In a patient with protracted active phase, oxytocin augmentation at maximum dose fails to achieve adequate uterine contractions (Montevideo units <200). Intrauterine pressure catheter confirms MVU = 150 mmHg×min in 10 minutes. What is the MOST accurate characterization of this finding?

• A Normal labor — Friedman curve allows progress up to 1 cm/hour regardless of contraction quality
• B Primary uterine inertia — failed to enter active labor despite adequate oxytocin
• C Adequate uterine activity — 150 MVU is sufficient for cervical dilation beyond 4 cm
• D Hypotonic uterine dysfunction — inadequate Montevideo units (< 200 MVU) despite maximum oxytocin ✓

Explanation

Montevideo units (MVU) represent the sum of peak uterine contraction amplitudes above baseline over 10 minutes. Adequate labor requires ≥200 MVU for effective cervical dilation. Less than 200 MVU despite maximum oxytocin represents hypotonic uterine dysfunction. This is important because it differentiates inadequate uterine force (treatable with oxytocin until maximum dose) from mechanical obstruction (cephalopelvic disproportion, requiring cesarean). When MVU remains < 200 on maximum oxytocin, cesarean section is indicated for failed induction/augmentation.

Reference: Williams Obstetrics, 26th ed.

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