Acid-Base Disorders

Question 1

Major types of acid-base disorders

Answer 1

1. Respiratory alkalosis (decrease in CO₂ resulting in a increase in pH)
2. Respiratory acidosis (increase in CO₂ resulting in a decrease in pH)
3. Metabolic alkalosis (increase in HCO₃ resulting in an increase in pH)
4. Metabolic acidosis (decrease in HCO₃ resulting in decrease in pH)

Question 2

Respiratory alkalosis: Definition/differenital dx

Answer 2

• respiratory process caues primary decrease in PCO₂
• DDx:
  
  • always due to hyperventilation
  • pulmonary disease
  • hypoxemia
  • mechanical ventilation

Question 3

Respiratory alkalosis: compensation

Answer 3

• decreased HCO₃ due to:
  
  • H⁺ release from cells (acute)
  • renal H⁺ retention (chronic)
    
    • takes 3-5 days to complete
• compensation rules:
• ##### Acute: ΔHCO₃^- = ↓ 2 meq/L for every 10 mmHg ↓ in PCO₂ [↓(2:10)]
• ##### Chronic (3 to 5 days): ΔHCO₃^- = ↓4 meqL for every 10 mmHg ↓ in PCO₂ [↓(4:10)]

Question 4

Respiratory alkalosis: Signs, Sx, Tx

Answer 4

• Lab abnormalities: decreased potassium (small); decreased phosphorus (may be large).
• Symptoms: neurologic (paresthesias, carpopedal spasms).
• Consequences: decreased intracranial pressure, cardiac arrhythmias.
• Tx: treat underlying cause; can depress ventilation w/sedative

Question 5

Respiratory acidosis: definition, DDx

Answer 5

• respiratory process causes a primary increase in the PCO₂
• DDx:
  
  • always due to inadequate respiration @ 4 steps of respiration: sensing/signaling, muscles, free flow, gas exchange

Question 6

Respiratory acidosis: compensation

Answer 6

• increased HCO₃ due to:
  
  • cell buffering (acute)
    
    • H+ reabsorbed by intracellular buffers
  • renal H⁺ excretion (chronic)
    
    • 3-5 days
• pH should not fall below 7.20 in appropriately compensated chronic respiratory acidosis
• compensation rules:
• Acute: ΔHCO3- = ↑ 1 meq/L for every 10 mmHg ↑ in PCO2 [↑ (1:10)]
• Chronic (3 to 5 days): ΔHCO3- = ↑4 meqL for every 10 mmHg ↑ in PCO2 [↑ (4:10)]

Question 7

Respiratory acidosis: Signs, Sx, Tx

Answer 7

• Symptoms: Neurologic: headache, decreased arousal/sleepiness (aka CO₂ narcosis)
• Consequences: Increased intracranial pressure, cardiac arrhythmias, hypotension from peripheral vasodilatation
• **Treatment: **
  
  • ​Treat underlying cause
  • Pay attention to PO₂

Question 8

Metabolic alkalosis: definition, DDx

Answer 8

• metabolic process causes primary increase in HCO₃
• DDx:
  
  • generation of bicarb increase
    
    • addition of HCO₃
    • loss of H⁺
    • loss of fluid w/chloride
    • post-hypercapneia
    • hypokalemia
  • maintenance of met alk is always due to renal inability to excrete excess HCO₃

Question 9

Main mechanisms that generate metabolic alkalosis

Answer 9

1. Added HCO₃ via:
   
   1. direct admin of bicarb
   2. direct admin of substrate metabolized to bicarb
2. Loss of H⁺ via:
   
   1. GI loss: vomiting
   2. Renal loss: loop, thiazide diuretics, mineralcorticoid excess
3. Loss of chloride rich fluid
   
   1. loop diuretics
   2. cystic fibrosis
4. post-hypercapnia
   
   1. metabolic alkalosis in patient w/chronic respiratory acidosis + mechanical ventilation
5. hypokalemia

Question 10

Mechanisms of maintenance of metabolic alkalosis

Answer 10

• chloride depletion ==> increased resorption of bicarb (chloride vs. non-chloride responsive met alk)
• potassium depletion
• increased mineralocorticoid activity ==> stimulate H⁺ ATPase to secrete H⁺ into tubule ==> bicarb resorption
• hypovolemia ==> release of aldosterone ==> increased Na resorption + bicarb (to maintain electroneurtrality)

