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INTERN BASICS Acid-Base August 9, 2005 Jeremy Marcus MD.

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1 INTERN BASICS Acid-Base August 9, 2005 Jeremy Marcus MD

2 ACID BASE – Why it’s “hard” -It’s Math -Everyone does it differently -Everyone thinks they know the “best” way to do it and/or teach it -Emphasis on numbers instead of clinical correlation

3 A word about “internal consistency” pH = log ([HCO3]/0.03 x pCO2) [H+] = 24 x pCO2/[HCO3] What is the equation getting at? e.g. Pt with COPD, acute-on-chronic tachypnea & dyspnea; team got ABG: 7.48/87/56, arterial HCO3 = 63

4 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders

5 Acidosis or alkalosis pH < 7.38 Acidosis pH > 7.42Alkalosis

6 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders

7 Primary disorder respiratory or metabolic? In respiratory acidosis, pCO2 and HCO3 both go up In metabolic acidosis, pCO2 and HCO3 both go down In respiratory alkalosis, pCO2 and HCO3 both go down In metabolic alkalosis, pCO2 and HCO3 both go up Examples: 7.32/28/83 HCO /70/75HCO /50/55 HCO3 15

8 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders

9 If respiratory, is it acute or chronic? Two numbers to remember: 0.08 and 0.03 (works for respiratory acidosis or alkalosis) Acute resp acidosis: for every 10 incr pCO2, pH decr 0.08 Chronic resp acidosis: for every 10 incr pCO2, pH decr 0.03 Acute resp alkalosis: for every 10 decr pCO2, pH incr 0.08 Chronic resp alkalosis: for every 10 decr pCO2, pH incr 0.03 Example: 7.31/70/78HCO3 34

10 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders

11 Appropriate compensation Metabolic acidosis: Winter’s formula pCO2 = 1.5 [HCO3] + 8  2 Alternative: For every HCO3 decr 1, pCO2 decr 1 Respiratory acidosis/alkalosis: Acute resp acidosis: for every 10 incr pCO2, HCO3 incr 1 Chronic resp acidosis: for every 10 decr pCO2, HCO3 incr 4 Acute resp alkalosis: for every 10 incr pCO2, HCO3 decr 2 Chronic resp alkalosis: for every 10 decr pCO2, HCO3 incr 5 Metabolic alkalosis: pCO2 should never be >55 If pCO2 is elevated, pH should be alkalemic For every HCO3 incr 10, pCO2 should incr 7

12 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders

13 Calculate the anion gap … every time! Unmeasured Anions Unmeasured Cations Proteins (albumin) 15 mEq/L Calcium 5 mEq/L Organic acids 5 mEq/L Potassium 4.5 mEq/L Phosphates 2 mEq/L Magnesium 1.5 mEq/L Sulfates 1 mEq/L Totals: 23 mEq/L 11 mEq/L Difference = 12 mEq/L = normal anion gap -Correct for albumin (2.5 for every drop of 1 below 3.0) -If anion gap > 20 with metabolic alkalosis, there’s an additional acidosis

14 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders

15 Assess for complex (triple) disorders Corrected bicarbonate = (AG-12) + HCO3 If < 24, suggests concurrent acidosis If > 24, suggests concurrent alkalosis (Yes, this is the same as “delta delta.”)

16 Stepwise approach 1.Acidosis or alkalosis 2.Primary disorder respiratory or metabolic 3.If respiratory, is it acute or chronic? 4.Appropriate compensation? 5.Calculate anion gap 6.Assess for complex (triple) disorders 7.CORRELATE CLINICALLY!

