# Situation: A 52-year-old woman taking estrogen-containing hormone therapy is admitted to the intensive care unit (ICU) 3 days after a 14-hour flight. Computed tomography pulmonary angiography confirms pulmonary embolism (PE). She weighs 75 kg and has no history of bleeding, stroke, recent surgery, or head injury. An arterial blood gas drawn while she breathed room air shows: pH 7.49, partial pressure of arterial carbon dioxide (PaCO2) 29 mmHg, bicarbonate (HCO3−) 23 mEq/L, partial pressure of arterial oxygen (PaO2) 58 mmHg. Normal: pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3− 22–26 mEq/L. Which interpretation is correct?

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> subject: Nursing Practice V — Care of Clients with Maladaptive Patterns of Behavior; Care of Clients with Life-Threatening Conditions, Acute Multi-Organ Problems, High Acuity and Emergency Situations

## 문제

Situation: A 52-year-old woman taking estrogen-containing hormone therapy is admitted to the intensive care unit (ICU) 3 days after a 14-hour flight. Computed tomography pulmonary angiography confirms pulmonary embolism (PE). She weighs 75 kg and has no history of bleeding, stroke, recent surgery, or head injury.

An arterial blood gas drawn while she breathed room air shows: pH 7.49, partial pressure of arterial carbon dioxide (PaCO2) 29 mmHg, bicarbonate (HCO3−) 23 mEq/L, partial pressure of arterial oxygen (PaO2) 58 mmHg. Normal: pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3− 22–26 mEq/L. Which interpretation is correct?

## 보기

1. Mixed respiratory and metabolic alkalosis with hypoxemia
2. Metabolic alkalosis with respiratory compensation and hypoxemia
3. Respiratory alkalosis with full renal compensation and hypoxemia
4. Acute respiratory alkalosis with hypoxemic respiratory failure **✔ 정답**

**정답: 4**

## 해설

A high pH with a low PaCO2 and a normal HCO3− is an acute, uncompensated respiratory alkalosis from hyperventilation. A PaO2 below 60 mmHg on room air with a low PaCO2 is hypoxemic (type 1) respiratory failure, the typical pattern of ventilation–perfusion mismatch in PE.

## 심화 해설

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.
References (research sources)

- [1]Acid-Base Interpretation: A Practical Approach.Research articleMorikawa MJ, Ganesh PR (2025)

## 임상 시나리오

Acute Respiratory Alkalosis with Type 1 Hypoxemic Failure in PEStepwise acid-base and oxygenation interpretation at the bedside
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.

CautionDo 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.

## 핵심 개념

- **Acute respiratory alkalosis** — High pH with low PaCO2 and normal HCO3−; kidneys have not yet compensated by excreting bicarbonate
- **Type 1 hypoxemic respiratory failure** — PaO2 below 60 mmHg on room air with normal or low PaCO2; oxygenation failure without hypercapnia
- **V/Q mismatch** — Ventilation–perfusion mismatch; in PE, blood flow is blocked to ventilated alveoli, impairing oxygenation
- **Compensation** — Renal response to respiratory alkalosis is bicarbonate excretion, lowering HCO3− over hours to days
- **Uncompensated** — Normal HCO3− indicates the kidneys have not yet responded to the primary respiratory disorder

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