A 29-year-old G1P0 at 39 weeks gestation undergoes an emergent cesarean delivery. She pres… — USMLE Step 2 CK

A 29-year-old G1P0 at 39 weeks gestation undergoes an emergent cesarean delivery. She presented to the labor triage unit 20 minutes prior with painful uterine contractions and a history of a sudden gush of fluid at home. Upon admission, her cervix was 6 cm dilated, and the fetal heart rate was found to be persistently in the 60s, prompting immediate surgical intervention. A male neonate is delivered and requires positive pressure ventilation.

Umbilical artery blood gas analysis is obtained immediately after delivery:

Parameter	Value	Reference Range
$pH$	$6.96$	$7.20 - 7.30$
$pCO_2$	$88$ mm Hg	$40 - 50$ mm Hg
$pO_2$	$14$ mm Hg	$15 - 25$ mm Hg
Bicarbonate	$21$ mEq/L	$20 - 24$ mEq/L
Base excess	$-4$ mEq/L	$> -12$ mEq/L

Which of the following is the most likely primary mechanism for this neonate's acid-base derangement?

Acute occlusion of umbilical cord vesselsAnswer
B
Prolonged anaerobic fetal tissue metabolism
C
Chronic uteroplacental microvascular disease
D
Fetal systemic inflammatory response syndrome
E
Maternal-to-fetal transfer of unmeasured anions

Answer

Acute occlusion of umbilical cord vessels

The correct answer identifies that the neonate is suffering from a pure, acute respiratory acidosis. Because

CO_2

is highly lipid-soluble, it diffuses across the placenta extremely rapidly under normal conditions. Any abrupt cessation of umbilical blood flow—such as acute occlusion of umbilical cord vessels following a cord prolapse—causes an immediate spike in fetal

pCO_2

and a corresponding drop in

pH

. Because the hypoxic event in this scenario occurred so acutely (within 20 minutes prior to delivery), there was inadequate time for fetal tissues to transition to anaerobic glycolysis and accumulate lactic acid. As a result, the base excess remains normal.

Step-by-Step Solution

Analyze the umbilical artery blood gas parameters.

The

pH

6.96

(severe acidemia),

pCO_2

is markedly elevated at

88

mm Hg (hypercapnia), while both bicarbonate (

21

mEq/L) and base excess (

-4

mEq/L) remain within normal limits.

To identify the primary nature of the acid-base derangement.

Classify the type of fetal acidemia.

The presence of severe hypercapnia with a normal base excess (no base deficit) defines an uncompensated, isolated acute respiratory acidosis.

To differentiate between an acute event (no time for lactic acid accumulation) and a prolonged event (which would cause a metabolic lactic acidosis).

Correlate the acid-base diagnosis with the clinical scenario.

An acute respiratory acidosis corresponds to a sudden, catastrophic interruption of umbilical blood flow that rapidly halts

CO_2

clearance. The sudden gush of fluid followed by profound fetal bradycardia points directly to umbilical cord prolapse.

To connect the pathophysiologic mechanism to the correct answer choice.

Key Concept

Fetal umbilical cord blood gas analysis can differentiate acute intrapartum events from chronic hypoxic stress. An isolated acute respiratory acidosis (low pH, high pCO2, normal base excess) indicates a sudden interruption of umbilical blood flow, such as umbilical cord prolapse.

Estimated Time:2m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic