Awam Medical

1. What is plasma osmolality?

Plasma osmolality is a function of the ratio of body solute to body water; it is regulated by changes in water balance. Water intake is derived primarily from three sources: Ingested water, water contained in food, and water produced from oxidation of carbohydrates, proteins, and fats. Water losses occur in the urine and stool, as well as evaporation from the skin and respiratory tract. Alterations in plasma osmolality of as little as 1% - 2% are sensed by osmoreceptors in the hypothalamus. These receptors initiate mechanisms that affect water intake (via thirst) and water excretion (via antidiuretic hormone [ADH]) to return plasma osmolality to normal.

2. Define "effective circulating volume".

Effective circulating volume is defined as that part of the extracellular fluid (ECF) that is in the vascular space and effectively perfusing tissues. It varies directly with ECF volume and also with total body sodium, since sodium salts are the primary ECF solutes holding water in the extracellular space. Therefore, regulation of sodium balance, by changes in renal sodium ion, and the maintenance of effective circulating volume, are closely related.

3. What are the major effectors of effective circulating volume?

Three major effectors alter effective circulating volume: 1) The sympathetic nervous system, 2) angiotensin II, and 3) renal sodium excretion. Volume depletion, sensed by arterial baroreceptors as hypotension, causes an increase in peripheral sympathetic tone. Increased sympathetic tone returns volume to normal by initiating specific compensatory changes. These compensatory changes include the following:

o Venous constriction: Increased venous return

o Increased myocardial contractility and heart rate: Increased cardiac output

o Arterial vasoconstriction: Increases systemic vascular resistance and blood pressure

o Increased renin secretion: Increases levels of angiotensin II which is a potent vasoconstrictor

o Increased renal tubular sodium resorption (due to increased levels of angiotensin II and aldosterone).

Sympathetic tone induced changes in effective circulating volume are transient and compensatory; appropriate changes in renal sodium excretion are required to restore normal volume.

4. What is the body’s main defense against hyperosmolality?

The major defense against hyperosmolality (accumulation of solute in excess of body water) is increased thirst. Although the kidney can minimize water losses via the action of ADH, water deficits can be corrected only by increased dietary intake.

5. When can hypo-osmolality result?

Hypoosmolality can result from excessive body water retention with subsequent dilution of body solutes or from solute loss in excess of water loss (e.g., diarrhea). Because the kidney excretes large volumes of water daily, persistent water retention resulting in hypoosmolality occurs only in the presence of decreased renal water excretion. In patients with normal renal function, hypoosmolality must therefore be due to solute loss in excess of body water loss.

6. How does hypovolemia (i.e., dehydration) increase the circulating volume?

Hypovolemia causes an increase in renin secretion. The subsequent increase in angiotensin II causes an increase in blood pressure (as a result of arterial vasoconstriction), as well as renal sodium retention (this is both a direct effect and also the result of increased aldosterone secretion). With sodium retention, water is also retained.

7. How do you determine the degree of dehydration in an animal?

Clinical assessment of dehydration is best accomplished by serial body weight monitoring. Experience has shown that the physical examination findings often underestimate the degree of dehydration. During the acute phase of volume depletion, these classical physical examination findings are all that are available. The chart below is offered as a general guideline and assumes more serious hypovolemia is present.

TABLE 1. Estimating the percentage dehydration based upon physical examination findings.

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.

8. When and how much fluid can be given via the subcutaneous route?

In mild dehydration, subcutaneous fluids are useful. Isotonic fluids should be used and no more than 5 to 10 ml/lb should be given at each injection site. The rate of subcutaneous fluid flow usually is governed by patient comfort. These fluids are aseptically administered and multiple sites are required to provide adequate fluid volume. Generally, all subcutaneous fluids are resorbed within 6 to 8 hours. If fluids are still noted subcutaneously after this time, the use of intravenous fluids to reestablish peripheral perfusion should be considered.

9. How about using the intraperitoneal route for fluid administration?

The intraperitoneal route is quick, easy, and the fluids will generally be reabsorbed thus increasing the circulating volume. However, there is the potential of bacterial peritonitis, perforating viscera, and decreasing ventilation from impeding diaphragmatic excursion. Experience with peritoneal dialysis in dogs has shown that peritoneal fluids often traverse the diaphragm, entering the thoracic space, and further affecting ventilation. Currently, intraperitoneal fluids cannot be recommended.

10. When and how are you going to administer the intravenous fluids?

In general, intravenous fluid administration is indicated in dogs and cats with 7% or greater dehydration. There are numerous potential routes for intravenous fluid administration:

o Peripheral veins

o Jugular veins

o Intraosseous

11. When and how do you estimate the volume of fluids to be given an animal?

The amount of fluid needed for replacement depends on the patient's status. Of primary concern is the status of the blood volume and later concern is directed to restoration of total body water and electrolytes.

