1. Venous Blood Gas Versus Arterial Blood Gas Analysis
Ping-Wei Chen
PGY-2
Emergency Medicine

2. It’s Go Time… 25 yo female Single vehicle rollover near Sundre
Intubated for deteriorating GCS
In the ED:
BP 70 palp, HR 122
Not responding to painful stimuli
01:23 - VBG ordered as part of workup
01:41 – ABG ordered as part of workup
I’m not really going to focus on the trauma aspect of this case because I want to get to the main topic of my talk, but at 1:23 am, a VBG is ordered as part of the workup for the patient not responding to painful stimulus greater than one hour after being RSI’d. There is some concern over the use of a tourniquette to obtain the VBG, and an ABG is subsequently ordered following.
This is just one example of why one might run into conflict between VBGs and ABGs, however, I’ve had numerous other instances where I’ve encountered resistance for ordering a VBG.

3. Objectives Controversy Can VBGs replace ABGs?
When are VBGs and ABGs different?
When might I want an ABG?
NOT covered
Electrolytes
Lactate

4. What’s all the fuss about?
Arterial Blood Gas
Venous Blood Gas
PAINFUL
Arterial injury
Thrombosis with distal ischemia
Hemorrhage/hematoma
Aneurysm formation
Median nerve damage
Infection
Needlestick injury
Reflex sympathetic dystrophy
Samples can be drawn simultaneously at time of venipuncture
Should be done without tourniquette
More difficult to obtain in pulseless patients
Controversy regarding level of agreement with arterial values
Question: What has been some of the hassles that you people have experienced in ordering VBGs?
Question: What are some of the consequences of obtaining ABG samples?
Question:How would we expect values for pH, CO2, O2, and HCO3 to compare in VBG VS ABG?
Largely a remnant of the past when pulse oximetry was not available to measure systemic oxygen levels.

5. Prospective, observational study 218 subjects, ED population
dyspnea, DKA, renal failure, seizures, ↓LOC, ingestions, ischemic colitis
A priori definition of clinically important difference
Pearson correlation coefficient
Bland-Altman plots
There are few large scale studies looking at this topic in ED populations, but many smaller studies looking at specific subsets of patients.
Elegant study – Collected ABG and VBG in series in patients at Kingston General Hospital and recorded pH, PCO2, and calculated HCO3. Then, they surveyed 45 physicians to determine the level of difference that they would consider clinically important.
Bland-Altman analysis: plots the difference between two methods of measurement (y-axis) against the average of the values obtained by the two methods. Limits of agreement are then determined to be the mean difference between the two values +/- 2SDs, determined to be the 95% confidence interval. Often used when two methods of measurement are being evaluated against each other to determine if they can be used interchangeably.
87% samples within 5 mins, 96% samples within 10 mins, 100% samples within 30 mins
Rang et al Can J Emerg Med 4(1):7

6. Results pH (r= 0.913) pCO2 (r=0.921) calculated HCO3 (r=0.953)
Excellent correlation
pH (r= 0.913)
pCO2 (r=0.921)
calculated HCO3 (r=0.953)
In this study, they found excellent correlation between the values of those measured by ABG and VBG.
However, correlation by itself is not good enough for one method of measurement to replace the other.

7. Results Clinically Important Differences 26/45 physicians responded
Here are the Bland-Altman plots for this study which you recall show the difference between two methods of measurement (y-axis) against the average of the values obtained by the two methods (x-axis). The one difference between these graphs and true Bland-Altman plots is that the dotted lines represent what the physicians thought were clinically significant differences rather than the values around the mean difference that would capture 95% of the differences between ABG and VBG values.
Of the 26/45 physicians who responded to the survey, they defined the above as what they considered to be clinically important or “I would feel uncomfortable using only the venous value for clinical decisions if it was more than ___units away from the arterial value”.
Physicians opinions:
pH ±0.05 (66% study values) VS 0.08
PCO2 ±6.6 (51% study values) VS 13.9
HCO3 ±3.5 (87% study values) VS 3.5

8. Result Mean Differences pH 0.036 (0.030-0.042)
pCO2 6.0 mm Hg ( )
HCO3 1.5 mEq/L ( )
The mean differences found in this study were the above. The values in the brackets indicate the 2 standard deviations around the mean difference required to capture 95% of the differences between the VBG and ABG measurements.
Bland and Altman state that if the mean difference between 2 measurements ±2SD is not clinically significant, then the two measurements could be used interchangeably.
Therefore, from these results, VBG and ABG results are not equivalent, and cannot be used interchangeably, however, correlate well with each other.
The authors’ conclusions were that although the values measured on VBG cannot be used interchangeably with those of an ABG, they can be used to follow trends or for broader use with a refined set of normal values.
Therefore, the question you have to ask yourself is: are these differences enough to make you change your management?

