Notez On Nursing....: Some Fluid/Electrolyte Balance Info........

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Compartments of
Body and Distribution of Water by Weight

Plasma 5%
Interstitial 15%

Intracellular 40%

Total 60 % Water

Solids - 40%
fat, protein, carbohydrates,
minerals

Fluid and Electrolyte Balance

The main fluid in the body is water. Total body water is 60% of body weight. The water is distributed in three main compartments separated from each other by cell membranes. The intracellular compartment is the area within the cell. The extracellular compartment consists of the interstitial area (between and around cells) and the inside of the blood vessels (plasma).

ELECTROLYTE DISTRIBUTION
Electrolyte	Extracellular meq/liter	Intracellular meq/liter	Function
Sodium	142	10	fluid balance, osmotic pressure
Potassium	5	100	Neuromuscular excitability acid-base balance
Calcium	5	-	bones, blood clotting
Magnesium	2	123	enzymes
Total Positive ions	154	205

Electrolytes are the chemicals dissolved in the body fluid. The distribution has important consequences for the ultimate balance of fluids.
Sodium chloride is found mostly in extracellular fluid, while potassium and phosphate are the main ions in the intracellular fluid.

Electrolyte Distribution
Electrolyte	Extracellular meq/liter	Intracellular meq/liter	Function
Chloride	105	2	fluid balance, osmotic pressure
Bicarbonate	24	8	acid-base balance
Proteins	16	55	osmotic pressure
Phosphate	2	149	energy storage
Sulfate	1	-	protein metabolism
Total Negative ions	154	205

Introduction

Acid-base balance is critical for maintaining the narrow pH range that is required for various enzyme systems to function optimally in the body.⁴ Normal blood pH ranges from 7.3-7.4.³ Decreased pH is termed acidemia and is caused by an increase in the concentration of hydrogen ions ([H⁺]). Increased blood pH is termed alkalemia and is caused by a decrease in the [H⁺].

The buffer systems that maintain this pH balance are bicarbonate, phosphates, and proteins.⁴ Bicarbonate is the most important extracellular buffer, while phosphates and proteins contribute mostly to intracellular acid-base balance.² The bicarbonate system is the only buffer measured for the calculation of acid-base status in patients and is represented by the equilibrium equation: CO₂ + H₂O <—> H₂CO₃ <—> H⁺ + HCO₃^-. This equation allows one to visualize what effects the addition of carbon dioxide (CO₂) or bicarbonate (HCO₃^-) will have on the buffer system and the blood pH. Addition of CO₂ to the system will cause the equation to shift to the right, increasing the [H⁺] and, therefore, lowering the pH. Addition of HCO₃^- to the system will cause the equation to shift to the left, lowering the [H⁺] and increasing the pH. Another way to conceptualize this information is to simply think of CO₂ as an acid and HCO₃^- as a base. If CO₂ is increased it will tend to cause acidemia. If HCO₃^- is increased, then alkalemia is the expected result.

In addition to buffers, the lungs and kidneys play a major role in acid-base homeostasis.¹ The lungs function in ventilation and they are responsible for regulating the amount of CO₂ present in plasma. The kidneys are responsible for controlling the amount of HCO₃^- in the blood by resorbing or excreting it in the proximal tubule.¹ Abnormalities in acid-base status are classified as to whether the primary abnormality lies with the CO₂ concentration or the HCO₃^- concentration ([HCO₃^-]). If CO₂ is primarily affected, then a respiratory disturbance is present. If HCO₃^- is primarily affected, then a metabolic disturbance is present.

Simple Acid-Base Disorders

Simple acid-base disorders are those that are confined to one primary alteration in CO₂ or HCO₃^- with or without a compensatory response. There are four simple acid-base disorders: respiratory acidosis, metabolic acidosis, respiratory alkalosis and metabolic alkalosis. The primary alterations will be discussed in this section, compensatory responses will be discussed later.

