As presented at EUSEM 2015, this presentation discusses how venous blood gas analysis fits into clinical care in emergency departments. The evidence is correct as of Sept 2015
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Venous Blood Gases in the ED: EuSEM15
1. Anne-Maree Kelly
Professor and Director
Joseph Epstein Centre for Emergency Medicine
Research @Western Health, Australia
OR
Can venous blood gas analysis
replace ABG in the ED?
2. I have not received industry funding for any of my blood gas research.
3. I am a ‘woose’
◦ I dislike needles and am averse to pain
My experience
◦ Late presentation of asthma and DKA because of fear of ABG
◦ More severe illness, that was potentially preventable
Was there another way?
◦ Searched the literature – limited data in paeds for VBG but no
data in adults
◦ Decided to generate data to test my clinical questions
4. To understand the agreement performance of
variables on arterial and venous blood gas analysis
To be aware of how venous blood gas analysis can be
safely used in clinical decision-making
To be aware of grey areas and unanswered questions
5. Discussion will be limited to comparisons between
arterial and peripheral venous samples as these are
the most relevant to Emergency Medicine practice
6. Establishing acid-base status
◦ Mainly pH; but also bicarbonate
Measuring respiratory function/ ventilation
Mainly pCO2; but also pH
‘Quick check’ potassium, haematocrit, some
electrolytes
7. Less pain for patients
Fewer complications, especially vascular and
infection
Fewer needle-stick injuries to staff
Easier blood draw
Minimal training requirement
8. Respiratory Disease Metabolic 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 acidotic/
alkalotic?
What sort of acid-base
disturbance do they have?
Is my treatment working?
9. ELISSA WOULD YOU?
18 year old
Known asthmatic, previous
admissions
2 day exacerbation
Arrival by ambulance
Pulse 120, SpO2 on oxygen
93%, able to speak in short
phrases, tight wheeze
Obtain an ABG for pO2,
pCO2 and pH?
Obtain a VBG for pCO2 and
pH?
Obtain a VBG for pH and
hypercarbia screen?
Proceed without blood gas
based on clinical
assessment
10. Clinical features VBG RESULT
18 year old
Known asthmatic, previous
admissions
2 day exacerbation
Arrival by ambulance
Pulse 120, SpO2 on oxygen
93%, able to speak in short
phrases, tight wheeze
VBG result
◦ pH 7.35
◦ pCO2 35mmHg (4.7 kPa)
Is this data enough to guide
initial clinical decision-
making?
◦ Yes
◦ No
◦ Unsure
11. TRAN WOULD YOU?
74 year old
Known COPD
Acute respiratory distress
Pulse 118, BP 140
Respiratory rate 35
SpO2 (air) 86%
Obtain an ABG for pO2,
pCO2 and pH?
Obtain a VBG for pCO2 and
pH?
Obtain a VBG for pH and
hypercarbia screen?
Proceed without blood gas
based on clinical
assessment
12. Clinical features VBG result
74 year old
Known COPD
Acute respiratory distress
Pulse 118, BP 140
Respiratory rate 35
SpO2 (air) 86%
VBG result
◦ pH 7.16
◦ pCO2 82.6mmHg (11 kPa)
◦ Bicarbonate 28.8mmmol/l
Is this data enough to guide
initial clinical decision-
making?
◦ Yes
◦ No
◦ Unsure
13. Clinical features A different VBG result
74 year old
Known COPD
Acute respiratory distress
Pulse 118, BP 140
Respiratory rate 35
SpO2 (air) 86%
VBG result
◦ pH 7.45
◦ pCO2 42mmHg (5.6 kPa)
◦ Bicarbonate 28.8mmmol/l
What about this?
14. JANE WOULD YOU?
26 year old
Insulin dependent
diabetic
2 days of vomiting and
diarrhoea
Pulse 125, BP 100
Bedside glucose ‘Hi’
Obtain an ABG for pO2,
pCO2 and pH?
Obtain a VBG for pCO2 and
pH?
Obtain a VBG for pH and
hypercarbia screen?
Proceed without blood gas
based on clinical
assessment
15. Clinical features VBG result
26 year old
Insulin dependent
diabetic
2 days of vomiting and
diarrhoea
Pulse 125, BP 100
Bedside glucose ‘Hi’
VBG result:
◦ pH: 7.26
◦ pCO2 16mmHg (2.1 kPa)
◦ Bicarbonate 7.1 mmol/l
◦ Potassium 3.8 mmol/l
◦ Base excess -14
Is this data enough to guide
initial clinical decision-
making?
◦ Yes
◦ No
◦ Unsure
16. 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
17. 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 considerable variation between clinicians regarding
this tolerance!
