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Approach to Acid Base

Approach to Acid Base. Jeff Kaufhold, MD FACP 2013 Source: The ICU Book Chapter 36-38. Acid Base Disorders. Basic Physiology Approach to A-B disorders Equations Cases Electrolytes. Acid Base Physiology. Balance based on Henderson-Hasselbach equation

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Approach to Acid Base

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  1. Approach to Acid Base Jeff Kaufhold, MD FACP 2013 Source: The ICU Book Chapter 36-38

  2. Acid Base Disorders • Basic Physiology • Approach to A-B disorders • Equations • Cases • Electrolytes

  3. Acid Base Physiology • Balance based on Henderson-Hasselbach equation • Hydrogen Ion measured in NanoEquivalents , i.e. very tightly regulated. • Buffers for acid: • Bicarbonate, Hemoglobin, Albumin, Bone • Acid excretion: Lungs and Kidneys

  4. Acid - Base Physiology • Sources of Acid: • 70 mEq daily as inorganic acids • H2SO4, H3PO4 • Excreted by kidney • 16,000 mMole as CO2 • Excreted by lungs • 32,000 mEq organic acid which gets metabolised

  5. Acid – Base Physiology • Lung blows off acid as CO2 • H2CO3  H2O + CO2 • Kidney • Proximal tubule has lumenal carbonic anhydrase to reclaim bicarb from urine • Distal tubule has basolateral CA to pull H+ out of blood, complexed to NH3 to form NH4Cl (ammonium chloride) • Can measure activity by urinary Anion Gap (Na+K – Cl)

  6. ABG reporting • Normals: • pH 7.36 – 7.44 • pCO2 36 – 44 mmHg • pO2 over 60 • HCO3 (bicarb) 22 – 26 • Reported as pH / pCO2 / pO2 / HCO3 • Example 7.40 / 40 / 80 / 23

  7. Disorder Resp Acidosis Resp Alkalosis Met Acidosis Met Alkalosis End point is a constant ratio of: Primary / Compens Pco2 up HCO3 up Pco2 down, HCO down HCO down, Pco2 down HCO up Pco2 up PCO2 / HCO3 Compensatory Mechanisms

  8. Rules for interpretation • 1. Primary Metabolic disorders: • PCO2 and pH move in same direction • Can be quickly identified if last 2 digits of pH = pCO2 • i.e 7.20 / 20 / 70 • 7.51 / 50 / 120 • If 7.20 / 40 / 70, not simple disorder • Respiratory compensation not adequate

  9. Rules for interpretation • 3. Primary Respiratory disorders: • PCO2 and pH move in opposite direction • Resp acidosis: 7.20 / 58 / 68 • Resp alkalosis: 7.50 / 25/ 80 • 7.25 / 48 / 52 Not a simple disorder • Neither is 7.40 / 25 / 68

  10. Rules for interpretation • Rule 5 Mixed Disorders: • Compensatory responses do not completely correct primary problem • Can have BOTH metabolic acidosis and metabolic alkalosis simultaneously (sepsis plus vomiting is common) • But can only hyperventilate (resp alk) OR hypoventilate (resp acidosis). • Usually due to failure to adequately lower pCO2 (resp fatigue)

  11. Expected ChangesRules of Thumb • Met Acidosis • Pco2 = 1.5 * bicarb + 8 +/- 2 • 7.20 / 20 / 68 / HCO3 = 8 • Met Alkalosis • Pco2 = 0.7 * HCO3 + 21 +/- 2 • 7.50 / 50 / 120 / HCO3 = 40

  12. Expected Changes • Acute Resp Acidosis • Change in pH = 0.008 * change pCO2 • 7.30 / 50 / 68 / HCO3 = 22 • Acute Resp Alkalosis • Change in pH = 0.008 * change pCO2 • 7.50 / 20 / 120 / HCO3 = 20 • Shortcut: D pH = 0.08 for each 10 mmHg change in pCO2

  13. Expected Changes • Chronic Resp Acidosis • Change in pH = 0.003 * change pCO2 • 7.33 / 50 / 68 / HCO3 = 32 • Chronic Resp Alkalosis • Change in pH = 0.0017 * change pCO2 • 7.48 / 25 / 120 / HCO3 = 20

  14. Expected Changes • Note that the chronic respiratory conditions induce chronic changes in bicarbonate handling by the kidney, which takes time.

