stand with his legs together and join along, so that he will not stand out as one who does not participate in matters of sanctity with the congregation. Similarly, if they arrive at Modim d’Rabbanan or Barchu, he bows slightly and answers with them. If he is praying in a minyan in which the people are reciting Pesukei d’Zimrah and he hears Kedushah, Modim, or Barchu being recited in a different minyan, he is permitted to remain seated and say Pesukei d’Zimrah, since by continuing to pray in an orderly fashion he does not stand out as one who separates himself from the congregation.
One who is reciting Pesukei d’Zimrah should not be called up to the Torah. He may only be called if he is a Kohen or a Levi and there are no other Kohanim or Levi’im present. Additionally, since he is in the middle of Pesukei d’Zimrah he may not interrupt his prayer to ask the gabbai (synagogue coordinator) to say a prayer on his behalf (Mishebeirach). However, in a situation in which the gabbai mistakenly calls a Yisrael who is still in the middle of reciting Pesukei d’Zimrah, he must go up out of respect for the Torah and the congregation (Mishnah Berurah 51:10).
. Mishnah Berurah 51:8 rules that one should recite the blessing immediately and so does the Aruch HaShulchan. Kaf HaChaim 51:28 writes that one should recite the blessing after Yishtabach, and according to the Eshel Avraham, one should recite the blessing after the Amidah. The Igrot Moshe 4:14 writes that it is preferable to say it after the Amidah. However, one who prefers to recite it immediately may do so. It is best that he recite the blessing immediately since there is concern that he might forget to recite the blessing altogether.
.Shut Tzitz Eliezer 11:3 and Halichot Shlomo 6:12 explain that all the laws of interruption concerning matters of sanctity in Pesukei d’Zimrah are optional, meaning there is no obligation to stop in order to respond. Therefore, if responding disturbs his kavanah, it is best that he does not answer. In any case, one should take care not to appear as separating himself from the congregation; therefore, he stands at Kedushah, stands and bows at Modim, and bows slightly at Barchu, and since he already interrupted to stand and bow, it seems best that he respond as well. However, if he is part of another minyan, he does not appear to be separating himself from the congregation and therefore it is not necessary for him to stand and he may continue praying as he is (see the laws of Keriat Shema 16:5 and note 4).
Since a person is not obligated to respond, there is no great need to discuss the laws of interruption at length, so we will briefly mention only a few. The Mishnah Berurah 51:8 and Bei’ur Halachah write that for responding Amen to berachot one may make an interruption even in the middle of a verse, provided that a theme of the verse has been concluded. However, for Kaddish, Kedushah, and Modim (these parts of prayer are recited in a minyan, and according to Minhag Ashkenaz, Amen after HaKel HaKadosh and Shome’a Tefillah as well), he may respond even in a place in which a theme has not yet ended. Many Acharonim do not mention this distinction at all and maintain that it is permissible to interrupt to answer Amen at any point in Pesukei d’Zimrah. One may respond to all the Amens in Kaddish until “D’Amiran B’Alma,” each according to his minhag. However, the law concerning the subsequent Amens is like that of “Baruch Hu u’varuch Shemo,” that we do not interrupt in order to respond in the middle of Pesukei d’Zimrah. Regarding Modim d’Rabbanan, the Mishnah Berurah 51:8 implies that one may answer and recite all of it, yet in Yabia Omer, part 6, 4, it is written that one only says the words “Modim anachnu Lach.” Regarding a person who hears thunder or sees lightning, according to the Chayei Adam 20:3 he is permitted to interrupt and recite a berachah on them, and so write most poskim, although there are some who disagree (Kaf HaChaim 51:23).