Fluid, Electrolyte, and Acid-Base Balance Flashcards
Starling equation and +/- signs
Jv = Kf [(Pc - Pi) - (pic - pii)]
If driving force is + then filtration (movement of fluid out into interstitum)
If driving force is - then reabsorption (movement of fluid into capillaries)
Pc + means pressure out of capillaries (favor filtration)
Pi - means pressure out of capillaries
pic + means fluid into capillaries
pii + means fluid out of capillaries (favor filtration)
How would we increase filtration with Starling forces?
Increase Pc or decrease pic
Edema
When the volume of interstitial fluid exceeds the ability of lymphatics to return it to the circulation
Get edema when (1) increased filtration, or (2) impairment of lymphatic drainage
Types of capillaries
Continuous (capillaries of muscle; extreme case is BBB)
Fenestrated (capillaries of kidney, intestine, endocrine gland; special case is glomerulus–specialized for high filtration)
Discontinuous (capillaries of bone marrow, spleen, liver)
What determines Pc (pressure inside the capillary)?
Venous pressure, NOT mean arterial pressure! This is because of autoregulation of arteriolar sphincter tone
What 3 safety factors minimize potential edema?
1) When filtration increases, lymph flow increases to keep up with it and carry off extra fluid
2) When filtration increases, that dilutes interstitial proteins, which causes water to flow back into capillary (increased difference in oncotic pressures)
3) When filtration increases, Pi increases, which reduces difference in pressure and causes water to flow back into capillary
Note: water does not necessarily flow BACK into capillary as I stated, but it does at least reduce the filtration rate
When does increased Pc cause edema?
Remember, Pc increases when VENOUS pressure increases, not when MAP increases!
Increased renal retention of Na+ causes increased plasma volume
Obstruction such as hepatic cirrhosis or deep venous thrombosis
Static exercising muscle can block flow
When does decreased oncotic pressure in the capillary cause edema?
Hypoalbuminemia could happen because of leaky glomeruli or decreased albumin synthesis by the liver
Note: have to get really low albumin (2 gm/dL compared to normal 3.7-4.6) in order to see obvious edema
When does increased water conductivity (Lp) and/or decreased protein selectivity cause edema?
Causes more protein to leak into interstitum, reducing oncotic pressure
Burns, trauma, inflammation, sepsis and allergic reactions cause this
When does lymphatic obstruction cause edema?
When lymphatics obstructed, get fluid filtered into interstitum that is not removed. Also because of stagnation, plasma proteins leave capillary and equilibrate across capillary wall
Cancer itself or biopsy/removal of lymph nodes can cause this.
Causes of hypovolemia
Hemorrhage
GI losses (vomiting, diarrhea)
Renal losses (diuresis, adrenal insufficiency, salt wasting nephropathy)
Skin losses (burns, sweat)
“Third-spacing” (internal bleeding, peritonitis)
Causes of hypervolemia
Congestive heart failure
Nephrotic syndrome
Cirrhosis
Can plasma osmolality of [Na+] tell you about a person’s volume status?
NO!
Weight, CV status (blood pressure, HR, etc) can
Hypovolemic hyponatremia
Decreased volume state and decreased [Na+]
Vomiting, diarrhea, hemorrhage, third-spacing
Since you have severe water loss, volume stimuli for ADH release override osmotic stimuli AND this water loss is stimulus for thirst, so you get volume increase without [Na+] increase
When get back to a good volume, Na+ input will volume expand the ECF (using aldosterone?)
Hypervolemic hyponatremia
Increased volume and decreased {Na+]
Severely reduced EFFECTIVE circulatory volume: body THINKS there is decreased volume when there’s not (CHF), body has decreased oncotic pressure (hypoalbuminemia in nephrotic syndrome), or get too much vasodilation (cirrhosis)
Volume stimuli for ADH release override osmotic stimuli AND this water loss is stimulus for thirst, so you get volume increase without [Na+] increase
Euvolemic hyponatremia
Volume is normal but decreased [Na+]
Syndrome of inappropriate ADH secretion (SIADH) causes increased water reabsorption without Na+ reabsorption so you become hyponatremic but have normal volume (volume stimuli still working to keep volume normal)
SIADH because of ectopic production or hypothalamic production, or potentiation of ADH on kidney
Also could be drugs causing increased ADH (analgesics), compulsive water drinking
Primary polydipsia (water intoxication)
Free water intake exceeds rate at which kidney can excrete dilute urine
Schizophrenia, water drinking competitions!
