1. ED use of blood gases
AWH Teaching Program 2013

2. ABG or VBG Treatment is based on clinical parameters
i.e. real time observations and response to treatment
Almost never a need to do ABG in ED.
VBG provides all the information you might need.
SaO2 provides the rest - won’t rule out hyperoxia
see the next slide

3. oxyhaemoglobin dissociation curve
A known saturation will reasonably provide you with the PaO2
90% being roughly equivalent to 60mmHg - the point at which the curve flattens
oxyhaemoglobin dissociation curve

4. Oxygen Measurement
Joseph Priestly first extracted oxygen from blood (see diagram)
Krogh used aerotonomoter to measure rabbit arterial blood oxygen tension
Clark (and others) created an oxygen electrode

5. Pulse Oximetry Method invented in 1972 by Takuo Aoyagi
byproduct of research into non-invasive measurement of cardiac output
finger probes developed in 1979
SaO2 accurate to within 2.75% of PaO2 in sepsis
Wilson et al. The accuracy of pulse oximetry in emergency department patients with severe sepsis and septic shock: a retrospective cohort study
BMC Emergency Medicine 2010, 10:9

6. Values commonly measured from a VBGpH
PaO2
PaCO2
HCO3-
Base excess
COHb
Na+ K+
Cl-
Ca++ - ionised
lactate
Hb/Creatinine

7. STEADY STATE VALUES PaO2 PaCO2 HCO3- ABG VBG pH 7.35 - 7.45
PaO2
SaO2
96-100%
>75%
PaCO2
HCO3-

8. Venous pH in the ED
Kelly AM, McAlpine R, Kyle E. Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department.
EMERG MED J SEP;18(5):340-2
Good correlation between values in range of disease states
multiple small studies
first large study was performed in Australia 2001
approx. 250 patients had simultaneous ABG and VBG
200 with respiratory disease
50 suspected of metabolic derangement
pH values differed by 0.4

9. Value of ABG in the ED to diagnose dyspnoea
Burri E, Potocki M, Drexler B, et al Value of arterial blood gas analysis in patients with acute dyspnea: an observational study Crit Care. 2011;15(3):R145. doi: /cc Epub 2011 Jun 9
Retrospective study of prospectively collected data
approx 1150 patients presenting to Basel ED with dyspnoea
diagnoses include APO, COPD, Asthma, Pneumonia and Hyperventilation
No ability to differentiate between major diagnoses
ICU admissions were greater with pH <7.33
mortality was greater with lower pH

10. Predicting ABG values in COPD
Ak, A., Ogun, C., Bayir, S. et al Prediction of Arterial Blood Gas Values in Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease
Tohoku J. Exp. Med., 2006, 210(4),
study of 144 comparing ABG and VBG values
good correlation between pH, pCO2, HCO3-
100% negative predictive value of venousPaCO2 <46 for arterialPaCO2 <46
poor correlation between PaO2 and SaO2

11. using VBG instead of ABG in DKA
KELLY AM. The case for venous rather than arterial blood gases in diabetic ketoacidosis. Emerg Med Aust (2006) 18, 64-67
Review article
attempted to correlate ABG with VBG values in DKA
found good correlation between
pH unit difference
HCO difference
data based on small studies
uncertain if true in haemodynamic instability or respiratory failure (not often the case in DKA)

12. calculations or corrections
Compensation is really the physiological response to the primary acid/base disorder
It is possible to determine the presence of a mixed or combined acid/base disorder
Following are some formulae to help with that
Follow the links to some more detailed explanations

13. Calculations Expected values: Rule of thumb calculations Anion gap
Delta ratio

14. RULE OF THUMB In acute respiratory disease
for every ↑ in CO2 of 10mmHg the pH ↓ is 0.1 units
for every ↓ in CO2 of 10mmHg the pH ↑ is 0.1units
true for the range of pH

15. Concepts to help explain
Henderson-Hasselbach equation
law of mass action:
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

