Respiratory system: ventilation in normal pregnancy: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Respiratory system: ventilation in normal pregnancy

Sample item

A woman at 30 weeks of gestation is comfortable at rest. Arterial blood gas shows pH 7.44, PaCO2 30 mmHg and bicarbonate 20 mmol/L. How should this be interpreted?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Normal pregnancy: progesterone-driven hyperventilation with renal bicarbonate compensation

Progesterone sensitises the respiratory centre to CO2, so minute ventilation rises and PaCO2 falls to about 30 mmHg. The kidneys excrete bicarbonate to keep pH near normal. A normal PaCO2 of 40 mmHg in late pregnancy should therefore raise concern about fatigue or respiratory compromise.

Option by option

  • A. Normal pregnancy: progesterone-driven hyperventilation with renal bicarbonate compensation (correct)

    Correct. A low PaCO2 with a low bicarbonate and near-normal pH is the expected compensated state.

  • B. Pulmonary embolism with respiratory acidosis

    Embolism would not be expected to cause hypercapnic acidosis with a normal pH.

  • C. Acute respiratory acidosis

    Acute respiratory acidosis would have a high PaCO2 and a low pH.

  • D. Compensated metabolic acidosis

    The PaCO2 is low and the pH is above 7.40, so this is not acidosis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Normal pregnancy: progesterone-driven hyperventilation with renal bicarbonate compensation. Progesterone sensitises the respiratory centre to CO2, so minute ventilation rises and PaCO2 falls to about 30 mmHg.

Why is "Pulmonary embolism with respiratory acidosis" not the answer?

Embolism would not be expected to cause hypercapnic acidosis with a normal pH.

Why is "Acute respiratory acidosis" not the answer?

Acute respiratory acidosis would have a high PaCO2 and a low pH.

Why is "Compensated metabolic acidosis" not the answer?

The PaCO2 is low and the pH is above 7.40, so this is not acidosis.

What concept does this question test?

It tests Respiratory system: ventilation in normal pregnancy in Physiology. The kidneys excrete bicarbonate to keep pH near normal. A normal PaCO2 of 40 mmHg in late pregnancy should therefore raise concern about fatigue or respiratory compromise.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Respiratory system: ventilation in normal pregnancy?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.