Acid Base Disorders: Key Core Concepts. Thomas DuBose M.D., MACP, FASN ASN Board Review Course Online Resource Material 2014

Transcription

1 Acid Base Disorders: Key Core Concepts Thomas DuBose M.D., MACP, FASN ASN Board Review Course Online Resource Material 2014

2 Speaker Disclosure I, Thomas DuBose, M.D., have no financial relationships or affiliations with industry to disclose.

3 Basic Concepts of Acid-Base Balance Henderson-Hasselbach Equation: ph a = log 10 (HCO 3- /P a CO 2 X ) ph maintained between 7.35 & 7.45 and ph i Fine tuned regulation occurs in face of continuous production of acid metabolites and is accomplished by intracellular and extracellular buffers in conjunction with respiratory and renal regulatory mechanisms.

4 Primary Disturbance and Compensatory Responses ph pco 2 HCO - 3 Metabolic acidosis 2 o 1 o Metabolic alkalosis 2 o 1 o Respiratory acidosis 1 o 2 o Respiratory alkalosis 1 o 2 o

5 Compensatory Responses for Simple Acid- Base Disorders* (Winter Equation) *DuBose TD, Acidosis and Alkalosis, in HPIM ED 19, Chapt 66, 2014

6 Derivation of Winter Equation

7 A Simpler Approach to Predict Respiratory Compensation in Metabolic Acidosis and Metabolic Alkalosis In range of serum bicarbonate of meq/l Add 15 to patient s [HCO 3- ] to predict PCO 2 Examples: HCO 3 - Predicted PCO Compare predicted and measured values

8 Types of Acid-Base Disturbances 1. Simple Respiratory acidosis alkalosis Metabolic acidosis alkalosis 2. Mixed

9 Mixed Acid-Base Disturbances Definition Combination of two or more of the 4 simple disturbances Examples Mixed respiratory-metabolic disorders Mixed metabolic disorders

10 Key Board Review Point: Step-wise evaluation of acid-base disorders 1. Always analyze acid-base disturbance with both ABG and Venous Electrolyte Panel (BMP) 2. Verify accuracy Compare calculated [HCO 3- ] on arterial blood gas with measured [HCO 3- ] on electrolyte panel 3. Calculate anion gap; but correct for deviation of P alb from normal (4.0 Gm/dL) 4. Calculate predicted respiratory or metabolic compensation 5. Know Causes of HAG and NAG acidosis 6. Compare Δ HCO 3 - with D AG 7. Compare D Na + with D Cl - 8. Calculate Serum Osmolar Gap when cause of HAG not known or toxic alcohol ingestion suspected

11 The Anion Gap AG = Na + (Cl - + HCO 3 - ) Normal Value 6-12 meq/l Represents unmeasured anions present in serum including anionic proteins, phosphate, sulfate, and organic anions. Major assumption is that ECFV is normal or that Hct and P alb are normal. If not must correct AG for deviation of P alb from normal value of 4.0 Gm/dL.

12 Anatomy of the Anion Gap K 5 Ca 5 Mg 2 meq/l Protein 16 Organic acid 4 PO 4 /SO 4 2 HCO 3 25 These cations = 12 These anions = 22 Anion gap is Unmeasured anions - Unmeasured cations or = 10 Na Cl Anion gap is calculated as Na - (Cl + HCO 3 ) or = 10 Cations Anions

13 Correction of the AG for P Alb For each 1 Gm/dL DECREASE in albumin below 4.0mEq/L the reported AG will be factitiously reduced by 2.5mEq/L For each 1 Gm/dL INCREASE in albumin above 4.0mEq/L the reporteded AG will be factitiously increased by 2.5mEq/L Example: if P Alb is 2 and AG is 15, to correct AG add 2.5 X 2 = 5 to 15 Corrected AG = 20 meq/l

14 Illustration of Method Na 140, K 4.9, Cl 106, HCO 3 14, BUN 23, Cr 1.1, Alb 4.0 ph 7.39, PaCO 2 24, PaO 2 90, HCO 3 13 Apply the step-wise approach to answer next question:

15 Question: What is the precise acidbase diagnosis in the previous example? 1. Metabolic acidosis with overcompensation 2. High anion gap metabolic acidosis 3. Mixed metabolic acidosis + respiratory alkalosis 4. Mixed metabolic acidosis + respiratory acidosis

