Step 1: Identify the primary disorder from pH, PaCO₂, and HCO₃⁻
The pH is 7.49, which is above the normal range of 7.35–7.45, indicating alkalosis. The PaCO₂ is 29 mmHg, which is below the normal range of 35–45 mmHg. A low PaCO₂ drives the pH upward, so the primary process is respiratory alkalosis. The HCO₃⁻ is 23 mEq/L, which falls within the normal range of 22–26 mEq/L. This means the kidneys have not yet had time to compensate by excreting bicarbonate, so the disorder is acute and uncompensated.
A high pH with a low PaCO₂ and a normal HCO₃⁻ is the classic pattern of acute, uncompensated respiratory alkalosis. In a systematic approach to acid-base interpretation, the first step is to determine the primary process from the pH, PaCO₂, and HCO₃⁻ measurements, and the second step is to evaluate for compensation [1]. Here, the normal bicarbonate confirms that renal compensation has not occurred.
Step 2: Interpret the oxygenation status
The PaO₂ is 58 mmHg while the patient is breathing room air. A PaO₂ below 60 mmHg on room air defines hypoxemic respiratory failure. Because the PaCO₂ is low rather than elevated, this is specifically type 1 respiratory failure, meaning oxygenation is impaired but ventilation is adequate or even increased.
A PaO₂ below 60 mmHg on room air with a low PaCO₂ is hypoxemic (type 1) respiratory failure. This is the expected pattern in pulmonary embolism, where blood flow is blocked to ventilated alveoli, creating a ventilation–perfusion (V/Q) mismatch. The patient hyperventilates in an attempt to improve oxygenation, which lowers PaCO₂ and produces the respiratory alkalosis.
Step 3: Rule out the other options
| Option | Why it is incorrect |
|---|---|
| 1. Mixed respiratory and metabolic alkalosis | Metabolic alkalosis would require an elevated HCO₃⁻ above 26 mEq/L. The HCO₃⁻ of 23 mEq/L is normal, so there is no metabolic component. |
| 2. Metabolic alkalosis with respiratory compensation | The primary disorder is not metabolic because HCO₃⁻ is normal. Also, respiratory compensation for metabolic alkalosis would raise PaCO₂, not lower it. |
| 3. Respiratory alkalosis with full renal compensation | Full renal compensation would lower HCO₃⁻ below 22 mEq/L. A normal HCO₃⁻ means no renal compensation has occurred, indicating an acute process. |
Watch out! Do not confuse a low PaCO₂ with respiratory compensation for a metabolic problem. In this case, the low PaCO₂ is the primary abnormality, not a compensatory response, because the pH is alkalotic and the HCO₃⁻ is normal.
Key point! In PE, the acid-base disturbance is typically acute respiratory alkalosis from hyperventilation, and the oxygenation defect is type 1 respiratory failure from V/Q mismatch. Both findings point to the same underlying pathophysiology: a sudden increase in dead space and a strong drive to breathe.
The combination of acute respiratory alkalosis and hypoxemic respiratory failure in a patient with confirmed PE reflects the body’s immediate response to impaired gas exchange. The low PaCO₂ is not a sign of compensation but of the primary hyperventilation triggered by hypoxemia and increased dead space ventilation.
First, identify the primary disorder using pH, PaCO2, and HCO3−. A pH of 7.49 with PaCO2 of 29 mmHg and normal HCO3− of 23 mEq/L is acute uncompensated respiratory alkalosis.
Next, assess oxygenation. PaO2 of 58 mmHg on room air is below 60 mmHg, defining hypoxemic respiratory failure. With low PaCO2, this is type 1 respiratory failure, the classic V/Q mismatch pattern in pulmonary embolism.
Do not label this as compensated or metabolic alkalosis; normal HCO3− means no renal compensation has occurred yet. Serial ABGs are needed to track compensation over time.
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