Acidosis is a physiologic condition that tends to increase the concentration of hydrogen ions, which will decrease the pH.⁴ This condition can be respiratory or metabolic in origin. An increase in the concentration of CO₂, expressed as pCO₂, is known as respiratory acidosis. Alternatively, a decrease in the [HCO₃^-] is known as metabolic acidosis. Both of these situations cause the buffer equation to shift to the right, causing an increased [H⁺] and a decreased pH. Alkalosis is a physiologic condition that tends to decrease the [H⁺], which will increase the pH.⁴ Like acidosis, alkalosis can be respiratory or metabolic in origin. A primary condition that results in decreased pCO₂ is termed respiratory alkalosis and a primary condition that results in increased [HCO₃^-] is termed metabolic alkalosis. Both situations shift the bicarbonate equation to the left, resulting in a decreased [H⁺] and an increased pH.

Metabolic Alkalosis

Decreased

Increased

Increased [HCO₃^-]

An important distinction needs to be made at this point with respect to certain definitions in acid-base disorders. Acidemia and alkalemia are alterations in the pH of the blood. Acidosis and alkalosis (respiratory or metabolic) are the disorders that will cause these pH alterations. Most cases of simple acidosis or alkalosis, such as those described above, will result in acidemia or alkalemia, respectively. However, there are compensatory mechanisms that the body will employ in an attempt to maintain a normal pH. In these situations it is possible to have an acidosis or alkalosis disorder and maintain a normal blood pH. There are also mixed acid-base disorders in which opposing primary disorders can effectively cancel each other out and produce a normal pH balance.

Etiology

Respiratory acidosis is caused by any condition which increases the pCO₂ (hypercapnia).² While increased production of CO₂ (hyperthermia, cardiopulmonary arrest) is a possible cause of hypercapnia, the vast majority of cases are due to impaired removal of CO₂ through the lungs. Hypoventilation, ventilation-perfusion mismatch and impaired alveolar gas exchange can all lead to hypercapnia. Therefore, the broad categories of disease which can lead to respiratory acidosis include: respiratory center depression, neuromuscular disease, restrictive extrapulmonary disease, intrinsic pulmonary and small airway disease, large airway obstruction, and increased CO₂ production with impaired alveolar ventilation.²

Respiratory alkalosis is caused by conditions that will decrease the pCO₂ (hypocapnia).² Hyperventilation will lead to hypocapnia, and it can be caused by hypoxemia, pulmonary disease, direct activation of the respiratory center in the brainstem, overzealous mechanical ventilation, or situations causing pain, fear, or anxiety.²

Causes of metabolic alkalosis include loss of acidic chloride-rich fluids from the body and chronic administration of alkali. In small animal practice, most cases of metabolic alkalosis are caused by vomiting of stomach contents.² Abomasal reflux of hydrochloric acid (HCl) into the rumen will cause metabolic alkalosis in ruminants.³
There are two types of metabolic acidosis. Both are characterized by a decrease in the [HCO₃^-] but they differ in how that decrease occurs. Secretional metabolic acidosis is caused by a direct loss of bicarbonate-rich fluid such as diarrhea or saliva. Titrational metabolic acidosis is caused by the presence of non-CO₂ acids that titrate bicarbonate causing a decreased [HCO₃^-].

Titration-type metabolic acidosis is the result of increased endogenous or exogenous acids in the plasma.⁴ Exogenous acids include ethylene glycol metabolites and salicylate. Endogenous acids include lactic acid, uremic acids, and ketones. It follows then that shock, renal failure, and diabetic ketoacidosis, respectively are common causes of titration-type metabolic acidosis.⁴ In cases of hypovolemic shock, perfusion is decreased. Anaerobic metabolism is a consequence of decreased perfusion, so lactic acid accumulates. In cases of renal failure, uremic acids will accumulate since the kidneys can not effectively excrete them. In cases of undiagnosed or uncontrolled diabetes mellitus, cells cannot utilize glucose due to a lack of insulin. Therefore, the body must metabolize its adipose tissue. Ketones are the byproduct of oxidation of free fatty acids by the liver.²

