Put 175 lbs on a barbell. Do a 1/4 squat on the left leg only. Then do a 1/4 squat on the right leg only. Repeat that back and forth, left and right, over and over, squatting every second - 60 squats a minute for five minutes - 300 squats - nonstop.

Don't you think that would tire you out?

That is kind of what a stair machine is like if you weight 175 lbs. Lifting your body weight on one leg repeatedly for 100s of repetitions.

Look at it this way - five minutes at one step per minute is 300 steps. The average story is 12 steps. So in five minutes you have walked up 25 stories. The amount of work you have done is lift your body weight up 25 stories or 200 feet into the air. That is a lot of sustained work.

When we lift weights, we change the weight to improve ourselves. On a stair machine, we change the speed. Try slowing the pace, and as you get better, going at a faster pace (1/second) for 30 seconds, followed by 30-60 seconds of a slower pace. Later, you can consider a weight vest, but speed is really all that is needed for most of us.

(I assume by 'palpitations' you mean that you can feel your heart pounding/racing, not that you feel irregular heart beats or beats skipping. Consider getting a heart rate monitor - even something simple. Many stair machines have them built in. Do not push yourself past a safe range for your age (Google it). If you are pushing yourself too hard and feeling your heart pounding and getting out of breath, you are likely working too hard (e.g. 90% of your max heart rate), which cannot be sustained. Manage the work (speed) to keep you at a more comfortable level and let your ability grow by working in fast work and recovery periods.)