Electrolytes and Acid-Base Disorders Flashcards
TBW of term infants
75%
TBW from the first year of life until puberty
60%
TBW at the end of puberty, males
60%
TBW at the end of puberty, females
50%
ICF is ___% of TBW
30-40
ECF is ___% of TBW
20-25
Plasma is ___% of TBW
5
IF is ___% of TBW
15
Normal plasma osmolality
285-295 mOsm/kg
Formula for osmolality
2Na + Gluc/18 + BUN/2.8
Formula for effective osmolality
2Na + Gluc/18
Effective osmolality is aka
Tonicity
Determines the osmotic force that is mediating the shift of water between the ICF and ECF
Effective osmolality (tonicity)
Formula for corrected Na in hyperglycemia
Measured sodium + [1.6 (glucose – 100) / 100]
Osmolal gap is a clinical clue to
Presence of unmeasured osmoles and may indicate poisoning with methanol or ethylene glycol
Osmolal gap is present if
Measured osm exceeds calculated osm bby >10 mOsm/kg
Elevated effective osmolality leads to secretion of what hormone
ADH
Most important determinant of renal Na excretion
Volume status of the child
Main sites for precise regulation of Na balance in the kidney
Distal tubule and collecting duct
Accounts for elevated BUN and uric acid in dehydration
Resorption of uric acid and urea in the proximal tubule when Na retention increases
Increase in blood volume stimulates release of what hormone
ANP –> increase in GFR –> inhibition of Na resorption in the medullary portion of the collecting duct
Na intake is recommended not to exceed
2500mg/day
T/F Presence of glucose enhances Na absorption in the GIT
T
Most devastating consequence of hypernatremia
Brain hemorrhage
Goal in hypernatremia is to decrease Na by ___
<12meq/L every 24 hrs or 0.5meq/L/hr
In hypernatremic dehydration, first priority is to
Restore intravascular volume with ISOTONIC solution, preferably normal saline
Why is NSS>LR in restoration of intravascular volume in hypernatremic dehydration
Low Na concentration of LR can cause serum Na to decrease too rapdily
Formula for water deficit
Weight x 0.6 (1-145/Na)
MCC of hypovolemic hyponatremia
Diarrhea
Type of hyponatremia seen in heart failure and renal failure
Hypervolemic hyponatremia
Type of hyponatremia seen in SIADH
Euvolemic hyponatremia
Responsible for most of the symptoms of hyponatremia
Brain cell swelling
Traditional first step in the diagnostic process in hyponatremia
Determination of plasma osmolality
Low vs normal osmolality vs high osmolality: True hyponatremia
Low
Low vs normal osmolality vs high osmolality: Pseudohyponatremia
Normal
Low vs normal osmolality vs high osmolality: Elevation of another effective osmole, e.g. glucose
High
To prevent central pontine myelinosis, correction of hyponatremia should not be more than ___meq/L in 48 hours
18
Each mL of 3% NaCl (HTS) increases Na by approximately
1 meq/L
Insulin increases movement of K into the cells by activating
Na-K-ATPase
T/F Decrease in pH drives potassium extracellularly
T
Mechanism of beta agonist in cases of hyperkalemia
Increases movement of K into the cells by activating Na-K-ATPase
T/F α-agonists causes a net movement of K out of the cell
T
T/F Exercise causes a net movement of K out of the cell
T
Principal hormone regulating potassium secretion
Aldosterone
Most important effects of hyperkalemia are due to
Role of K in membrane polarization
ECG changes in hyperkalemia begins with
Peaking of T waves –> OTHER: ST depression, increased PR, flattening of P, widening of QRS –> Vfib
Useful method to evaluate renal response to hyperkalemia
TTKG (Transtubular potassium gradient)
Formula for TTKG
Kurine/Kplasma x (plasma osm/urine osm)
TTKG value when there is normal renal excretion of K
> 10
TTKG value when there is a defect in renal excretion of K
<8 (such as in lack of aldosterone)
Treatment of hyperkalemia that RAPIDLY decreases the risk of life-threatening arrhythmias: Shift potassium intracellularly
Insulin, beta agonist, NaHCO3
Treatment of hyperkalemia that RAPIDLY decreases the risk of life-threatening arrhythmias: Cardiac membrane stabilization
IV Calcium
Treatment of hyperkalemia that removes potassium from the body
Loop diuretic, SPS (Kayexalate), dialysis
MOA of SPS
Na is exchanged for K and K-containing resin is excreted from the body
Met alk + hypoK + high urine chloride + normal BP
Bartter, Gitelman, diuretic use
ECG changes in hypoK
Flattened T, ST depression, U wave
Paralysis is possible only at K level of
<2.5meq/L
TTKG of ___ in the presence of hypoK suggests excessive urinary losses of K
> 4
4th MC cation in the body
Mg
3rd MC intracellular cation
Mg
MCC of hypomag
GI and renal losses
Drugs that cause significant Mg wasting
Amphotericin and Cisplatin
