# Situation: A 34-year-old man is admitted to the intensive care unit (ICU) after an intentional overdose of an unknown drug with aspiration of gastric contents. He is intubated and receiving mechanical ventilation. The consultation–liaison psychiatry team will see him once he is awake. On a fraction of inspired oxygen (FiO2) of 0.40, his oxygen saturation by pulse oximetry (SpO2) is 97% with a clear waveform, and his breathing and color are unchanged. A new arterial blood gas (ABG) sample was drawn by radial puncture and analyzed within 10 minutes. Results (normal ranges: pH 7.35–7.45; partial pressure of arterial carbon dioxide, PaCO2, 35–45 mmHg; partial pressure of arterial oxygen, PaO2, 80–100 mmHg; bicarbonate 22–26 mEq/L): - 06:00: pH 7.41, PaCO2 40 mmHg, PaO2 92 mmHg, bicarbonate 24 mEq/L - 10:00: pH 7.36, PaCO2 47 mmHg, PaO2 38 mmHg, bicarbonate 26 mEq/L Which explanation of the 10:00 result is MOST likely?

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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 34-year-old man is admitted to the intensive care unit (ICU) after an intentional overdose of an unknown drug with aspiration of gastric contents. He is intubated and receiving mechanical ventilation. The consultation–liaison psychiatry team will see him once he is awake.

On a fraction of inspired oxygen (FiO2) of 0.40, his oxygen saturation by pulse oximetry (SpO2) is 97% with a clear waveform, and his breathing and color are unchanged. A new arterial blood gas (ABG) sample was drawn by radial puncture and analyzed within 10 minutes. Results (normal ranges: pH 7.35–7.45; partial pressure of arterial carbon dioxide, PaCO2, 35–45 mmHg; partial pressure of arterial oxygen, PaO2, 80–100 mmHg; bicarbonate 22–26 mEq/L):
- 06:00: pH 7.41, PaCO2 40 mmHg, PaO2 92 mmHg, bicarbonate 24 mEq/L
- 10:00: pH 7.36, PaCO2 47 mmHg, PaO2 38 mmHg, bicarbonate 26 mEq/L
Which explanation of the 10:00 result is MOST likely?

## 보기

1. The client has new severe hypoxemia
2. Excess liquid heparin diluted the sample
3. It was a venous sample drawn in error **✔ 정답**
4. An air bubble was left in the syringe

**정답: 3**

## 해설

An oxygen pressure of 38 mmHg cannot coexist with an SpO2 of 97% and an unchanged clinical picture. Venous blood typically shows an oxygen pressure near 40 mmHg, a carbon dioxide pressure several mmHg higher and a slightly lower pH, which matches the 10:00 result. The nurse reports it as a probable sampling error and repeats the arterial sample.

## 심화 해설

Clinical picture versus ABG result

The most important first step is to compare the arterial blood gas (ABG) values with the patient’s clinical presentation. At 10:00, the ABG shows a partial pressure of arterial oxygen (PaO2) of 38 mmHg, which would indicate severe hypoxemia. However, the patient’s oxygen saturation by pulse oximetry (SpO2) is 97% on FiO2 0.40, with a clear waveform and no change in breathing or color. A PaO2 of 38 mmHg cannot physiologically coexist with an SpO2 of 97% and an unchanged clinical picture. This mismatch is the key clue that the sample itself is the problem, not the patient’s oxygenation.

Key point! Always interpret ABG results in the context of pulse oximetry and the patient’s appearance. A laboratory value that contradicts a reliable clinical finding should prompt you to suspect a preanalytical or sampling error before assuming a sudden change in the patient’s condition.

Why the 10:00 sample looks venous

The 10:00 ABG shows a pH of 7.36, a partial pressure of arterial carbon dioxide (PaCO2) of 47 mmHg, and a bicarbonate of 26 mEq/L. These values are typical of venous blood: venous blood normally has a PaO2 near 40 mmHg, a PaCO2 several mmHg higher than arterial blood, and a slightly lower pH. The 06:00 sample was clearly arterial, with a PaO2 of 92 mmHg. The shift from 92 mmHg to 38 mmHg over four hours, without any clinical deterioration, is best explained by inadvertent venous sampling during the radial puncture.

