Question:
A 28-year-old primigravida woman is admitted to the hospital at 10 weeks gestation. Temperature is 36.7°C (98°F), pulse is 96/min, and respirations are 12/min. Arterial blood gas laboratory results are as follows:

• pH: 7.49

• PaCO2: 54 mm Hg

• Bicarbonate: 44 mEq/L.

Which of the following is the most likely cause of this patient's abnormal arterial blood gas?

A. Asthma exacerbation
B. Hyperemesis gravidarum
C. Normal phenomenon of pregnancy
D. Obesity hypoventilation
E. Pulmonary embolism
F. Severe diarrhea

Answer:
B. Hyperemesis gravidarum

🧠 Pregnancy-related Hormonal Changes (e.g., Increased hCG)
⬇️
Increased vomiting frequency and severity
⬇️
Gastric content loss (rich in hydrochloric acid)
⬇️
Metabolic alkalosis (due to loss of HCl)
⬇️
🧠 Volume depletion (fluid loss from vomiting)
⬇️
Renal compensation: ↑ bicarbonate retention to maintain pH
⬇️
Blood gas abnormalities:

pH ↑ (alkalosis)

PaCO2 ↑ (compensatory metabolic alkalosis)

Bicarbonate ↑ (renal compensation)
⬇️
🧠 Key Symptoms:

Temperature 36.7°C (98°F) (normal)

Pulse 96/min (mild tachycardia due to dehydration)

Respirations 12/min (normal respiratory rate)
⬇️
🧠 Clinical Examination

Signs of dehydration (dry mouth, reduced skin turgor)

Weight loss (due to sustained vomiting)
⬇️
Severe Vomiting → Disruption of normal metabolic processes → Alkalosis and Volume Depletion

Explanation of Incorrect Answers

A. Asthma exacerbation
⬇️
Asthma typically causes respiratory acidosis, not alkalosis, due to air trapping and CO2 retention.
⬇️
Asthma symptoms also include wheezing and dyspnea, which are absent in this patient.

C. Normal phenomenon of pregnancy
⬇️
Pregnancy does cause mild respiratory alkalosis due to increased ventilation but not such significant metabolic changes like in this case (severe vomiting and metabolic alkalosis).

D. Obesity hypoventilation
⬇️
This condition is characterized by respiratory acidosis due to hypoventilation and not alkalosis. This patient does not have features like obesity or hypoventilation.

E. Pulmonary embolism
⬇️
Pulmonary embolism typically leads to respiratory alkalosis but also hypoxia, tachycardia, and chest pain, which are not present in this case. Additionally, ABG changes in PE usually show low PaO2, which is absent here.

F. Severe diarrhea
⬇️
Diarrhea usually causes metabolic acidosis due to loss of bicarbonate, not alkalosis. This patient's findings are consistent with metabolic alkalosis, not acidosis.

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