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