Approach to Acid-Base Disorders (Selby) Flashcards
definition metabolic acidosis
low serum HCO3-
definition metabolic alkalosis
highs serum HCO3-
definition respiratory acidosis
high PCO2
definition respiratory alkalosis
low PCO2
Anion gap formula
= Na - (HCO3 + Cl)
When is anion gap used?
to differentiate HAGMA vs.
NAGMA
what will falsely lower anion gap?
hypoalbuminemia
what is correction factor for anion gap with hypoalbuminemia?
for every 1 g/dL drop in albumin, the AG drops by 2.5 mEq/L
normal serum osmolality
275- 290 mosm/L
formular for serum osmolality
= 2(Na) + (Glucose/18) + (BUN/2.8)
formular for osmolar gap
measured serum osmolality- calculated serum osmolality
what is osmolar gap clinically useful for?
- screening for EtOH ingestions (esp. HAGMA cases)
- screening for ketoacidosis
- screening for lactic acidosis
When is delta delta gap used for?
in patients with HAGMA to determine if there is coexisting NAGMA or metabolic alkalosis present
delta delta gap formulas
delta gap = calculated AG -normal AG (always 12)
delta HCO3 = normal HCO3 (always 24) - delta gap
what if measured HCO3 is equal to delta delta gap?
then no additional acid base disorder is present
what if measured HCO3 is greater than delta delta gap?
metabolic alkalosis is present in addition to HAGMA
what if measured HCO3 is less than delta delta gap?
NAGMA is present in addition to HAGMA
normal pH on arterial blood gas
pH 7.35-7.44
normal HCO3 on ABG
24 mEq/L
normal PCO2 on ABG
40 mmHg
normal anion gap on ABG
12
normal osmolality gap on ABG
10 mmol/L
Differential Dx HAGMA
G- glycols (ethylene and propylene) O- oxoproline (acetamino tox) L- lactic acidosis (L-isomer) D- lactic acidosis (D-isomer) M- methanol A- aspirin R- renal failure K- ketoacidosis (EtOHism, DM, starvation)
Who is pyroglutamic (5-oxoproline) acidosis seen in?
women who are malnourished or critically ill
DDx of increased osmolar gap
M- methanol E- ethanol D- diethylene glycol I- isopropyl EtOH E- ethylene glycol - propylene glycol - keto/lactic acidosis
what cause of increased osmolar gap is not associated with metabolic acidosis?
isopropyl alcohol
What level of potassium is acidosis associated with ?
hyperkalemia
what level of potassium is alkalosis associated with?
hypokalemia
NAGMA DDx
D- diarrhea U- ureteral diversion R- renal tubular acidosis H- hyperalimentation A- acetazolamide A- addison's disease M- miscellaneous (glue sniffing)
respiratory alkalosis DDx
anything that increases respiratory rate or tidal volume (including pregnancy)
respiratory acidosis DDx
anything that lowers respiratory rate/tidal volume, increases dead space, or worsens airway obstruction
what is urine anion gap used for
clinically used to differentiate renal from nonrenal causes of NAGMA
what is UAG a marker of
NH4Cl excretion, which indicates proper urinary acidification
formula urine anion gap
UAG = (urine sodium + urine potassium) - urine chloride
negative UAG meaning
appropriate distal nephron urinary acidification
postive UAG meaning
inappropriate distal nephron urinary acidification
problem in proximal RTA (type 2)
decreased capacity of proximal tubule to reabsorb HCO3, eventually some reabsorption occurs in other parts of tubule, but at a lower level (–> acidosis)
Secretion of what leads to HCO3 reabsorption?
H+
what is most common cause of RTA type 2 in children?
cystinosis
Common cause of RTA type 2 in adults?
Fanconi syndrome (likely due to multiple myeloma)
Problem in distal RTA (type 1)
patients are unable to acidify their urine; lack of net H+ ion secretion prevents urinary acidification and excretion of ammonium
common etiologies type 1 RTA
Sjogren’s syndrome, glue sniffing
clinical manifestations type 1 RTA
nephrolithiasis or nephrocalcinosis
4 things for diagnosis RTA type 1
1) NAGMA
2) unable to acidify urine pH <5.5
3) hypokalemia, usually severe
4) UAG is positive
Etiology hyperkalemic RTA (type 4)
- deficiency circulating aldosterone
- aldosterone resistance in collecting ducts
- BOTH lead to impaired Na+ reabsorption by principle cells (leads to hyperkalemia)
clinical manifestations type 4 RTA
- usually asymptomatic
- NAGMA
- hyperkalemia
- most patients in 50-70s with h/o DM or CKD
Diagnosis type 4 RTA
- variable urine pH (usually >5.5)
- UAG is positive
DDx metabolic alkalosis (5 things)
- hypokalemia
- vomiting or nasogastric tube suctioning
- diuretics (loop/thiazide)
- volume depletion
- mineralocorticoid excess (Conn syndrome)
General cause of metabolic alkalosis
factors that stimulate Na+ reabsorption, secondarily increase H+ secretion and stimulate HCO3- reabsorption
what cell is a mirror image of alpha intercalated cells?
beta intercalated
Why is there hypochloremia with volume depletion (contraction alkalosis)?
The HCO3-Cl exchanger in beta intercalated cells must replete Cl- to help with HCO3- secretion
winters formula
PCO2 = 1.5 [HCO3]+8 +/-2