16 Stepwise solution for diagnosis Measured and calculated HCO 3 similar AG = 20, defines presence of a high anion gap acidosis What is predicted respiratory compensation in this case?: Winter equation: 1.5 X = Therefore, correct answer? (next slide)

17 Correct Answer, # 3 1. Metabolic acidosis with overcompensation 2. High anion gap metabolic acidosis 3. Mixed metabolic acidosis respiratory alkalosis 4. Mixed metabolic acidosis respiratory acidosis

18 Example of stepwise approach using D values (step 6) A patient is admitted with a history of vomiting for 2 days and orthostatic hypotension. PMH of CKD secondary to DM, baseline Cr 3. Laboratory: Na 140, K 3.7, Cl 95, HCO 3-25, BUN 80, Cr 7.9, glucose 130, ph 7.40, P a CO 2 39, PO 2 92, Albumin 4.0 Consider the AG, and the DHCO 3 vs. DAG

19 Using D values AG = 20, DAG = 10 DHCO 3 = 0 Therefore, DAG > DHCO 3, defines a mixed high anion gap metabolic acidosis plus metabolic alkalosis

20 Mixed Metabolic Acidosis- Alkalosis Electrolyte Values (meq/l) Serum Electrolytes Normal High Gap MA MA and MA (Vomiting) Sodium Chloride Bicarbonate Anion gap DAG D C

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22 Step 5: Causes of High Anion Gap Ketoacidosis Acidosis Diabetic ketoacidosis Alcoholic ketoacidosis Starvation ketoacidosis Lactic Acidosis L-Lactic acidosis Type A Type B D-Lactic acidosis

23 Causes of High Anion Gap Acidosis - continued Renal Failure Acute and Chronic Toxins Ethylene glycol High Osmolar Gap Methyl alcohol High Osmolar Gap Propylene glycol High Osmolar Gap Salicylates high salicylate level Pyroglutamic acid or 5- oxoprolene (glutathione depletion)

24 Gold Mark: Modern Mnemonic for Anion Gap Acidosis GOLD MARK Glycols (ethylene and propylene) Oxoproline L-lactate D-lactate Methanol Aspirin Renal failure Ketoacidosis Mehta, AN, Emmett,JB and Emmett, M. The Lancet, 2008; 372: 892

25 Useful Ancillary Tests in the Diagnosis of High AG Metabolic Acidosis Serum and urine ketones Serum creatinine Serum L-lactate (consider D-lactate) Serum osmolality to calculate osmolar gap Serum toxic alcohols Pyroglutamic acid (5-oxoproline) Urine microscopy for crystals

26 Osmolar Gap in Diagnosis of Toxin- Induced Anion Gap Acidosis Compare measured and calculated osmolality Gap osm = P osm det - P osm cal P osm cal = 2Na + + BUN/2.8 + Glu/18 Key Point: Gap osm > 10 mosm/kg. in setting of possible toxin ingestion suggests methyl alcohol, ethylene glycol, or propylene glycol intoxication

27 UNDERSTANDING NON-GAP METABOLIC ACIDOSIS Overview of Renal Pathophysiology Distinguishing Renal from Non-Renal Forms Role of the Kidney in the Defense Against Metabolic Acidosis

28 Role of the Kidney in Regulation of Acid-Base Balance: 2 Components Reclamation of Filtered Bicarbonate Proximal Tubule Distal Nephron Regeneration of ECF [HCO 3- ] consumed by net acid production Ammonia production and excretion increases with dietary acid load

29 Definition of Non-Gap Acidosis Low Bicarbonate, low ph Normal Anion Gap (~8-10 meq/l) Note Albumin correct to 4 Gm/dL Compensatory decrease in PCO 2 Predicting Respiratory Compensation: Winter Equation: PCO 2 = 1.5 (HCO 3- ) + 8 ± 2 Add 15 to Patient s [HCO 3- ] Example:

30 Clinical Examples: NAG Acidosis Electrolyte Values ( meq/l) NAG - MA Serum Electrolytes Normal NAG - MA + High AG - MA S o dium Chlo r ide Bicarbonate Ani o n gap AG C

