are venous and arterial blood gas analysis interchangeable in ed assessment of acute respiratory...
DESCRIPTION
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.TRANSCRIPT
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ARE VENOUS AND ARTERIAL BLOOD GAS ANALYSIS INTERCHANGEABLE IN ED ASSESSMENT OF ACUTE
RESPIRATORY DISEASE?
Anne-Maree KellyProfessor and DirectorJoseph Epstein Centre for Emergency Medicine Research @Western Health
@kellyam_jec
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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.
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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
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Caveats
Discussion will be limited to comparisons between arterial and peripheral venous samples Not arterial vs central venous/ mixed venous, etc
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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
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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?
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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
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Can venous blood gas answer the question?
Using a venous blood gas, can I answer the 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)
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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
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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
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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
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Does he have acute respiratory acidosis?
pH=7.26
pCO2=66mmHg
VBG
•64 year old man•Infective exacerbation COAD
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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
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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!
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Does he have acute respiratory acidosis?
pH=7.26
pCO2=66mmHg
pH=7.30
pCO2=58mmHg
VBG ABG
YES
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Is this patient a CO2 retainer?
pH=7.35
pCO2=45mmHg
VBG
•58 year old man•Long smoking history•Chest infection
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Venous pCO2: A screening test for hypercarbia?
Author, year
No.
Screening cut-off
Sens. Spec. NPV %ABG avoide
d
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
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Is this patient a CO2 retainer?
pH=7.35
pCO2=45mmHg
pH=7.42
pCO2=39mmHg
VBG ABG
NO
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Do I need to provide additional ventilatory
support?
pH=7.4
pCO2=50mmHg
VBG
•40 year old female•Exacerbation of asthma
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Do I need to provide additional ventilatory
support?
pH=7.4
pCO2=50mmHg
pH=7.44
pCO2=56mmHg
VBG ABG
?
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Blood gas are only part of the puzzle
Pulse rate 125 Respiratory rate 40 Extreme accessory muscle use Looks tired
What do you think now?
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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
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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
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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
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Can venous blood gas answer the question?
Using a venous blood gas, can I answer the 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?
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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!
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Another approach
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
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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.
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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):268-72Rees et al. Eur Respir J. 2009
May;33(5):1141-7.
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Monitoring over time: Example
Red=measured arterialBlack dots =calculated arterialBlue dashes=measured venous
pH pCO2
Courtesy of SE Rees (unpublished)
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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
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Questions?
Questions?Questions?