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Fluid Balance and Shock

Outline
Fluid Balance

A. Fluid Compartments

1. osmolarity of all three fluid compartments is always the same,
         normally 280 -295mosm. (it may be more or less, but will always
         equilibrate until it is the same in all three compartments

    2. each compartment is separated from each other by a
       semipermeable membrane

For example in the diagram above, the cell membrane of the red
         blood cells separates intravascular from intracellular, the capillary
         membrane separates intravascular from interstitial, the cell membrane
         of cells located in the tissue separates intracellular from interstitial

B. Principles (Factors) Governing Movement of Fluid Between Compartments
    1. Osmosis ( definition of osmosis-  movement of water across a
         semipermeable membrane from an area of lesser concentration to an
         area of greater concentration)

a..Role of Na+
                1. volume of ECF depends chiefly upon concentration of this ion
                (about 1/2 of osmotic pressure of ECF due to concentration of
                this ion)

2. if Na+ increases in intravascular then  (because of osmosis) fluid will be pulled from interstitial and intracellular. If this continues the shift of fluid  may cause
                     a. hypervolemia of intravascular compartment
                     b. dehydration of the interstitial and intracellular

              3. if Na+ decreases in intravascular then (because of osmosis)
                      fluid will shift from intravascular into interstitial and
                     intracellular. If this is significant enough it will cause 
                         a. hypovolemia of intravascular compartment
                          b. edema of interstitial and intracellular

             4. the kidneys play an important role in regulating the
                     concentration of  Na+ via the renin angiotensin mechanism (see diagram in HTN leture)

            b.Role of Colloids
                 1. colloids are large proteins
                 2. albumin, synthesized by liver, mostly responsible for
                      maintaining colloidal osmotic pressure in intravascular fluid
                      compartment.

            c. Role of glucose

2. Balance between hydrostatic and colloidal osmotic pressure (cop)
       a. hydrostatic pressure is a mechanical force. It is greatest closest to
             the heart and then drops off as distance from heart increases,
             hence the proximal ends of capillaries have a higher hydrostatic
             pressure then do the distal ends .Hydrostatic pressure works
             within the intravascular compartment to push fluid out into the
             interstitial

        b. as blood enters capillary fluid is pushed out into interstitial tissue
         (because hydrostatic pressure is greater than cop at this point.)
             at the end of the capillary, most of the fluid is pulled back in
             (because hydrostatic pressure has dropped and cop is now
                 greater)

3. Role of Lymph system

4. Altered Capillary Permeability.       

Shock

A.Def. a pathologic syndrome characterized by abnormal cell metabolism that results from inadequate perfusion and oxygenation or cellulardysfunction

B. Cardiogenic
    1. more than 80% of pts who develop cardiogenic shock after an MI
         will die

    2. due to extensive loss of ventricular myocardium (usually 40% or
         more), cardiac output drops.

    3. a vicious cycle ensues - because of decreased C.O. and decreased
         BP get increase in SNS activity causing peripheral vasoconstriction
        (increases afterload) and renin angiotensin system activated causing
         increase in preload. Increased afterload and preload  just further
         aggravate situation

C. Circulatory Shock (actual or relative loss of intravascular volume)
    1. Hypovolemic
        a. signs of shock occur when vascular volume depleted 15 -25%

        b. may be due to hemorrhage or shift of fluid out of intravascular
            fluid compartment (third spacing)

        c. signs and symptoms
            1. mild
                a. minimal tachycardia
                b. slight decrease in BP
                c. mild peripheral vasoconstriction (cool hands and feet)

            2. moderate
                a. HR= 100 -120 bpm
                b. decrease in pulse pressure
                c.systolic pressure = 90 -100 mm Hg
                d. restlessness
                e. sweating, pallor
                f. oliguria

    3. severe
                a. HR > 120 bpm
                b. BP < 60 mm Hg systolic
                c. mental stupor   
                d. anuria
                e. acidosis

