Changing ventilatory rate while holding power output constant on a bike can cause heart rate to spike or decrease due to changes in CO2 levels and pH.

“so if you have a heart rate monitor and a power meter next time you're on your bike make a deliberate effort to change your ventilatory rate so vent a lot and vent a little and watch what the heart rate does even as you hold the power constant and of course this is because that CO2 is very soluble and CO2 is tracked by the brain very closely as a proxy for pH and the body is is very particular about keeping the pH at 7.4 and so as you're as you slow down your ventilation rate and hold your breath and your carbon dioxide levels rise and your pH Falls you will see that heart rate start to spike with no additional input in power output and of course the reverse is true”