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

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

30.  COAD
◦ 2 studies (643 patients)
◦ Weighted mean difference= -1.34 mmol/l
◦ 95% limits of agreement: none reported

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

42. Questions?Questions