ED use of blood gases AWH 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 oxyhaemoglobin dissociation curve

Oxygen Measurement 1774 - 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 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

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

STEADY STATE VALUES PaO2 PaCO2 HCO3- ABG VBG pH 7.35 - 7.45 7.40 - 7.50 PaO2 80 - 100 40 - 50 SaO2 96-100% >75% PaCO2 35 - 45 HCO3- 22 - 26 20 - 24

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. 2001 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

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: 10.1186/cc10268. 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

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), 285-290 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

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 - 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)

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

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 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

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

HCO3- in respiratory acidosis Chronic expected HCO3- = 24 + 4([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) http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

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- = 24 - 5([CO2]- 40) 10 Acute - not <18mmHg = 24 - 2([CO2]- 40) http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

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) http://www.anaesthesiamcq.com/AcidBaseBook/ab3_2.php

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]) http://www.anaesthesiamcq.com/AcidBaseBook/ab3_3.php

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

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