  15. Common Causes of A-B Disorders • Met Acidosis MUD PILES • Met Alkalosis Vomiting, NG suction, IV infusion of bicarb or Acetate (TPN), Diuretics • Resp Acidosis : respiratory failure, COPD exacerbation, Narcotics • Resp Alkalosis: Hyperventilation/anxiety, Pregnancy, Early sepsis, early stage asthma, chronic alcoholism

  16. Anion Gap • Useful for evaluation of metabolic acidosis. • Calculate AG every time you see electrolytes, esp on exams. • AG = Anions – Cations • AG = Na + K – (CL + HCO3) • AG = 140 + 4 – (102 + 25) • = 8 to 16 • > 18 nephrologist gets excited

  17. Anion Gap • Low AG suggests extra plasma proteins, such as Myeloma • Non Anion Gap acidosis suggests loss of bicarbonate through either: • Diarrhea • Renal Tubular Acidosis (RTA) • Or Infusion of acid via NS, TPN • High AG means accumulation of acid • MUDD PILES or other nemonic

  18. Anion Gap • High AG means accumulation of acid • Organic acid can be metabolised back to bicarbonate • So AG over 16 or so can represent “potential Bicarb” or “delta gap”. • Calculating the potential bicarb is useful for identifying mixed disorders, since both met acidosis and met alkalosis can be present at same time. • Delta Gap = AG – 12 (extra anions) • Potential bicarb = HCO3 + delta gap

  19. Potential Bicarb HCO3 + Delta gap=Potential bicarb

  20. Gapped Metabolic Acidosis • Methanol (wood alcohol) • Uremia/acute renal failure • DKA • D lactate (Metformin/blind loop in bowel) • Paraldehyde • Isopropyl alcohol • Lactic acidosis (hypoxia/hypoperfusion) • Ethanol • Starvation Ketosis

  21. Osmolal Gap • Used to help identify presence of exogenous acids which may be causing gapped metabolic acidosis • Measured Osm – calculated Osm • Osm calc = 2(Na) + Glucose/18 + BUN/30 • OG > 20 suggests Ethanol, Methanol

  22. Clinical Problems in A-B • 1. Calculate AG and potential bicarb • 2. Is pH acidic or alkalotic • 3. Is pCO2 alkalotic or acidotic • < 40 > 40 • 4. Is the bicarb measured by the ABG machine same as that on lytes? • If not the samples are not simultaneous • 5. Apply the rules of thumb • If values are consistent, Simple disorder • If values are not consistent: Mixed.

  23. Case 1 • 38 y.o. male with chronic GN. Admitted with weakness, fatigue. BUN 100 cr 6 • Labs: Na 134, K 5.6, Cl 100, CO2 14. • ABG: 7.26 / 27 / 72 / 14 • What is A-B disturbance? • AG = • Delta gap/Potential bicarb = • Acidosis or alkalosis ? • Direction of CO2 ? • MA rule: pCO2 = 1.5*HCO3 + 8 +/-2

  24. Case 1 • Labs: Na 134, K 5.6, Cl 100, CO2 14. • ABG: 7.26 / 27 / 72 / 14 • What is A-B disturbance? • AG = 20 • Delta gap/Potential bicarb = 8 / 22 • Acidosis or alkalosis ? acidosis • Direction of CO2 ? alkalosis • MA rule: pCO2 = 1.5*HCO3 + 8 +/-2 works • So simple metabolic acidosis with appropriate respiratory compensation

  25. Case 2 • 23 y.o. garage mechanic presents with acute confusion after drinking antifreeze. • Lab: Na 137 K 5.4 Cl 105 HCO 5 • ABG: 6.95 / 15 / 80 / 4 • What is A-B disturbance? • AG = • Delta gap/Potential bicarb = • Acidosis or alkalosis ? • Direction of CO2 ? • MA rule: pCO2 = 1.5*HCO3 + 8 +/-2

