1. ABG Interpretation Dr Abdollahi Afshar Hospital

2. Information Obtained from an ABG:  Acid base status  Oxygenation Dissolved O2 (pO2) Saturation of hemoglobin  CO2 elimination  Levels of carboxyhemoglobin and methemoglobin

3. Indications:  Assess the ventilatory status, oxygenation and acid base status  Assess the response to an intervention

4. Why an ABG instead of Pulse oximetry?  Pulse oximetry does not assess ventilation (pCO2) or acid base status.  Pulse oximetry becomes unreliable when saturations fall below 70-80%.  Technical sources of error (ambient or fluorescent light, hypoperfusion, nail polish, skin pigmentation)  Pulse oximetry cannot interpret methemoglobin or carboxyhemoglobin.

5. Which Artery to Choose?  The radial artery is superficial, has collaterals and is easily compressed. It should almost always be the first choice.  Other arteries (femoral, dorsalis pedis, brachial) can be used in emergencies.

8. Preparing to perform the Procedure:  Make sure you and the patient are comfortable.  Assess the patency of the radial and ulnar arteries.

10. Collection Problems:  Type of syringe Plastic vs. glass  Use of heparin  Air bubbles  Specimen handling and transport

11. Contraindications:  Bleeding diathesis  AV fistula  Severe peripheral vascular disease, absence of an arterial pulse  Infection over site

12. Why an ABG instead of Pulse oximetry?  Pulse oximetry uses light absorption at two wavelengths to determine hemoglobin saturation.  Pulse oximetry is non-invasive and provides immediate and continuous data.

14. Type of Syringe  Glass- Impermeable to gases Expensive and impractical  Plastic- Somewhat permeable to gases Disposable and inexpensive

15. Transport  After specimen collected and air bubble removed, gently mix and invert syringe.  Because the wbcs are metabolically active, they will consume oxygen.  Plastic syringes are gas permeable.  Key: Minimize time from sample acquisition to analysis.

16. Transport  Placing the AGB on ice may help minimize changes, depending on the type of syringe, pO2 and white blood cell count.  Its probably not as important if the specimen is delivered immediately.

17. Performing the Procedure:  Put on gloves  Prepare the site Drape the bed Cleanse the radial area with a alcohol  Position the wrist (hyper-extended, using a rolled up towel if necessary)  Palpate the arterial pulse and visualize the course of the artery.

18. Performing the Procedure:  If you are going to use local anesthetic, infiltrate the skin with 2% xylocaine.  Open the ABG kit  Line the needle up with the artery, bevel side up.  Enter the artery and allow the syringe to fill spontaneously.

19. Performing the Procedure:  Withdraw the needle and hold pressure on the site.  Protect needle  Remove any air bubbles  Gently mix the specimen by rolling it between your palms  Place the specimen on ice and transport to lab immediately.

20. غلظت يون H+ در ECF حاصل تعادل بين فشار نسبي HCO3,PCO2 است [H+]=24 × PCO2/HCO3=24 × 40/24=40 neq/lit چون غلظت برحسب neq/lit بوده و بقيه يون ها بر اساس meq/lit ‌ است برا ي جلوگيري از سردرگمي از لگاريتم منفي كه همان PH باشد استفاده مي شود معادله هندرسون هاسلباخ : PH=PK+ log HCO3/0.03 × PCO2 = 7.4 ضريب تجزيه PK=6/1

21. pH  Normal pH is 7.35-7.45  Value <7.35 is acidotic  Value >7.45 is alkalotic  Acidosis & Alkalosis can be caused by a problem with the respiratory system or a metabolic cause  Can also have combined respiratory/metabolic states

22. Is it Respiratory or Metabolic? 1. Respiratory Acidosis 2. Respiratory Alkalosis 3. Metabolic Acidosis 4. Metabolic Alkalosis  Increased pCO2 >50  Decreased pCO2<30  Decreased HCO3 <18  Increased HCO3 >30

23. Compensated or Uncompensated—what does this mean? 1. Evaluate pH—is it normal? Yes 2. Next evaluate pCO2 & HCO3 pH normal + increased pCO2 + increased HCO3 = compensated respiratory acidosis pH normal + decreased HCO3 + decreased pCO2 = compensated metabolic acidosis

24. Compensated vs. Uncompensated 1. Is pH normal? No 2. Acidotic vs. Alkalotic 3. Respiratory vs. Metabolic pH 50 + normal HCO3 = uncompensated respiratory acidosis pH<7.30 + HCO3<18 + normal pCO2 = uncompensated metabolic acidosis pH>7.50 + pCO2<30 + normal HCO3 = uncompensated respiratory alkalosis pH>7.50 + HCO3>30 + normal pCO2 = uncompensated metabolic alkalosis

25. Causes of Acidosis  Respiratory Hypoventilation Impaired gas exchange  Metabolic Ketoacidosis  Diabetes Renal Tubular Acidosis  Renal Failure Lactic Acidosis  Decreased perfusion  Severe hypoxemia

26. Causes of Alkalosis  Respiratory Hyperventilation due to:  Hypoxemia  Metabolic acidosis  Neurologic Lesions Trauma Infection  Metabolic Hypokalemia Gastric suction or vomiting Hypochloremia

27. Assessing Oxygenation  Normal value for arterial blood gas 80-100mmHg  Normal value for venous blood gas 40mmHg  Normal SaO2 Arterial: 97% Venous: 75%

28. Important points for assessing tissue oxygenation  This is the O2 that’s really available at the tissue level.  Is the THb normal? Low THb means the ability of the blood to carry the O2 to the tissues is decreased  Is perfusion normal? Low perfusion means the blood isn’t even getting to the tissues

29. Let’s Practice

30. 12 year old diabetic presents with Kussmaul breathing  pH: 7.05  pCO2: 12 mmHg  pO2: 108 mmHg  HCO3: 5 mEq/L  BE: -30 mEq/L Severe partly compensated metabolic acidosis without hypoxemia due to ketoacidosis

31. 17 year old w/severe kyphoscoliosis, admitted for pneumonia  pH: 7.37  pCO2: 25 mmHg  pO2: 60 mmHg  HCO3: 14 mEq/L  BE: -7 mEq/L Compensated respiratory alkalosis due to chronic hyperventilation secondary to hypoxia

32. 9 year old w/hx of asthma, audibly wheezing x 1 week, has not slept in 2 nights; presents sitting up and using accessory muscles to breath w/audible wheezes  pH: 7.51  pCO2: 25 mmHg  pO2 35 mmHg  HCO3: 22 mEq/L  BE: -2 mEq/L Uncompensated respiratory alkalosis with severe hypoxia due to asthma exacerbation

33. 7 year old post op presenting with chills, fever and hypotension  pH: 7.25  pCO2: 32 mmHg  pO2: 55 mmHg  HCO3: 10 mEq/L  BE: -15 mEq/L Uncompensated metabolic acidosis due to low perfusion state and hypoxia causing increased lactic acid