Hypomag causes secondary hypocal bby
Impairing release of PTH and blunting of tissue response to PTH
Symptoms of hypermag do not occur until plasma Mg is
> 4.5
Effect of hypermag on muscles and reflexes
Inhibits Ach release at the NMJ –> hypotonia, hyporeflexia, weakness
Effect of hypermag on BP
Associated with hypotension because of vascular dilation
Level of hypermag that causes complete heart block and cardiac arrest
> 15mg/dL
Aprox 65% of plasma phos is in
Phospholipids
Most plentiful intracellular anion
Phosphate
Effect of PTH on serum phos
Decreases serum and increases urinary phosphate by decreasing resorption in the kidneys
Body’s compensatory mechanism for hypophos
Stimulates 1α-hydroxylase –> 25-D to 1,25-D (calcitriol) –> increase intestinal absorption and renal resorption of phos
Effect of Phosphatonin
Inhibits renal resorption of phos causing phosphaturia and hypophosphatemia
Mechanism of hypophos in refeeding syndrome
Anabolism –> significant cellular demand for phos
Mechanism of hypophos in respi alk
Glycolysis > intracellular use of phos
Avid uptake of phos along with Ca and Mg which can produce plasma deficiency of all 3 ions
Hungry bone syndrome (seen after parathyroidectomy)
MCC of severe hypophos in adults
Alcoholism
Severe hypophos is defined as a level of
1-1.5mg/dL
MC complication of acute hypophos
Rhabdomyolysis
MCC of hyperphos
Renal insufficiency
Principal clinical consequence of hyperphosphatemia
Hypocalcemia and systemic calcification
Mechanism of Sevelamer
An ORAL phosphate binder given in significant hyperphosphatemia that prevents absorption of dietary phosphorus by binding it in the GIT
Henderson-Hasselbbach equation
pH = 6.1 + log [HCO3-]/[CO2]
Rapid pulmonary response to changes in CO2 concentration occurs via
Central sensing of pCO2 and subsequent increase or decrease in ventilation
3 principal sources of hydrogen ions
1) Dietary protein metab 2) Incomplete metab of carbb (lactic acid) and fats (ketones) 3) Stool losses of bicarb
Necessary 1st step in renal regulation of acid-base balance
Resorption of filtered bicarbonate
Reclaims ~85% of filtered HCO3
Proximal tubule
Most important regulator of renal acid excretion
Extracellular pH
Respiratory compensation for a metabolic process happens quickly and is complete within
12-24 hours
Renal compensation for a metabolic process is completed within
3-4 days
Expected compensation: Met acid
pCO2 = 1.5 x HCO3 + 8±2
Expected compensation: Met alk
pCO2 increases by 7mmHg for every 10meq/L increase in serum HCO3
Expected compensation: Respi acid, acute
HCO3 increases by 1 for each 10mmHg increase in pCO2
Expected compensation: Respi acid, chronic
HCO3 increases by 3.5 for each 10mmHg increase in pCO2
Expected compensation: Respi alk, acute
HCO3 falls bby 2 for each 10mmHg decrease in pCO2
Expected compensation: Respi alk, chronic
HCO3 falls by 4 for each 10mmHg decrease in pCO2
MC etiology of met acid
Diarrhea
Serum pH ___ may impair cardiac contractility and increase risk of arrhythmia
<7.2
Effect of acidemia on cardiovascular response to catecholamines
May decrease
Effect of acidemia on pulmonary vasculature
VASOCONSTRICTION
Acute effect of acidemia on action of insulin
Insulin resistance
Acute effect of acidemia on ATP
Reduced ATP synthesis
Anion gap formula
Na - Cl - HCO3
Normal AG
4-11
Met acid resulting from increase in unmeasured anions
HAGMA
Met acid resulting from decrease in bicarbonate concentration without an increase in unmeasured anions
NAGMA
Approx 11 meqs of anion gap is normally sec to
Albumin
1g/dL decrease in albumin decreases anion gap by
~2.5meq/L
Met alkalosis is MC secondary to
Emesis or diuretic use
pCO2 increases by 7mmHg for each 10meq/L increase in serum HCO3 but pCO2 never exceeds
55-60
Etiologies of met alkalosis are divided into 2 categories babsed on
Urinary chloride level
Etiology of met alkalosis: Does not respond to volume repletion
Elevated urinary chloride
Etiology of met alkalosis: Responds to volume repletion
Low chloride level
Mechanism of tetany in alkalemia
iCa decreases as a result of increased binding of Ca to albumin
Intervention is usually necessary with moderate or severe met alkalosis, that is with a HCO3 level of
> 32
T/F In a patient breathing room air, hypoxia is always present if a respiratory acidosis is present
T
Effect of hypercapnia on pulmonary vasculature
Vasoconstriction
Effect of hypercapnia on cerebral vasculature
Vasodilation
T/F In patient with chronic respi acid, respiratory drive is often less responsive to hypercarbia and more responsive to hypoxia
T
Effects of hypoxemia on ventilation begins when O2sat decreases to approx ___ and pO2 ___
90%, 60mmHg