# Situation: A 68-year-old man with chronic obstructive pulmonary disease (COPD) and a 45 pack-year smoking history is admitted to the medical ward with an exacerbation: 3 days of increased breathlessness and more purulent sputum. His records show chronic carbon dioxide retention. On room air, his oxygen saturation by pulse oximetry (SpO2) is 84%. On day 3 he is on a nasal cannula at 1 L/min with an SpO2 of 90%. A worried relative turns the flowmeter up to 6 L/min. Two hours later the nurse finds him hard to rouse. He complains of headache, his skin is warm and flushed, his respiratory rate has fallen from 22 to 10/min, and his SpO2 is 98%. What is the MOST likely explanation?

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> subject: Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations

## 문제

Situation: A 68-year-old man with chronic obstructive pulmonary disease (COPD) and a 45 pack-year smoking history is admitted to the medical ward with an exacerbation: 3 days of increased breathlessness and more purulent sputum. His records show chronic carbon dioxide retention. On room air, his oxygen saturation by pulse oximetry (SpO2) is 84%.

On day 3 he is on a nasal cannula at 1 L/min with an SpO2 of 90%. A worried relative turns the flowmeter up to 6 L/min. Two hours later the nurse finds him hard to rouse. He complains of headache, his skin is warm and flushed, his respiratory rate has fallen from 22 to 10/min, and his SpO2 is 98%. What is the MOST likely explanation?

## 보기

1. Better oxygenation has let him rest after days of breathlessness
2. Worsening hypoxia of the brain from the exacerbation
3. Fatigue of his breathing muscles from a new pneumonia
4. Too much oxygen has raised his blood carbon dioxide level **✔ 정답**

**정답: 4**

## 해설

In a client with chronic carbon dioxide retention, oxygen above the target worsens ventilation–perfusion matching and reduces carbon dioxide carriage by hemoglobin, so the arterial carbon dioxide level rises. Drowsiness, headache, warm flushed skin, and a falling respiratory rate with an SpO2 above target are signs of carbon dioxide narcosis. The nurse returns the flow to the prescribed rate without stopping oxygen, obtains an arterial blood gas, and notifies the physician.

## 심화 해설

Why the patient became drowsy after the oxygen was increased

The most likely explanation is that the high oxygen flow raised his arterial carbon dioxide level, producing carbon dioxide narcosis. This patient has chronic CO₂ retention from long-standing COPD. His respiratory drive no longer depends primarily on a falling PaO₂; instead, it is driven partly by a chronically elevated PaCO₂ and partly by hypoxemic stimulation of peripheral chemoreceptors. When a relative increased the nasal cannula from 1 L/min to 6 L/min, the inspired oxygen concentration rose sharply and his SpO₂ climbed to 98%. That level is well above the target range for a known CO₂ retainer.

In a patient with chronic hypercapnia, excessive oxygen worsens ventilation–perfusion matching and reduces carbon dioxide carriage by hemoglobin, so the PaCO₂ rises further. Two mechanisms are involved. First, high inspired oxygen abolishes the hypoxic pulmonary vasoconstriction that had been directing blood away from poorly ventilated lung units. Perfusion then increases to areas with poor ventilation, creating more dead-space ventilation and allowing CO₂ to accumulate. Second, when hemoglobin is already highly saturated, less reduced hemoglobin is available to carry CO₂ as carbaminohemoglobin, and the Haldane effect shifts CO₂ transport so that more CO₂ remains dissolved in plasma.

The clinical picture supports hypercapnia rather than improved rest or worsening hypoxia. The patient is difficult to rouse, complains of headache, has warm flushed skin, and his respiratory rate has fallen from 22 to 10/min. Drowsiness, headache, warm flushed skin, and a falling respiratory rate with an SpO₂ above target are signs of CO₂ narcosis. If the problem were simply better oxygenation allowing rest, the respiratory rate would not drop to 10/min with obtundation. Worsening brain hypoxia is also unlikely because the SpO₂ is 98%. Respiratory muscle fatigue from a new pneumonia would typically present with tachypnea and increased work of breathing, not bradypnea and depressed consciousness.

