ARE VENOUS AND ARTERIAL BLOOD GAS ANALYSIS
INTERCHANGEABLE IN ED ASSESSMENT OF ACUTE
RESPIRATORY DISEASE?
Anne-Maree Kelly...
Conflicts of interest
 I received financial support for travel and accommodation from Radiometer Pty
Ltd to present a sim...
Objectives
 After this presentation, participants will:
 Understand the agreement performance of variables
on arterial a...
Caveats
 Discussion will be limited to comparisons between
arterial and peripheral venous samples
 Not arterial vs centr...
Why venous rather than arterial?
 Less pain for patients
 Fewer complications, especially vascular and infection
 Fewer...
Key questions in acute respiratory
disease
 Is my patient hypoxic?
 Does this patient have respiratory failure?
 Is thi...
Is my patient hypoxic?
 VBG no good for this.
 In patients with adequate perfusion, pulse
oximetry is accurate
 If the ...
Can venous blood gas answerthe question?
Using a venous blood gas, can I answerthe question Yes/No/Sometimes
Does this pat...
Statistical considerations
 Outcome of interest is how closely
venous and arterial values agree, not
how well they correl...
Clinical considerations
 There is limited data about the tolerance clinicians have
with respect to agreement between arte...
Issues with the evidence
 Patient cohorts highly varied
 Patient groups of real interest are those at
high risk of acido...
Does he have acute respiratory acidosis?
 pH=7.26
 pCO2=66mmHg
VBG
•64 year old man
•Infective
exacerbation COAD
Does this patient have respiratory
failure?
 Interested in pH and pCO2 (and HCO3)
 pH
 5 studies (643 patients)
 Weigh...
HCO3 in respiratory disease
 2 studies (643 patients)
 Weighted mean difference - -1.34 mmmol/l
 No data re 95% limits ...
Does he have acute respiratory acidosis?
 pH=7.26
 pCO2=66mmHg
 pH=7.30
 pCO2=58mmHg
VBG ABG
YES
Is this patient a CO2 retainer?
 pH=7.35
 pCO2=45mmHg
VBG
•58 year old man
•Long smoking
history
•Chest infection
Venous pCO2: A screening test forhypercarbia?
Author, year No. Screeni
ng cut-
off
Sens. Spec. NPV %ABG
avoided
Kelly, 200...
Is this patient a CO2 retainer?
 pH=7.35
 pCO2=45mmHg
 pH=7.42
 pCO2=39mmHg
VBG ABG
NO
Do I need to provide additional
ventilatory support?
 pH=7.4
 pCO2=50mmHg
VBG
•40 year old female
•Exacerbation of
asthma
Do I need to provide additional
ventilatory support?
 pH=7.4
 pCO2=50mmHg
 pH=7.44
 pCO2=56mmHg
VBG ABG
?
Blood gas are only part of the puzzle
 Pulse rate 125
 Respiratory rate 40
 Extreme accessory muscle use
 Looks tired
...
Is my treatment working?
 Time 1
 pH=7.16
 pCO2=83mmHg
 Time 2
 pH=7.28
 pCO2=62mmHg
VBG
•75 year old man
•Mixed COA...
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
 ...
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 agreeme...
Can venous blood gas answerthe question?
Using a venous blood gas, can I answerthe question Yes/No/Sometimes
Does this pat...
Mixed acid-base disorders
 No attempt (yet) to determine if VBG can
accurately classify mixed disorders
 Apply calculati...
Anotherapproach
 Team from Center for Model Based Medical Decision
Support Systems, Dept of Health Science and
Technology...
The model
 The method calculates arterial values
using mathematical models to
simulate the transport of venous
blood back...
Validations
 Respiratory patients
 N=40 (55% acute
admissions)
 Arterial-calculated pH
difference = -0.001pH units
(95%...
Monitoring overtime: Example
Red=measured arterial
Black dots =calculated arterial
Blue dashes=measured venous
pH pCO2
Cou...
Take home messages
 Arteriovenous agreement for pH is good – clinically
interchangeable
 Arteriovenous agreement for pCO...
Questions?
Questions?
Questions?
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Are venous and arterial blood gas analysis interchangeable in ED assessment of acute respiratory disease?

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Ever wondered if you can use a venous blood gas instead on an arterial analysis to guide management of patients with acute respiratory disease in the eemergency department? This presentation will try to answer the key questions including does my patient have acute respiratory failure, is my patient a CO2 retainer, do I need to provide additional ventilatory support and is my treatment working.

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

  1. 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. 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. 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. 4. Caveats  Discussion will be limited to comparisons between arterial and peripheral venous samples  Not arterial vs central venous/ mixed venous, etc
  5. 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. 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. 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. 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. 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. 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. 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. 12. Does he have acute respiratory acidosis?  pH=7.26  pCO2=66mmHg VBG •64 year old man •Infective exacerbation COAD
  13. 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. 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. 15. Does he have acute respiratory acidosis?  pH=7.26  pCO2=66mmHg  pH=7.30  pCO2=58mmHg VBG ABG YES
  16. 16. Is this patient a CO2 retainer?  pH=7.35  pCO2=45mmHg VBG •58 year old man •Long smoking history •Chest infection
  17. 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. 18. Is this patient a CO2 retainer?  pH=7.35  pCO2=45mmHg  pH=7.42  pCO2=39mmHg VBG ABG NO
  19. 19. Do I need to provide additional ventilatory support?  pH=7.4  pCO2=50mmHg VBG •40 year old female •Exacerbation of asthma
  20. 20. Do I need to provide additional ventilatory support?  pH=7.4  pCO2=50mmHg  pH=7.44  pCO2=56mmHg VBG ABG ?
  21. 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. 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. 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. 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. 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. 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. 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. 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. 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. 30. Monitoring overtime: Example Red=measured arterial Black dots =calculated arterial Blue dashes=measured venous pH pCO2 Courtesy of SE Rees (unpublished)
  31. 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. 32. Questions? Questions? Questions?

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