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Thread: When Is Venous Blood Gas Analysis Enough?

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    When Is Venous Blood Gas Analysis Enough?


    Since venipuncture is less painful, less costly, and safer for the practitioner, do you really need that arterial blood sample?

    The authors explain why, in many cases, the answer is no.

    Case Report:
    A 34-year-old man with a history significant for diabetes mellitus presents to the emergency department after experiencing progressive abdominal pain and vomiting for three days. His medications include insulin, but he states that he has missed several doses. A bedside glucose level taken in triage is 532 mg/dl. The patient appears to be in moderate discomfort with dry mucous membranes. His blood pressure is 118/80 mmHg; his heart rate, 124. He is in no apparent respiratory distress, but his respiratory rate is 22. Pulse oximetry reading on room air is 97%.

    You order routine lab tests and a fluid bolus, then search for an arterial blood gas (ABG) kit. Just as you are about to draw blood from the patient’s radial artery, the nurse asks if you would like to order a venous blood gas (VBG) along with the cell count and chemistries.
    CORNERSTONE OF MANAGEMENT
    Arterial blood gas analysis has been a cornerstone in the management of acutely ill patients with presumed acid-base imbalances since automated blood gas analyzers first became available in the early 1960s. Decades of experience with ABG analysis have provided emergency physicians with an armamentarium of equations to diagnose multiple complicated and often concurrent metabolic derangements.

    Recent studies have questioned this dogma, however. In specific scenarios, a VBG analysis may provide enough information to make the correct diagnosis and begin appropriate therapy.
    In this article, we will review the practical advantages of VBG analysis and discuss its utility in the management of both metabolic and respiratory acid-base disorders.
    Advantages of vbg analysis
    Blood gas analysis provides a great deal of data in a short period of time. When it has been set up in the emergency department as a point-of-care test, results can be obtained in about two minutes. In addition, most analyzers give more information than just the patient’s pH, PO2, PCO2, and HCO3. Other data include hemoglobin, sodium, potassium, glucose, methemoglobin, carboxyhemoglobin, ionized calcium, and lactate levels.

    The most obvious advantage to obtaining a VBG instead of an ABG is decreased pain for the patient. A 1996 double-blinded study by Giner found that pain scale ratings were reduced by almost half with VBG compared to ABG. (There was no significant difference in the ratings when a local anesthetic had been used prior to an arterial blood draw, but this is not always done.)

    Also, a VBG sample can be drawn using the same intravenous (IV) line that is used to draw blood for other lab tests, thus necessitating only one puncture. This translates into decreased costs, labor, and risk of needlestick injury to the health care provider. Furthermore, complications such as arterial laceration, hematoma, and thrombosis are all but negated with venous blood sampling.

    CRITICAL ILLNESS AND CARDIAC ARREST

    So VBG analysis has several obvious advantages over ABG.

    But what about the data obtained with ABG analysis?
    Is it unique? Is it diagnostically superior to the data obtained with venous blood sampling?


    Normal values for pH, PO2, PCO2, and HCO3 in healthy individuals are listed in the table (below). However, because emergency department patients are generally not healthy, it is important to compare arterial and venous values in acutely ill individuals. A 1985 study by Gennis of critically ill emergency department patients revealed a breakdown in the linear relationship between arterial and venous values for pH, PCO2, and HCO3 seen in healthy individuals. However, the study did demonstrate that normal venous pH, PCO2, and HCO3 ruled out severe acid-base disturbances. For example, a venous pH of 7.25 or higher predicted an arterial pH of 7.2 or higher in 98% of all cases, which makes VBG testing valuable as a screening procedure. If the results are normal, ABG analysis should not be necessary. Conversely, abnormal venous levels predicted abnormal arterial values, but again in a nonlinear fashion. A venous pH of 7 or lower, for example, predicted an arterial pH of 7.2 or lower in 98% of cases.

    In cardiac arrest victims, the disparity between arterial and venous values is even greater. During cardiac arrest, tissue hypoxia is all but a certainty and is reflected by the lower pH and higher PCO2 on the venous side. A 1986 study by Weil demonstrated a significantly lower pH in venous samples (mean, 7.15 vs 7.41 in arterial samples) and a significantly elevated PCO2 (mean, 74 mm Hg vs 32 mm Hg) in these patients. In clinical practice, however, knowledge of either the arterial or venous pH or PCO2 during cardiac arrest does not alter management, making the debate less relevant.
    A similar 1989 study by Androque confirmed the above findings in cardiac arrest, but it also included patients in severe hemodynamic failure not related to cardiac arrest. The study found that as cardiac output declines, the differences between arterial and venous measurements increase. These authors concluded that VBG analysis in cardiac arrest provides values more indicative of the true cellular environment.


    lactate levels
    Elevated arterial lactate levels are an early sensitive marker of tissue hypoperfusion, predicting both the severity of hemodynamic compromise and overall patient prognosis. Unlike the pH during cardiac arrest, interpreting a venous lactate level in relation to an arterial value could be useful in the workup and management of critically ill patients. Based on the available evidence, the correlation between arterial and venous lactate values appears to be quite close.

    A 1996 study by Younger of 48 emergency department patients in whom concurrent arterial and venous blood gas analyses were performed demonstrated that an abnormally elevated venous lactate level (1.6 mmol/L or greater) was 100% sensitive and 89% specific in predicting elevated arterial lactate levels. A 2000 study by Lavery bolstered these results, demonstrating a close correlation between arterial and venous lactate levels in a population of 375 hypovolemic trauma patients. A venous lactate level above 2 mmol/L predicted an elevated injury severity score, ICU admission, and length of stay.
    METABOLIC CONDITIONS
    Using ABG measurements and serum electrolyte levels, the emergency physician can perform the six steps involved in calculating mixed acid-base disturbances (see box below). Venous pH values can aid in the first step because they closely mirror arterial values for several metabolic conditions, including diabetic ketoacidosis (DKA) and uremia.


    RESPIRATORY ILLNESS

    In physiologic terms, ABG composition reflects the relationship between ventilation and perfusion and thus is an important reflection of overall pulmonary function. In clinical practice, respiratory insufficiency can be broken down into two categories—namely, ventilatory failure (inadequate clearance of CO2, or hypercarbia) and oxygenation failure (inadequate plasma oxygenation, or hypoxemia). Arterial blood gas analysis has long been used to diagnose and quantify the severity of either or both of the above conditions in patients with apparent respiratory distress.

    A close correlation between arterial and venous PCO2 that would decrease dependency on an ABG does not exist. However, a venous PCO2 value that predicts significant arterial hypercarbia might be useful. In a study published in 2002 by Kelly, a venous PCO2 level above 45 mm Hg predicted an arterial PCO2 above 50 mm Hg (the designated value for significant hypercarbia) with a sensitivity of 100% and specificity of 57%. In this study, a venous PCO2 value above 45 mm Hg detected all cases of significant arterial hypercarbia (negative predictive value, 100%) and reduced the requirement for arterial blood sampling in 41% of cases.

    Venous PO2 values do not provide any significant reflection of arterial PO2 levels and are therefore a poor surrogate to quantify oxygen delivery to target tissues. However, the widespread availability of pulse oximetry makes it an attractive alternative. A 2001 study by Witting of more than 700 emergency department patients showed that an oxygen saturation level of 96% or less on room air predicted a PO2 below 70 mm Hg with a sensitivity of 100% and a specificity of 54%.
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    CASE RESOLUTION

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    Last edited by trimurtulu; 01-23-2009 at 03:52 PM.

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