Radial/Brachial/Femoral Arterial Puncture: blood passively enters the heparinized syringe, which is put immediately on ice and transferred promptly to the blood gas laboratory for analysis
Venous Blood Gas (VBG)
Technique for Drawing VBG from a Peripheral Blood Draw Using a Tourniquet: it is recommended to release the tourniquet 1 min before drawing the blood to avoid ischemia-related changes in the measured parameters
Normal Values for Blood Gases
Arterial Blood Gas (ABG)
pH: 7.40
pCO2: 40 mmHg
pO2: age-dependent
Predicted Room Air pO2 = 104.2 – (0.27 x Age)
Alternatively, pO2 can be evaluated by calculating the alveolar-arterial O2 gradient (A-a gradient) (see Hypoxemia)
HCO3: 24 mEq/L
Differences Between Arterial Blood Gas (ABG) and Venous Blood Gas (VBG) Values (see Venous Blood Gas)
General Comments
Differences Between Arterial and Venous Values are Due to the Uptake and Buffering of Metabolically-Produced CO2 in the Capillaries and the Addition of Organic Acids Produced by the Tissue Bed Drained by the Vein
Venous pH
pH from Arterial and Venous Samples Correlate Reasonably Well (Respirology, 2014) [MEDLINE] and (Emerg Med J, 2014) [MEDLINE]: with agreement being highest at normal values
Venous pH is Approximately 0.03 Lower than the Arterial pH (95% Confidence Interval: 0.039 to 0.027) (Emerg Med J, 2014) [MEDLINE]
Venous pCO2
pCO2 from a Venous Sample is Approximately 4.4 mm Hg Higher Than the pCO2 from an Arterial Sample: for this reason, a normal venous pCO2 has a good negative predictive value for a normal arterial pCO2
Venous pCO2 is Approximately 4.4 mm Hg Higher than the Arterial pCO2 (95% Confidence Interval: 2.55-6.27) (Eur J Emerg Med, 2014) [MEDLINE]
Venous pO2
pO2 from Arterial and Venous Samples Do Not Correlate with Each Other
Arterial PO2 is Approximately 36.9 mm Hg Higher than the Venous pO2, with Significant Variability (95% Confidence Interval: 27.2-46.6 mm Hg) (Respirology, 2014) [MEDLINE]
Venous Bicarbonate
Bicarbonate from Arterial and Venous Samples Correlate Reasonably Well
Venous Bicarbonate is Approximately 1.03 mmol/L Higher than the Arterial Bicarbonate (95% Confidence Interval 0.56-1.50) (Eur J Emerg Med, 2014) [MEDLINE]: agreement is highest at normal values
Venous Carboxyhemoglobin (by Co-Oximetry)
Carboxyhemoglobin from Arterial (ABG) and Venous (VBG) Samples Correlate Well (Ann Emerg Med, 1995) [MEDLINE] and (Crit Care Med, 2000) [MEDLINE]
Lactate from Arterial and Venous Samples Correlate Poorly at Abnormal Levels: however, this agreement is closer at normal levels such that, if the venous lactate is normal, the arterial lactate is generally also normal
Venous Lactate is Approximately 0.25 mmol/L Higher Than the Arterial Lactate (95% Confidence Interval: 0.15-0.35) (Eur J Emerg Med, 2014) [MEDLINE]
Cautions
While Blood Gas Analyzers May Report Potassium Values, These Analyzers Do Not Typically Report if the Sample Has Been Hemolyzed (as Clinical Laboratories Routinely Do): for this reason, use of a VBG sample to assess potassium must be interpreted with caution
Principles of Arterial Blood Gas (ABG) Analysis
Parameters
pH: measured using a pH electrode
pCO2: measured using a chemical reaction that consumes CO2 and produces a hydrogen ion -> this is sensed as a change in pH
pO2: measured using oxidation-reduction reactions that generate electric currents
Temperature Dependence: pH increases and both pCO2 and pO2 decrease as the temperature decreases
For this Reason, ABG Analysis Must Account for Either the Patient’s Temperature (or Use 37 Degrees C as a Standardized Procedure)
HCO3: calculated
SaO2: in a standard blood gas analyzer (without co-oximetry), this is usually calculated from the pO2 (see below)
Methods to Determine the SaO2 from an Arterial Blood Gas Sample
General Comments
It is Critical to Know Which Device is Being Used by Your Specific Laboratory to Report the SaO2, Since the Presence of Dyshemoglobinemias Can Lead to Misinterpretation of the Data
