Heart medication question. what happens in heat if on diuretic?

Increase H2O or redu: You have to compensate the water loss via sweat due to excess heat by either increase oral replacement of water/fluids, or reduce/stop the diuretic, or u may need to do both depending on severity of heat and duration of exposure. It is a rather complicated balance and u must do that diligently or you may get in trouble with dehydration or fluid overload. Use a weight-scale. Good luck.