  26. Case 2 • Lab: Na 137 K 5.4 Cl 105 HCO 5 • ABG: 6.95 / 15 / 80 / 4 • What is A-B disturbance? • AG = 27 • Delta gap/Potential bicarb = 15 / 20 • Acidosis or alkalosis ? acidosis • Direction of CO2 ? alkalosis • MA rule: pCO2 = 1.5*HCO3 + 8 +/-2 works so simple met acidosis with appropriate resp compensation.

  27. Case 3 • 65 y.o. recently discharged after pneumonia. Presents with C dif Diarrhea. • Lab: Na 132 K 2.4 Cl 105 HCO 15 • ABG: 7.30 / 30 / 80 / 14 • What is A-B disturbance? • AG = • Delta gap/Potential bicarb = • Acidosis or alkalosis ? • Direction of CO2 ? • MA rule: pCO2 = 1.5*HCO3 + 8 +/-2

  28. Case 3 • Lab: Na 132 K 2.4 Cl 105 HCO 15 • ABG: 7.30 / 30 / 80 / 14 • What is A-B disturbance? • AG = 12 • Delta gap/Potential bicarb = 0 / 15 • Acidosis • Direction of CO2 ? alkalotic • MA rule: pCO2 = 1.5*HCO3 + 8 +/-2 • Works so Non Gap Metabolic Acidosis

  29. Case 4 • 40 y.o. with status asthmaticus. • Lab: Na 135 K 3.4 Cl 100 HCO 21 • ABG: 7.50 / 30 / 80 / 20 • What is A-B disturbance? • AG = • Delta gap/Potential bicarb = • Acidosis or alkalosis ? • Direction of CO2 ? • Acute RA rule: DpH = 0.008 * DpCO2

  30. Case 4 • 40 y.o. with status asthmaticus. • Lab: Na 135 K 3.4 Cl 100 HCO 21 • ABG: 7.50 / 30 / 80 / 20 • What is A-B disturbance? • AG = 14 • Delta gap/Potential bicarb = 23 • alkalosis • Direction of CO2 ? alkalosis • Acute RA rule: DpH = 0.008 * DpCO2 • works

  31. Case 5 • 32 y.o. chronic pyelo, CRF, pleural effusion. Admitted for thoracentesis • Lab: Na 130 K 5.0 Cl 94 HCO 15 • ABG: 7.32 / 32 / 55 / 15 • Attempted thoracentesis unsuccessful, became dyspneic and coded as blood drawn: • Na 131/k 7.8/cl 92/CO 9/ 6.9/50/40

  32. Case 5CRF with pneumothorax • Admission AG = 20 • Appropriate resp compensation • Uremic metabolic acidosis • After thoracentesis: • Resp acidosis, plus met acidosis • Met acidosis worse, gap up to 30. • Due to uremia plus lactic acidosis.

  33. Case 6 • 44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion. • Lab: Na 142 K 3.4 Cl 98 HCO 20 • ABG: 7.28 / 41 / 58 / 20 • What is A-B disturbance? • AG = • Delta gap/Potential bicarb = • Acidosis or alkalosis ? • Direction of CO2 ?

  34. Case 6 • 44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion. • Lab: Na 142 K 3.4 Cl 98 HCO 20 • ABG: 7.28 / 41 / 58 / 20 • What is A-B disturbance? • AG = 24 • Delta gap/Potential bicarb = 12 / 32 • Acidosis or alkalosis ? Acidosis • Direction of CO2 ? Not appropriate • What is going on?

  35. Case 6 • 44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion. • Lab: Na 142 K 3.4 Cl 98 HCO 20 • ABG: 7.28 / 41 / 58 / 20 • Pneumonia is interfering with resp compensation. • Also see this with mechanical problems of ventilation such as flail chest, paralyzed hemidiaphragm, abdominal compartment syndrome.

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