(2.1) Acid/Base Disorders [DSA-Selby] Flashcards
Bicarbonate reaction
CO2 + H2O <=> H2CO3- via carbonic anhydrase
H2CO3- <-> H+ + HCO3-
What is the Henderson Hasselbach equation? What is the relationship between pH and PCO2?
HCO3- and pH directly related
PCO2 and pH inversely related
How do the lungs regulate pH?
Control concentration of PCO2 by increasing or decreasing respiratory rate
Inc RR = more CO2 blown off
Dec RR= less CO2 blown off
How do the kidneys regulate pH?
Excreting acidic or alkaline urine
How is acid excreted by the kidneys?
Renal epithelial cells secrete large amounts of H+ into the tubular lumen => acid removed from ECF
What are the 4 acid-base disturbances and their relationship to the buffer system?
Metabolic Acidosis
– Low serum HCO3-
Metabolic Alkalosis
– High serum HCO3-
Respiratory Acidosis
– High PCO2
Respiratory Alkalosis
– Low PCO2
Normal anion gap metabolic acidosis (NAGMA) is also referred to as _____ acidosis
Hyperchloremic acidosis
How is base excreted by the kidneys?
Large amounts of HCO3- continuously filtered into the urine
If not reabsorbed, will be excreted => base removed from ECF
How are the acid-base disturbances compensated
Lung compensates for kidney disturbance
Kidney compensates for lung disturbance
Compensation formula for metabolic acidosis
Winter’s formula: PCO2 = 1.5[HCO3-] + 8 +/- 2
Compensation formula for metabolic alkalosis
Dr. Karius’s formula: pCO2 = 0.7 [HCO3-24] + 20 mmHg ± 5
Dr. Selby: PCO2 increases by 0.7 mmHg for ever 1.0 mEq/L increase in HCO3- from normal (normal HCO3- is 24)
Compensation formula for:
Acute respiratory acidosis
Chronic respiratory acidosis
Dr. Karius:
Acute: [HCO3-] = PCO2/10 +/- 3
Chronic: 24 + (PCO2 - 40)/10 = [HCO3-]
Dr. Selby:
Acute: HCO3- increases by 1 mEq/L for every 10 mmHg increase in PCO2 from normal (normal is 40)
Chronic: HCO3- increases by 3.5 mEq/L for every 10 mmHg increase in PCO2 from normal (40)
Compensation formula for:
Acute respiratory alkalosis
Chronic respiratory alkalosis
Dr. Karius:
Acute: 24 - 2(40 - PCO2)/10 = [HCO3-]
Chronic: 24 - 5(40 - PCO2)/10 = [HCO3-]
Dr. Selby:
Acute: HCO3- will decrease by 2 mEq/L for every 10 mmHg decrease in PCO2 from normal (normal is 40)
Chronic: HCO3- will decrease by 5 mEq/L for every 10 mmHg decrease in PCO2 from normal
How many acid-base disturbances can someone have at one time?
3
(4 is impossible, cannot breathe fast and slow simultaneously)
What is the anion gap equation?
AG = Na+ - (HCO3- + Cl-)
What is a normal anion gap?
Normal AG = 12 +/- 2
Why does the anion gap remain normal in NAGMA?
H+ is buffered by HCO3-, which drops HCO3- =>
Cl- levels rise and offset HCO3- => normal AG
How does hypoalbuminemia affect anion gap? How is this corrected?
Hypoalbuminemia falsely lowers AG
For every 1 g/dL drop in albumin, calculated AG decreases by 2.5 mEq/L and may mask an elevated AG
Correct by adding back the deficit to get the correct AG
Albumin-Corrected Anion Gap = Anion Gap + 2.5 x ([Normal Albumin] - [Observed Albumin])
What is the serum osmolality equation? What is the normal?
Calculatedulated serum osmolality = 2(Na) + (Glucose/18) + (BUN/2.8)
Normal = 275 - 290 mosm/L
What is the osmolar gap? What is the normal?
Osmolar gap = Measured serum osmolality - calculated serum osmolality
Normal osmolar gap < 10 mosm/L
If osmolar gap >10 mosm/L, what is that suggestive of?
Additional solutes in blood
What level of anion gap is highly suspicious for alcohol ingestion?
AG >20
What is osmolar gap most clinically useful for?
Screening for alcohol ingestion, particularly in HAGMA
(can also screen ketoacidosis and lactic acidosis)
What is the Delta-Delta gap used for?
Used in pts with HAGMA to determine if there is coexistent NAGMA or metabolic alkalosis
How do you calculate the Delta-Delta Gap?
Every increase in AG, there should be an equal decrease in serum HCO3-
Delta gap = calculated AG - normal AG (12)
Delta HCO3- = normal HCO3- (24) - Delta gap
What is the range for normal pH?
7.35-7.44
Normal HCO3-?
24 mEq/L
Normal PCO2?
40 mmHg
Preferred Ddx for HAGMA?
GOLD MARK (previously MUDPILES)
Glycols (ethylene and propylene)
Oxoproline (Pyroglutamic acid), tylenol toxicity
L-Lactic acidosis
D-Lactic Acidosis - only seen with colonic metabolization
Methanol
Aspirin
Renal failure
Ketoacidosis (alcoholic, diabetic, starvation)
45 yo female presents to the ED. Upon examination, she appears malnourished and weak. Pt states that she takes acetaminophen daily for headaches. What kind of acid-base disturbance might you see?