Question 11

Definition/causes of chloride responsive metabolic alkalosis

Answer 11

• [U_Cl] < 20 mEq/L => chloride depletion is major maintence factor
• major causes:
  
  • diuretics
  • vomiting, gastric drainage
  • villous adenomas
  • congenital chloride-losing diarrhea
  • cystic fibrosis
  • post-hypercapnia

Question 12

Definition/causes of chloride resistant metabolic alkalosis

Answer 12

• [UCl] > 20 mEq/L
• causes:
  
  • excess mineralocorticoids
    
    • hyperaldosteronism
    • cushing’s syndrome
  • licorice ingestion

Question 13

Metabolic alkalosis: compensation

Answer 13

• rise in pH ==> decrease in ventilation ==> rise in CO₂
• compensation rules:
• ΔCO₂ (in mmHg) = 0.25 – 1.0 XΔHCO₃

Question 14

Metabolic alkalosis: treatment

Answer 14

• possible severe consequences:
  
  • cardiac arrhythmias
  • hypocalcemia => neuromuscular irrability => tetany
• hypoventilation via mechanical vent
• Chloride responsive = Infusions of NaCl or KCl
• Chloride resistant = block mineralocorticoid effect w/sprionolactone

Question 15

Metabolic acidosis: definition, DDx

Answer 15

• metabolic process causes primary decrease in HCO₃
• DDx/major mechanisms:
  
  • loss of bicarbonate = normal anion gap
  • addition of acid = increased anion gap
  • anion gap = [Na] - [Cl] - [HCO₃]
    
    • normal = 9 +/- 3

Question 16

Characteristics of non-anion gap metabolic acidosis

Answer 16

• caused by loss of bicarb
• loss of bicarb is replaced by an increase in chloride

Question 17

Characteristics of anion gap metabolic acidosis

Answer 17

• caused by addition of acid ==> decrease in bicarb/loss of anion from plasma
• anions other than chloride increase to maintain electroneutrality
• anion gap is “increased” when > 18

Question 18

Causes of non-anion gap metabolic acidosis

Answer 18

• loss of bicarb
• GI loss: diarrhea
  
  • urine pH < 5.3
  • urine anion gap < 0
• renal loss: renal tubular acidosis = due to defect in renal bicarbonate or hydrogen ion handling
  
  • proximal, distal, hyperkalemic
  • urine pH > 5.3
  • urine anion gap > 0

Question 19

Causes of anion gap metabolic acidosis

Answer 19

• MUD PILES:
• Methanol
• Uremia (renal failure)
• DKA
• propylene Glycol
• INH (isoniazid)
• Lactate
• Ethylene Glycol
• Salicylates

Question 20

Physiologic effects of metabolic acidosis

Answer 20

• myocardial contractility decreases + peripheral resistance falls ==>
• hypotension, pulmonary edema, vfib
• respirations = deep and rapid (Kussmaul’s)
• chronic ==> hypercalcuira + bone disease

Question 21

Metabolic acidosis: compensation

Answer 21

• increased ventilation ==> fall in PCO₂
  
  • = incomplete compensation
• appropriate compensation:
  
  • Δ PaCO₂ (in mmHg) = 1.0 – 1.5 x Δ **[HCO₃^-] (in mEq/L) **
  • quick approximation: PCO₂ = last 2 digits of pH
• both HCO₃ and PCO₂ should be decreased; if this does not occur likely a mixed disorder__​

Question 22

Mixed disorder: primary respiratory alkalosis + metabolic acidosis

Answer 22

• Δ PaCO₂ (in mmHg) > 1.0 – 1.5 x Δ [HCO₃^-] (in mEq/L)

Question 23

Metabolic acidosis: Tx

Answer 23

• correct underlying disorder
• chronic = oral sodium bicarbonate

Question 24

Characteristics of mixed disorders

Answer 24

• coexistence of two+ primary acid-base disorders
• expected compensation will be less than or greater than expected
• PCO₂ and HCO₃ differ from normal in opposite direction