Veterinary Clinical Medicine-I (General and Systemic)

DEHYDRATION - CONCEPTS AND ASSESSMENT

Dehydration is one of the most commonest general systemic state, encountered in majority of the diosders /diseases and Fluid therapy is the most imporntant therapeutic strategy, not only in day to day veterinary practice, but also as a cornerstone in emergency and critical care medicine practice.

Body Fluid Distribution

• The total body water ranges from 55-70% of the lean body weight. In the average adult dog the total body water is about 60%. Thus in a 15 Kg dog the total body water will equal about 9 liters.  
• Total body water is distributed into 2 main compartments:
  • The intracellular fluid space, and
  • The extracellular fluid space.
• About 66% of the total body water resides in the intracellular fluid space and 33% in the extracellular fluid space.  
• The extracellular fluid space is further subdivided into two fluid containing compartments:
  • The interstitial space (containing 75% of the extracellular fluid space water) and
  • The intravascular space (containing 25% of the extracellular fluid space water).
• When water is added to one compartment, it distributes evenly across the total body water and the amount of volume added to any given compartment, is proportional to its fractional representation of the total body water. Thus, if one liter of free water is placed in the intravascular space, there will be a minimal increase in the intravascular volume after equilibrium takes place. In fact, approximately 30 minutes after rapid volume infusion of free water, only 1/10th of the volume infused remains in the intravascular space. 

FLUID MOVEMENT

• A. Into and out of cells. Determined by concentration gradient in freely diffusible substances (ie: urea)
  • Tonicity dictates water movement in “nonpermeable” substances. (ie: Na/K, proteins)
• B. Between vascular and interstitial spaces
• C. Governed by starling forces
• D. Influenced by integrity of capillary endothelium (inflammation causes increased vascular permeability).
• E. Forces favoring fluid into vessel (reabsorbtion)
  • Tissue hydrostatic pressure
  • Plasma oncotic pressure
• F. Forces favoring fluid out of vessel (filtration)
  • Vascular hydrostatic pressure
  • Tissue oncotic pressure
• G. Net: Filtration at arteriolar end of capillary, reabsorption at venule end (also some fluid goes into lymphatic system).

DETERMINING THE PERCENTAGE OF DEHYDRATION

• The percentage of dehydration can be subjectively estimated based on the presence and degree of loss of body weight, mucous membrane dryness, decreased skin turgor, sunken eyes, and altered mentation. These parameters are largely subjective because they can also be affected by decreased body fat and increased age.

Estimated Percentage Dehydration	Physical Examination Findings
<5	History of fluid loss but no findings on physical examination
5	Dry oral mucous membranes but no panting or pathological tachycardia
7	Mild to moderate decreased skin turgor, dry oral mucous membranes, slight tachycardia, and normal pulse pressure.
10	Moderate to marked degree of decreased skin turgor, dry oral mucous membranes, tachycardia, and decreased pulse pressure.
12	Marked loss of skin turgor, dry oral mucous membranes, and significant signs of shock.

• The more severe stages of dehydration are also accompanied by signs of hypovolemic shock. Other factors, including hemorrhage and third spacing of body fluids, can also result in a decrease in intravascular circulating volume, resulting in signs of hypovolemia.
• Severe hypovolemia resulting in more than a 15% depletion of effective circulating volume leads to a transcompartmental fluid shift from the interstitial to the intravascular compartments, which occurs within one hour of fluid loss.6 When fluid loss is so severe that intravascular fluid volume is affected, hypovolemia can result in tachycardia, prolonged capillary refill time, decreased urine output, and hypotension.  
• The vascular space is sensitive to changes in the amount of circulating volume. During states of normovolemia, baroreceptors in the carotid body and aortic arch sense vascular wall tension and send pulsatile continuous feedback via vagal afferent stimuli to decrease heart rate.
• In the early stages of hypovolemic shock, the baroreceptors sense a decrease in vascular wall stretch or tension and blunt the tonic vagal stimulation. This allows sympathetic tone to increase heart rate and contractility in an attempt to normalize cardiac output.
• Later, decreased blood flow and sodium delivery to receptors in the juxtaglomerular apparatus activate the renin-angiotensin-aldosterone axis, stimulating sodium and fluid retention to replenish intravascular fluid volume.