    2. Obstructive
        a. inability of heart to fill properly (e.g. cardiac tamponade) or

        b. obstruction to outflow from the heart (e.g. pneumothorax)

3. Distributive (also called normovolemic shock since although
         decreased C.O., not decreased total blood volume as occurs in
          hypovolemic)
        a. Neurogenic
            1. caused by decreased sympathetic control of blood vessel tone
                 due to defect in vasomotor center of brain or sympathetic
                 outflow to blood vessels due to
                    a. brain injury
                    b. drugs
                    c. hypoxia
                    d. hypoglycemia
                    e. general anesthesia

            2. loss of vascular tone causes pooling of blood in arterioles
                 and veins

        b. Anaphylactic
            1. caused by immunological reaction, release of histamine and other
                 vasodilator substances acting systemically
            2. get  pooling of blood
            3. caused by allergic response, e.g. bee sting
            4. accompanied by bronchospasm, contraction of GI and uterine
                 smooth muscle and urticaria or angioedema

        c. Septic Shock
            1. associated with infection and body's systemic response
                 to infection
            2. most often caused   by infection with bacteria
            3. bacteria releases endotoxins which triggers off immune reactions
                    a. activates inflammatory reactions- side effect is
                         degranulation of mast cells, release of histamine, other
                         inflammatory substances, blockade of SNS induced
                         vasoconstriction in early stages- resulting in pooling
                         of blood and altered capillary permeability
                    b. interferes with coagulation

            4. Two stages
                    a. Early ( (also called warm or pink or hyperdynamic)
                        1. total peripheral resistance decreased (because of SNS
                             blockade on peripheral blood vessels) (decreased
                             afterload)
                        2. blood pooling not returned to heart (decreased preload),
                             but heart maintains C.O. by increasing HR and
                             contractility

                    b. Late (also called cool, blue or hypodynamic)
                        1. C.O. drops, now resembles hypovolemic shock

D. Summary of Hemodynamic Pattern in Shock

    Discussion

Edema is an increase in fluid volume, mostly in the interstitial fluid compartment, but it may also occur in the intracellular fluid compartment. It is very important, clinically, to be able to describe edema as thoroughly and as detailed as possible. Therefore, many  adjectives and terms can be used to describe differing conditions. Generalized edema means an expansion of both the interstitial and intracellular fluid compartments. Anasarca is a term used to mean a generalized massive edema involving all parts of the body, including the genitalia, chest wall and arms. Third spacing means a shift of large amounts of fluid from the intravascular to the interstitial. If edema in the interstitial space is caused because the lymphatic vessels, which normally drain the tissues,  are blocked, then you have lymphedema. If the area of edema, after being palpated by an examiner, leaves a pit, then it is called pitting edema and can be scored on a scale of 1 - 4. Sometimes edema, upon palpation, does not produce a pit, then you have non pitting edema. If edema develops when an extremity is in one position but then disappears when placed in another position (for example, the ankle- edema appears when standing, disappears when the leg is elevated), then you have dependent edema. If the fluid in the interstitial space is being pushed out through the pores of the skin when pressed, then you have weeping edema. Edema occurring in the sacral area is called sacral edema, around the eyes it is periorbital edema.  Large amounts of fluid accumulating in the peritoneal cavity (classic example of third spacing) is called ascites.

Clinically  

    Ascites is a frequent complication of liver disease. Three factors that occur in association with a damaged liver cause ascites: 1. the liver is unable to synthesize adequate amounts of albumin ( hence colloidal osmotic pressure drops, and fluid shifts into the interstitium), 2. the liver is congested, so blood returning to the liver is backed up, resulting in increased portal pressure, a type of hydrostatic pressure which pushes fluid out of blood vessels and 3. the liver is unable to metabolize aldosterone adequately, so the levels of aldosterone build up. Working at the kidney level this causes re absorption of sodium, pulling fluid from other tissues back into circulation where it is then shifted out into the peritoneal space.