The document discusses whether venous and arterial blood gas analysis are interchangeable for assessing acute respiratory disease in the emergency department. It finds that while pH agreement is good, pCO2 agreement has wide limits. Venous pCO2 can screen for hypercarbia but not rule it out. Clinical context is important. Venous values may monitor trends if considered with symptoms. New methods calculate arterial values from venous samples plus oximetry with promising validation results. However, evidence for mixed disorders is limited.

1. ARE VENOUS AND ARTERIAL BLOOD GAS ANALYSIS
INTERCHANGEABLE IN ED ASSESSMENT OF ACUTE
RESPIRATORY DISEASE?
Anne-Maree Kelly
Professor and Director
Joseph Epstein Centre for Emergency Medicine
Research @Western Health
@kellyam_jec

2. Conflicts of interest
 I received financial support for travel and accommodation from Radiometer Pty
Ltd to present a similar presentation at 4th
International Symposium on Blood Gas
and Critical Care in France in 2008.
 I am undertaking some research with A/Prof Rees into calculated values which
may be commercialised. I have no pecuniary interest in this program.
 I have not received industry funding for any of my blood gas research projects.

3. Objectives
 After this presentation, participants will:
 Understand the agreement performance of variables
on arterial and venous blood gas analysis, in particular
 pH
 pCO2
 Be aware of new approaches being taken to improve
accuracy of prediction of arterial values from venous
blood gas samples

4. Caveats
 Discussion will be limited to comparisons between
arterial and peripheral venous samples
 Not arterial vs central venous/ mixed venous, etc

5. Why venous rather than arterial?
 Less pain for patients
 Fewer complications, especially vascular and infection
 Fewer needle-stick injuries
 Easier blood draw
 Minimal training requirements

6. Key questions in acute respiratory
disease
 Is my patient hypoxic?
 Does this patient have respiratory failure?
 Is this patient a CO2 retainer?
 Do I need to provide additional ventilatory support?
 Is my treatment working?

7. Is my patient hypoxic?
 VBG no good for this.
 In patients with adequate perfusion, pulse
oximetry is accurate
 If the picture doesn’t add up, do an ABG

8. Can venous blood gas answerthe question?
Using a venous blood gas, can I answerthe question Yes/No/Sometimes
Does this patient have respiratory failure?
Is this patient a CO2 retainer?
Do I need to provide additional ventilatory support?
Is my treatment working?
In groups of 2-3, try to answer the questions if necessary
putting caveats/ conditions on your answer. (You have 2
minutes)

9. Statistical considerations
 Outcome of interest is how closely
venous and arterial values agree, not
how well they correlate
 Weighted mean difference gives an
estimate of the accuracy between
the methods
 95% limits of agreement give
information about precision
Arterial value
Venous value
95% LoA

10. Clinical considerations
 There is limited data about the tolerance clinicians have
with respect to agreement between arterial and venous
values of blood gas parameters
 Depending on this tolerance, the degree of agreement
may be acceptable or unacceptable
 Known variation between clinicians re this
 Not known how tolerance of emergency physicians
compares to respiratory physicians or ICU specialists

11. Issues with the evidence
 Patient cohorts highly varied
 Patient groups of real interest are those at
high risk of acidosis or hypercarbia
 Reporting does not always report this detail
 Data may to be dominated by patients with
normal pH, pCO2 and blood pressure
 Need for more work in high risk patient groups

12. Does he have acute respiratory acidosis?
 pH=7.26
 pCO2=66mmHg
VBG
•64 year old man
•Infective
exacerbation COAD

13. Does this patient have respiratory
failure?
 Interested in pH and pCO2 (and HCO3)
 pH
 5 studies (643 patients)
 Weighted mean difference= 0.034 pH units
 95% limits of agreement generally +/- 0.1
 pCO2
 4 studies (452 patients)
 Weighted man difference = 7.26 mmHg
 95% limits of agreement: up to -14 to +26mmHg
 All 3 studies reporting LoA report LoA band >20mmHg

14. HCO3 in respiratory disease
 2 studies (643 patients)
 Weighted mean difference - -1.34 mmmol/l
 No data re 95% limits of agreement
Interpret with caution!

