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Poll: Which of the following Starling force changes is the primary cause of the edema seen in patients wi

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  1. #1
    Anonymous is offline Unregistered Guest
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    Jan 2003

    of the following Starling force changes is the primary cause of the edema seen in patients with nephrotic syndrome?

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    Since we're studying physiology this week, I ran into this question and thought I'd post it. Try to answer it first, then read through the answer and explanation below. No peeking

    The rate of exchange, in either direction, is determined by physical factors: hydrostatic pressure, oncotic pressure, and the physical nature of the barrier separating the blood and the interstitium of the tissue (i.e., the permeability of the capillary wall).

    There are two important and opposing hydrostatic forces: capillary hydrostatic pressure (PC) and tissue hydrostatic pressure (PT). Because PC is normally much greater than PT, the net hydrostatic pressure gradient across the capillary is positive, meaning that hydrostatic forces are driving fluid out of the capillary and into the interstitium. There are also two opposing oncotic pressures influencing fluid exchange: capillary plasma oncotic pressure (pC) and tissue (interstitial) oncotic pressure (pT). pC is much greater than pT, therefore the oncotic pressure gradient across the capillary, if unopposed by hydrostatic forces, would reabsorb fluid from the interstitium into the capillary. The oncotic pressure difference (pC - pT) should be multiplied by the reflection coefficient that represents the permeability of the capillary barrier to the proteins responsible for generating the oncotic pressure. Because both hydrostatic and oncotic forces are normally expressed in units of mmHg. The net driving force (NDF) for fluid movement is the net pressure gradient determined by the sum of the individual hydrostatic and oncotic pressures.

    There are several factors that can precipitate edema:

    • Increased capillary hydrostatic pressure (as occurs when venous pressures become elevated by gravitational forces, in heart failure or with venous obstruction)

      Decreased plasma oncotic pressure (as occurs with hypoproteinemia)

      Increased capillary permeability caused by proinflammatory mediators (e.g., histamine, bradykinin) or by damage to the structural integrity of capillaries so that they become more "leaky" (as occurs in tissue trauma, burns, and severe inflammation)

      Lymphatic obstruction (as occurs in filariasis)

    The treatment for edema involves modulating one or more of the physical factors that regulate fluid movement. For example, in edema (pulmonary or systemic) secondary to heart failure, diuretics are given to reduce blood volume and venous pressure. If a patient suffers from ankle edema, that person will be instructed to keep their feet elevated whenever possible (to diminish the effects of gravity on capillary pressure), use tight fitting elastic hose (to increase tissue hydrostatic pressure), and possibly be prescribed a diuretic.


    The correct answer is B. This question illustrates an important strategy: knowing what you're looking for before you consider the answer choices. If you thought about the answer before considering the choices, this question was very straightforward and simple. If, on the other hand, you considered each answer choice in turn, you no doubt got pretty confused and wasted a lot of precious test time.
    The first thing to remember is that nephrotic syndrome is defined as proteinuria (over 3.5 gm/day) with concurrent hypoalbuminemia and hyperlipidemia. The loss of protein in the urine results in a decreased oncotic pressure in the vascular space (decreased pc). This decrease in capillary oncotic pressure promotes movement of fluid into the interstitium and the development of edema. This is also the cause of edema in patients with liver disease.
    Decreased interstitial oncotic pressure (pi; choice D) would actually promote the movement of fluid into the vasculature; it would not lead to edema. The same thing would occur with decreased capillary hydrostatic forces (Pc; choice A).
    While decreased interstitial hydrostatic pressure (Pi; choice C) would lead to edema, it is not the mechanism of action in nephrotic syndrome.
    While increased capillary hydrostatic pressure (choice E) does lead to edema, it is not the mechanism at work in nephrotic syndrome. It is, however, the mechanism of edema in the setting of congestive heart failure (increased capillary hydrostatic pressure due to inefficient pumping of the heart, leading to pooling) and in glomerulonephritis (increased intravascular volume due to inefficient excretion by the kidney).
    Increased capillary oncotic pressure (choice F) would not lead to edema.
    Increased interstitial hydrostatic pressure (choice G) would not lead to edema.
    Increased interstitial oncotic pressure (choice H) would cause edema, but not in the setting of nephrotic syndrome. Instead, this is the mechanism of edema (typically localized) in the setting of burns and inflammation (increased capillary permeability allows protein to leak into interstitium and increase oncotic pressure).

    ref: oucom.ohiou.edu, kaplan

  2. #2
    Anonymous is offline Unregistered Guest
    Join Date
    Jan 2003

    edema flashcards

    some additional stuff on this that you might find useful:

