Chapter 5

Water, Electrolyte and acid-Base Balance

 

 

 

                      

 5.1 REQUIRED READING ASSIGNMENT:

Veterinary Clinical Pathology - Coles

5.2 objectives and/or study questions:

Students should be answer the following questions at the completion of this unit.

Questions

  • 1. What is the distribution of fluid in an animal's body? What are the 2 main sources of body fluid?
  • 2. What is dehydration and how can it be detected clinically? What is the significance of PCV, Total protein, and BUN in the dehydrated animal?
  • 3. What are the main electrolytes (cations and ions) in the ECF? ICF? What is the relationship between these ions and the tonicity of the body fluid? What is hyper-and hyponatremia? What is hyper- and hypokalemia?
  • 4. What is a possible sequelae to severe hyperkalemia?
  • 5. What is the most important buffer system in the body fluid? How does this system interact with the respiratory system? The kidney?
  • 6. What are the 4 main acid-base disturbances? What are some causes of each of these disturbances? What ion disturbances are seen in each? Characterize each of these disturbances according to pH, pCO2 and HCO3 findings (i.e., increased, decreased, normal) in both uncompensated and partially compensated forms.
  • 7. What the mechanisms (e.g., ion regulation) that the kidney uses to compensate for acidosis? alkalosis? How does the respiratory system compensate for these conditions?
  • 8. What the the nine tests which can be used to evaluate the fluid, acid-base and electrolyte status of an animal? What anticoagulant must be used for this panel?
  • 9. What are 4 basic questions that need to be answered by the clinical evaluation of a patient before the administration of fluid therapy?
  • 10. What does measurement of "total CO2" assess?
  • 11. What % body weight is a fluid deficit representative of slight dehydration? Severe dehydration?
  • 12. What is the calculation for determining the amount of fluid needed for replacement therapy in an animal?
  • 13. What are the calculations for determining the amount of electrolyte (e.g., HCO3) needed in replacement therapy?

 

5.3 Introduction: 

A knowledge of the normal state is essential for formulation of a logical and accurate plan of therapy for the patient with abnormalities in water, electrolyte and/or acid-base balance. This fundamental knowledge coupled with an understanding of the basic compensatory mechanisms utilized by the body to correct these alterations is the basis for effective fluid therapy. The goal of this fluid therapy is to re-establish and/or maintain in the body certain basic conditions which favor normal cellular metabolism. These basic conditions are not those required for normal functioning of one biochemical reaction, or one body organ, but are basic conditions required for all biochemical processes and all organs if normal function is to be restored. Specifically, the goals of this kind of fluid therapy are to insure that normal conditions prevail with respect to body water, tonicity of body fluids, specific body electrolytes and acid-base status.

5.4 GENERAL INFORMATION

5.4.1 Body Water (Body Fluid)

5.4.2 Electrolytes

Substances that become ionized when placed in water

TABLE V.1

REPRESENTATIVE ELECTROLYTE CONCENTRATIONS

IN THE BODY FLUID COMPARTMENTS

(mEg/L)

Electrolytes

Intracellular Fluid

Extracellular Fluid

Interstitial

ntravascular

Cations

-

-

-

-

Sodium

15

147

142

-

Potassium

155

4

5

5

Calcium

2

2.5

-

-

Magnesium

27

1

2

-

Anions

-

-

-

-

Bicarbonate

10

30

27

-

Chloride

1

114

103

-

Phosphate

100

2

2

-

Sulfate

20

1

1

-

Organic acids

1

7.5

-

5

Protein

62

0

16

-

 
  • b. The total concentration of cations always equals that of the anions in all body fluids.

5.4.3 Distribution of fluid among the two body fluid compartments

 

5.5 ACID-BASE REGULATION IN THE BODY FLUIDS

Normal metabolic processes in an animal body result in the production of relatively large quantities of acids. These acids are transported to the excretory organs, i.e., the lungs and the kidneys, without causing marked alterations in blood pH. This sensitive control of blood pH in the normal range of 7.3 to 7.5 is accomplished by the combined effects of the blood buffer system, the respiratory system, and the renal system. The body responds quickly to alterations in blood pH. Correction in these alterations occurs in steps, with buffer systems providing the immediate response to any pH alteration, followed quickly by the respiratory response. Later, the kidney mechanism is initiated and sustains the corrective activity for a longer time span.

A buffer is a mixture of a weakly dissociated acid and a salt of that acid. The blood buffers that play a role in control of blood pH are:

The bicarbonate/carbonic acid buffer system is the single most important buffer system in the body fluids and our discussion will be restricted to this system.

5.5.1 The Bicarbonate/Carbonic Acid Buffer System

The equation for the bicarbonate/carbonic system is:

 

pH = 6.1 + log [bicarbonate]

[carbonic acid]

 

Therefore, the pH of plasma is dependent upon the ratio of HCO3- to H2CO3. As can be seen in Figure V.1, the normal ratio between these substances is 20:1. when one changes without a comparable change in the other, the pH becomes abnormal.

