Fluid Therapy Recap Flashcards
Primary Extracellular Cations
Na+
Primary Extracellular Anions
Cl-
HCO3-
Primary Intracellular Cations
K+
Mg+
Primary Intracellular Anions
Phos
Proteins
Define Osmolality
particles/molecules (osmoles) / kg
Define Osmolarity
particles/molecules (osmoles) / L
Equation for calculated osmolarity
Calculated osmolarity = 2 (Na+K) + glucose/18 + BUN/2.8
Equation for effective osmolarity
Effective osmolarity = 2 (Na+K) + glucose/18
What is the difference between calculated and effective osmolarity/osmolality?
BUN distributes equally between intracellular/extracellular space (freely diffusible), so is not effective at being an osmole (draws water equally to both places). However, the brain has relatively low urea permeability (BBB), so BUN is a more effective osmole in the brain.
Write out and explain Starling’s Equation (traditional).
Net filtration = Kf [(Pcap-Pif) – σ (πp –πif)]
- Kf: Net permeability of the capillary wall (leaky or not leaky to water).
- Pcap: Pressure in capillary (generated by heart/BP)
- Pif: Pressure of interstitial fluid.
- πp: Oncotic pressure (pull) of plasma
- πif: Oncotic pressure (pull) of interstitial fluid
- σ : “reflection coefficient” for a particular solute (permeability of that capillary to that solute)
Explain the modifications/revisions to Starling’s Law taking into account our more current knowledge of vascular fluid dynamics.
- Revised law replaces (πif) with the subglycocalyx space (πg)- oncotic pressure gradient is between the plasma and the space between the endothelial cell wall and the endothelial surface layer (glycocalyx)
- This space is nearly protein free (so interstitial protein content doesn’t matter much in reality)
- The glycocalyx is the semi-permeable membrane (rather than the endothelium)
- Different products have a different ability to move through the glycocalyx due to charge, so have different physiologic effects despite a similar COP (hetastarch moves easily, Albumin doesn’t)
What factors can damage the glycocalyx?
- Ischemia/Reperfusion/Oxidant injury
- Inflammation
- Cytokines
- Hyperglycemia
- Hypercholesterolemia
- Hypervolemia/excessive fluid therapy
- Hypoalbuminemia
Name 10-12 negative effects of excessive fluid therapy (positive fluid balance)
- Tissue and interstitial edema → poor oxygen diffusion, tissue distortion, obstructed capillary flow/drainage → organ dysfunction
- Increased duration of ICU stay
- Increased mortality
- Increased ICU infections (including lungs)
- Impaired tissue healing
- Compromised gas exchange in lungs
- Increased post operative ileus/delayed gastric emptying
- Impaired healing of GI anastamosis
- Impaired renal function and blood flow (encapsulated), decreased urine output
- Impaired hepatic function and blood flow (encapsulated)
- Decreased cardiac function including decreased contractility, impaired myocardial oxygenation, impaired conduction, increased cardiac morbidity
- Impaired neurologic function
Explain how Aldosterone affects sodium/water regulation:
Increases Na reabsorption by increasing # and activity of open Na channels in collecting ducts, increases synthesis of Na+/K+ ATPase for insertion into basal membranes
Explain how Catecholamines affect sodium/water regulation:
vasoconstriction of efferent > afferent arterioles → increased filtration fraction → increased water and Na reabsorption. Directly stimulate Na reabsorption in proximal tubule.
Explain how Angiotensin II affects sodium/water regulation:
vasoconstriction of efferent > afferent similar to catecholamines → increased water/Na reabsorption, directly stimulates Na/H antiporter in proximal tubules, stimulates aldosterone release
Explain how ANP affects sodium/water regulation:
dilates afferent and constricts efferent arterioles to increase GFR, inhibits Na reabsorption in CD, inhibits renin and aldosterone secretion 🡪 increased Na/H20 excretion
Explain how ADH/Vasopressin affects sodium/water regulation:
increases water reabsorption in collecting ducts by insertion of aquaporins into luminal membrane of principal cells
What are the 3 broad causes of hypernatremia? Give an example of each.
1) Pure water deficit
Example: hypodypsia, diabetes insipidus, fever, inadequate access to water
2) Hypotonic fluid loss
Example: GI losses, 3rd space losses, cutaneous losses (burns), renal losses
3) Salt gain
Example: Salt ingestion, cathartic administration, hyperosmolar fluid administration, hyperaldosteronism,
What are the 3 main categories of hyponatremia? Give an example of a cause of each.
1) Normal plasma osmolality
Example: pseudohyponatremia (not clinically relevant)
2) Increased plasma osmolality
Example: hyperglycemia, mannitol administration
3) Decreased plasma osmolality (“true” hyponatremia)
Example: Loss of higher sodium fluids (hypovolemic)- ie Addison’s, GI; CHF with RAAS activation (hypervolemic); psychogenic polydipsia (normovolemic)
What effect would Acute hypernatremia have on the brain?
cerebral dehydration, hemorrhage +/- demyelination
What effect would Acute hyponatremia have on the brain?
cerebral edema
What effect would rapid correction of hypernatremia have on the brain?
