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.
Call Girls Frazer Town Just Call 7001305949 Top Class Call Girl Service Avail...
Venous and arterial blood gas analysis in the ED: What we know and what we don't
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