16. How to pick the major disorder
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-
Acidosis - rise in [H+]
Metabolic
Respiratory
pH↓
Bicarb. ↓
CO2 ↑
CO2 ↓
Bicarb. ↑
Alkalosis - fall in [H+]
Metabolic
Respiratory
pH↑
Bicarb. ↑
CO2 ↓
CO2 ↑
Bicarb. ↓ pH
Primary Change
Physiological response

17. CO2 in metabolic acidosis
expected CO2 = 1.5[HCO3-]+ 8
12-24 hrs to stabilise
limit of ‘compensation’ - 10mmHg

18. CO2 in metabolic alkalosis
expected CO2 = 0.7[HCO3-]+20

19. HCO3- in respiratory acidosis
Chronic
expected HCO3-
= ([CO2]- 40) 10
Acute
= 24 + ([CO2]- 40)
4:1 RULE - the rise in bicarbonate in stable chronic respiratory acidosis (2-3 days) is 4 times higher than in acute respiratory acidosis (immediate)

20. HCO3- in respiratory alkalosis
5:2 RULE - the fall in bicarbonate in stable chronic respiratory alkalosis (2-3 days) is higher than in acute respiratory alkalosis
Chronic - not <15mmHg
expected HCO3-
= ([CO2]- 40) 10
Acute - not <18mmHg
= ([CO2]- 40)

21. anion gap explained
excess of measured positively charged ions - cations
calculated by the formula:
(Na++K+)-(HCO3-+Cl-)
normal range (12-16 if K+ not included)

22. when is it real? at 20-29 metabolic acidosis present - in 2/3 patients
>29 considered a wide anion gap acidosis
you can use delta ratio to discover further acid/base disorders

23. Delta ratio
compares the relative difference between the change in the anion gap with the change in HCO3-
as acidity rises (↑anion gap) bicarbonate should fall
calculate ∆ Anion gap = [18 - measured Anion Gap]
calculate ∆ HCO3-
divide ∆ Anion gap by ∆ HCO3- - the ‘delta ratio’

24. now what? < 0.4 - Hyperchloraemic normal anion gap acidosis
0.4 to 0.8  - Combined high AG and normal AG acidosis
1  - Common in DKA due to urinary ketone loss
1 to 2  - Typical pattern in high anion gap metabolic acidosis
> 2 Check for either a:
co-existing Metabolic Alkalosis (which would elevate [HCO3])  
or a co-existing Chronic Respiratory Acidosis (which results in compensatory elevation of [HCO3])

25. Recap
The clinical scenario, pH and bicarb/CO2 are all needed to determine the primary acid/base disorder
Use of calculations will determine if a secondary acid/base disorder exits
The delta ratio can be used to ‘discover’ additional acid/base disorders - beware of over-interpretation

26. Examples
The following examples have no workings and are presented for you to have a go.....

27. example #1 26 year old male type 1 diabetes
moderately unwell with vomiting

28. example #2 56 year old female
under investigation for endocrine disorder
shocked on arrival

29. example #3
56 year old female
Known COPD
Drowsy

30. Resources
The accuracy of pulse oximetry in emergency department patients with severe sepsis and septic shock: a retrospective cohort study
Wilson et al.
BMC Emergency Medicine 2010, 10:9
Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department.
Kelly AM, McAlpine R, Kyle E.
Emerg Med J Sep;18(5):340-2
Value of arterial blood gas analysis in patients with acute dyspnea: an observational study.
Burri E, Potocki M, Drexler B, Schuetz P, et al
Crit Care. 2011;15(3):R145. doi: /cc Epub 2011 Jun 9
Prediction of Arterial Blood Gas Values in Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease
Ak, A., Ogun, C., Bayir, S. et al
Tohoku J. Exp. Med., 2006, 210(4),
The case for venous rather than arterial blood gases in diabetic ketoacidosis.
Kelly AM.
Emerg Med Australas Feb;18(1):64-7. Review