Sinusoidal heart rate pattern is seen in:
Sinusoidal Pattern:

Stable baseline heart rate of 120 to 160 beats/min with regular oscillations.
Amplitude of 5 to 15 beats/min (rarely greater).
Long-term variability frequency of 2 to 5 cycles per minute.
Fixed or flat short-term variability.
Oscillation of the sinusoidal waveform above or below a baseline.
Absence of accelerations.

Causes of Sinusoidal Pattern:

Serious fetal anemia due to Rh-isoimmunisation/rupture vasa previa / feto maternal hemorrhage / twin to twin transfusion.
Drugs — Meperidine, morphine, alpha prodine and butorphanol.
Amnionitis.
Fetal distress (+/–).
Umbilical cord occlusion.
Placenta previa
Vasa previa
Battledore placenta
Succenturiate placenta