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.
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Are venous and arterial blood gas analysis interchangeable in ED assessment of acute respiratory disease?
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
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.
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