Secretional and titrational metabolic acidosis can be differentiated by their effects on the anion gap (AG). The anion gap is a calculated value based on the principle of electroneutrality which states that the total anions in the body must always be equal to the total cations. We regularly measure the most significant ions: Na⁺, K⁺, Cl^- and HCO₃^-. The ions we do not regularly measure are referred to as unmeasured ions. There are unmeasured cations (Ca⁺², Mg⁺², and gammaglobulins) and unmeasured anions (albumin, phosphates, sulfates, and organic acids). The unmeasured anions outnumber the unmeasured cations and the difference is the anion gap (Figure 1). The anion gap is easily calculated from the ions we do measure: AG = (Na⁺ + K⁺) – (Cl^- + HCO₃^-).⁴ Unmeasured cations do not undergo significant changes in health or disease and so changes in the anion gap are almost always associated with changes in the unmeasured anions.

With secretion-type metabolic acidosis, the anion gap is normal.⁴ The body compensates for the increased loss of bicarbonate by a retaining Cl-. As such, as the [HCO₃^-] decreases, the [Cl^-] increases and the anion gap remains normal (Figure 2). With titrational metabolic acidosis, the anion gap is increased. Remember that titration type metabolic acidosis is associated with increased levels of exogenous or endogenous acids. Since HCO₃^- is consumed to buffer these organic acids and there is no effect on the [Cl-], the anion gap increases (Figure 3). The anion gap is therefore very useful in determining a possible etiology for metabolic acidosis that can be confirmed based on history and clinical signs.

With secretion-type metabolic acidosis, the anion gap is normal.⁴ The body compensates for the increased loss of bicarbonate by a retaining Cl-. As such, as the [HCO₃^-] decreases, the [Cl^-] increases and the anion gap remains normal (Figure 2). With titrational metabolic acidosis, the anion gap is increased. Remember that titration type metabolic acidosis is associated with increased levels of exogenous or endogenous acids. Since HCO₃^- is consumed to buffer these organic acids and there is no effect on the [Cl-], the anion gap increases (Figure 3). The anion gap is therefore very useful in determining a possible etiology for metabolic acidosis that can be confirmed based on history and clinical signs.

Compensation

Compensation is the process whereby the body attempts to restore the normal blood pH during an acid-base disorder. Overcompensation does not occur. As with the primary disorders, compensation can be metabolic or respiratory in origin.

Respiratory compensation is a rapid process in which ventilation is adjusted to alter the pCO₂ in response to a primary alteration in the [HCO₃^-]. Increased HCO₃^- levels will stimulate hypoventilation and a subsequent rise in pCO₂. Decreased HCO₃^- levels will produce the opposite effect. Respiratory compensation begins within seconds and can reach maximum effectiveness within 12-24 hours.² Respiratory compensation can never completely regain a normal blood pH. Other factors involved in ventilation control, especially the need for oxygen, will not allow for full compensation.⁷

Metabolic compensation is a slower process that occurs in two phases. Intracellular non-bicarbonate buffers are responsible for the first phase, which occurs immediately. The kidneys are responsible for the second phase, which begins within hours, but takes 2 to 5 days to reach maximal effectiveness. The kidneys achieve compensation by altering net bicarbonate reabsoprtion and net acid excretion into the urine.² Given enough time (up to 4 weeks) metabolic compensation may be able to return the blood pH to normal in chronic respiratory disorders.²

Diagnosis

The values that need to be examined to determine if there is an acid-base imbalance are: blood pH, pCO₂, [HCO₃^-], and the anion gap. If any of these values are outside of the reference range, an acid-base abnormality is present.⁴ The blood gas analysis reports pH, pCO₂, [HCO₃^-], and pO₂. Blood gas analysis is completed on whole blood collected in heparin, and the sample should be collected anaerobically and processed as soon as possible after collection.⁴ The serum chemistry profile reports the anion gap and total CO₂ (TCO₂). TCO₂, not to be confused with pCO₂, is another measurement of [HCO₃^-] and is often used synonomously.

Most simple acid-base disorders are associated with a change in pH. Once acidemia or alkalemia is detected, the next step is to determine if the primary cause is respiratory or metabolic in nature. This requires analysis of the pCO₂ and HCO₃- (TCO₂) values. Changes in the HCO₃^- value that correspond to the change in pH reflect a primary metabolic disorder while changes in the pCO₂ that correspond to the change in pH reflect a primary respiratory disorder. The next step is to determine if there is secondary compensation by the body to try to correct the primary disturbance. The following algorithm presents this information in the form of a flow chart.

