ICL 2.6: Approach to Acid Base

Question 1

what is metabolic vs respiratory acidosis?

Answer 1

metabolic = presence of process which leads to gain of H+ or loss of HCO3-.

respiratory = presence of process which leads to retention of CO2

Question 2

what is metabolic vs respiratory alkalosis?

Answer 2

metabolic = presence of process which leads to loss of H+ or addition of HCO3-

respiratory = presence of process which leads to excessive elimination of CO2

Question 3

how does the body compensate for acid-base disturbances?

Answer 3

metabolic acidosis or alkalosis result in immediate respiratory compensations by alterations in ventilation

however, respiratory acidosis or alkalosis result in renal compensation which takes 2 to 5 days

Question 4

how much does PCO2 compensate during metabolic acidosis vs. alkalosis?

Answer 4

in metabolic acidosis, for each HCO3 decrease, pCO2 will decrease 1.-1.5

in metabolic alkalosis, for each HCO3 increase, PCO2 will increase.25-1

Question 5

how much does HCO3 compensate during respiratory acidosis vs. alkalosis?

Answer 5

in acute respiratory acidosis, HCO3 will increase 1 for each 10 pCO2 increase

in chronic respiratory acidosis, HCO3 will increase 4 for each 10 pCO2 increase

in acute rep alkalosis, HCO3 will decrease 1-3 for each 10 pCO2 decrease

in chronic respiratory alkalosis, HCO3 will decrease 2-5 for each 10 pCO2 decrease

Question 6

how does the kidney handle acid?

Answer 6

1. HCO3- reabsorption coupled to Na reabsorption
2. NH4+ synthesis sensitive to pCO2.
3. aldosterone influence: H+ secretion and NH4+ trapping in the kidney to get rid of acid

Question 7

how much acid does your body make in a day?

Answer 7

the Western diet obligates approximately 1 mmol/Kg acid excretion per day

most of this acid occurs from the metabolism of sulfur containing amino acids to SO42-

Question 8

what is the major component of the normal anion gap?

Answer 8

albumin

Question 9

what is normal anion gap acidosis?

Answer 9

there is a failure to excrete H+ without retention of an organic anion; as a result Cl- compensates and there’s increased Cl-

or HCO3- is lost

Question 10

what is increased anion gap acidosis?

Answer 10

addition of organic acid leads to generation of an organic anion and H+ simultaneously and both are retained

the proton (H+) consumes HCO3- in the ECF & there is no change in Cl-

increase in anion gap should be equal to decrease in HCO3-

Question 11

what happens to the anion gap and bicarbonate levels in DKA?

Answer 11

ketoacids are neutralized by HCO3 so HCO3 levels drop

there’s also ketones floating around contributing to the unaccounted anions so the anion gap increases

so you’ll get metabolic acidosis with increased anion gap

Question 12

what happens to the anion gap and bicarbonate levels with diarrhea?

Answer 12

HCO3 decreases because it’s being lost through the diarrhea but there no anion replacing it, it’s being replaced by Cl- so the anion gap doesn’t change

Question 13

what conditions cause metabolic acidosis with a normal anion gap?

Answer 13

1. diarrhea
2. fistulae
3. ill loop
4. renal tubular acidosis
5. carbonic anhydrase inhibitor
6. post hypocapnia

Question 14

what conditions cause metabolic acidosis with an increased anion gap?

Answer 14

1. salicylates
2. methanol/ethanol
3. ketoacidosis = diabetic or starvation
4. lactic acidosis
5. uremia

Question 15

what does it mean if the increase in the serum anion gap is greater than the decrease in serum HCO3?

Answer 15

combined metabolic acidosis and metabolic alkalosis

this can happen in someone who is developing lactic acidosis and also vomitting and diarrhea

Question 16

what does it mean if the decrease in the serum HCO3 is greater than the increase in serum anion gap?

Answer 16

it indicates combined increased anion gap metabolic acidosis and normal anion gap metabolic acidosis

Question 17

why would you measure the urine anion gap?

Answer 17

calculate urine anion gap if normal anion gap metabolic acidosis is present

urine cations (Na + K + NH4) = urine anion (Cl)

since urine NH4 is not measured, normal urine anion gap [measured urine cations (Na + K) - measured urine anions (Cl)] will be a negative number (< 0 meq/L)

impaired NH4 excretion in Renal tubules in Renal Tubular acidosis will lead to loss of negative value of urine anion gap

urine anion gap helps to differentiate Renal Tubular Acidosis from non renal causes (e.g., diarrhea) of normal anion gap metabolic acidosis

Question 18

what are the consequences of acidosis on the heart?

Answer 18

1. impairment of cardiac contractility
2. arteriolar dilatation
3. venoconstriction
4. decrease in pulmonary vascular resistance
5. decreased sensitization to arrhythmia
6. decreased sensitivity to catecholamines

Question 19

what are the consequences of acidosis on the lungs?

Answer 19

1. hyperventilation

2. decreased strength of respiratory muscles

Question 20

what are the consequences of acidosis on the metabolism?