Blood Gas Analyzer without Co-Oximetry
Principle: pO2 is measured by the analyzer and the SaO2 is calculated using a standard equation
Technical Issues
Blood Gas Analyzer Uses a Calculated or Default Hemoglobin Value
Blood Gas Analyzer Assumes a Normal Hemoglobin Value and the Absence of Dyshemoglobinemias (Such as Methemoglobin, Carboxyhemoglobin, Sulfhemoglobin)
Clinical Scenarios
Methemoglobinemia (see Methemoglobinemia): pO2 and (calculated) SaO2 will both be reported as normal
Functional Hemoglobin Saturation from Simple Co-Oximetry
Principle: determination of SaO2 utilizing measurement of oxyhemoglobin (O2Hb) and deoxyHb (DeO2Hb) only
FO2Hb is Usually Expressed as a Percentage: typically ranges from 90-95% in healthy normals
Clinical Scenarios
Methemoglobinemia (see Methemoglobinemia): the reported FO2Hb will be considerably lower than the SaO2 reported by the blood gas analyzer
Co-Oximetry on Arterial Blood Gas Sample
Principle: co-oximeter is a simplified spectrophotometer which measures light absorbance at various different wavelengths of light
Early Co-Oximetry Devices Were Capable of Measuring Light Absorbance at Four Wavelengths of Light
Modern Co-Oximetry Devices (Continuous Wave Spectrophotometers) are Capable of Measuring Absorbance at >100 Different Wavelengths of Light: additional wavelengths improve accuracy, minimize artifacts from interfering substances, and enable reporting of additional components
More Complex Modern Co-Oximetry Devices Can Measure Absorbance at 128 Wavelengths, Allowing Determination of Total Hemoglobin Concentration, SaO2, Fractional Oxyhemoglobin, and Fractional Carboxyhemoglobin, Fractional Methemoglobin, and Fractional Sulfhemoglobin
Hemoglobin Species Detected by Co-Oximetry Devices
Oxyhemoglobin
Deoxyhemoglobin
Carboxyhemoglobin
Methemoglobin: peak absorbance at 630 nm
Sulfhemoglobin: peak absorbance at 614 nm (overlaps to 630 nm and may be reported as methemoglobin on older machines)
Clinical Scenarios Where Discordance May Occur Between Pulse Oximetry Saturation (SpO2) and pO2 from Arterial Blood Gas (see Pulse Oximetry)
Pulse Oximeter Equipment Malfunction: in this case, SpO2 may vary widely from the pO2
Venous Blood Sample Inadvertently Drawn (Instead of Arterial Blood Sample): in this case, pO2 would be lower than one would expect from an arterial blood sample
Arterial Blood Sample Drawn from Ischemic Body Site (Such as an Ischemic Limb): in this case, the arterial pO2 would be lower than the SpO2 would predict (and would be lower than the arterial pO2 obtained at another body site)
Pseudohypoxemia: in cases with very high WBC count, in vitro consumption of oxygen can occur in the arterial blood gas sample during transit (prior to processing) on the blood gas machine -> results in artifactually low pO2, as compared to the SpO2
Methemoglobinemia (see Methemoglobinemia): in this case, SpO2 will give an abnormally low reading, as compared to a normal pO2
References
Relationship between venous and arterial carboxyhemoglobin levels in patients with suspected carbon monoxide poisoning. Ann Emerg Med. 1995;25:481–483 [MEDLINE]
Relationship between arterial, mixed venous, and internal jugular carboxyhemoglobin concentrations at low, medium, and high concentrations in a piglet model of carbon monoxide toxicity. Crit Care Med 2000 Jun;28:1998–2001 [MEDLINE]
Oxygen Saturation. A Guide to Laboratory Assessment, 2006 [LINK]
Agreement between mathematically arterialised venous versus arterial blood gas values in patients undergoing non-invasive ventilation: a cohort study. Emerg Med J. 2014;31(e1):e46–e49 [MEDLINE]
Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014;19:168–175 [MEDLINE]
The role of venous blood gas in the emergency department: a systematic review and meta-analysis. Eur J Emerg Med. 2014 Apr;21:81–88 [MEDLINE]