Mixed acid-base disorders are more difficult to diagnose and are discussed in the next section.

Mixed acid-base disorders

A mixed acid-base disorder is one in which two different primary conditions are acting at the same time. Mixed disorders can be a combination of metabolic and respiratory disorders or a combination of different metabolic disorders. The separate processes may have either a neutralizing or additive effect on the pH.

First, there are mixed disorders which have a neutralizing effect on pH.² In these cases, the body may appear to be overcompensating because the pH is normal or close to normal.⁴ Since the body does not overcompensate, a mixed disorder should be suspected. A classic example of this type of disorder is a vomiting dog who becomes dehydrated. The loss of stomach acid leads to alkalosis while the dehydration and subsequent lactic acid buildup leads to acidosis.

Second, it is possible to have mixed disorders which have an additive effect on pH.² For example, a respiratory acidosis and metabolic acidosis can occur concurrently in a dog with thoracic trauma that also has lactic acidosis due to shock. In this case the pH would be dangerously low. Mixed disorders that have an additive effective on the pH will always have an abnormal pH.²

Acidosis will cause more potassium ions to be moved extracellularly in exchange for hydrogen ions. Hyperkalemia may result.

Clinical Implications

Many of the clinical signs observed in animals with acid-base disturbances are the result of the primary disease process but there are also clinical signs that can develop as a result of the acid-base disturbance itself. Most notably, changes in neural function, cardiac output and concentrations of calcium and potassium may all occur as a direct result of acid-base abnormalities.
Severe acidosis will impair the ability of the brain to regulate its volume, resulting in obtundation and coma. Severe metabolic alkalosis can induce agitation, disorientation, stupor and coma. Cardiac output is also compromised by severe acidosis. Myocardial contractility decreases when the blood pH falls below 7.2 and acidosis may predispose the heart to ventricular arrythmias or ventricular fibrillation.²

Serum ionized calcium concentration can be affected by acid-base abnormalities. Acidosis causes displacement of calcium ions from their binding sites on albumin as the binding sites become protonated and an increase in ionized calcium concentration results. Conversely, alkalosis will cause a decrease in ionized calcium concentration and may lead to muscle twitching.²

The distribution of potassium ions between the intracellular and the extracellular fluids (and therefore the blood [K⁺]) may be affected by acid-base disorders as well. As the blood [H⁺] rises in cases of acidosis, more H⁺ ions are pumped intracellularly in exchange for K⁺ ions that are pumped extracellularly (Figure 4). This exchange of ions is necessary to maintain electrical neutrality. The result is a rise in serum [K⁺]. This situation has only been observed in cases of titration-type metabolic acidosis caused by non-organic (mineral) acids like hydrochloric acid and secretion-type metabolic acidosis.^2,5 Even though hyperkalemia can develop, it is not likely to occur if renal function and urine output are normal. Clinical signs of hyperkalemia can include muscle weakness and cardiac conduction disturbances.²

Just as hyperkalemia can accompany acidosis, hypokalemia can accompany alkalosis.³ Hydrogen ions moves out of the cell to increase the [H⁺] extracellularly, so potassium ions move inside the cell to maintain electrical neutrality (Figure 5). Clinical signs of hypokalemia can include muscle weakness, polyuria, polydipsia, impaired urinary concentrating ability, and cardiac arrhythmias.²

The exchange of potassium and hydrogen ions that can lead to hypokalemia in cases of alkalosis

Treatment

Treatment of respiratory acidosis and respiratory alkalosis is aimed at correcting hypercapnia and hypocapnia, respectively.² Therefore, causes of hypoventilation and hyperventilation need to be investigated in order to determine the underlying cause of hypercapnia and hypocapnia, respectively.

