This presentation delivered at the International Conference on Emergency Medicine in Dublin summarises agreement between venous and arterial blood gas parameters and utility of venous blood gas analysis in emergency department clinical practice. It also highlights important gaps in our knowledge on this topic.

1. VENOUS AND ARTERIAL BLOOD GAS
ANALYSIS IN EMERGENCY
DEPARTMENTS:
WHAT WE KNOW AND WHAT WE DON’T!
Anne-Maree Kelly
Professor and Director
Joseph Epstein Centre for Emergency Medicine
Research @Western Health

2. Permissions
 This presentation can be used in part or whole for
educational purposes on the condition that the following
appears on each slide used:
‘Re-produced with permission of Professor Anne-Maree
Kelly, Joseph Epstein Centre for Emergency Medicine
Research, Melbourne, Australia’
@kellyam_jec

3. 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.

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

5. Caveats
 Discussion will be limited to comparisons between
arterial and peripheral venous samples
 Data is up-to-date as of publications to May 2012
 Includes some of data only ‘published’ as abstract in
2012

6. Blood gases in emergency medicine
 Establishing acid-base status
 Mainly pH; but also bicarbonate
 Measuring respiratory function/ ventilation
 Mainly pCO2; but also pH
 ‘Quick check’ potassium, haematocrit, some
electrolytes
 Not addressed in this presentation

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

8. Setting the context
 JANE
 26 year old, insulin
dependent diabetic
 2 days of vomiting and
diarrhoea.
 Pulse 120 bpm, BP
100/-, BSL ‘Hi’
 TRAN
 74 year old COAD
 Acute respiratory distress.
 Pulse 110, BP 140/-,
SpO2 (air) 88%

9. The clinical questions
 Can we
 Exclude / diagnose
 Monitor progress of
 Base therapeutic decisions for (eg use and settings of
NIV)
Metabolic acidosis or acute respiratory failure
using venous blood gas analysis rather than
arterial?

10. 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

11. 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
 There is known variation between clinicians re this

12. Issues with the evidence
 Patient cohorts highly varied
 Patient groups of 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 research focussed on high risk patient groups

13. pH
 13 studies
 Range from 44 to 346 patients
 Various conditions
 DKA (3), COAD (4), trauma (1)
 2009 patients
 Weighted mean difference of 0.033 pH units
 95% limits of agreement (7 studies) generally within +/-
0.1 pH units

14. pH in illness subgroups
 DKA
 3 studies (265
patients)
 Weighted mean
difference = 0.02 pH
units
 95% limits of
agreement = -0.009 to
0.02 pH units (1 study)
 COAD
 5 studies (643 patients)
 Weighted mean
difference= 0.034 pH
units
 95% limits of agreement
generally +/- 0.1 (3
studies)

15. pH- Other
 One ICU-based study suggests that as
hypotension increases, AV pH agreement
deteriorates
 Very small patient numbers
 Finding not yet validated

16. What we know & evidence gaps
 We know:
 Generally close AV agreement in both respiratory and
metabolic disease
 Evidence gaps:
 AV agreement in various levels and types of shock
 AV difference in toxicology scenarios (1 small study in
TCA OD only)
 AV difference in mixed acid-base disease

17. Bicarbonate
 8 studies
 1211 patients
 Various conditions (COAD 2)
 Weighted mean difference = -1.3mmol/l
 95% limits of agreement : up to +/- 5mmol/l (3
studies)

18. Bicarbonate in illness subgroups
 DKA
 1 study (21 patients)
 Weighted mean
difference = -1.88
mmol/l
 95% limits of
agreement = -2.8 to
0.9 mmol/l
 COAD
 2 studies (643 patients)
 Weighted mean
difference= -1.34 mmol/l
 95% limits of
agreement: none
reported