15. Does he have acute respiratory acidosis?
 pH=7.26
 pCO2=66mmHg
 pH=7.30
 pCO2=58mmHg
VBG ABG
YES

16. Is this patient a CO2 retainer?
 pH=7.35
 pCO2=45mmHg
VBG
•58 year old man
•Long smoking
history
•Chest infection

17. Venous pCO2: A screening test forhypercarbia?
Author, year No. Screeni
ng cut-
off
Sens. Spec. NPV %ABG
avoided
Kelly, 2002 196 45 100 57 100 43
Kelly, 2005 107 45 100 47 100 29
Ak, 2006 132 45 100 * 100 33
McCanny,
2011
94 45 100 34 100 23
POOLED
DATA
529 45 100
(95% CI 97-
100)
53
(95% CI
57-58)
100
(95% CI
97-100)
35%
(95% CI
32-41)
Data limited to studies in cohorts with respiratory disease

18. Is this patient a CO2 retainer?
 pH=7.35
 pCO2=45mmHg
 pH=7.42
 pCO2=39mmHg
VBG ABG
NO

19. Do I need to provide additional
ventilatory support?
 pH=7.4
 pCO2=50mmHg
VBG
•40 year old female
•Exacerbation of
asthma

20. Do I need to provide additional
ventilatory support?
 pH=7.4
 pCO2=50mmHg
 pH=7.44
 pCO2=56mmHg
VBG ABG
?

21. Blood gas are only part of the puzzle
 Pulse rate 125
 Respiratory rate 40
 Extreme accessory muscle use
 Looks tired
 What do you thinknow?

22. Is my treatment working?
 Time 1
 pH=7.16
 pCO2=83mmHg
 Time 2
 pH=7.28
 pCO2=62mmHg
VBG
•75 year old man
•Mixed COAD/
CHF
•On NIV

23. Is my treatment working?
 Time 1
 pH=7.16
 pCO2=83mmHg
 Time 2
 pH=7.28
 pCO2=62mmHg
 Time 1
 pH=7.23
 pCO2=61
 Time 2
 pH = 7.3
 pCO2=53mmHg
VBG ABG

24. Monitoring trend
pH:
Average difference:0.001
LoA -0.07 to +0.07
pCO2:
Average difference:0.4
LoA -17.3 to 18.2
pH agreement is good; pCO2 direction same but magnitude varies

25. Can venous blood gas answerthe question?
Using a venous blood gas, can I answerthe question Yes/No/Sometimes
Does this patient have respiratory failure?
Is this patient a CO2 retainer?
Do I need to provide additional ventilatory support?
Is my treatment working?
What do you think now?

26. Mixed acid-base disorders
 No attempt (yet) to determine if VBG can
accurately classify mixed disorders
 Apply calculations to assess this with caution
as is evidence-free zone!

27. Anotherapproach
 Team from Center for Model Based Medical Decision
Support Systems, Dept of Health Science and
Technology, Aalborg University, Denmark (A/Prof
Steven Rees)
 Developed venous to arterial conversion method using
venous blood gas variables and pulse oximetry
 Designed to be incorporated into blood gas analysers

28. The model
 The method calculates arterial values
using mathematical models to
simulate the transport of venous
blood back through the tissues until
simulated arterial oxygenation
matches that measured by
 Constant value of the respiratory
quotient of 0.82
 Change in base excess from arterial
to venous blood is 0 mmol/l
Rees SE, Toftegaard M, Andreassen S. A method for calculation of arterial acid–base and blood gas status from measurements in the peripheral venous
blood. Comp Methods Programs Biomed. 2006, Vol 81, 18-25.

29. Validations
 Respiratory patients
 N=40 (55% acute
admissions)
 Arterial-calculated pH
difference = -0.001pH units
(95% LoA -0.026 to
+0.026)
 Arterial-calculated pCO2
difference = -0.68mmHg
(95% LoA -4.81 to +3.45
mmHg)
 Respiratory/ICU
 N=103
 Arterial-calculated pH
difference = -0.002pH units
(95% LoA -0.029 to
+0.025)
 Arterial-calculated pCO2
difference = 0.3mmHg
(95% LoA -3.58 to +4.18
mmHg)
Toftegaard et al. Emergency Medicine Journal. 2009
Apr;26(4):268-72
Rees et al. Eur Respir J. 2009
May;33(5):1141-7.

30. Monitoring overtime: Example
Red=measured arterial
Black dots =calculated arterial
Blue dashes=measured venous
pH pCO2
Courtesy of SE Rees (unpublished)

31. Take home messages
 Arteriovenous agreement for pH is good – clinically
interchangeable
 Arteriovenous agreement for pCO2 has wide 95% limits
of agreement
 Venous pCO2 can be used to screen for arterial
hypercarbia
 The clinical picture is more important than the numbers
 Venous values can probably be used to monitor trend, if
interpreted in conjunction with the clinical picture
 Limitation: No data on agreement in mixed disease

32. Questions?
Questions?
Questions?