Treatment of metabolic alkalosis is aimed at replacing the chloride deficit in cases where vomiting of stomach contents or diuretic administration is the underlying cause.² However, potassium and sodium deficits are likely to be present also, so these must be addressed simultaneously. Therefore, the treatment of choice for the correction of metabolic alkalosis in dogs and cats is intravenous administration of a sodium chloride (NaCl) solution with added potassium chloride (KCl). If ongoing gastric losses are present, H₂-blocking agents can be useful to decrease gastric acid secretion. In cases where a gastric foreign body is causing vomiting, definitive treatment consists of surgical removal of the foreign body.²

Treatment of metabolic acidosis is dependent upon the underlying etiology. Fluid therapy to correct dehydration and electrolyte disturbances, if present, is warranted. However, specific treatments will vary for ethylene glycol intoxication, lactic acidosis, renal failure, and diabetic ketoacidosis.²

Fluid Exchange Processes

Exchange Processes Between Fluid Compartments

The movement of water and electrolytes between fluid compartments takes place by a variety of processes. Movement of water and electrolytes occurs through membranes and cell walls. The permeability of membranes is controlled by the size of the "pores" or "holes". The size of the pores can be changed in response to pressure and hormones. Some membranes selectively allow passage of certain ions or molecules and exclude all others. A semi-permeable membrane allows only water through it

l. DIFFUSION (Df): Ions and molecules flow from an area of higher concentration to
and area of lower concentration. Dialysis is the specific process of separating diffusible from nondiffusible ions and molecules.

QUES. 15: Hemodialysis is the artificial cleansing of blood when kidneys are not working efficiently. The blood is diverted through a cellophane tube that serves as a dialyzing membrane. On the outside of the cellophane tube is an isotonic solution to all materials that are to be left in the blood. This solution, however, doe not contain waste products. Where do the waste products go by the process of diffusion or dialysis?

2. FILTRATION (FL): Pressure causes water, ions, and molecules to move from an area of higher pressure to an area of lower pressure. The pumping heart causes a blood pressure. The height of a column of water causes hydrostatic pressure.

3. ACTIVE TRANSPORT (AcT): Water and/or ions and molecules are carried by a larger molecule or a vacuole on a cell wall. This requires specific enzymes and energy. This transport is in the manner of a "pump" against a concentration difference.

As you look at the table of Electrolyte Distributions, you may wonder how the potassium and phosphate ions get from the extracellular compartment into the cells. Active transport is needed to get potassium ions into the cells since diffusion will not work as the concentration of potassium is highest in the cells.
Anytime there is a concentration differential where you need to go from a low concentration to a high concentration, then active transport mechanisms are needed. The cells have developed many types of these.

4. OSMOSIS (OS): Water only flows from a dilute solution to a more concentrated solution.
Osmotic pressure is defined as the pressure required to be placed on a solution separated from water by a membrane to prevent osmosis from taking place. Osmosis occurs because there are more molecules of water bombarding the membrane on the pure water side than on the side containing a solution (water molecules plus dissolved molecules).

OSMOTIC PRINCIPLE: Water only flows from a dilute solution to a more concentrated solution. Water flows from a solution of low osmotic pressure (Hypotonic) to a solution of higher osmotic pressure (hypertonic).

If two solutions have identical osmotic pressures, they are isotonic. If one solution has a lower osmotic pressure (lower concentration of salts), it is hypotonic with respect to the other.
In the opposite situation a solution of higher osmotic pressure is hypertonic with respect to the other. In clinical situations the concentrations of solutions used for replacement of body fluids must be carefully controlled. A 0.9% NaCl (normal saline solution) or 5% glucose solutions are isotonic with body fluids.

QUES. 13: If red blood cells are placed in a solution of low osmotic pressure (hypotonic), which direction will water flow in the red blood cells? (Called hemolysis)
QUES. 14: If red blood cells are placed in a l.3% NaCl saline solution (hypertonic), which direction will the water flow in the red cells? (Called crenation)

QUES. 13: If red blood cells are placed in a solution of low osmotic pressure (hypotonic), which direction will water flow in the red blood cells? (Called hemolysis)
QUES. 14: If red blood cells are placed in a l.3% NaCl saline solution (hypertonic), which direction will the water flow in the red cells? (Called crenation)

Wednesday, May 4, 2011

Some Fluid/Electrolyte Balance Info........

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