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

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.

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

Caveats

Discussion will be limited to comparisons between arterial and peripheral venous samples Not arterial vs central venous/ mixed venous, etc

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

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?

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

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)

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

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

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

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) 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

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!

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 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

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

What do you think now?

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

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

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

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?

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!

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

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.

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.

Monitoring over time: Example

Red=measured arterialBlack dots =calculated arterialBlue dashes=measured venous

pH pCO2

Courtesy of SE Rees (unpublished)

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

Questions?

Questions?Questions?