Pyroglutamic (5-oxoproline) acidosis
(Seen more in women who are malnourished or critically ill)
What mechanism is responsible for pyroglutamic acidosis?
Acetaminophen depletes glutathione =>
Increased formation of pyroglutamic acid and accumulation
What diagnostic test would you order for pyroglutamic (5-oxoproline) acidosis?
Urinary organic acid screen
What is the treatment for pyroglutamic (5-oxoproline) acidosis?
Discontinue acetaminophen
IVF
N-acetylcysteine
Ddx for increased osmolar gap?
ME DIE (same…)
Methanol
Ethanol
Diethylene glycol - Mannitol diuretic
Isopropyl alcohol (rubbing alcohol)
Ethylene glycol
(also propylene glycol, ketoacidosis, and lactic acidosis)
If a patient has metabolic acidosis, association of which alcohol can be ruled out?
Isopropyl alcohol (NOT associated with metabolic acidosis)
What is the relationship between acidosis and serum potassium?
Acidosis is associated with hyperkalemia
(H+ enter cells, K+ exit)
What is the relationship between alkalosis and serum potassium?
Alkalosis is associated with hypokalemia
(H+ exits the cells, K+ enter)
Ddx for normal anion gap metabolic acidosis (NAGMA)?
DURHAM
Diarrhea
Ureteral diversion (ileal conduit) or fistula
Renal tubular acidosis (RTA)
Hyperalimentation (Enteral nutrition)
Acetazolamide (carbonic anhydrase inhibitor)
Addison’s disease (adrenal insufficiency)
Miscellaneous (toulene toxicity - glue sniffing)
Type 2 RTA has decreased capacity in the ____ to reabsorb ____
Decreased capacity in the PT to reabsorb HCO3-
(results in low serum HCO3-, will eventually stabilize at the lower level)
Where is the majority of HCO3- reabsorbed in the kidney?
Proximal tubule (80-90%)
8 year old patient presents with type 2 RTA, what is their most likely diagnosis?
Cystinosis
(Most common cause of Type 2 RTA in children)
An adult presenting with type 2 RTA likely has what etiology? With what secondary cause?
Fanconi syndrome with secondary Multiple Myeloma
A patient with proximal RTA may present with what clinical manifestations?
(Type 2 RTA)
NAGMA w/ or w/o PT dysfunction
Hypokalemia (milder than Type 1)
How do you diagnose a proximal (Type 2) RTA?
Urine pH high or low depending on serum HCO3-
Can have urine pH <5.5 when in new steady state (normal H+ secretion in distal nephron)
Urine anion gap positive or negative
What is the urine anion gap (UAG) clinically used for?
Clinically used to differentiate renal from non-renal causes of NAGMA
What is the difference between a positive and a negative urine anion gap?
Negative = appropriate distal nephron urinary acidification
Positive = inappropriate distal nephron acidification
What is urine anion gap a marker of?
NH4Cl (ammonium chloride) excretion => appropriate urinary acidification
(will increase in the setting of metabolic acidosis)
What are the two main causes of distal (Type 1) RTA?
Unable to acidify urine due to:
1) Decreased H+ ion secretion due to channel defects
2) Gradient defect: abnormally permeable distal tubule and collecting duct allows H+ ions to flow back into tubular cell
(lack of H+ ion secretion prevents urinary acidification and excretion of ammonium)
A patient taking amphotericin may be at risk for what type of RTA?
Distal (Type 1) RTA
(May cause gradient defect)
What clinical manifestations may be seen with distal (Type 1) RTA?
Nephrolithiasis
Nephrocalcinosis
Which RTA presents with systemic disease?
Distal (Type 1 RTA)
(usually presents with Sjogren’s disease)
Which RTA can be caused by glue sniffing?
Type 1 (Distal) RTA due to toulene
What diagnostic criteria may present for Type 1 (distal) RTA?
NAGMA
Unable to acidify urine pH <5.5
Hypokalemia (severe)
UAG positive
Type 4 RTA is also known as _____ RTA
Hyperkalemic
Characterized by distal nephron dysfunction from impaired H+ and K+ excretion => NAGMA and hyperkalemia
What are the two main theories of hyperkalemic RTA?
Deficiency of circulating aldosterone (DM, NSAIDS, Bblockers, ACEi/ARB, high dose heparin)
Aldosterone resistance in collecting ducts (interstitial renal disease, K+ sparing drugs)
60 yo male w/ hx of DM presents for a wellness check. Upon lab study, pt has NAGMA and hyperkalemia but does not appear symptomatic. What type of RTA is this pt likely to have?
Type 4 (hyperkalemic)
(most pts in 50s-70s w/ hx of DM or CKD)
What diagnostic presentations might a patient with Type 4 RTA have?
Variable urine pH >5.5
Positive UAG
In general, what factors may lead to a metabolic alkalosis?
Factors that stimulate Na+ reabsorption
Increase H+ secretion and stimulate HCO3- reabsorption
Ddx for metabolic alkalosis?
Hypokalemia
Vomiting/nasogastric tube suctioning
Diuretics (loop and thiazide)
Volume depletion (Contraction alkalosis)
Mineralocorticoid excess (appropriate or inappropriate)
Contraction alkalosis would occur in what setting?
Volume depletion in the setting of Cl- depletion => RAAS activation and aldosterone secretion
What is the relationship between an alpha-intercalated cell and a beta-intercalated cell?
Beta-intercalated cell is a mirror image of alpha-intercalated
HCO3- exits into tubular lumen
In contraction alkalosis, must replete Cl- to help HCO3- secretion
