Effects of exogenous lipids on blood gas measurements

Numerous reports have appeared describing the effects of intravenous lipid administration on the pulmonary function of the critically ill patient. Our study was undertaken to determine whether the lipid content of an arterial blood gas specimen affects the measurement of arterial pH, PaO2, PaCO2, or arterial oxygen saturation.

Prospective, in vitro controlled study.

Medical and cardiac intensive care units.

Critically ill patients undergoing clinically-directed blood gas sampling via indwelling arterial catheters.

None.

Arterial blood gas specimens were modified in vitro by dividing the sample and adding a known amount of lipid emulsion to half of the sample, resulting in a difference between the plasma triglyceride concentrations of the two halves. Two series of experiments were run: one series was run with a predicted plasma triglyceride difference of 400 mg/dL (4.5 mmol/L) between the two samples; the other series was run with a predicted plasma triglyceride difference of 800 mg/dL (9.0 mmol/L) between the two samples. Blood gas measurements were performed on each half of a sample, and the results were compared. Because some studies have only noted changes in patients with the adult respiratory distress syndrome (ARDS), samples from these patients were also analyzed as a separate group.

No significant changes were found in arterial pH, PaO2, PaCO2, or arterial oxygen saturation between the two halves of the sample. With 95% confidence, differences as small as 1.5 torr (0.2 kPa) for PaO2 and PaCO2, 0.5% for arterial oxygen saturation, and 0.005 for pH, would have been detected. No differences were found in the ARDS subgroup.

The addition of clinically relevant amounts of lipid to blood samples does not affect blood gas measurements. Any observed changes in blood gas values after lipid feeding are presumably due to products of lipid metabolism or alterations in pulmonary function.