ICL 2.6: Approach to Acid Base Flashcards
what is metabolic vs respiratory acidosis?
metabolic = presence of process which leads to gain of H+ or loss of HCO3-.
respiratory = presence of process which leads to retention of CO2
what is metabolic vs respiratory alkalosis?
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
how does the body compensate for acid-base disturbances?
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
how much does PCO2 compensate during metabolic acidosis vs. alkalosis?
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
how much does HCO3 compensate during respiratory acidosis vs. alkalosis?
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
how does the kidney handle acid?
- HCO3- reabsorption coupled to Na reabsorption
- NH4+ synthesis sensitive to pCO2.
- aldosterone influence: H+ secretion and NH4+ trapping in the kidney to get rid of acid
how much acid does your body make in a day?
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-
what is the major component of the normal anion gap?
albumin
what is normal anion gap acidosis?
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
what is increased anion gap acidosis?
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-
what happens to the anion gap and bicarbonate levels in DKA?
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
what happens to the anion gap and bicarbonate levels with diarrhea?
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
what conditions cause metabolic acidosis with a normal anion gap?
- diarrhea
- fistulae
- ill loop
- renal tubular acidosis
- carbonic anhydrase inhibitor
- post hypocapnia
what conditions cause metabolic acidosis with an increased anion gap?
- salicylates
- methanol/ethanol
- ketoacidosis = diabetic or starvation
- lactic acidosis
- uremia
what does it mean if the increase in the serum anion gap is greater than the decrease in serum HCO3?
combined metabolic acidosis and metabolic alkalosis
this can happen in someone who is developing lactic acidosis and also vomitting and diarrhea
what does it mean if the decrease in the serum HCO3 is greater than the increase in serum anion gap?
it indicates combined increased anion gap metabolic acidosis and normal anion gap metabolic acidosis
why would you measure the urine anion gap?
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
what are the consequences of acidosis on the heart?
- impairment of cardiac contractility
- arteriolar dilatation
- venoconstriction
- decrease in pulmonary vascular resistance
- decreased sensitization to arrhythmia
- decreased sensitivity to catecholamines
what are the consequences of acidosis on the lungs?
- hyperventilation
2. decreased strength of respiratory muscles
what are the consequences of acidosis on the metabolism?
- insulin resistance
- hyperkalemia
- decreased metabolic demand
what are the consequences of acidosis on the brain?
- inhibition of metabolism and volume regulation
2. coma
how do you treat metabolic acidosis?
- treat the underlying disorder
- 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:
- 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
what are 4 of the problems associated with giving bicarbonate replacement during metabolic acidosis?
- volume expansion; avoid by giving isotonic
- hypernatremia (you’re giving NaHCO3
- paradoxical worsening of intracellular acidosis due to buffering by HCO3-
- stimulates 6-phosphofructokinase activity and organic acid production
why does paradoxical worsening of intracellular acidosis happen if you give bicarbonate during metabolic acidosis?
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
how do you treat metabolic acidosis caused by toxic alcohol ingestion?
- dialysis –> it helps remove unmetabolized alcohol and possibly toxic metabolites
- gastric lavage, induced emesis, or activated charcoal
- ethanol or fomepizole to delay or prevent generation of toxic metabolites –> decreases metabolism of ethylene glycol and methanol!
what is fomepizole?
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
what are the 2 phases of metabolic alkalosis?
- initiation phase
- 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
what are the adverse effects of metabolic alkalosis on the heart?
- arteriolar constriction
- reduced coronary flow
- arrhythmias
what are the adverse effects of metabolic alkalosis on the lungs?
hypoventilation
what are the adverse effects of metabolic alkalosis on the metabolism?
- hypokalemia
- decreased ionized Ca+2
- decreased Mg+2, PO4-2
what are the adverse effects of metabolic alkalosis on the brain?
- decreased CBF
- tetany
- seizures
- delirium
- stupor
how do you treat metabolic alkalosis?
you want to reduce HCO3 to under 40 mEq/L
- most are chloride responsive
- in case of vomiting, decrease chloride loss in gastric secretion by inhibiting acid secretion
- administration of HCO3 or precursors should be stopped
- carbonic anhydrase inhibitor (Acetazolamide) which interferes with HCO3 reabsorption in the kidney
what are the causes of respiratory acidosis?
- decreased alveolar ventilation
obstruction like bronchospasm or emphysema, drugs, trauma, infection, mechanical or neuromuscular defects, sleep apnea
- decreased tissue removal or pulmonary exchange of CO2
severe pulmonary edema, pneumonia, cardiac arrest
how do you treat respiratory acidosis?
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)
what are the causes of respiratory alkalosis?
- CNS respiratory stimulation –> anxiety, pain, fever, head trauma, brain tumors, vascular accidents, salicylates, pregnancy
- peripheral respiratory stimulation –> pulmonary emboli, CHF, pneumonia
- mechanical hyperventilation
- early gram negative sepsis
- hepatic failure
how do salicylates cause respiratory alkalosis?
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!!**
how do you treat respiratory alkalosis?
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
