Upper UTIs are infections of the kidneys. These are potentially life threatening, if bacteria move from the infected kidney into the blood. This condition is called sepsis. Sepsis can cause dangerously low blood pressures, shock, and death.
So whe you hold your breath, you stop getting oxygen in your lungs that go to your heart. The oxygen level in the blood drops, so your muscles need more oxygen, so the heart pumps faster. This is like running, your muscles need more oxygen so heart pumps faster
If you cannot hold your breath at full lung capacity for 20 seconds, then hold your breath for as long as you can and then increase duration gradually. This will set up your lungs to receive more and more air each time. Repeat 3 or 4 times in one series.