Is venous blood an acceptable alternative to arterial blood for blood gas analysis – and can pulse oximetry help make it so?

Blood gas analysis, which involves measurement of three parameters: blood pH, pCO₂ and pO₂, is almost unique among routine blood tests in its requirement for arterial blood. All other blood tests are performed on venous blood or occasionally, capillary blood.

Collection of arterial blood is technically more demanding and associated with more pain and risk to the patient than collecting venous blood. Although arterial blood is the gold standard sample for blood gases, it would be logistically easier for clinical staff and safer and more comfortable for patients if venous blood were used.

This study is the most recent of a number that have addressed the question: is venous blood an acceptable alternative to arterial blood for blood gas analysis? Previous study has demonstrated good correlation and acceptable agreement between arterial and venous pH, and arguably, not quite as good agreement between arterial and venous pCO₂.

All studies have, however, demonstrated predictably poor agreement between arterial and venous pO₂. Arterial blood is clearly essential if the parameter of blood oxygenation (pO₂) is required, but may not be essential if only measures of acid-base and ventilation (pH and pO₂) are of interest.

Previous studies have investigated the relationship between arterial and venous pH and pCO₂ among discrete, defined populations of acutely ill patients who require blood gases, e.g. patients with diabetic ketoacidosis or patients with acute exacerbation of chronic obstructive pulmonary disease (COPD).

It remains unclear if the level of agreement between arterial and venous pH and pCO₂ seen in these patient populations applies across the board to the generality of patients who require blood gases.

The authors of this most recent study sought to address this knowledge gap by selecting patients for study only on the basis that their attending physician deemed arterial blood gas analysis necessary for their clinical care.

In the event the study population comprised 129 patients with diverse diagnoses, of whom 53 were in emergency room, 41 were in medical ICU and 35 were in surgical ICU. The primary goal of this study was to compare venous pH and pCO₂ to arterial pH and pCO₂. Additionally they wanted to compare SpO₂, the parameter of blood oxygenation generated by non-invasive pulse oximetry, to arterial pO₂.

Arterial and venous blood, sampled simultaneously from each study patient, was submitted for blood gas analysis. SpO₂ at the time of blood sampling was recorded.

Statistical analysis of paired results revealed good correlation and close agreement between venous and arterial pH: Pearson correlation coefficient (r) 0.94; mean difference (arterial minus venous) was 0.03 pH units (95 % confidence interval 0.03-0.04); and Bland-Altman analysis revealed 95 % limits of agreement (LOA) –0.05 to 0.12.

Results for pCO₂ were equally indicative of good correlation and acceptably close agreement: Pearson correlation (r) 0.93; mean difference (arterial-venous) was –4.88 mmHg [–0.64 kPa] (95 % confidence interval –3.7-6.0 mmHg [–0.48-0.80 kPa]); and 95 % LOA –17.82 to 8.05 mmHg [–2.37 to +1.07 kPa].

Subgroup analysis revealed that these levels of correlation and agreement between venous and arterial pH and pCO₂ held irrespective of initial working diagnosis, irrespective of patient disposition (emergency room, medical ICU or surgical ICU), and irrespective of whether the venous blood was sampled from a peripheral vein or via central venous catheter.

Study authors chose to assess correlation between SpO₂ and arterial pO₂ by use of the clinical dictum – based on the oxygen-hemoglobin dissociation curve – that when SpO₂ is >90 %, then arterial pO₂ is >60 mmHg (8 kPa). By this measure SpO₂ correlated with arterial pO₂ in 122 of 129 (94.6 %) study patients.

The authors conclude that their study results indicate that venous blood gases plus pulse oximetry provide sufficiently accurate information to make clinical decisions regarding acid-base, ventilation and oxygenation status for the generality of critically ill patients in emergency room and intensive care units.

In discussion of their study, the authors reflect on the limitations of using pulse oximetry (SpO₂) rather than arterial pO₂ to assess patient oxygenation status.