By breathless you mean you feel choking or that you don't recognise the breath? If it's the second one, no problem, just leave it as it is, it is a sign of calmness. If the first one, observe what changes, what makes you feel in need of air, try to breathe through the mouth.
"If the Buddha-Nature is seen, there will be no seeing of a nature in any thing."
(Nirvana Sutra, T12n374p521b3; tr. Yamamoto)
"Neither cultivation nor seated meditation — this is the pure Chan of Tathagata."
(Mazu Daoyi, X1321p3b23; tr. Jinhua Jia)
"With sudden enlightenment to Tathagata Chan,
The six paramitas and myriad means are complete within that essence."
(Yongjia Xuanjue, T2014p395c14; tr. from "The Sword of Wisdom")