USMLE Step 2 CK Lecture Notes 2019: Obstetrics/Gynecology (Kaplan Test Prep)

OB TRIAD

Variable    Decelerations

Variable deceleration: A visually apparent abrupt decrease in FHR. These are
mediated by umbilical cord compression.

Sinusoidal pattern: A visually apparent, smooth, sine wave-like undulating
pattern in FHR baseline with a cycle frequency of 3–5/min which persists for
≥20 min.

Abrupt  drop    of  FHR

Sudden  return  of  FHR

Variable    in  relation    to  contractions

An  abrupt  FHR decrease    is  defined as  from    the onset   of  the deceleration    to  the

beginning   of  the FHR nadir   of  <30 seconds.

The decrease    in  FHR is  calculated  from    the onset   to  the nadir   of  the

deceleration.

The decrease    in  FHR is  ≥15 beats   per minute, lasting ≥15 seconds,    and <2

minutes in  duration.