I'm not really on board with chasm54's suggestion about going to harder gears if out of breath.
It makes sense depending on the cause. Reading "out of breath" as "high heart rate and blood pressure" anyway. When you push a muscle for too much force output then pushing blood through it becomes more difficult - this is why weightlifters can go read faced. So dropping to a gear where power output is the same and force per cycle is less can reduce this.