HYPOVOLEMIA

A. Causes

Volume depletion generally results from a deficit in total body Na+ content. This may result from renal or extrarenal losses of Na + from the ECF. Water losses alone can also cause volume depletion, but the quantity required to do so is large, as water is lost mainly from the ICF and not the ECF, where volume contraction can be assessed.

Renal losses may be secondary to enhanced diuresis, salt-wasting nephropathies, mineralocorticoid deficiency, or the resolution of obstructive renal disease.

Extrarenal losses include fluid loss from the GI tract (vomiting, nasogastric suction, fistula drainage, diarrhea), respiratory losses, skin losses (especially with burns), hemorrhage, and severe third spacing of fluid in critical illness.

1. GI losses due to vomiting, nasogastric suction, diarrhea, fistula drainage, etc.2. Third-spacing due to ascites, effusions, bowel obstruction, crush injuries, burns. 3. Inadequate intake4. Polyuria for example, DKA. 5. Sepsis, intra-abdominal and retroperitoneal inflammatory processes.6. Trauma, open wounds, sequestration of fluid into soft tissue injuries.7. Insensible losses evaporatory losses through the skin (75%) and the respiratory tract (25%)

B. Clinical features

SYMPTOMS: include complaints of thirst, fatigue, weakness, muscle cramps, and postural dizziness. Sometimes, syncope and coma can result with severe volume depletion.

SIGNS: low jugular venous pressure, postural hypotension, postural tachycardia, and the absence of axillary sweat. Diminished skin turgor and dry mucous membranes are poor markers of decreased interstitial fluid. Mild degrees of volume depletion are often not clinically detectable, while larger fluid losses can lead to mental status changes, oliguria, and hypovolemic shock.

1. CNS findings: mental status changes, sleepiness, apathy, coma. 2. Cardiovascular findings (due to decrease in plasma volume): orthostatic hypotension, tachycardia, decreased pulse pressure, decreased central venous pressure (CVP), and pulmonary capillary wedge pressure (PCWP).3. Skin: poor skin turgor, hypothermia, pale extremities, dry tongue. 4. Oliguria5. Ileus, weakness6. Acute renal failure due to prerenal azotemia (fractional excretion of sodium 20.)

C. Diagnosis

Laboratory studies are often helpful but must be used in conjunction with the clinical picture.

Urine Sodium is a marker for Na+ avidity in the kidney. -Urine Na+ < 15mEq is consistent with volume depletion, as is a fractional excretion of sodium (FeNa) < 1%. The latter can be calculated as [(Urine Na+ x Serum Cr)/(Urine Cr x Serum Na+)] x 100-Concomitant metabolic alkalosis may increase urine Na+ excretion despite volume depletion due to obligate excretion of Na+ to accompany the bicarbonate anion. In such cases, a urine Chloride of 20:1 suggest hypoperfusion to the kidneys, which usually (not always) represents hypovolemia.

3. Increased hematocrit: 3% increase for each liter of deficit.

4. The concentration of formed elements in the flood (RBC, WBCs, platelets, plasma proteins) increases with an ECF deficits and decreases with an ECF excess.

D. Treatment

It is often difficult to estimate to volume deficit present, and therapy is thus largely empiric, requiring frequent reassessments of volume status while resuscitation is under way.

Mild volume contraction can usually be corrected via the oral route. However, the presence of hemodynamic instability, symptomatic fluid loss, or intolerance to oral administration requires IV therapy.

The primary therapeutic goal is to protect hemodynamic stability and replenish intravascular volume with fluid that will preferentially expand the ECF compartment. This can be accomplished with Na+ - based solutions, since the Na+ will be retained in the ECF.

-Isotonic fluid, such as NS (.9% NaCl), contains a Na+ content similar to that of plasma fluid in the ECF and thus remains entirely in the ECF space. It is the initial fluid of choice for replenishing intravascular volume.

The administration of solute-free water is largely ineffective, since the majority of water will distribute to the ICF space. Half-normal saline (.45% NS) has 77 mEq of Na+ per liter, roughly half the Na+ content of an equal volume in the ECF. Thus, half of this solution will stay in the ECF, and half will follow the predicted distribution of water.

-Fluids can be administered as a bolus or at a steady maintenance rate. In patients with symptomatic volume depletion, a 1 to 2-L bolus is often preferable to acutely expand the intravascular space. This should be followed by a careful reassessment of the patients volume status. The bolus can be repeated if necessary, although close attention should be directed toward possible signs of volume overload. Smaller boluses should be used for patient with poor cardiac reserve or significant edema. Once the patient is stable, fluids can be administered at a maintenance rate to replace ongoing losses. In patients with hemorrhage or GI bleeding, blood transfusion can accomplish both volume expansion and concomitant correction of anemia.

1. Correct volume deficitA. Use bolus to achieve euvolemia. Begin with isotonic solution (lactated Ringers or NS).B. Again, frequent monitoring of HR, BP, urine output, and weight is essential. C. Maintain Urine output at .5 to 1 mL/kg/hourD. Blood loss replace blood loss with crystalloid at a 3:1 ratio

2. Maintenance fluidA. D51/ 2NS solution with 20mEq KCL/L is the most common adult maintenance fluid. (Dextrose is added to inhibit muscle breakdown).B. There are two methods of calculating the amount of maintenance fluid.

100 /50 / 20 Rule:

1. 100ml/kg for first 10kg, 50ml/kg for next 10kg, 20ml/kg every 1 kg over 20.2. Divide total by 24 for hourly rate

Example: 70kg man100 X 10 = 100050 X 10 = 50020 X 50 =1000Total = 2500Divide by 24 hrs 104 ml/hr

4 / 2 / 1 Rule:

4ml/kg for first 10kg, 2ml/kg for next 10kg, 1ml/kg for every 1kg over 20. For example, for a 70kg man: 4 x 10 = 40 2 x 10 = 201 x 50 = 50Total = 110 ml/hr