A short fun reveiw of fluid and electroltyes
Great discusion on ABGs
Introduction
Two important parts of the body are the fluid and electrolyte components. The
two important body fluids are intracellular fluid (ICF), and extracellular fluid
(ECF). Plasma, and interstitial fluid are the two types of extracellular fluid.
Some important electrolytes in the body are sodium ion, potassium ion,
calicum ion, magnesium ion, chloride ion, bicarbonate ion, phosphate ions,
sulfate ion, organic acids and proteins. These electrolytes are dissolved in the
body fluid. They serve various important functions in the body. For instance,
sodium and potassium ions are used for the creation of action potentials.
The big deal with body fluid and electrolytes is homeostasis. It is important
that electrolyte concentration be maintained at certain levels. This is were the
function of the kidney becomes so important. If electrolyte balance is not
maintained there can serious effects.
Ways to maintain osmolarity are antidiurectic hormone, thirst,
aldosterone, and the
sympathetic nervous system.
Water had several functions in the body including temperature regualtion, as a protective cushion, lubricant, reactant, solvent and transporter.
There are three key concepts in consideration of fluid and electrolyte
management: cell membrane permeability, osmolarity, and electroneutrality. Cell
membrane permeability refers to the ability of a cell membrane to allow certain
substances such as water and urea to pass freely, while charged ions such as
sodium cannot cross the membrane and are trapped on one side of it. Osmolarity
is a property of particles in solution. If a substance can dissociate in
solution, it may contribute more than one equivalent to the osmolarity of the
solution. For instance, NaCl will dissociate into two osmotically active ions:
Na and Cl. One millimolar NaCl yields a 2 milliosmolar solution. Finally, the
principle of electroneutrality means that the overall number of positive and
negative charges balances. For instance, in conditions like renal tubular
acidosis where HCO3- is lost, chloride is retained leading
to a hyperchloremic state.
The expected osmolarity of plasma can be calculated according to the
following formula. As is evident, the concentration of sodium is the major
determinant:
| Osmolarity (mOsm/kg) = 2*[mEq/L Na+] + (mg/dL glucose)/18 + (mg/dL BUN)/2.8 |
Normal serum osmolarity ranges from about 280 to 295 mOsm/kg. This equation is
valid only when other significant particles are not present (for instance,
alcohol, ethylene glycol, mannitol).
A number of fluid and electrolytes disturbances are reviewed here. The most common disturbance is dehydration which is divided into subclasses on the next page. Other disturbances include altered metabolic states such as diabetic ketoacidosis and diabetes insipidus, SIADH, and renal tubular acidosis. All of these conditions require fluid management. In working up a patient with suspected fluid/electrolyte problems, the following tests may be helpful:
| Electrolytes | This provides information about serum Na+, K+, Cl-, HCO3- |
| BUN and Cr | These measures provide an indication of renal perfusion. An elevated BUN generally reflects intravascular depletion. Creatinine is a useful indicator of acute renal failure. |
| CBC | The CBC may provide some indication of hemoconcentration in cases of dehydration. The WBCs and differential cell count are useful indicators of infection. Platelets can elevate as acute phase reactants. |
| UA | The specific gravity of the urine is related to the patient's hydration state. In cases of renal disease, it can help classify the condition. Urine ions can be specifically requested, and are helpful in determining whether sodium is being retained or not. |
| Serum/Urine Osmolarity |
A true measure of serum osmolarity can be compared to the calculated osmolarity. Normally, true osmolarity is about 10 mEq/L higher than calculated due to the presence of particles which are not in the basic osmolarity equation. If there is a greater "osmolar gap" than this, the presence of additional particles should be considered (such as alcohol or mannitol). The osmolarity of serum determines whether a patient is in an isotonic state or if this state has been disturbed. Urine osmolarity is helpful in determining if the kidney is doing its job of concentrating urine. |
| Total Protein | Total protein, and sometimes albumin levels, are indirect measures of both liver function (where they are produced), dietary protein intake, and renal loss. If serum protein levels fall, the intravascular oncotic pressure falls and fluid migrates to "third spaces". This can be seen in liver disease, nephrotic syndromes, malnutrition and other cases. |
| Arterial Blood Gas |
In addition to providing information about the patient's blood gases and assisting in classification of acidosis or alkylosis, the ABG yields information about bicarbonate levels. Usually, STAT electrolytes can also be obtained from a blood gas sample, with turn around time better than serum chemistry. |