Vol. 14 •Issue 13 • Page 26
Venous Blood Gas Samples: ABG Alternative?
Arterial blood gases are still a mainstay of respiratory care practice. But are they the only way we can effectively manage a ventilator patient or assess an individual’s acid base balance? Is there a way we could utilize blood being drawn for other purposes to also evaluate the patient’s pulmonary status? Nurses routinely draw venous blood for labs. What if therapists could use a small part of that blood to assess ventilatory status too? Would it work?
Here’s what we know on that issue so far. Umbilical cord blood gas and pH analysis is routinely done in many high-risk deliveries and whenever newborn depression may be suspected. Umbilical cord blood gas analysis assists the clinician in excluding the diagnosis of birth asphyxia in approximately 80 percent of depressed term newborns.1 Usually arterial blood is used because arterial acidemia may occur with a normal venous pH. It is also believed the sampling of arterial blood is more representative of the fetal metabolic condition. But there are a growing number of studies that list tables and correlations for venous and arterial blood gas ranges.
McGillivray et al.2 studied 78 children aged one month to 18 years to determine whether there was a correlation between venous and arterialized capillary blood gas values. They found correlation coefficients between the venous and capillary samples to be .92 (pH), .80 (PCO2 ), and .67 (PO2 ). They also looked at whether or not these values could be used for clinical management decisions and found that concordance values of capillary and venous blood gas values, with 95 percent confidence intervals (CIs) were respectively for physician A and B interpretations: .61 (.47 to .73) and .48 (.41 to .55); need for bicarbonate, .85 (.73 to.97) and .80 (.72 to.88); and need for intubation .73 (.64 t .82), and .83 (.75 to .91).
Basically, the researchers concluded that venous samples were an acceptable alternative to capillary blood samples for making clinical management decisions for well-perfused children.
We can hardly use this data to establish acceptable venous ranges at this time, but the studies are out there, and they are starting to define relationships between the numbers that may be clinically relevant.
MEANWHILE IN THE ER…
Brandenburg and Dire of the University of Oklahoma Health Sciences Center, Oklahoma City,3 compared arterial and venous blood gas values in the initial evaluation of diabetic patients presenting to their emergency department with ketoacidosis. They wanted to assess whether venous blood gas values could replace arterial gas values for these individuals. Samples were collected and analyzed in 44 episodes of diabetic ketoacidosis (38 patients).
They found (mean +/- SD): arterial pH, 7.20 +/- 14; venous pH, 7.17 +/- 13; serum glucose, 33.8 +/- 16 mmol/L (609 +/- 288mg/dL); arterial HCO3, 11.0 +/- 6.0 mmol; venous HCO3, 12.8 +/- 5.5 mmol/L; serum CO2, 11.8 +/- 5.0 mmol/L; and anion gap, 26.7 +/- 7.6 mmol/L. or more precisely the arterial and venous pH results (r = .9689) and arterial and venous HCO3 results (r = .9543) correlated “highly.”
They found a mean difference between arterial and venous pH values of 0.03 (range 0.0 to 0.11) or, in a nutshell, the venous blood gases could be used to accurately assess these patients.
In the research facilities of New York’s Mt. Sinai Medical Center, Oropello et al. are currently evaluating the role of continuous fiberoptic arterial and venous blood gas monitoring in hemorrhagic shock4.
They are working with pigs to determine whether continuous fiberoptic blood gas monitoring correlates with intermittent sampling of both arterial and venous blood during severe hemodynamic shifts. Granted they are not specifically assessing the correlation of arterial to venous values. But their data are available, and information could probably be taken from it quite easily.
They found a comparably accurate correlation between the continuous and intermittent samples during severe hemodynamic shifts. In other words, they took healthy pigs, intubated them, induced shock and then resuscitated them and all three (continuous, arterial and the venous) samples could be correlated. They noted other studies have shown changes in venous PCO2 correlated with changes in global tissue perfusion and cardiac output, leading this group to conclude continuous venous blood gas monitoring may be useful for monitoring shock and the response to resuscitation.
At this point, it would be difficult to assess the clinical applications of these diverse studies. But they are only the tip of the iceberg. The point is there is a growing interest in the use of venous blood gases, and researchers are rethinking their usefulness and reassessing their clinical application.
RT INVOLVEMENT
Can RTs get involved in the process? RTs have had the capability to draw and run venous blood samples for years. They usually discount them as being of little or no value, but they may have to change their mindset and start thinking this blood may be of value in certain cases. Soon caregivers may have a correlation factor such as: ” If the venous is this, add 5 for every X to X of BP over…” You get the picture.
That is too much trouble, you say. Too primitive. That will never happen. When I started in the field, I was taught to draw a line from pH to PCO2 to figure out HCO3 using the Siggaard Anderson model.
Computers do it for us now, but maybe we will be teaching RT students the venous equivalents sometime soon.
Margaret Clark, Boston, is a practitioner and freelance writer.
References
1. Thorp JA, Rushing RS. Obstet Gynecol Clin North Am (1999 Dec;26(4):695-709).
2. McGillivray D, Ducharme FM, Charron Y, Mattimoe C, Treherne S. Clinical decision- making based on venous versus capillary blood gas values in the well-perfused child. Ann Emerg Med (1999 Jul;34(1):58-63).
3. Brandenburg MA, Dire DJ. Comparison of arterial and venous blood gas values in the initial emergency department evaluation of patients with diabetic ketoacidosis. Ann Emerg Med (1998 Apr;31(4):459-65).
4. Oropello JM, Manasia A, Hannon E, Leibowitz A, Benjamin E. Continuous fiberoptic arterial and venous blood gas monitoring in hemorrhagic shock. Chest (1996 Apr;109(4):1049-55).