19. What we know and evidence gaps
 We know:
 Limited data suggests good agreement
 Very little data re limits of agreement
 ? +/- 5mmol/L
 Evidence gaps:
 AV agreement in specific disease states
 AV agreement in various levels and types of shock
 AV difference in toxicology scenarios
 AV difference in mixed acid-base disease

20. pCO2
 8 studies
 965 patients
 Various conditions (COAD 4)
 Weighted mean difference = 6.2 mmHg
 95% limits of agreement: up to -17.4 to +23.9
mmHg
 5/7 studies reporting LoA report LoA band >20mmHg

21. pCO2 in COAD
 4 studies
 452 patients
 Weighted man difference = 7.26 mmHg
 95% limits of agreement: up to -14 to +26
 All 3 studies that reported LoA report LoA band
>20mmHg

22. Venous pCO2: A screening test for hypercarbia?
Author, year No. Screening
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
52
9
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

23. Using venous pH and CO2 to track progress?
 Preliminary data presented at this meeting as a
poster
 41 comparisons in 29 patients
 Arteriovenous difference for change in pH =0.004 (95%
LoA -0.09 to 0.1)
 Arteriovenous difference for change in pCO2 = 0.55mmHg
(95% LoA -16.6 to 17.6mmHg)

24. What we know & evidence gaps
 We know:
 AV agreement is NOT good enough for clinical inter-
changeability
 Wide limits of agreement
 Venous pCO2 has potential as a screening test for hypercarbia
 Excellent NPV
 AV agreement in change in pCO2 is NOT good enough for clinical
inter-changeability (pilot data only)
 Wide limits of agreement
 Evidence gaps:
 Whether trend in venous pCO2 and pH can safely drive a care pathway
for COAD
 Subject of current international research project

25. Base excess
 Two studies only
 In a sample of 103 patients (various conditions), they
report:
 mean difference of 0.089mmol/L
 95% limits of agreement -0.974 to +0.552 mmol/L
 In 326 trauma patients
 mean difference -0.3 BE units
 95% limits of agreement -4.4 to +3.9 BE units
 20% did not fall within pre-defined clinical equivalence threshold
Current view: LOA too wide. If accuracy needed in critically ill,
need ABG

26. Clinical application
 JANE
 DKA
 AV agreement is
acceptable; at least
in non-shocked
patients
 Can use venous pH
to diagnose/
monitor
 TRAN
 Acute respiratory
distress
 pH agreement good but
pCO2 has considerable
imprecision
 Can use venous pCO2
as a screening test for
hypercarbia

27. 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

28. The model
 The method calculates
arterial values using
mathematical models
 Assumes:
 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;26:268-72Rees et al. Eur Respir J. 2009;33:1141-7.

30. Validations
 Emergency dept patients
 N=148 patients (47 clinical need
for ABG, 101 without)
 pH can be calculated to within
0.02 pH units (95% LoA)
 pCO2 can be calculated to within
4mmHg (0.5kPa)
Tygesen et al. Eur J Emerg Med. 2011 Nov 11. [Epub ahead of print]

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

32. Comments
 Hard to know how many patients were acidotic or
hypercarbic
 No validation in patients undergoing respiratory support
e.g. NIV
 Model undergoing commercialisation
 Add on licence to blood gas machine
 No app planned at this stage (personal communication)

33. Take home messages
 pH and bicarbonate
 probably close enough agreement for clinical
purposes in DKA, acute respiratory failure, isolated
metabolic acidosis
 More work needed in toxicology, shock, mixed
disease

34. Take home messages
 pCO2
 NOT enough agreement for clinical purposes, either as
one-off or to monitor change
 Data suggests venous pCO2 is useful as a screening test
 Base excess
 Probably not enough agreement for clinical purposes

35. Take home messages
 Mathematical modelling approaches might be more
accurate especially for pCO2
 For broad applicability an app/ similar would be more
feasible than integration into blood gas machines
 More work needed to prove accuracy and precision in high
risk groups

36. Questions?
Questions?
Questions?
@kellyam_jec