Cerebral edema
What effect would rapid correction of hyponatremia have on the brain?
cerebral dehydration, demyelination +/-hemorrhage
How would you treat this patient:
2.3 kg, 17yo FS DSH who has been eating less for a few weeks and losing weight, presents obtunded, 10% dehydrated. Na is 173
Chronic hypernatremia (symptomatic):
If hypotensive- bolus 0.9% saline until BP corrected. 7-10 ml/kg/hr D5W for the first 2-3 hrs until neuro improves. Then slow correction of free water deficit over 48 hours, dropping Na by no more than 0.5-1 mEq/L/hr to avoid cerebral edema, monitor Na q 4 hours.
7ml/kg/hr = 16ml/hr D5W x 2-3 hrs
FWD = 0.6 x 2.3kg x [(173/150) – 1] = 0.2 L / 48 hrs = 4ml/hr D5W
+ maintenance fluids
How would you treat this patient:
32kg ,4yo MN Lab, vomiting, ataxic, very mentally inappropriate, was playing at the beach 1 hour earlier. Na is 165.
Acute hypernatremia- can correct more rapidly with combination of D5W + furosemide (likely to be hypervolemic, increases Na excretion). Can correct as fast as possible.
10ml/kg/hr = 320ml/hr D5W
Furosemide 2mg/kg
Or
FWD = 0.6 x 32 x [(165/140) -1] = 3.4 liters
Would you give mannitol to an acute or chronic hypernatremic mentally inappropriate patient? Explain.
NO!!! Hypernatremia leads to cerebral dehydration- giving mannitol would make this worse!
What is SIADH?
Syndrome of inappropriate antidiuretic hormone secretion.
ADH secretion is inappropriate because it occurs in a situation of decreased plasma osmolality.
Patient is hyponatremic and euvolemic with high urine osmolality.
Explain & Draw the basic mechanisms of electrolyte reabsorption/secretion in the kidneys. This does not have to be a specific site or electrolyte.
- Active transport at the basolateral membrane (3Na/2K/ATPase pump) creates a gradient (Na out of cell into interstitium, K into cell)
- Passive transcellular transport at the luminal membrane driven by concentration gradient, partially created by the active pump (cotransporters, antitransporters) and partially created due to lower concentration of electrolytes in blood (highly filtered at glomerulus)
- Paracellular transport driven by transepithelial (transmembrane) potential difference (electrical charge)- lumen positive or lumen negative
- Paracellular solute drag along with water (aids with reabsorption)
- “Sink” effect from rapid tubular flow (aids excretion)
Label the following diagram with the diuretic that acts at each site. Explain the net effect of each diuretic (inhibition, stimulation, electrolyte changes in the patient). Which is the most potent and why?
- Diuretic: Furosemide (loop)
Net effect: inhibits Na/K/2Cl cotransporter, leading to increased loss of Na/K/Cl in urine 🡪 increased water loss. MOST POTENT- loop of henle is the main site of creation of the urine concentration gradient
- Diuretic: Thiazides
Net effect: inhibits Na/Cl cotransporter leading to increased Na/Cl and water loss in the urine. Less potent because the DCT is mostly for fine tuning (not large volume of electrolyte movement).
- Diuretic: K sparing (Amiloride, triamterene)
Net effect: block Na channels in the CD, so less Na reabsorption. Aldosterone antagonists (like spironolactone) antagonize the effect of aldosterone on these channels. Loss of Na and water, but not K.
Explain the effect of hypochloremia on acid-base status.
Chloride and HCO3 are both negatively charged and must be kept in balance. Decreased Cl → Increased HCO3. Hypochloremia increases the strong ion difference and causes a metabolic alkalosis that cannot be corrected without providing chloride.
Name 3 causes of Corrected hypochloremia:
- GI losses
- Renal losses (ie thiazide/loop diuretics)
- Hyperadrenocorticism, chronic resp acidosis, administration of high Na/low Cl solution
Name 3 causes of Corrected hyperchloremia:
- Pseudohyperchlormemia: bromide, hemolysis, lipemia, high bili
- High Cl administration
- Renal Cl retention (Addison’s)
Name 3 causes of hypokalemia:
- Decreased intake
- Translocation: alkalemia, insulin/glucose, albuterol, catecholamines, refeeding syndrome
- Increased renal or GI losses
Name 4 causes of hyperkalemia:
- Pseudohyperkalemia: thrombocytosis, hemolysis
- Increased intake or oversupplementation (unlikely)
- Translocation: acidosis, tumor lysis syndrome, reperfusion injury
- DECREASED URINARY EXCRETION!!!