17 Differential diagnosis: metabolic acidosis Anion gap acidosisNon-anion gap acidosis K etoacidosisU reterosignoidostomy U remiaS aline S alicylatesE arly renal failure M ethanolD iarrhea E thanol, ethylene glycolC arbonic anhydrase inhibitors L actateA mino acids R enal tubular acidosis S upplements (TPN) P ancreatic fistula USUALLY saline or diarrhea

18 Differential diagnosis: metabolic alkalosis Volume contraction (vomiting, overdiuresis, ascites) Hypokalemia Alkali ingestion (bicarbonate) Excess gluco- or mineralocorticoids Bartter's syndrome USUALLY vomiting or overdiuresis

19 Differential diagnosis: respiratory acidosis Central Nervous System Depression (Sedatives, CNS disease, Obesity Hypoventilation syndrome) Pleural Disease (Pneumothorax) Lung Disease (COPD, pneumonia) Musculoskelatal disorders (Kyphoscoliosis, Guillain-Barre, Myasthenia Gravis, Polio) Practically, think about “tiring” (even a little respiratory acidosis in asthma is often a harbinger of badness)

20 Differential diagnosis: respiratory alkalosis Catastrophic CNS event (CNS hemorrhage) Drugs (salicylates, progesterone) Pregnancy (especially the 3 rd trimester) Decreased lung compliance (interstitial lung disease) Liver cirrhosis Anxiety/Pain

21 Examples 21 yo woman presents with confusion, fever, flank pain, “breathing heavy” 7.32/

22 Examples 21 yo woman presents with confusion, fever, flank pain, “breathing heavy” 7.32/ Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 29 (ok) AG = 22; expected HCO3 = = 24 (ok)

23 Examples 21 yo woman presents with confusion, fever, flank pain, “breathing heavy” 7.32/ Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 29 (ok) AG = 22; expected HCO3 = = 24 (ok) Anion gap metabolic acidosis DKA with pyelonephritis

24 Examples 58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol. 7.31/

25 Examples 58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol. 7.31/ Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 15, so concurrent respiratory alkalosis AG = 19; expected HCO3 = = 12, so non-anion gap metabolic acidosis

26 Examples 58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol. 7.31/ Primary disorder = metabolic acidosis Winter’s formula: expected pCO2 = 15, so concurrent respiratory alkalosis AG = 19; expected HCO3 = = 10, so non-anion gap metabolic acidosis Anion gap metabolic acidosis, non-anion gap metabolic acidosis, respiratory alkalosis Alcoholic ketoacidosis, diarrhea, pneumonia

27 Examples 56 yo man found vomiting on the street 7.40/

28 Examples 56 yo man found vomiting on the street 7.40/ Can’t tell primary disorder by pH… but AG = 21 Expected HCO3 = = 33 so concurrent metabolic alkalosis

29 Examples 56 yo man found vomiting on the street 7.40/ Can’t tell primary disorder by pH… but AG = 21 Expected HCO3 = = 33 so concurrent metabolic alkalosis Metabolic alkalosis and metabolic acidosis Vomiting in the setting of worsening uremia due to CKD

30 Examples 58 yo man with 4d cough, vomiting, altered mental status 7.50/

31 Examples 58 yo man with 4d cough, vomiting, altered mental status 7.50/ Respiratory alkalosis AG = 30, so concurrent anion gap metabolic acidosis Expected HCO3 = = 33 so concurrent metabolic alkalosis

32 Examples 58 yo man with 4d cough, vomiting, altered mental status 7.50/ Respiratory alkalosis AG = 30, so concurrent anion gap metabolic acidosis Expected HCO3 = = 33 so concurrent metabolic alkalosis Respiratory alkalosis, anion gap metabolic acidosis, metabolic alkalosis Pneumonia, alcoholic ketoacidosis, vomiting

33 Examples 35 yo woman presents obtunded 7.10/

34 Examples 35 yo woman presents obtunded 7.10/ Primary respiratory acidosis AG = 30 so concurrent primary metabolic acidosis Expected HCO3 = = 33 so concurrent metabolic alkalosis

35 Examples 35 yo woman presents obtunded 7.10/ Primary respiratory acidosis AG = 30 so concurrent primary metabolic acidosis Expected HCO3 = = 33 so concurrent metabolic alkalosis Same as last patient but obtunded so hypoventilating! Hypoventilation due to altered mental status, DKA, vomiting

36 Landing safely Do the exercise on every ABG for practice

37 Landing safely Do the exercise on every ABG for practice

38 Landing safely Do the exercise on every ABG for practice Ask questions


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