Question: What interruptions, if any, are permitted during Pesukei d’Zimrah?
Answer: Almost all mitzvah responses are permitted. These include:
a) Responding to ‘Kaddish’ (p. 82 and others)
b) Answering to ‘Kedusha’ (p. 100)
c) Answering to ‘Borchu’ (p. 84)
d) Reciting the silent ‘Modim’ along with the chazzan (p. 112)
e) Reciting the first ‘pasuk’ (verse) of ‘Krias Shema’ along with the congregation, if that is what they are up to, and you are behind in ‘Pesukei d’Zimrah’. (Ed: Try coming earlier next time!)
f) Answering ‘amein’ to any bracha
g) Reciting the entire ‘Krias Shema’, if one realized while in the middle of ‘Pesukei d’Zimrah’ that the latest time for Shema was fast approaching, and that he would not make it if he would continue in order.
h) ‘Birchos HaTorah’ (Brachos on Torah Study), if one realized that he did not make them before davening.
i) Getting an ‘aliyah’ to the Torah, if he was called up already, or if he was the only Kohein or Levi, etc. Even then, he should limit his interruptions to the brachos only and not offer names for a ‘Mi-she-beirach’ etc.
j) Timely brachos of ‘shevach v’hoda’ah’ (praise and thanks), such as ‘Asher yatzar’ (after using the bathroom), ‘She-kocho u’gevuraso amlei olam’ (upon hearing thunder).
Whenever one is permitted to make a mitzvah-response, he may do so even in the middle of a ‘pasuk’, but preferably in-between two phrases. (Ed: Use the commas for guidance.) Furthermore, one is permitted to answer ‘amein’ even if he is between the bracha of ‘Baruch She’amar’ and the beginning of the next paragraph (i.e. ‘Hodu’, according to Ashkenazic custom.) Ordinarily, one is forbidden to interrupt, even to answer ‘amein’, if he has just made a mitzvah-bracha and has not yet begun the mitzvah he is coming to perform, or if he has just made a bracha and has not yet tasted the food he is coming to eat. The reason why this is an exception is because here the mitzvah he is coming to perform is to praise Hashem through ‘Tehillim’ (Psalms). Well, guess what? The ‘amein’ response is also a praise of Hashem, so it is not an interruption – it is precisely the same mitzvah!
One should not say ‘Baruch Hu u’Varuch Shemo’ to a bracha when he is in the middle of ‘Pesukei d”Zimrah’, because that response is not as prominent as the ‘amein’ response, as it is not sourced in the Gemara.
From all of the above, it is self-evident that non-mitzvah responses should not be made, unless there is an emergency. Remember, the entire ‘Pesukei d’Zimrah’ is sandwiched between two brachos, not really any different from being in the middle of ‘Hallel’ or ‘Birkas Hamazon’, etc. In the event that one is forced to speak, there is a “trick” on how to temporarily remove oneself from the midst of this “bracha sandwich”: there is a paragraph in the ‘Pesukei d’Zimrah’ (found on p. 74) called ‘Baruch Hashem l’Olam…’ (Blessed is Hashem forever…) This paragraph is a kind of “pseudo-bracha”. If one says it immediately before he makes his interruption, and then again immediately before resuming, he has thus split his ‘Pesukei d’Zimrah’ into two mini-sandwiches, and he is simply making his interruption in-between the two of them, not during one big one.