Pseudohyponatremia
Person has normal Na balance even though [Na+] is low (<135 mmol/L)
Hyperglycemia causes this by pulling more water out of cells and reducing plasma [Na+] (because glucose is taking up the rest of the “osmole” space in the blood)
Uremia does not have this osmotic effect because urea does not affect water movement (ineffective osmole)
Hypernatremia due to non-renal water loss
Sweating, burns, diarrhea
Losing water but main problem is that person is not drinking enough water
1) Inability to access water
2) Altered thirst mechanism (elderly, hyperaldosteronism resets osmostat, reduced osmoreceptor sensitivity, CNS lesions)
Hypernatremia due to renal water loss
1) Diabetes insipidus (ADH not signaling, so get lots of dilute, sugar free urine)
2) Interference with urine concentrating mechanisms (osmotic diuresis, loop diuretics, renal failure)
Acute hyponatremia
Less than 48 hours duration
No RVD yet, risk of increase in intracranial pressure causing herniation
Seizures, coma
Administer hypertonic saline
Examples: stress causes ADH over-secretion and more water consumption; infants w/diarrhea fed sugar water instead of milk
Chronic hyponatremia
Greater than 48 hours duration
Common electrolyte abnormality in hospitalized patients
Do not have symptoms!
Have done RVD
If you over-treat them, will shrink the brain and cause irreversible demyelination
Dextrose solutions
Add free water to the body when you have had insensible losses or hypernatremia
Added sugar (glucose) is rapidly metabolized, leaving only water behind
Examples: D5W (~280 mOsm)
Note: D10W (~560 mOsm) is actually hypertonic because glucose sticks around!
Saline solutions
Only contain NaCl unless otherwise indicated (3% is 3x normal, normal is isotonic, half normal is half isotonic)
Ringer’s solution: isotonic with physiological conc of KCl, CaCl2, NaCl, lactate (which is converted to bicarb)
Where does normal saline go?
ECF only
Where does D5W go?
D5W (water!) goes 1/3 to ECF and 2/3 ICF (follows volume distribution of body and distributes freely)
Osmolality decreases
Where does half normal saline go?
All the saline part goes to ECF (1/2 of solution) and water part distributes freely (1/6 solution to ECF and 2/6 to ICF).
Overall: 2/3 to ECF and 1/3 to ICF
Osmolality decreases
(Equal parts saline and water)
Where does 3% normal saline go?
All (1L) goes into ECF and also brings in 1.4L from ICF to ECF
Osmolality increases
Treatment for isotonic hypovolemia
Isotonic saline and D5W or half-normal saline to account for insensible losses
Treatment for hypernatremia
Hypotonic fluids (water, D5W) but must take into account chronic vs acute
Treatment for hypovolemic hyponatremia
If chronic: isotonic fluids to replace lost volume (turn off excess ADH), then let normal function (aldosterone?) restore osmolality
If acute: hypertonic fluids (cautiously, if necessary), otherwise isotonic
Treatment for euvolemic hyponatremia
If not emergent situation, don’t need to do anything
Figure out what is causing hyponatremia and treat that
Treatment for hypervolemic hyponatremia
If not emergent situation, don’t need to do anything
Figure out what is causing hyponatremia and treat that (example: CHF–could be made worse by increasing volume further)
Normal plasma [Na+]
135 - 145 mmol/L H2O
What equation do you use when deciding how much water to add to get to certain [Na+]?
Total body osmoles *proportional to* (TBW)(plasma[Na+])
TBW1 x Na+1 = TBW2 x Na+2
What equation do you use when deciding how much Na+ to add to get to certain [Na+]?
Extra amount of Na+ to be added = TBW x ([Na+]2 - [Na+]1)
Why would the kidney retain enough excess salt and water (3 liters!!!) to cause edema?!
1) Volume receptors (arterial side) sense effective circulating volume is low so signal kidney to retain more (CHF)
2) Kidney itself responds abnormally and retains salt and water even when effective circulating volume is normal (nephrotic syndrome)