Answer 20

1. insulin resistance
2. hyperkalemia
3. decreased metabolic demand

Question 21

what are the consequences of acidosis on the brain?

Answer 21

1. inhibition of metabolism and volume regulation

2. coma

Question 22

how do you treat metabolic acidosis?

Answer 22

1. treat the underlying disorder
2. treatment with bicarbonate should be reserved for severe metabolic gap acidosis.

if the pH <7.20, correct with sodium bicarbonate

the total replacement dose of [HCO3-] can be calculated as shown below:

3. replace with one-half the total amount of bicarbonate over 8 -12 hours and reevaluate

be aware of sodium and volume overload during replacement.

normal or isotonic bicarbonate drip is made with 3 ampules NaHCO3 (50 mmol NaHCO3 / ampule) in 1 Liter of D5W

Question 23

what are 4 of the problems associated with giving bicarbonate replacement during metabolic acidosis?

Answer 23

1. volume expansion; avoid by giving isotonic
2. hypernatremia (you’re giving NaHCO3
3. paradoxical worsening of intracellular acidosis due to buffering by HCO3-
4. stimulates 6-phosphofructokinase activity and organic acid production

Question 24

why does paradoxical worsening of intracellular acidosis happen if you give bicarbonate during metabolic acidosis?

Answer 24

because when you give HCO3- you shift the equilibrium to the left which increases CO2 levels

CO2 then enters the cells and causes paradoxical intracellular acidosis = increased contractility of the heart

Question 25

how do you treat metabolic acidosis caused by toxic alcohol ingestion?

Answer 25

1. dialysis –> it helps remove unmetabolized alcohol and possibly toxic metabolites
2. gastric lavage, induced emesis, or activated charcoal
3. ethanol or fomepizole to delay or prevent generation of toxic metabolites –> decreases metabolism of ethylene glycol and methanol!

Question 26

what is fomepizole?

Answer 26

an inhibitor of alcohol dehydrogenase and is approved for treatment of methanol and ethylene glycol intoxication

so it delays metabolism of ethanol and ethylene glycol so that toxic metabolites aren’t formed

Question 27

what are the 2 phases of metabolic alkalosis?

Answer 27

1. initiation phase
2. maintenance phase

so when you’re throwing up, you’re in the initiation phase because you’re throwing up stomach acid

then, when you’re dehydrated, aldosterone kicks in and secretes K+ and H+ so that you can reabsorb Na to help with the dehydration – >this is the maintenance phase

Question 28

what are the adverse effects of metabolic alkalosis on the heart?

Answer 28

1. arteriolar constriction
2. reduced coronary flow
3. arrhythmias

Question 29

what are the adverse effects of metabolic alkalosis on the lungs?

Answer 29

hypoventilation

Question 30

what are the adverse effects of metabolic alkalosis on the metabolism?

Answer 30

1. hypokalemia
2. decreased ionized Ca+2
3. decreased Mg+2, PO4-2

Question 31

what are the adverse effects of metabolic alkalosis on the brain?

Answer 31

1. decreased CBF
2. tetany
3. seizures
4. delirium
5. stupor

Question 32

how do you treat metabolic alkalosis?

Answer 32

you want to reduce HCO3 to under 40 mEq/L

1. most are chloride responsive
2. in case of vomiting, decrease chloride loss in gastric secretion by inhibiting acid secretion
3. administration of HCO3 or precursors should be stopped
4. carbonic anhydrase inhibitor (Acetazolamide) which interferes with HCO3 reabsorption in the kidney

Question 33

what are the causes of respiratory acidosis?

Answer 33

1. decreased alveolar ventilation

obstruction like bronchospasm or emphysema, drugs, trauma, infection, mechanical or neuromuscular defects, sleep apnea

2. decreased tissue removal or pulmonary exchange of CO2

severe pulmonary edema, pneumonia, cardiac arrest

Question 34

how do you treat respiratory acidosis?

Answer 34

renal response to increase in pCO2 takes 3-5 days hence life threatening acidosis occurs easily during respiratory decompensation

ventilation is the answer!

permissive hypercapnia may need IV sodium bicarbonate or other agent(s)

Question 35

what are the causes of respiratory alkalosis?

Answer 35

1. CNS respiratory stimulation –> anxiety, pain, fever, head trauma, brain tumors, vascular accidents, salicylates, pregnancy
2. peripheral respiratory stimulation –> pulmonary emboli, CHF, pneumonia
3. mechanical hyperventilation
4. early gram negative sepsis
5. hepatic failure

Question 36

how do salicylates cause respiratory alkalosis?

Answer 36

they directly engage the brain to increase respiratory ventilation

they also cause a metabolic anion gap acidosis

the anion with salicylates is lactic acid

if someone has a tooth ache and takes lots of aspirin they’ll develop a combines respiratory alkalosis and metabolic gap acidosis!!**

Question 37

how do you treat respiratory alkalosis?

Answer 37

find the underlying cause and treat the underlying cause

if respiratory alkalosis is life threatening, may need to control pCO2 while maintaining adequate pO2 with mechanical ventilation