Interpretation of Arterial Blood Gases (ABGs)

Acid-Base Disorder analysis and interpretation through ABGs.
Step wise approach to analysis

1. ABG analysis & Acid-Base Disorders Dr. Haseeb Ahmed 2008-98 ZMU

2. Steps for ABG analysis What is the pH? Acidemic or Alkalemic ? What is the primary disorder present? Is there appropriate compensation? Is the compensation acute or chronic? Is there an anion gap? If there is a AG, what is the delta gap? What is the differential for the clinical processes?

3. Step 1: Look at the pH: is the blood acidemic or alkalemic ? Variable Normal Range pH - 7.37 - 7.43 pCO2 - 35-45 - Bicarbonate - 22-26 Normal Values

4. Step 2: What is the primary disorder? What disorder is present? pH pCO2 or HCO3 Respiratory Acidosis pH low pCO2 high Metabolic Acidosis pH low HCO3 low Respiratory Alkalosis pH high pCO2 low Metabolic Alkalosis pH high HCO3 high

5. Step 3: Is there appropriate compensation? Respiratory Acidosis Acute : for every 10 increase in pCO2 -> HCO3 increases by 1 Also know for every acute increase of 10 in pCO2 there is a decrease of 0.08 in pH. Chronic : for every 10 increase in pCO2 -> HCO3 increases by 4 Also know for every chronic increase of 10 in pCO2 there is a decrease of 0.03 in pH Respiratory Alkalosis Acute: for every 10 decrease in pCO2 -> HCO3 decreases by 2 Chronic: for every 10 decrease in pCO2 -> HCO3 decreases by 5

6. Step 3: Is there appropriate compensation? Metabolic Acidosis Winter’s formula : pCO2 = 1.5[HCO3] + 8 ± 2 MEMORIZE Winter’s formula calculates the expected pCO2 in the setting of metabolic acidosis. If the serum pCO2 > expected pCO2 then there is additional respiratory acidosis in which the etiology needs to also be determined. Metabolic Alkalosis For every 10 increase in HCO3 -> pCO2 increases by 6

7. Step 4: Calculate the anion gap AG = Na – Cl – HCO3 (normal 12 ± 2) AG corrected = AG + 2.5[4 – albumin] If anion gap is greater than 20, a metabolic acidosis is always present regardless of the pH or serum bicarbonate concentration because the body is not able to physically generate such a large anion gap via purely compensatory mechanisms (i.e. even in the setting of chronic respiratory alkalosis). Therefore, there must be a primary metabolic disorder present.

8. Differential for Anion Gap Metabolic Acidosis - MUDPILERS Methanol Uremia Diabetic ketoacidosis , starvation ketoacidosis , EtOH ketoacidosis Paraldehyde INH, iron toxicity Lactic acidosis Ethylene glycol Rhabdomyolysis Salicylates

9. Step 5: Calculate the delta gap Only need to calculate delta gap (excess anion gap) when there is an anion gap present to determine additional hidden metabolic disorders ( nongap metabolic acidosis or metabolic alkalosis) Delta gap = AG – 12 + HCO3 (normal 22-26) If delta gap > 26 -> additional metabolic alkalosis If delta gap < 22 -> additional nongap metabolic acidosis If delta gap 22 – 26 -> no additional metabolic disorders

10. The Delta Gap Delta gap is equivalent to excess anion gap. The principle behind this formulation is that for each mMol of acid titrated by the carbonic acid buffer system, 1 mMol of bicarbonate is consumed as water and carbon dioxide and 1 mMol of sodium salt of acid is formed. Therefore, each mMol decrease in bicarbonate is accompanied by a mMol increase in the anion gap. Delta bicarb = Delta anion gap. The sum of the new (excess) anion gap and the remaining (measured) bicarbonate should be equal to a normal bicarbonate concentration. If the sum of the excess anion gap and the measured bicarbonate value exceeds the normal bicarbonate concentration, then an additional metabolic alkalosis must be present. If the sum is less than normal, there must be an additional nongap metabolic acidosis. If the delta gap is equal to expected, there is no additional metabolic disorders.