12. What are the three phases of fluid therapy?

o Emergency phase

o Replacement phase

o Maintenance phase

13. How much fluid should be given during replacement therapy?

The volume of fluid administered during the dehydration phase is based on an assessment of fluid needs for the following:

Returning the patient's status to normal (deficit volume)

Replacing normal ongoing losses (maintenance volume)

Replacing continuing abnormal losses (continuing losses volume)

14. How do you calculate the deficit volume?

The deficit volume is an estimate based on findings from the physical examination (Table I) or on known changes in body weight To make the calculation of deficit volume, the estimated dehydration is multiplied by the body weight. It must be remembered that it is difficult to replace all deficits in a 24-hour period. An attempt to do so may result in urinary losses furthering dehydration. Thus, it is recommended that only 75% to 80% of the deficit volume be replaced during the first 24 hours. Also, don’t forget that you must also add "daily maintenance volumes" to your calculated deficit volume if the animal is not eating nor drinking.

Example 1:

A 22-lb (10 kg) dog is assessed to be 7% dehydrated. What volume of fluid deficit should be given during the first 24 hours?

Total Deficit Replacement Volume = Deficit Volume PLUS Maintenance Volume

Deficit replacement volume (ml) = % dehydration x body weight (lb) x 454^a x 0.80

Deficit replacement fluid volume (ml) = 0.07 x 22 lb x 454^ax 0.80 = 560 ml

^{a = 454 mls = 1 pound}

or

Deficit replacement volume (ml) = % dehydration x body weight (kg) x

1000^b x 0.80

Deficit replacement fluid volume (ml) = 0.07 x 10 x 1000 x 0.80 = 560

ml

^{b = 1000 ml = 1 kilogram}

15. What are "maintenance volumes" for fluid therapy?

Maintenance volumes are normal ongoing losses. Ongoing losses are divided into sensible and insensible losses. Sensible losses can be measured and are water losses in the urine and feces. Insensible losses are normal but are not easily quantitated. These water losses occur during panting or sweating. One-third of the maintenance volume is made up of the insensibile volumes and two-thirds, sensible volumes.

16. How much fluid should you give for maintenance volume if the animal is not eating nor drinking?

There is a paucity of data regarding water needs for the dog and cat and many authors recommend dramatically different fluid and energy requirements. As you will recall, water and energy requirements are numerically the same (1 Kcal of energy = 1 ml of water). Unfortunately, many authors recommend dramatically different fluid and energy requirements. Data from recent research has been used to document that energy expenditure or consumption is less than published formulas and recommendations. Estimation of water needs include 50mls/kg/day, 132 kcal X Kg^0.75, 156 X Kg^0.667, (30 X kg) + 70, 70 X Kg^0.75. Indirect calorimetry is being used to estimate energy (and thus water) needs for the dog and cat. These studies reinforce that previously recommended formulae overestimate the energy (water) requirements of the cat and dog (Figures 1 and 2).

Figure 1. Maintenance fluid volumes for the cat. Recommended volumes are calculated from the formula (30 X BW_Kg) + 70.

Figure 2. Maintenance fluid volumes for the dog. Recommended volumes are calculated from the formula (30 X BW_Kg) + 70.

Example 2:

A 22-lb (10 kg) dog is assessed to be 7% dehydrated and has been vomiting. How much fluid should be given during the next 24 hours?

Volume (ml of fluid required) = deficit volume + maintenance volume

= [0.07 x 22 lb x 454 x 0.80] + [(10 x 30) + 70]

= [560] + [370] = 930 ml

or

= [0.07 x 10 kg x 1000 x 0.80] + [(10 x 30) + 70]

= [560] + [370] = 930 ml

17. Does the formula recommended above satisfy water needs in the sick animal?

The question regarding energy (water) requirements in sick animals continues to elicit controversy. Traditionally it has been taught that illnesses, injuries, and surgery resulted in the increased need for more energy (water). These teachings were extrapolated from human and rodent data. In the dog, there is mounting evidence that increased energy requirements are not common in the sick, injured, or surgical dogs. In fact, there are increased numbers of publications documenting the lower energy (water) requirements for both the normal and sick or traumatized dog. Additionally, from an evolutionary perspective, it seems logical to expect the dog to preserve available energy with illness or injury. The reserves are already minimal and it makes little sense to increase metabolic requirements in order to survive. It makes more sense to conserve available energy and to reduce metabolic (thus energy and water) requirements. Studies on dogs in a critical care unit have documented the presence of significant hypothyroid function. Thus, metabolic requirements are reduced.

The decision to change the formulae for calculating water requirements will only come with more objective evidence taken from normal and sick dogs and cats.

18. How do you account for continuing losses during the replacement phase of fluid therapy?

A crude but effective guideline for replacing continuing abnormal losses is to estimate the volume of fluid loss and then double this estimate. The result will be surprisingly close to the actual volume of vomitus, diarrhea, or urine.

