Venous versus arterial blood for gas analysis

Although arterial blood remains the gold standard sample for blood gas analysis, it is, compared with peripheral venous blood, a more difficult sample to obtain, and its collection is more painful and hazardous for the patient. These considerations have fuelled a growing interest in study aimed at establishing if venous blood might be a suitable alternative sample for blood gas analysis, in at least some circumstances.

Clinical studies, which have compared blood gas results obtained from an arterial sample with those obtained from a simultaneously collected venous sample, have found arguably clinically acceptable agreement for acid-base parameters (pH, pCO₂ and bicarbonate) but, as is predicted from physiology, very poor correlation between arterial and venous measures of oxygenation (pO₂, sO₂).

These studies suggest a possible role for venous blood gases in the assessment of patients in whom acid-base balance is the only concern (e.g. in diabetic ketoacidosis). It is clear from these studies that there is significant variability in the arterio-venous (A-V) difference for all blood gas parameters among patients requiring blood gas analysis.

Understanding the cause of this variability is important for more widespread adoption of venous blood for assessment of patient acid-base status. The notion that patient blood pressure affects A-V difference of blood gas parameters is explored by a recently published study from Iran.

The study population comprised 192 adult patients admitted to the emergency department and whose clinical condition demanded blood gas analysis. Of these 192, 78 were suffering hypotension - defined as systolic/diastolic blood pressure less than 90/60 - and the remaining 114 patients were normotensive.

Arterial and venous blood were collected simultaneously (within 3 minutes) from all 192 patients and submitted for blood gas analysis; the attending physician only received arterial blood gas results.

The mean A-V difference for pH was higher for the hypotensive group compared with those who were normotensive (–0.03 versus –0.016), indicating that hypotension is associated with poorer agreement between arterial and venous pH. However, the 95 % limits of agreement (LOA) were wider for the normotensive group (–0.1 to –0.068) than for the hypotensive group (–0.09 to –0.03).

With regard to pCO₂, both mean and 95 % LOA data indicated that hypotension is associated with greater A-V difference compared with normal blood pressure: mean A-V difference (95 % LOA) was 2.69 mmHg (–20.43 to +25.81) for the hypotensive group compared with 2.03 mmHg (–7.75 to +11.81) for the normotensive group.

A similar finding was evident when data relating to bicarbonate and base excess was considered. Overall, this study indicates that for all acid-base parameters generated during blood gas analysis, agreement between arterial and venous values is likely to be better among patients with normal blood pressure than among those who are hypotensive.