11. Nongap metabolic acidosis Causes of nongap metabolic acidosis - DURHAM Diarrhea, ileostomy , colostomy, enteric fistulas Ureteral diversions or pancreatic fistulas RTA type I or IV, early renal failure Hyperailmentation , hydrochloric acid administration Acetazolamide , Addison’s Miscellaneous – post - hypocapnia , toulene , sevelamer , cholestyramine ingestion For nongap metabolic acidosis, calculate the urine anion gap UAG = UNA + UK – UCL If UAG>0: renal problem If UAG<0: nonrenal problem (most commonly GI) In working kidneys: HCl + NH3 ↔ NH4CL, urine chloride increases, UAG <0.

12. Metabolic alkalosis Calculate the urinary chloride to differentiate saline responsive vs saline resistant Must be off diuretics in order to interpret urine chloride Saline responsive UCL <10 Saline-resistant UCL >10 Vomiting If hypertensive: Cushings , Conn’s, RAS, renal failure with alkali administartion NG suction If not hypertensive: severe hypokalemia , hypomagnesemia , Bartter’s , Gittelman’s Syndrome Over- diuresis Exogenous corticosteroid administration Post- hypercapnia

13. Causes of Respiratory Alkalosis Anxiety, pain, fever Hypoxia, CHF Lung disease with or without hypoxia – pulmonary embolus, reactive airway, pneumonia CNS diseases Drug use – salicylates , catecholamines , progesterone Pregnancy Sepsis, hypotension Hepatic encephalopathy, liver failure Mechanical ventilation Hypothyroidism High altitude

14. Causes of respiratory acidosis CNS depression – sedatives, narcotics, CVA Neuromuscular disorders – acute or chronic Acute airway obstruction – foreign body, tumor, reactive airway Severe pneumonia, pulmonary edema, pleural effusion Chest cavity problems – hemothorax , pneumothorax , flail chest Chronic lung disease – obstructive or restrictive Central hypoventilation, OSA

15. Case 1 65yo M with CKD presenting with nausea, diarrhea and acute respiratory distress. ABG 7.23/17/235 on 50% VM Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1

16. Case 1 answer Primary metabolic acidosis – gap of 19 (uremia/renal failure causing gap met acidosis), delta gap 14 -> additional non gap metabolic acidosis (diarrhea). Winter’s formula 18 -> no additional respiratory acidosis.

17. Case 2 60yo M with COPD on steroids presenting with worsening SOB, hypoxia, and hypotension ABG 7.38/54/ Na 134/ Cl 77/ HCO3= 33

18. Case 2 answer Primary respiratory acidosis with chronic metabolic compensation (COPD). Gap = 24. Gap metabolic acidosis (sepsis). Delta gap = 35 -> additional metabolic alkalosis from exogenous steroids. Triple disorder

19. Case 3 28yo F who is 28 weeks pregnant, diabetic, previous alcoholic who recently stopped insulin and started binge drinking ABG 7.60/21/ Na 136/ Cl 80/ HCO3 19

20. Case 3 answer Primary respiratory alkalosis with acute metabolic compensation (pregnancy, anxiety). Gap = 37 Gap metabolic acidosis (DKA, alcoholic ketoacidosis ). Delta gap 34 -> additional metabolic alkalosis (vomiting). Another triple ripple

21. Case 4 17yo F with a history of depression is brought in altered to the ED by her mother, who reports finding multiple empty medication bottles around her. ABG 7.50/20/ Na 140/ Cl 103/ HCO3 15

22. Case 4 answer Primary respiratory alkalosis with chronic metabolic compensation (hyperventilation). Gap = 22. Gap metabolic acidosis ( salicylates ). Delta gap 25 -> no additional metabolic disorders. ASA overdose

23. Case 5 A 45yo F with Type 1 Diabetes is admitted with a gastroenteritis, hyperglycemia and confusion. ABG 7.10/50/102 BMP Na 145/ Cl 100 / HCO3 15

24. Case 5 answer Primary respiratory acidosis (obtunded) with gap metabolic acidosis , gap = 30 (DKA). Delta gap 33 -> additional nongap metabolic alkalosis (vomiting).