Fluid homeostasis & chronic fluid therapy

Question 1

Define and differentiate hypoperfusion and dehydration

Answer 1

Hypoperfusion is a deficit of free water in the body. Dehydration is a deficit of sodium in the interstitial space.

Question 2

List the body fluid compartments and outline the underlying factors that determine fluid and electrolyte movement between compartments

Answer 2

60% of body weight is water. 40% of body weight is ICF, and 20% of body weight is ECF. IVF is approx ¼ of ECF

Water makes up approximately 60% of the adult animal’s mature body weight, two thirds is intracellular and one third is extracellular. Of the extracellular fluid portion - approximately 25% is in the intravascular space and 75% is in the interstitial space.
Endothelium (separating interstitial and intravascular) and cell membrane (separating intracellular and interstitial) are freely permeable to water molecules.
An osmole is one mole of any fully dissociated substance dissolved in water. The water permeable cell-membranes are impermeable to sodium and potassium, so these two osmoles are effective osmoles and exert osmotic pressure across the cell membrane. Osmotic pressure causes water to move from an area of low osmolarity to an area of high osmolarity to restore equal osmolarity between the two compartments.

Question 3

Explain how fluid is retained within the intravascular space

Answer 3

Fluid is retained within the intravascular space by Starlings Law, hydrostatic pressure and oncotic pressure. Osmolarity determines the fluid balance within the intravacular space, and osmoles such as Na (the main osmole) draw water with it.
The most abundant osmoles in the body are sodium (and its accompanying anions , chloride and bicarbonate), glucose and urea. Plasma sodium concentration is the main determinant of intravascular osmolarity.

Question 4

Differentiate the physiological mechanisms that control the volume of the extracellular space and the osmolality of the body

Answer 4

Hypothalamic osmoreceptors detect small changes in osmolarity and induce compensatory mechanisms to return osmolarity to its hypothalamic setpoint.

ADH is a peptide secreted by the posterior pituitary gland. Two main stimuli for ADH release are increased plasma osmolarity, and decreased circulating volume.

Osmoreceptors in the hypothalamus - when plasma osmolarity increases, water flows out of cells to restore equilibrium, these osmoreceptor cells in particular shrink, and when they do they send out a signal to the posterior pituitary, stimulating ADH release.
Baroreceptors in aortic arch and carotid - when effective circulating volume is low, these cells send out a signal to the posterior pituitary, also stimulating ADH release

Without ADH, renal tubular cells are quite impermeable to water. ADH changes this by causing aquaporin channels to be inserted into the cell’s luminal membrane, allowing water to move into the renal tubular cell. Water can then flow into the hyperosmolar renal medulla (down its osmotic gradient). Importantly, if the kidney is unable to generate a hyperosmolar renal medulla because of diuretic administration or disease, then water will not be reabsorbed even with high levels of ADH.
ADH is the main determinant of free water retention and excretion.

Thirst - is stimulated by hyperosmolarity and low effective circulating volume as well. The mechanism is similar to the release of ADh, and thirst is the main determinant of free water intake.

Circulating volume is controlled by the renin-angiotensin-aldosterone system, and circulating volume is always prioritised over osmolarity. So in a patient with poor circulating volume, they can have increased thirst and ADH release, regardless of their osmolarity. This is why you can see a patient with chronic congestive heart failure and hyponatraemia.

Question 5

Explain the differing effects of hypo-, iso- and hypertonic fluid losses on the intravascular volume

Answer 5

Hypotonic fluid losses are a loss of free water from the intravascular space, so sodium will

Question 6

Explain the concepts of replacement, maintenance and ongoing losses with respect to fluid deficits in an animal

Answer 6

Maintenance fluid rates for large dogs is 40ml/kg/day and 60ml/kg/day for smaller dogs

Urine output goal should be between 1-2ml/kg/hr with a USG of 1.026 (d) and 1.035 ©
Insensible losses estimated ¬20ml.kg/day
Weigh patient daily or twice daily - increased weight could indicate overzealous fluids, losing weight could be polyuria or inadequate fluid rate

Question 7

Explain the differences between a replacement fluid and a maintenance fluid

Answer 7

Replacement fluids have sodium content which is equivalent to that of plasma and are used to replace fluid and electrolyte loss from all fluid compartments, due to their high sodium content they rapidly replace intravascular volume but soon move out into the interstitial and intracellular spaces.

While maintenance fluids have a lower sodium content that is equivalent to daily sodium loss. They are often used once the patient’s fluid losses have been replaced and are designed to provide the patient’s daily fluid and electrolyte needs (ie replace normal losses)

Replacement fluids
0.9% sodium chloride
LRS
Plasmalyte 148
Normosol R
Maintenance fluids (can be isotonic or hypotonic)
0.45% NaCl + 2.5% dextrose)
Plasmalyte 56
Normosol M
0.45% NaCl (hypoT)