18. A number of relatively small studies
Patient cohorts are highly varied
Patient groups of interest are those at high risk of
acidosis or hypercarbia
◦ Reporting does not always provide this detail
◦ Data is often dominated by patients with normal pH, pCO2
and blood pressure
19. 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 generally within +/- 0.1 pH
units
20. COAD
◦ 5 studies (643 patients)
◦ Weighted mean difference= 0.034 pH
units
◦ 95% limits of agreement generally +/-
0.1
21. 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)
22. In patients without severe circulatory compromise,
agreement between arterial and venous values for
pH in both metabolic and respiratory conditions is
close.
Level of a agreement is probably clinically acceptable
to most clinicians.
23. 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
24. 4 studies
452 patients
Weighted man difference = 7.26 mmHg
95% limits of agreement: up to -14 to +26mmHg
◦ All 3 studies that report LoA have LoA band >20mmHg
25. Agreement between venous and arterial pCO2 is NOT
good enough for clinical inter-changeability
BUT WAIT ......
26. 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
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
27. 1 study
Average difference between change in pH (v-a) was 0.001
(LoA -0.7 to +0.7).
Average difference between change in pCO2 (v-a) was
0.04mmHg (LoA -17.3 to +18.2).
For both pH and pCO2, in the majority of cases the direction
of change was the same although the magnitude was
variable.
28. Agreement between venous and arterial pCO2 is NOT
good enough for clinical inter-changeability
pCO2 on VBG is a reliable screening test for clinically
relevant hypercarbia
In combination with clinical assessment, change in
venous pH and pCO2may be useful to monitor progress
but requires validation
29. 8 studies
1211 patients
Various conditions
◦ COAD =2
Weighted mean difference = -1.3mmol/l
95% limits of agreement : up to +/- 5mmol/l
31. DKA
◦ 1 study (21 patients)
◦ Weighted mean difference = -1.88 mmol/l
◦ 95% limits of agreement = -2.8 to 0.9 mmol/l
32. Limited data shows good agreement
Evidence regarding 95% limits of agreement is sparse
Probably close enough agreement for classification as
high, low or normal
Clinical acceptability may be context specific
33. Two studies only
◦ In a sample of 103 patients (various conditions), they
report:
mean difference of 0.089
95% limits of agreement -0.974 to +0.552
◦ 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: Agreement unclear. If accuracy is
needed in critically ill, need ABG.
34. 2 studies in DKA comparing BG vs serum K+
In both studies serum K+ is usually higher than BG
K+.
Fu et al.
◦ 95% limits of agreement -0.96 to +1.19mmol/l
◦ 80% of patients had agreement within +/- 0.5mmmol/L
Roblas et al.
◦ Mean difference 1.13mmol/l (serum higher)
◦ 34% of patients had agreement within +/- 0.5 mmol/L.
35. Conflicting data
No data in mixed acid-base disorders
Limited data in toxicological conditions
36. Clinical decision-making isn’t just about the numbers
Clinical aspects of assessment are also important
Particularly the case in acute respiratory disease
37. ELISSA VBG RESULT
18 year old
Known asthmatic, previous
admissions
2 day exacerbation
Arrival by ambulance
Pulse 120, SpO2 on oxygen
93%, able to speak in short
phrases, tight wheeze
VBG result
◦ pH 7.35
◦ pCO2 35mmHg (4.7 kPa)
Is this data enough to guide
initial clinical decision-
making?
◦Yes
◦ No
◦ Unsure
38. TRAN VBG result
74 year old
Known COPD
Acute respiratory distress
Pulse 118, BP 140
Respiratory rate 35
SpO2 (air) 86%
VBG result
◦ pH 7.16
◦ pCO2 82.6mmHg (11 kPa)
◦ Bicarbonate 28.8mmmol/l
Is this data enough to guide
initial clinical decision-
making?
◦ Yes
◦ No
◦ Unsure
39. TRAN A different VBG result
74 year old
Known COPD
Acute respiratory distress
Pulse 118, BP 140
Respiratory rate 35
SpO2 (air) 86%
VBG result
◦ pH 7.45
◦ pCO2 42mmHg (5.6 kPa)
◦ Bicarbonate 28.8mmmol/l
Is this data enough to guide
initial clinical decision-
making?
◦ Yes
◦ No
◦ Unsure
40. JANE VBG result
26 year old
Insulin dependent
diabetic
2 days of vomiting and
diarrhoea
Pulse 125, BP 100
Bedside glucose ‘Hi’
VBG result:
◦ pH: 7.26
◦ pCO2 16mmHg (2.1 kPa)
◦ Bicarbonate 7.1 mmol/l
◦ Potassium 3.8 mmol/l
◦ Base excess -14
Is this data enough to guide
initial clinical decision-
making?
◦ Yes
◦ No
◦ Unsure
41. 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
pCO2
◦ NOT enough agreement for clinical purposes, either as one-off or to monitor
absolute change
◦ Data suggests venous pCO2 is useful as a screening test
Base excess
◦ Agreement unclear
Potassium
◦ Beware the error margin at the extremes of the normal range