    What is edema?
    Soft tissue swelling due to expansion of interstitial volume=ECF only!!! (not cells)
    True or False, edema can be localized or generalized?
    True or False, generalized edema is due to retention of water only?
    False, if only water were retained, it would disperse among the other 60% (both ECF and ICF). When Na is retained, water stays in the ECF better (tries to reduce osmolality)
    True or False, generalized edema can occur with low, normal, or high serum Na?
    True, it really depends on the osmolality of the serum, not just Na [ ]
    How is the volume of interstitial fluid maintained?
    Net fluid movement out of capillary. (described by Starling's law)=10mmHg
    How does Starling's law describe the state of the volume of interstitial fluid?
    Net fluid movement=(net hydrostatic pressure)-(net oncotic pressure)
    What are 5 causes of generalized edema?
    1)increased hydrostatic pressure, 2)reduced oncotic pressure (1-less proteins made, 2-increased vascular permeability), 3)lymph flow obstructed, 4)inappropriate retention of Na and water
    What 2 things can cause hydrostatic pressure to increase?
    1)CHF (congestion back thru venous system), 2)cirrhosis
    What 3 things can cause reduced oncotic pressure?
    1)nephrotic syndrome, 2)cirrhosis, 3)malnutrition
    What can cause lymph flow obstruction?
    What can cause in increase in permeability to protein?
    Inflammation, angioneurotic edema
    What 2 things can cause inappropriate retention of Na and water?
    1)renal failure (not as much is filtered), 2)nephrotic syndrome
    True or False, almost all clinical cases of edema are due to body holding on to too much water and Na?
    What is effective arterial blood volume (effective circulating volume)?
    Adequate perfusion pressure. Normal EABV=cardiac output and venous return are normal
    How is EABV reduced?
    1)blood loss (bleeding, dehydration), 2)vasodilation (cirrhosis, sepsis), 3)reduced cardiac output (heart failure)
    True or False, reduced EABV can be in the setting of low, normal, or high actual blood volume?
    True, EABV is not merely a measure of volume, but volume needed to meet tissue demands
    How does the kidney respond to a drop in EABV?
    It tries to increase perfusion pressure by retaining Na and water
    How does the kidney retain Na and water when perfusion pressure drops?
    Renin is released. Ang II causes efferent arteriole vasoconstriction which causes more fluid to be filtered and its oncotic pressure to rise. Fluid in the proximal tubule is massively reabsorbed to reduce the peritubular capillary osmotic presssure. Aldosterone and ADH also help in the distal tubule
    What happens to blood flow in CHF?
    Heart doesn't pump well so vessels vasoconstrict to try to increase pressure. Most of blood is diverted to brain, heart and lungs. The kidneys see a drop in perfusion pressure and do what I just described
    What is a real danger of severe CHF regarding water/ion balance?
    Hyponatremia. If CHF is severe, body tries so hard to conserve water that urine is concentrated and Na can be lost
    How can liver cirrhosis lead to decreased EABV?
    1)portal hypertension-fibrosis blocks blood flow (increased hydrostatic pressure), 2)hypoalbuminemia (decreased osmotic pressure), 3)SPLANCHNIC VASODILATION=MAJOR FACTOR (perceived drop in perfusion pressure)
    How does cirrhosis lead to splanchnic (mesenteric) vasodilation?
    It either doesn't degrade or overproduce vasodilating factors=vasodilation=decreased perfusion pressure
    How does the body respond to the effects of cirrhosis?
    The same way as in CHF. It does whatever possible to hold on to H20 and Na (1)efferent vasoconstriction, 2)renin release, 3)aldosterone release, 3)ADH)
    What is one serious side effect (besides edema) of portal hypertension?
    It can cause ascites (really just a really severe edema of the peritoneum
    What is the nephrotic syndrome?
    Losing protein in the urine (>/= 3.5g/day) resulting in hypoproteinemia. Edema and hyperlipidemia are also present
    What are 2 mechanisms for edema in nephrotic syndrome?
    1)Na and H2O retention=MAJOR CAUSE (unknown why), 2)hypoproteinemia results in reduced oncotic pressure (H2O goes to interstitium)
    What is one plausible explanation as to why Na and H2O are retained in nephrotic syndrome?
    Hypoproteinemia-->reduced oncotic pressure-->edema (H2O leaves vessels). If enough water leaves ECV will drop (perfusion pressure) and the kidney tries to hold onto Na and water
    What are reasons for treating edema?
    1)painful, 2)impairs circulation/venous return, 3)impairs wound healing, 4)increases infection, 5)unattractive, 6)pul edema reduces gas exchange, 7)MEANS SOMETHING ELSE IS WRONG
    What is the FRACTIONAL EXCRETION OF NA (FENa) test?
    A test to see if the body is holding onto water (CHF, renal failure, cirrhosis, nephrotic syndrome, etc.)
    What is the formula for determining FENa?
    (Urine [Na]/Plasma [Na])/(Urine [creatinine]/Plasma [creatinine])
    What do values of FENa tell you?
    <1 means the body is retaining Na, >1 signifies a volume expanded state (too much Na/water intake)
    What are the 4 steps to treating edema?
    1)treat underlying cause, 2)reduce Na/water intake, 3)increase Na/water excretion (diuretics), 4)don't make it worse (diuretics may cause volume depletion=many other problems)
    True or False, reducing Na intake will reduce edema?
    False, it really just holds it in check so it doesn't get worse
    True or False, IV's can cause edema in hospital patients with problems excreting Na?
    True. the least osmotic solution will still give ~19mEq/day. A person with problems clearing Na may only clear 15mEq=EDEMA
    How do diuretics work?
    They block Na and water reabsorption

    ref: umed.med.utah.edu

  3. #3
    Anonymous is offline Unregistered Guest
    Join Date
    Jan 2003


    bumping for study group

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