Lecture 1 1/23/25 Flashcards
Which factors impact daily water intake?
-environment
-diet
-age
-preferences
What is the normal daily water intake?
40-60 mL/kg/day
What controls body water homeostasis?
-plasma osmolality
-kidneys
-vascular volume
-hypothalamus thirst center
-pituitary
What is osmolality?
-concentration of osmotically active particles in solution
-# of particles in 1 kg of solution
What does osmolality depend on?
size and number of molecules
What is osmolarity?
of particles per 1 L of solvent
Which factors impact plasma osmolality?
-sodium
-blood glucose
-BUN
What increases alongside an increase in plasma osmolality?
-thirst
-ADH secretion
Which types of receptors are found within the hypothalamus thirst center?
-osmoreceptors
-baroreceptors
What are the triggers for thirst?
-hyperosmolality
-decreased vascular volume
Where is ADH produced?
hypothalamus
What is the role of the posterior pituitary in terms of ADH?
stores ADH and releases it as necessary
What are the potential triggers for ADH release?
-hyperosmolality
-hypovolemia
-angiotensin II
-emotional states
What are the effects of ADH?
-increase water reabsorption
-concentrate the urine
How does ADH function?
causes an increase in aquaporins within the kidneys to allow for greater water reabsorption/urine concentration
Why is it important that sodium is the key determinator of plasma osmolality?
helps to keep plasma osmolality within a tight range
Where are the receptors for ADH?
distal renal tubule
What is the main mediator of the thirst center?
hypothalamic osmoreceptors
What happens when there is decreased renal perfusion?
RAAS is activated
What is the polydipsia rate?
greater than or equal to 100 mL/kg/day
What is the polyuria rate?
greater than or equal to 50 mL/kg/day
What should be gathered during a history to determine if a patient has PUPD?
-frequency
-volume
-consciousness of micturition
What counts as PUPD?
frequent, large amounts of urine with consciousness of micturition
Which disorders are commonly mistaken for PUPD?
-pollakiuria/dysuria/small, frequent amounts
-incontinence/unaware of urination
-behavior disorders
When should PUPD be considered?
-increased water intake
-increased urine output
-USG persistently less than fully concentrated
What is the USG range for hyposthenuria?
1.00 to 1.007
What is the USG range for isosthenuria?
1.008 to 1.012
What is the USG range for minimally concentrated urine?
-1.013 to 1.030 in dogs
-1.013 to 1.040 in cats
What is the USG range for concentrated urine?
-greater than 1.035 in dogs
-greater than 1.045 in cats
What is needed to produce concentrated urine?
-functioning nephrons
-hypertonic medullary gradient
-ADH
-response to ADH
What fraction of nephrons must be working in order to concentrate urine?
1/3
Which molecules contribute to the hypertonic medullary gradient?
sodium and urea
What percent of fluid filtered at the glomerulus is reabsorbed in the proximal tubule?
80-90%
Which solutes are reabsorbed with fluid in the proximal tubule?
sodium and glucose
Where does selective reabsorption of sodium occur?
thick ascending loop
What is the function of ADH on the distal nephron?
allows for reabsorption of up to 90% of the remaining fluid not absorbed in the proximal tubule
What can cause primary polyuria?
-osmotic diuresis
-reduced/absent ADH production
-reduced/absent ADH-receptor function
-medullary washout
What is osmotic diuresis?
impaired fluid reabsorption in the proximal tubule
Which solutes are poorly reabsorbed and instead draw water into the urine?
-mannitol
-urea
-glucose
What are the mechanisms of osmotic diuresis?
-presence of poorly reabsorbed solutes
-excessive salt intake
-increased fluid volume reaching distal tubule
-overwhelmed capacity to reabsorb water
What condition occurs when there is reduced/absent ADH production or ADH-receptor function?
diabetes insipidus
What is central diabetes insipidus?
reduced/absent ADH production stemming from the brain
What is nephrogenic diabetes insipidus?
reduced/absent ADH-receptor function in the kidneys
What are the causes of central diabetes insipidus?
