shortness of breath; panting; puffing
running out of steam (of the economy, a project, etc.); losing momentum; loss of efficiency
階段を登ったら息切れした。
I got out of breath climbing stairs.
運動不足で息切れしやすい。
Lack of exercise makes me easily winded.
息切れがするので休憩した。
I rested because I was short of breath.