Prior studies have concluded that a VBG can be used instead of an ABG with acceptable clinical accuracy for patients with diabetic ketoacidosis (DKA), general severe medical illness, and chronic obstructive pulmonary disease [6
]. By contrast, Malinoski et al [8
] showed that these were not interchangable for posttrauma ventilated patients in the intensive care unit [9
However, this is the largest study comparing ABG to VBG results in acute-phase trauma patients. We found that VBG and ABG results correlate well in trauma patients (r2 = 0.70 for pH and 0.75 for BE). However, their LOAs were broader than the a priori clinically equivalent ranges, and the predicted values fell outside these ranges too frequently (28% for pH and 20% for BE) to allow their clinical substitution.
Given that we did not study trends in acid base status or correlate them with clinical outcomes, we cannot say with certainty that clinical equivalence is absent. A prudent clinician, armed with this data, may choose to consider the 2 values as being more interchangeable than we did. Furthermore, it is not clear which values (ie, ABG vs. VBG) best reflect shock physiology, although one could posit that postcapillary acid-base status might better reflect tissue hypoperfusion.
Brandenburg and Dire [6
] found a strong correlation (r
= 0.97) between arterial and venous pH values with mean difference of 0.03 (range, 0-0.11) in 61 DKA patients, whereas Gennis et al [7
] found a pH difference of 0.056 (-0.07 to 0.35) in a group of 184 acutely ill medical patients [8
]. However, the study of Brandenberg et al [6
] had a nearly 50% drop out rate when patients were not determined to be in DKA and did not report LOAs. Similary, Elborn et al [12
] compared VBG with ABG samples in patients recovering from acute exacerbations of chronic obstructive pulmonary disease. He found a strong correlation between the tests (r
= 0.84), but again, they did not report the proportion of patients whose values fell outside a clinically interchangeable range.
McGillivray et al [5
] studied acutely ill pediatric patients to compare venous and capillary blood gas values. Intraclass correlation coefficients for capillary and venous pH, Pco2
, and Po2
were the main outcomes (0.92, 0.80, and 0.67, respectively). Although this study demonstrated that a venous sample was very well correlated with an arterial specimen, the subject population was well perfused, and a true ABG was not performed. Again, mere correlation, in our view, is insufficient to gauge the appropriateness of clinical substituion.
The study results of Gennis et al [7
] regarding pH were remarkably similar to ours [8
]. They reported that 95% of their ABG pH values were predicted by their regression equation from VBG results within +0.11 pH units. Our study's 95% LOA ranges from was −0.10 to 0.11. However, although Gennis et al [7
] concluded, therefore, that the results were clinically interchangeable, our a priori definition of clinical equivalence was narrower, at ±0.05 or less. Using this definition, only 72% of values in our study were interchangeable, and therefore, VBG cannot be used to approximate ABG pH accurately enough to allow clinical substitution. Furthermore, the study population of Gennis et al was poorly described and, therefore, likely heterogeneous (including a minority of cardiac arrest patients), compared with our relatively homogenous trauma victim group.
A pilot study by Gindi et al [13
] of 30 severely injured trauma patients compared near-simultaneous ABG and VBG and showed a Pearson correlation coefficient for BE of 0.922, indicating that 85% (R2
) of the variability in ABG base deficit can be accounted for by VBG results. However, this study did not report correlation data on pH nor LOAs. Although they concluded that VBG may be able to replace ABG, to substitute the venous test for the arterial, in our view, near equivalence would need to be shown for both components of acid-base status.
Although the explanation for these discrepant conclusions (apparent reliable substitution in medical patients vs insufficient clinical precision in trauma patients) is unclear, we postulate that the clinical and hemodynamic state in trauma would change more rapidly than in medical patients, due to ongoing hemorrhage. Hence, a longer interval between blood samples may reflect a greater change in values than for medical patients. However, we did not find that the discrepancy between ABG and VBG values varied significantly by time.
Furthermore, we had short transport times in an urban emergency medical services system and assessed patients essentially in the first hour after injury, when hemodynamic state would be most rapidly changing. This is contrary to the situation with acutely ill medical patients, whose illness more commonly develops over several hours to days. We believe that our study results will be applicable to most urban, ACS-verified centers that also have short emergency medical services transport times.
Our venous blood samples were drawn from a peripheral vein, whereas our arterial samples were drawn from a central source (femoral). This would have an unknown effect on the acid-base values and may account for our observed discrepancy. To address this, the study would have to be repeated with both samples drawn centrally, which is not feasible during the acute resuscitation.
Our study advanced the issue of ABG vs VBG results by studying BE. Schmelzer et al [14
] previously reported on 100 critically ill trauma patients (24 died) and found that central venous base deficit was associated with survival past 24 hours, whereas arterial base deficit was not. Traditional teaching has advocated acid-base determination via ABG, but high Fio2
can artificially drive Po2
, yet tissues may extract inadequate oxygen. With improvements in pulse oximetry making arterial Po2
less relevant, we believe, in concert with Schmelzer et al, that venous BE may be a more clinically important determinant of global perfusion and, hence, patient outcome. Hence, there may be additional reasons, beyond pain and risk of arterial injury to deemphasize ABG in favor of VBG.
Our objective to determine if ABG and VBG are clinically interchangable in trauma patients begs the question of whether VBG alone or serial VBG values are sufficient to guide resuscitation. As we drew only 1 pair of samples and did not follow up patients to clinical outcome, our study design does not answer this question of clinical relevance.
Future studies would improve on the current offering by enrolling a consecutive sample, shortening the time between blood draws, and following up patients clinically for outcome. Arterial blood gas on all trauma patients would be required to fully answer the clinical question of equivalence but would be unethical in patients with minor injuries. Lastly, assessment of central rather than peripheral venous pH and BE would likely be more physiologically relevant in early resuscitation of trauma patients but is inherently more difficult to study.