31 Causes of Non-Gap Acidoses (step 5) Diarrhea or other GI losses of alkali (e.g., tube drainage) Ureteral diversion (e.g., ileal loop, ureterosigmoidostomy) Posttreatment of ketoacidosis (dilutional) Progressive chronic kidney disease Toluene ingestion (excretion of hippurate) Drugs Carbonic anhydrase inhibitors: acetazolamide, topiramate, sulfamyalon Amphotericin B CaCl 2, MgSO 4, Cholestyramine Acid loads (NH 4 Cl, acidic amino acids -TPN, sulfur) For Hyperkalemia: amiloride, triamterenene, spironolactone, TMP Post - hypocapnic state RTA s proximal, classical distal, mixed, type 4

32 Mnemonic for Non-Gap Acidosis HAARDUPS Hyperalimentation Acetazolamide or any CA Inhibitor Amphotericin B RTA Diarrhea Ureterosigmoidostomy Post hypocapneic state, pancreatic fistula Sulfamyalon

33 Types of Renal Acidoses Hypokalemic Forms Proximal RTA (Type 2) Classical Distal RTA (Type 1) Hyperkalemic Forms Aldosterone Deficiency or Resistance (Type 4) Non-mineralocorticoid Voltage Defect Normokalemic RTA of CKD 2-4 Uremic Acidosis

34 Distinction between Non-Renal and Renal Origin of NAG Acidosis; Use of the Urine Anion Gap: Pathophysiological Response of Kidney to Acidosis

35 Renal Origin vs. Non Renal Origin Estimate Urine Ammonium (spot urine lytes) Urine Anion Gap is Surrogate for U Am UAG = [U Na + U K ] -U Cl Interpretation: Negative Value: Non-Renal Origin (Ammonium Adequate) Positive Value: Renal Origin (Ammonium Low)

36 Clinical Recognition of Renal Response To Non-Gap Metabolic Acidosis Non-Renal Increase in NH 4 + Excretion (kidney response appropriate) Negative Urine Anion Gap Acid urine ph (<5.5) - exceptions Renal Inability to increase NH 4 + Excretion (inappropriate kidney response) Positive Urine Anion Gap Urine ph typically > 5.5 but more variable in Type 4

37 Tests useful in the Differential Diagnosis of NAG Metabolic Acidosis Serum potassium Serum creatinine Urine electrolytes TTKG or FE K FE HCO3- Urine osmolality and urine osmolar gap Urine ph

38 Mixed High Anion Gap and Normal Anion Gap Metabolic Acidosis in a Patient with Severe Diarrhea Electrolyte Values (meq/l) NAG-MA Serum Electrolytes Normal NAG-MA + High AG-MA Sodium Chloride Bicarbonate Anion gap DAG D C

39 Adverse Consequences Severe Acidemia Cardiovascular Impaired contractility, vasodilatation, venoconstriction, decreased C.O., sensitization to arrhythmias, decreased responsiveness to pressors. Respiratory Hyperventilation, respiratory muscle fatigue, dyspnea Metabolic Insulin resistance, inhibition of anaerobic glycolysis, protein degradation, decreased ATP synthesis, hyperkalemia.

40 Complications of Bicarbonate Therapy Overshoot alkalosis: Exogenously administered NaHCO 3 must be added to endogenously produced by metabolism of ketones, lactate, etc. Increase in lactate generation Volume expansion with ARF or ESRD Increased CO 2 production Hypocalcemia Cardiac depression

41 When to Give NaHCO 3 In ESRD, CKD stage 2-4 keep HCO 3 >22 to avoid osteopenia, hypercalciuria, natriuresis, sarcopenia, and to help slow progression of CKD. In DKA, almost never; extreme acidosis? (ph<6.9), never for children? Treat the underlying cause with regular insulin + i.v. fluid replacement In L-lactic acid acidosis, ph < 7 Treat the underlying cause Give no more NaHCO 3 than needed to increase ph to 7.1 Consequence of NaHCO 3 in lactic acidosis: increase in lactate production

42 Summary: Solving Acid-base Problems If metabolic acidosis: For Non-gap Acidoses Distinguish renal from non-renal forms Calculate urine anion gap and/or urine osmolar gap and note urine ph For High AG Acidoses Know causes of high anion gap metabolic acidosis If toxin suspected: calculate Osmolar Gap If hypokalemia: Calculate TTKG or FE K + to determine if K loss of renal origin If metabolic alkalosis: Look at urine [Cl - ] and separate into two categories: Cl responsive and Cl unresponsive