Watch out! A falling PaO2 with a rising PaCO2 and falling pH can mimic hypoventilation or respiratory failure. But in this case, the SpO2 remains normal and the patient is stable, so the pattern is far more consistent with a venous sample than with a true arterial blood gas change.

Ruling out the other options

| Option | Why it is less likely |
| --- | --- |
| New severe hypoxemia | A true PaO2 of 38 mmHg on FiO2 0.40 would produce a much lower SpO2 and visible respiratory distress. The patient’s SpO2 of 97% and unchanged color exclude this. |
| Excess liquid heparin dilution | Heparin dilution can lower the measured PaCO2 and bicarbonate, but it does not selectively drop PaO2 to 38 mmHg while leaving the clinical picture normal. It also would not produce the venous-like pattern seen here. |
| Air bubble in the syringe | An air bubble typically causes the PaO2 to rise toward atmospheric oxygen levels, not fall to 38 mmHg. It may also alter PaCO2, but the direction of PaO2 change argues against this option. |

Preanalytical error and repeat sampling

Blood gas results are highly sensitive to how the sample is obtained, handled, and stored. A significant change in blood gas values should only be considered real if it exceeds certain thresholds: about 0.015 for pH, 3 mmHg for PaCO2, and 5 mmHg for PaO2 [1]. In this patient, the PaO2 change from 92 mmHg to 38 mmHg is far larger than 5 mmHg, but the clinical stability and normal SpO2 indicate that the change is not a true physiological shift. Instead, it reflects a preanalytical error, most likely drawing venous blood instead of arterial blood.

The appropriate nursing action is to recognize the probable sampling error and repeat the arterial blood gas sample rather than treating the patient for hypoxemia or adjusting the ventilator. This prevents unnecessary interventions and clarifies the true acid–base and oxygenation status. Plastic syringes and delays in analysis can also introduce errors, but the venous-like values here point specifically to a wrong-sample problem [2].References (research sources)

- [1]Sampling and storage of blood for pH and blood gas analysis.Research articleHaskins SC (1977)

- [2]Plastic Blood Gas Syringes and Measurement Error in Central Venous Oxygen Saturations.Research articleGhanpur R, Santamaria J, Dixon B (2016) · DOI: 10.1097/SHK.0000000000000622

## 임상 시나리오

ABG-SpO2 Mismatch: Suspect Sampling ErrorWhen the gas does not match the patient, question the sample
A PaO2 38 mmHg cannot coexist with an SpO2 97% and an unchanged clinical picture. Always interpret ABG results in the context of pulse oximetry and the patient's appearance before assuming a sudden change in condition.

The 10:00 sample shows pH 7.36, PaCO2 47 mmHg, and PaO2 38 mmHg—values typical of venous blood. Venous blood normally has a PaO2 near 40 mmHg, a PaCO2 several mmHg higher than arterial, and a slightly lower pH.

CautionDo not treat a number that contradicts a reliable clinical finding. Report the probable sampling error and repeat the arterial sample before making any oxygenation or ventilation changes.

## 핵심 개념

- **Preanalytical error** — Error occurring before sample analysis, such as wrong site draw, air bubble, or heparin dilution
- **Venous blood gas** — Typically PaO2 near 40 mmHg, PaCO2 several mmHg higher than arterial, slightly lower pH
- **Pulse oximetry** — Noninvasive SpO2 measurement that should correlate with PaO2 on ABG
- **PaO2/FiO2 ratio** — Oxygenation index; a PaO2 of 38 on FiO2 0.40 would indicate severe hypoxemia
- **Radial artery puncture** — Arterial sampling site; can inadvertently draw venous blood if needle placement is off

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