ED use of blood gases AWH Teaching Program 2013. ABG or VBG Treatment is based on clinical...

ED use of blood gasesAWH Teaching Program 2013

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.......

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

Oxygen Measurement1774 - Joseph Priestly first extracted oxygen from blood (see diagram)

1908 - Krogh used aerotonomoter to measure rabbit arterial blood oxygen tension

1958 - Clark (and others) created an oxygen electrode

Pulse OximetryMethod 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

Values commonly measured from a VBG

pH

PaO2

PaCO2

HCO3-

Base excess

COHb

Na+

K+

Cl-

Ca++ - ionised

lactate

Hb/Creatinine

STEADY STATE VALUES

ABG VBG

pH 7.35 - 7.45 7.40 - 7.50

PaO2 80 - 100 40 - 50

SaO2 96-100% >75%

PaCO2 35 - 45 40 - 50

HCO3- 22 - 26 20 - 24

Venous pH in the ED

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

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. 2001 SEP;18(5):340-2

Value of ABG in the ED to diagnose dyspnoeaRetrospective 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

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: 10.1186/cc10268. Epub 2011 Jun 9

Predicting ABG values in COPD

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

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), 285-290

using VBG instead of ABG in DKA

Review article

attempted to correlate ABG with VBG values in DKA

found good correlation between

pH - 0.02 unit difference

HCO3- - -0.18 difference

data based on small studies

uncertain if true in haemodynamic instability or respiratory failure (not often the case in DKA)

KELLY AM. The case for venous rather than arterial blood gases in diabetic ketoacidosis. Emerg Med Aust (2006) 18, 64-67

calculations or correctionsCompensation 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

Calculations

Expected values:

Rule of thumb

calculations

Anion gap

Delta ratio

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 7.2 - 7.6

Concepts to help explain

Henderson-Hasselbach equation

law of mass action:

CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

How to pick the major disorder

pH

Primary Change

Physiological response

Acidosis - rise in [H+]

MetabolicRespirato

ry

pH↓ pH↓

Bicarb. ↓ CO2 ↑

CO2 ↓ Bicarb. ↑

Alkalosis - fall in [H+]

MetabolicRespiratory

pH↑ pH↑

Bicarb. ↑ CO2 ↓

CO2 ↑ Bicarb. ↓

CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

CO2 in metabolic acidosis

expected CO2 = 1.5[HCO3-]+ 8

12-24 hrs to stabilise

limit of ‘compensation’ - 10mmHg

http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

CO2 in metabolic alkalosis

expected CO2 = 0.7[HCO3-]+20


HCO3- in respiratory

acidosis

Chronic

expected HCO3-

= 24 + 4([CO2]- 40) 10

Acute

expected HCO3-

= 24 + ([CO2]- 40) 10

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)


HCO3- in respiratory

alkalosis

Chronic - not <15mmHg

expected HCO3-

= 24 - 5([CO2]- 40) 10

Acute - not <18mmHg

• expected HCO3-

= 24 - 2([CO2]- 40) 10

5:2 RULE - the fall in bicarbonate in stable chronic respiratory alkalosis (2-3 days) is higher than in

acute respiratory alkalosis


anion gap explained

excess of measured positively charged ions - cations

calculated by the formula:

(Na++K+)-(HCO3-+Cl-)

normal range 16-20 (12-16 if K+ not included)


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

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’

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])


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

Examples

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

example #1

26 year old male

type 1 diabetes

moderately unwell with vomiting

example #2

56 year old female

under investigation for endocrine disorder

shocked on arrival

example #3

56 year old female

Known COPD

Drowsy

ResourcesThe 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. 2001 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: 10.1186/cc10268. 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), 285-290

The case for venous rather than arterial blood gases in diabetic ketoacidosis.Kelly AM.Emerg Med Australas. 2006 Feb;18(1):64-7. Review


ED use of blood gases AWH Teaching Program 2013. ABG or VBG Treatment is based on clinical...

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