Is supraventricular tachycardia (svt) in fetus deadly?

It depends. Whether supraventricular tachycardia (svt) causes problems in a fetus depends on how often and for how long the fetus remains in svt, and on the rate of the svt (how fast). Typically, if a fetus is having problems with svt, a fetal cardiac ultrasound will detect that the heart function is declining and there may be evidence for abnormal fluid collections (so called non-immune hydrops).
Fetal svt. It can be if it is fast and especially if it sustains and persistent .