9. Prospective, observational study 246 subjects, ED population
acute respiratory disease, suspected metabolic disorder
pH only
Results:
Excellent correlation r=0.92
Mean difference: 0.04 pH units (-0.11 to +0.04)
Another study by Anne-Maree Kelly out of Australia looked at agreement in pH values between VBG and ABG.
Kelly et al Emerg Med J. 18:340

10. Prospective, observational study 95 patients, ED population
AECOPD, pneumonia, sepsis, ARF/CRF, DKA, ACS, acute gastroenteritis, SLE, toxic ingestion
Bland-Altman Analysis
Results: ABG compared to VBG
The most recent study to look at ABG VS VBG is this one by Malatesha et al in the Journal of Emergency Medicine. They found the above results. From this, they concluded that agreement is excellent between pH values, and that there is acceptably narrow agreement in PCO2 and HCO3 values.
Mean Difference
95% Limits of Agreement pH
0.015
-0.1 to 0.13
PCO2 -3
-7.6 to 6.8
HCO3
-0.74
-5.8 to 4.3
PO2 65
-32.9 to 145.3

11. Review article: 6 studies pH and HCO3 Results:
Mean difference
pH: 0.02 ( to 0.021), n=258, DKA patients only
pH: (-0.11 to 0.04), n =763, respiratory/metabolic illness
HCO3: mEq/L (N/A), n =21, DKA patients only
HCO3: mEq/L (-2.73 to 5.13), n=763, respiratory/metabolic illness
Recently, Anne-Maree Kelly also reviewed 6 studies looking at the agreement between ABGs and VBGs in DKA patients. Some of the papers reviewed, however, did not specify DKA as the specific diagnosis, but only stated the diagnosis as mixed respiratory/metabolic disorder and as such were included in the review. They found the above.

12. When are they different?
Weil et al Difference in acid-base state between venous and arterial blood during cardiopulmonary resuscitation. NJEM. 315:153-6.
Prospective, observational study (n=16)
ICU/CCU patients
Arteriovenous gradient
The literature identifies one population where VBGs differ from ABGs
Critically ill patient population with major hemodynamic deficits prior to participation in the study. Already had pulmonary artery catheters in place.
Pre-Arrest
Arrest pH
0.06±0.02
0.30±0.05
pCO2
11±2 mmHg
36±6mmHg

13. So when might I want an ABG?
Unable to establish IV access
Inability to obtain sample
Inability to obtain O2 saturation by pulse oximeter
Peripheral vasoconstriction
Abnormal hemoglobins
Carboxyhemoglobin
Methemoglobin
Sickle hemoglobin
If you’re Mel Herbert or Michael Gauding of EMRap, the only time you would do a venous gas is if you couldn’t establish IV access!
The only time I argue that you might want an ABG is when you can’t rely on your pulse oximeter to screen for hypoxia, which has been shown in the literature to be very good at doing this.
Carboxyhemoglobin – artificially elevate O2 sat
Methemoglobin – artificially lowers O2 sat
Sickle hemoglobin – can raise or lower O2 sat
Question: (for staff) Are there any other situations in which you might order an ABG instead of a VBG?

14. N = 1 VBG at 01:32 ABG at 01:41 pH 7.11/pCO2 41/HCO3 14/lactate 6.6
In our N = 1 study with samples taken 9 minutes apart, we see that the correlation in pH, PCO2, and HCO3 are within the limits where our clinical decision making would not be affected. For interests sake, although this is not backed up by evidence, the lactates are not as far off as you might think even though a tourniquette was used to obtain the sample.

15. Conclusions VBGs not interchangeable with ABGs BUT Consider ABG in:
Excellent correlation with ABG values
Reasonable agreement on VBG for clinical decision making in ED
pH lower
PCO2 3-6 mmHg higher
HCO3 essentially the same
Consider ABG in:
Inability to obtain sample
Inability to utilize pulse oximeter

16. Questions?