Physiologic differences between arterial and venous blood for blood gas and acid–base measurements
More than any other analytes, p O 2 , p CO 2 , and pH change markedly from arterial to venous blood. While the pH apparently changes only slightly (i.e., 7.40 to 7.37 = 0.4% difference), the actual H ion concentration has a much greater relative change, going from 3.98 × 10 −8 to 4.27 × 10 −8 , a 7% increase. Arterial blood is nearly always preferred over venous blood for blood gas analysis of pH, p CO 2 , and p O 2 for these reasons:
Arterial p O 2 indicates the ability of the lungs to oxygenate, or equilibrate, the blood with alveolar air.
Arterial blood provides an index of the oxygen and nutrients that will be provided to the tissues and cells.
The composition of arterial blood is consistent throughout the circulatory system.
Because typical venous blood collected from an arm vein represents oxygen metabolism only in the arm, venous blood representing oxygen and carbon dioxide metabolism of a larger section of the body requires the collection of either “central” venous or “mixed” venous blood. Central venous blood is collected from a central venous catheter inserted either peripherally in the arm or directly into the subclavian or jugular vein and terminating either in the superior vena cava or the right atrium. A central venous line also allows an infusion of fluids, medications, or parenteral nutrition and monitoring of central venous pressure. Mixed venous blood is drawn from a pulmonary artery catheter that is inserted through one of the main central veins (subclavian, internal jugular, femoral) to ultimately reach the pulmonary artery. A pulmonary artery catheter samples the mixture of venous blood returning from the entire venous circulation: head and arms (via superior vena cava), the gut and lower extremities (via the inferior vena cava), and the coronary veins (via the coronary sinus) ( ) . A pulmonary artery catheter also allows for direct measurement of pulmonary artery pressure and pulmonary capillary wedge pressure, and it can be used to estimate cardiac output. Both mixed venous and arterial blood are needed when determining parameters such as P50 and oxygen consumption (VO 2 ).
Interpretation of venous blood gas values
Arterial blood collected anaerobically is the standard sample for blood gas measurements to interpret acid–base and oxygenation status. Because arterial punctures are invasive, painful, and require special skills, venous blood gases are often used for the acid–base interpretation of the pH and p CO 2 results. As described elsewhere, venous blood may be collected as peripheral blood from a standard venipuncture, or as central or mixed venous blood from an existing central venous catheter or pulmonary artery catheter.
Blood gas values on arterial blood have been validated by far more research, and clinicians are more familiar with the reference intervals on arterial blood ( ) . However, there is increased interest in utilizing venous blood gas results, especially for acid–base interpretation. The interest in venous blood gas testing comes both from the difficulties of obtaining arterial blood mentioned earlier and the availability of reliable noninvasive pulse oximetry for assessing arterial oxygenation. For several reasons, p O 2 results on venous blood are both inaccurate and have no clinical value because the oxygen has already been extracted by the tissues from the arterial blood ( ) . A general disadvantage of venous blood is that its composition differs slightly depending on the source.
Venous blood by peripheral venipuncture is the least invasive and represents metabolism and gas exchange in the arm. Central venous blood is a mixture of venous blood from the upper half of the body ( ) . Blood collected from a pulmonary artery catheter is the most invasive and is a mixture of venous blood returning from the entire circulatory system. Blood gas results on venous blood are best interpreted by using the venous blood gas results to estimate the arterial values, then using these estimated arterial values for clinical decisions ( ) . For this purpose the arterial-venous differences are key. Table 3.1 lists estimated differences between arterial and venous blood:
|Convert central venous to arterial||Convert peripheral venous to arterial||References|
|pH||Add 0.03–0.05||Add 0.02–0.04 units||( )|
|Add 0.03||( )|
|p CO 2 (mmHg)||Subtract 4–5||Subtract 3–8||( )|
|Subtract 4.5||( , , )|
|HCO 3 (mmol/L)||Subtract 0.8||( , )|
|BE (mmol/L)||Subtract 0.3||( , )|
|p O 2 (mmHg)||Arterial higher by 63 ± 59 (cannot convert venous p O 2 results to arterial p O 2 results)||( )|
To summarize, here are some points about the clinical utility of venous blood gas results compared to arterial results ( ) :
Central venous pH, p CO 2 , and HCO 3 are highly correlated to similar parameters in arterial blood. On average, the central venous pH is ∼0.03 units lower than arterial pH, and central venous p CO 2 is ∼5 mmHg higher than arterial p CO 2 ,
HCO 3 concentrations are nearly the same in central venous and arterial blood.
Correcting central venous results as noted above will agree even better with values on arterial blood. Used judiciously, pH, p CO 2 , and HCO 3 results on central venous blood are acceptable substitutes for similar results on arterial blood.
In no case should p O 2 results on venous blood be used to evaluate a patient’s oxygenation status. For many patients, noninvasive pulse oximetry is a sufficiently accurate substitute for arterial p O 2 . However, in critical situations, p O 2 on arterial blood is required ( ) .
Capillary blood gases (neonatal)
Capillaries are the very small blood vessels (∼8 μm diameter) that connect the smallest arteries (arterioles) to the smallest veins (venules) ( ) . In these capillaries, there is a vital gas exchange of oxygen, carbon dioxide, waste products, and essential nutrients with the tissue cells. With this exchange, there are gradients from arterial to venous blood: pH lower by 0.02–0.03; p CO 2 higher by 4–5 mmHg (0.6–0.7 kPa), and p O 2 lower by about 60 mmHg (8 kPa).
While arterial punctures or catheters provide the best sample, they are invasive and not always available. Capillary blood collection is a much less painful and safer mode of collection using a lancet or incision device that punctures the skin about 1 mm deep, making them much more appropriate for neonatal blood collections. Capillary blood from warmed heel sticks is easier and less invasive, but what about the reference values and clinical interpretation of capillary blood gas results?
As Higgins points out with his usual excellent commentary ( ) , “capillary blood” obtained by skin puncture is a mixture of blood from punctured arterioles, capillaries, and venules, along with small contributions from interstitial and intracellular fluids.
How much are capillary blood gas results compromised?
As with reference intervals for arterial blood, reference intervals for neonatal blood gases also present practical challenges because they should be collected on healthy neonates with otherwise no need for blood gas results. As shown in Table 3.2 , Cousineau et al. reported reference intervals on 118 healthy term babies between 36 and 60 h of age ( , ) .
|Parameter||Mean||Mean ± 2 SD interval|
|p CO 2 (mmHg)||38.7||28.5–48.7|
|p O 2 (mmHg)||45.3||33–61|
|Ion Ca (mmol/L)||1.21||1.06–1.34|
A relatively old study was done on 158 paired arterial and capillary blood samples from 41 sick preterm babies between 3 h and 7 days old ( ) . Although the values obtained would not be appropriate for reference intervals, the arterial-venous differences are useful. p CO 2 measurements in capillary blood were good approximations of p CO 2 in arterial blood, and 89% of pH values on warmed heel collections, differed by 0.05 units or less.
However, for p O 2 , significant differences were observed in many cases, with 75% of results differing by 8 mmHg or more, and 53% of results differing by at least 15 mmHg. These discrepancies were always greater for unwarmed heel stick collections and the differences increased with higher arterial p O 2 . However, as shown in Table 3.3 , because capillary p O 2 was always lower than arterial p O 2 , a normal capillary p O 2 value above, say 50 mmHg, confirms that the patient is not hypoxemic ( ) .