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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.

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Presentation on theme: "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."— Presentation transcript:

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 - wont rule out hyperoxia see the next slide

3 oxyhaemoglobin dissociation curve A known saturation will reasonably provide you with the PaO 2 90% being roughly equivalent to 60mmHg - the point at which the curve flattens

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 SaO 2 accurate to within 2.75% of PaO 2 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 VBG pH PaO 2 PaCO 2 HCO 3 - Base excess COHb Na + K + Cl - Ca ++ - ionised lactate Hb/Creatinine

7 STEADY STATE VALUES ABGVBG pH PaO SaO %>75% PaCO HCO

8 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.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

9 Value of ABG in the ED to diagnose dyspnoea 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 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 9Value of arterial blood gas analysis in patients with acute dyspnea: an observational study.

10 Predicting ABG values in COPD study of 144 comparing ABG and VBG values good correlation between pH, pCO 2, HCO % negative predictive value of venous PaCO 2 <46 for arterial PaCO 2 <46 poor correlation between PaO 2 and SaO 2 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),

11 using VBG instead of ABG in DKA 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) KELLY AM. The case for venous rather than arterial blood gases in diabetic ketoacidosis. Emerg Med Aust (2006) 18, 64-67

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 true for the range of pH

15 Concepts to help explain Henderson-Hasselbach equation law of mass action: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 -

16 How to pick the major disorder pH Primary Change Physiological response Acidosis - rise in [H + ] MetabolicRespiratory pH Bicarb. CO 2 Bicarb. Alkalosis - fall in [H + ] MetabolicRespiratory pH Bicarb. CO 2 Bicarb. CO 2 + H 2 O H 2 CO 3 H + + HCO 3 -

17 CO 2 in metabolic acidosis expected CO 2 = 1.5[HCO 3 - ] hrs to stabilise limit of compensation - 10mmHg

18 CO 2 in metabolic alkalosis expected CO2 = 0.7[HCO 3 - ]+20

19 HCO 3 - in respiratory acidosis Chronic expected HCO 3 - = ([CO 2 ]- 40) 10 Acute expected HCO 3 - = 24 + ([CO 2 ]- 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)

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

21 anion gap explained excess of measured positively charged ions - cations calculated by the formula: (Na + +K + )-(HCO 3 - +Cl - ) normal range (12-16 if K + not included)

22 when is it real? at 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 HCO 3 - as acidity rises (anion gap) bicarbonate should fall calculate Anion gap = [18 - measured Anion Gap] calculate HCO 3 - divide Anion gap by HCO the delta ratio

24 now what? < Hyperchloraemic normal anion gap acidosis 0.4 to 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/CO 2 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


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