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Difference Between Arterial Blood Gases and Venous Blood Gases

Discussion in 'Emergency Medicine' started by Dr.Scorpiowoman, Aug 27, 2019.

  1. Dr.Scorpiowoman

    Dr.Scorpiowoman Golden Member

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    Do we need to get ABGs in the ED or are VBGs enough?
    Blood gas testing can be an extremely valuable part of Emergency Department (ED) assessment of patients. Blood gasses rapidly give the clinician a host of vital information including pH, PCO2, lactate, electrolytes and hematocrit. These data can be used to determine acid-base status, anion gap and the presence of tissue hypoperfusion. Arterial blood gas (ABG) testing was the standard testing strategy for decades but venous blood gas (VBG) testing has emerged over the last 10–15 years. Do VBGs give accurate information for the clinician to act on?

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    ABGs are easy. Why not just do the ABG?
    It’s true that obtaining an arterial blood sample for ABG assessment is easy . . . for the physician. Unfortunately, they are quite painful for the patient. Senior physicians tell anecdotes about patients with asthma or diabetes who avoided trips to the ED because they were scarred by the pain inflicted upon them by blood gas sampling. In addition, there are a number of dangers associated with arterial sampling as well.

    The most common location for obtaining an ABG is the radial artery. Prior to doing an ABG at the radial artery, a modified Allen’s test should be performed assessing for dual arterial supply to the hand (ulnar and radial artery). Although rare, disruption of blood supply from the radial artery during ABG sampling can result in ischemia to the hand if the ulnar artery is not patent. Despite this recommendation, the modified Allen’s test is not routinely performed (at least not in our experience). In addition to the rare case of ischemia, there are more common complications including hematoma, radial neuropathy and radial artery aneurysm.

    Obtaining a VBG, on the other hand, does not run any of these risks. In fact, getting a VBG doesn’t even require an additional needle stick as it can be drawn off the IV you have placed for the rest of your labs.

    Okay, so VBGs are easier and more patient friendly but do they give accurate, usable information?
    As discussed earlier, a VBG gives you the same information as an ABG. However, this information is going to be different since it’s obtained from the venous as opposed to the arterial side. The most obvious difference will be in PO2 which, of course, will be markedly lower in the venous side than in the arterial side. Prior to the ubiquitous presence of O2 saturation monitors, this would have been an issue as an ABG was the only way to get an accurate assessment of the amount of O2 dissolved in the blood. Now, though, we have reliable and accurate saturation monitors that give us information about the patient’s oxygenation status non-invasively and in near real time (there is a delay between changes in O2 saturation and reflection of these changes on the saturation monitor).

    What about the other major blood gas outputs? How closely does the VBG pH, lactate, CO2 and HCO3match values obtained in an ABG? Much of the literature that exists on this topic comes from small studies looking at patients who present with diabetic ketoacidosis1–3 although there are studies that involve other pathologies4–5. The summary of these studies is:

    pH ABG vs. VBG: VBG pH 0.03–0.05 lower
    HCO3 ABG vs. VBG: VBG HCO3 1.5–2.0mEq/L lower
    CO2 ABG vs. VBG: VBG CO2 6mm Hg higher

    It’s highly unlikely that any of these differences would significantly affect patient management. In fact, the Joint British Diabetes Society 2011 Guidelines endorse the use of a VBG instead of an ABG for initial assessment of acid/base status as well as continued monitoring during management.6

    Lim and Kelley additionally found that pH and HCO3 from VBG samples correlate well with ABG samples in patients with COPD exacerbations. However, in their meta-analysis, they found that pCO2 was not comparable between the two.7 Although they did not demonstrate good correlation between the two, it’s unclear how the difference would affect patient outcomes as this was not measured.

    Finally, let’s touch on lactate. There are fewer studies investigating this topic but the evidence again suggests no clinically significant difference. Younger and colleagues found that “an abnormally elevated peripheral venous lactate was 100% sensitive . . . in detecting arterial hyperlactacidemia.”8 The mean difference between ABG and peripheral VBG lactates in their study was 0.22mg/dl (venous lactate was higher). Jones and colleagues determined that tourniquet time, a factor often thought to be associated with elevations in lactates obtained in peripheral venous samples, did not in fact significantly affect lactate levels.9 Middleton found that central venous lactates correlated closely with ABG values as well (difference of 0.08 mg/dl).5

    So what’s the bottom line?
    ABGs are painful and are associated with rare, but serious complications. VBGs, on the other hand, can be obtained along with routine blood sampling or IV placement. Values for pH, HCO3 and lactate do not differ significantly between the two modalities. The pCO2 in COPD patients does not appear to correlate well between ABG and VBG but it is unclear if this affects patient management. Based on the best available evidence, VBGs can be used instead of ABGs in the majority of patients to guide management.

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