Measuring acid-base variables – is arterial blood essential?

Conventionally, arterial blood is sampled for blood gas analysis, but if the purpose is to assess acid-base balance, venous blood may be an acceptable alternative. That is the main conclusion of a recently published study conducted at the intensive care units of two Melbourne hospitals. 

The objective of the study was to assess the level of agreement between arterial and central venous blood for four parameters of acid-base balance: pH, bicarbonate, base excess and lactic acid. The study population comprised 110 intensive care patients who required arterial blood gas analysis. From each of these patients blood was sampled simultaneously via an in situ arterial line and in situ venous line. 

In total, the investigators obtained 168 matched arterial and venous sample pairs from the 110 patients. Both arterial and venous samples were analyzed using the same blood gas analyzer located within the intensive care units.

All four variables showed close agreement. The mean differences between arterial and venous blood for each of the analytes were: pH 0.03 pH units, bicarbonate 0.52 mmol/L, bases excess 0.19 mmol/L and lactate 0.08 mmol/L. The ranges of difference (95 % limits of agreement) were: pH –0.07 to 0.01, bicarbonate –1.81 to 2.85, bases excess –1.86 to 2.24 and lactate –0.27 to 0.42.

Although the authors concede that for certain subsets of intensive care patients the arteriovenous difference may be larger than the pooled results of this study indicate, they conclude that they have demonstrated a high level of agreement between central venous and arterial blood for all four parameters. 

It remains to be seen if this level of agreement is sufficient for widespread acceptance among clinicians of the notion that venous blood is an acceptable alternative to arterial blood for assessment of acid-base balance.