19. How do you tell when an animal is receiving an inadequate fluid volume?

Any acute change in body weight results from losses or gains in water. If the animal is losing body weight while being given crystalloid fluids, the animal is likely receiving inadequate volumes of fluid. One group of patients where body weight may fool you is in animals that are third-spacing fluids (peritonitis, pyometritis, pleural effusions). In these animals the animal may still be dehydrated but the body weight may not have changed. Monitoring the central venous pressure will result in a value which is well below 5 cm H₂0. Additionally, if renal function is adequate, an animal which is dehydrated will have a urine specific gravity above 1.025.

20. What are the clinical signs of overhydration?

Classically, pulmonary edema is associated with overhydration. Clinically, pulmonary edema is the terminal event of overhydration! Before pulmonary edema results you will first note an increased serous nasal discharge, followed by chemosis, and finally pulmonary congestion will be ausculated before edema ensues.

21. List the common crystalloid fluids, their electrolyte composition, pH, and osmolality.

						Buffer	Calories	Osmolality
Solution	Na+	K+	Cl-	Ca++	Mg++	mEq/L	Kcal/L	mOsm/L
Dextrose 5% in Water	-	-	-	-	-	-	170	278
Dextrose 2.5% in 0.45% Saline	77	-	77	-	-	-	85	280
Ringer’s Lactate Solution	130	4	109	3	-	Lactate 28	9	272
Ringer’s Solution	147	4	156	4.5	-	-	-	309
Normosol-R (Multisol-R)	140	5	109	-	3	Acetate 27 Gluconate 23	15	294
Dextrose 5% in Ringer’s Lactate	130	4	109	3	-	Lactate 28	179	525
Normal Saline (0.9%)	154	-	154	-	-	-	-	308
Dextrose 50%	-	-	-	-	-	-	1700	2525
Dextrose 5% in Saline (0.9%)	154	-	154	-	-	-	170	-
Potassium Chloride	-	2	2	-	-	-	-	-

22. How do you select the parenteral fluid to be given?

In selecting a fluid, it is important to know which electrolytes are lost and to institute replacement therapy based on knowledge of the pathophysiology of the disease. Table II provides an overview of electrolyte changes and replacement recommendations.
After you have studied Table II, you might want to test your knowledge by taking a quick quiz on selecting the appropriate fluid (http://www.cvmbs.colostate.edu/clinsci/wing/fluids/fluids.htm).

Table II. Selection of fluids for selected diseases.

			SERUM
CONDITION	Na+	Cl-	K+	HCO3-	Volume	FLUID OF CHOICE
Diarrhea	D	D	D	D	D	NORMOSOLR-R + KCl or Lactated Ringer's + KCl
Pyloric obstruction	D	D	D	I	D	0.9% NaCl + KCl
Dehydration	I	I	N	N/D	D	NORMOSOLR-R + KCl, Lactated Ringer's + KCl, 0.9% NaCl + KCl, 5% dextrose
Congestive heart failure	N/D	N/D	N	N	I	0.45% NaCl + 2.5% dextrose + KCl, 5% dextrose
End-stage liver disease	N/I	N/I	D	D	I	0.45% NaCl + 2.5% dextrose + KCl
Acute renal failure -Oliguria -Polyuria	I D	I D	I N/D	D D	I D	0.9% NaCl, NORMOSOLR-R + KCl, Lactated Ringer's + KCl,
Chronic renal failure	N/D	N/D	N	D	N/D	NORMOSOLR-R, Lactated Ringer's solution, 0.9% NaCl
Adrenocortical insufficiency	D	D	I	N/D	D	0.9% NaCl
Diabetic ketoacidosis	D	D	N/D	D	D	0.9% NaCl (+ KCl)

D = Decreased I = Increased N = Normal

Case Studies in Fluid Therapy

Suggested Reading

Aberman A: The ins and outs of fluids and electrolytes. Emerg Med 14(7):121-127, 1982.

Adams LG, Polzin DJ. Mixed acid-base disorders. Vet Clin N Amer: Small Anim Pract 19(2):307-326, 1989.

Bonner CW, Stidham GL, Westenkirchner DF, Tolley EA: Hypermagnesemia and hypocalcemia as predictors of high mortality in critically ill pediatric patients. Crit Care Med 18:921-928, 1990.

Concannon KT. Colloid oncotic pressure and the clinical use of colloidal solutions. J Vet Emer Crit Care. 3:49-62, 1993.

Dubick MA, Wade CE: A review of the efficacy and safety of 7.5% NaCl/6% dextran-70 in experimental animals and in humans. J Trauma 36:323-330, 1994.

Duval D. Use of hypertonic saline solutions in hypovolemic shock. Compend Contin Educ Pract Vet 17(10):1228-1231, 1995.

Garvey MS: Fluid and electrolyte balance in critical patients. Vet Clin North Am: Small Anim 19:1021-1057, 1989.

Haskins SC: A simple fluid therapy planning guide. Sem Vet Med Surg