Watch out! A high SpO₂ in a known CO₂ retainer is not reassuring. The target SpO₂ for most COPD patients with chronic hypercapnia is 88–92%. A reading of 98% signals oxygen overshoot.

Key point! The nurse should return the flow to the prescribed rate without stopping oxygen completely. Abruptly removing oxygen can cause dangerous hypoxemia. An arterial blood gas should be obtained, and the physician must be notified.

The case reports reinforce this concern. One describes a patient with COPD who self-administered high-flow oxygen and developed hypercapnic respiratory failure with severe respiratory acidosis requiring ICU admission and non-invasive ventilation [1]. Another case series notes that high-dose oxygen therapy with FiO₂ greater than 50% can induce significant hypercapnia in elderly patients even without an identifiable additional cause [2]. These reports support the principle that unsupervised increases in oxygen delivery are hazardous in patients with chronic CO₂ retention.

| Finding | Supports CO₂ narcosis | Does not support |
| --- | --- | --- |
| SpO₂ 98% on 6 L/min | Oxygen overshoot above target | Rules out worsening brain hypoxia |
| Respiratory rate 10/min | Depressed respiratory drive from rising PaCO₂ | Not typical of fatigue or pneumonia |
| Drowsiness, headache, flushed skin | Cerebral vasodilation and CO₂ narcosis | Not explained by restful sleep |

The priority nursing action is to reduce the oxygen flow to the prescribed rate while maintaining some oxygen delivery, assess the patient’s level of consciousness and respiratory effort, obtain an arterial blood gas to confirm the PaCO₂ and pH, and notify the physician for further management such as consideration of non-invasive ventilation if respiratory acidosis is severe.References (research sources)

- [1]Oxygen Overshoot: A Case Report Navigating the Perils of Unsupervised Supplementation in Chronic Obstructive Pulmonary Disease (COPD).Case reportYetiskul E, Lisle S, Rizvi TA, Khan S, Maniatis GA. (2023) · DOI: 10.7759/cureus.50274

- [2]High-Dose Oxygen Therapy and Acute Hypercapnia in Elderly Patients: A Case Series Analysis.Case reportSeery JP. (2024) · DOI: 10.12659/ajcr.945044

## 임상 시나리오

Oxygen Safety in Chronic CO2 RetentionPreventing CO2 Narcosis in COPD Exacerbation
In a patient with chronic carbon dioxide retention, oxygen above the prescribed target worsens ventilation–perfusion matching and reduces CO2 carriage by hemoglobin, so PaCO2 rises. Drowsiness, headache, warm flushed skin, and a falling respiratory rate with an SpO2 above target indicate CO2 narcosis.

The target SpO2 for a known CO2 retainer is typically 88–92%. An SpO2 of 98% on 6 L/min is well above the safe range and suppresses the hypoxic drive that was helping maintain ventilation.

CautionDo not stop oxygen abruptly. Return the flow to the prescribed rate, obtain an arterial blood gas, and notify the physician immediately. Abrupt withdrawal can cause dangerous hypoxemia.

## 핵심 개념

- **CO2 narcosis** — A depressed level of consciousness caused by severe hypercapnia, presenting with drowsiness, headache, flushed skin, and reduced respiratory rate.
- **Haldane effect** — Oxygenated hemoglobin has a reduced capacity to carry CO2, so high PaO2 shifts CO2 into plasma, raising PaCO2.
- **Hypoxic pulmonary vasoconstriction** — A physiologic mechanism that shunts blood away from poorly ventilated alveoli; high oxygen abolishes it, worsening V/Q mismatch.
- **Target SpO2 in chronic CO2 retention** — Typically 88-92% to avoid suppressing hypoxic respiratory drive and worsening hypercapnia.

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