Figure V.1

The Biocarbonate/Carbonic Acid Buffer system

 

 CO2 which is in turn controlled by ventilation. Abnormalities in carbonic acid or carbon dioxide, therefore, always result from abnormalities in ventilation.

  • a. When ventilation is decreased, carbon dioxide accumulates, carbonic acid is increased, and the condition is called respiratory acidosis (seen in pneumonia, pulmonary edema, etc.)
  • b. When hyperventilation occurs, carbon dioxide is reduced, carbonic acid is decreased below normal and the condition is respiratory alkalosis.

 

TABLE V.2

LABORATORY FINDINGS IN CLASSI UNCOMPENSATED ACID-BASE IMBALANCES(ACUTE) 
 

 * BE = Base excess

N = Normal


5.5.2 Respiratory Control of Acid-Base Balance

5.5.3 Renal Control of Acid-Base Balance

NOTE: In compensated acidosis or alkalosis, absolute concentrations of bicarbonate ions and carbonic acid may be changed, but as long as the ratio remains in the range of approximately 20:1, the pH may be in the normal range.

5.6 ACID-BASE DISTURBANCES

5.6.1 Respiratory Acidosis - carbonic acid excess (hypoventi-lation)

1. Causes

2. Clinical Signs

3. Laboratory findings

4. Pathogenesis

 
 

Partial Compensation 

 

Complete Compensation 

 

5.6.2 Respiratory Alkalosis - carbonic acid deficit (hyperventilation)

1. Causes - increased rate and depth of breathing

2. Clinical signs

3. Laboratory Findings

4. Pathogenesis

 
 

b. Respiratory alkalosis - carbonic acid deficit due to hyperactive breathing which results in an increased loss of carbon dioxide from the lungs.

 

 c. Body compensatory action

1) The compensatory mechanisms are principally renal. Renal control is manifested by a decrease in ammonia formation, a decrease in bicarbonate reabsorption, retention of hydrogen ions (exchanged for sodium) and an increase in excretion of bicarbonate instead of chloride.

  
 

5.6.3 Metabolic (Nonrespiratory) Acidosis - bicarbonate deficit

1. Causes

2. Clinical Signs

3. Laboratory Findings

4. Pathogenesis

a. Normal balance

  
 

 b. Metabolic acidosis - ketone and/or excess chloride ions replace bicarbonate ions.

 

    
 

 c. Body compensatory action

     
 

 5.6.4 Metabolic Alkalosis - bicarbonate excess

1. Causes

Accumulation of bicarbonate in extracellular fluid as a result of excessive acid loss.

2. Clinical signs

3. Laboratory Findings

4. Pathogenesis

a. Normal balance 

      
 

 b. Metabolic alkalosis - bicarbonate ion increased due to loss of chloride ion or to excess ingestion of bicarbonate.

 

       
 

 c. Body compensatory action

 
 

 

5.7 EVALUATION OF CLINICAL PATIENTS

5.7.1 Some basic questions to be answered.

5.7.2 Patient evaluation without laboratory tests.

1. The Clinical Examination

2. The shortcomings of patient evaluation without laboratory tests.

3. More objective evaluation of the patient is indicated when:

4. Patient Therapy

When patient needs are poorly defined, therapy must be general and conservative. When patient needs are well defined, therapy can be more specific, more radical and more successful.

5.7.3 Patient evaluation by laboratory tests.

5.8 FLUID THERAPY PRODUCTS

A. General

B.Common Fluids

Common fluids can be "spiked" with additional quantities of electrolytes especially needed, e.g., potassium or bicarbonate.

C. Increase of Bicarbonate

C.Since bicarbonate cannot be autoclaved, most solutions do not contain this substance. Instead, they contain lactate or acetate ions which can be metabolized by the body. When this happens, endogenous bicarbonate replaces the acetate or lactate given. In this way, stable sterile solutions are available which, indirectly, make it possible to increase the bicarbonate in the body.

5.9 SELECTION AND USE OF FLUIDS AND ELECTROLYTES

5.9.1 Estimating the quantity of fluid required.

a.Example of severe dehydration:

500 kg horse with a fluid deficit of 10% body weight = 10% x 500 kg = 50 kg (50 liters) of fluid needed.

b.Example of slight dehydration:

500 kg horse with a fluid deficit of 6% body weight = 6% x 500 kg = 30 kg (30 liters) of fluid needed.

5.9.2 The kind of fluid to give.

Patient HCO3- = 12 mEq/L

Normal HCO3- = 24 mEq/L

HCO3- deficit = 12 mEq/L

ECF est. - 0.3 x 450 kg = 135 L

HCO3- needed = 12 mEq/L x 135 L = 1620 mEq.

  • c)Patient needs are estimated as 40-50 liters of fluid and 1500-2000 mEq of bicarbonate.
  • d)Treatment Method #1: 3 liters of 5% NaHCO3 (1800 mEq) + 37 liters of balanced electrolyte solution.
  • e)Treatment Method #2: 40 liters of balanced electrolyte solution supplemented with 3-4 grams of NaHCO3/L = 1500-2000 mEq NaHCO3.

5.9.3 The route for fluid administration

5.9.4 Miscellaneous Topics

5.9.5 Summary