-congenital/primary
-acquired/secondary
-idiopathic
What can cause acquired diabetes insipidus?
-neoplasia
-trauma
-radiation
How can central diabetes insipidus be treated?
administration of exogenous ADH
What are the characteristics of congenital nephrogenic diabetes insipidus?
-very rare
-more common in male dogs
-defect in cellular mechanism that opens water channels
-cannot respond to exogenous ADH
-obligate water drinker; will die rapidly without access
What are the characteristics of acquired nephrogenic diabetes insipidus?
-something interferes with ADH receptor
-most common cause of DI in dogs and cats
-potentially reversible
What can interfere with the ADH receptor?
-cortisol
-endotoxins
-electrolytes/hypercalcemia
What can lead to impaired medullary gradient?
-impaired reabsorption of sodium and urea due to increased urine and/or blood flow
-decreased urea due to liver insufficiency/low protein diet
-decreased sodium due to hypoadrenocorticism, loop diuretics, or diarrhea
What are the potential causes of primary polydipsia?
-psychogenic: pain, stress, insufficient exercise
-neurologic
-GI disease
-hepatic encephalopathy
What are the characteristics of primary polydipsia diagnosis?
-diagnosis of exclusion
-can conc. urine with water restriction
-serum sodium may be low/diluted
What is the first step to addressing PUPD?
-confirm PUPD with good history, including diet and medications
-perform a physical examination
What are the possible conclusions if an animal appears to have PUPD and has concentrated urine?
-not consistent with obligatory PUPD
-normal animal
-primary polydipsic - conc. with water restriction
What are the possible conclusions if an animal appears to have PUPD and has dilute (USG < 1.008) urine?
-diabetes insipidus
-primary polydipsia
What are the possible conclusions if an animal appears to have PUPD and has isosthenuric urine?
-CKD
-nephrogenic diabetes insipidus secondary to an underlying cause
-partial central diabetes insipidus
What should be evaluated on a urinalysis?
-evidence for tubular dysfunction; glucose, protein, pH
-active sediment
-systemic disease
What are the characteristics of urine culture in PUPD diagnosis?
-recommended for all PUPD animals
-impaired conc. can predispose to UTI
What is the minimum database that should be run in PUPD patients?
-CBC
-Chem panel
-T4 in cats
Which endocrine screening tests can be done in patients with PUPD?
-Cushing’s/ACTH stim. test
-Addison’s/low dose dexamethasone suppression test
Which imaging techniques should be done in PUPD patients?
-thoracic rads
-abdominal rads
-ultrasound
What are tests that can be run to identify less common causes of PUPD?
-bile acids
-SDMA
-lepto. testing
What is the thought process when an animal has decreased serum sodium?
-decreased serum sodium indicates psychogenic PUPD
-animal should undergo modified water deprivation test
What is the thought process when an animal has normal to increased serum sodium?
-normal to increased serum sodium indicates central DI
-animal should undergo desmopressin trial
How does an animal’s response to synthetic ADH indicate the disease process?
-concentrating with synthetic ADH indicates CDI
-partial concentration with synthetic ADH indicates secondary NDI or partial CDI
-not concentrating with synthetic ADH indicates primary NDI or psychogenic cause
What are the steps of the desmopressin trial?
-measure water intake for 2-3 days prior to trial
-check USG before trial
-administer desmopressin for 5 to 7 days
-recheck; want to see decreased drinking and increase of USG
When should a water deprivation test NOT be performed?
-animal can concentrate greater than 1.030
-animal has pre-existing dehydration, azotemia, and/or hyponatremia
-unable to fully monitor animal
What are the steps of a modified water deprivation test?
-check blood and USG prior to trial
-weigh patient
-check USG and weight every 30-60 minutes once beginning trial
-if patient concentrates over 1.030, animal is normal and has psychogenic cause
-if patient concentrates less than 1.030 and/or loses 3% or greater body weight, administer desmopressin and check USG 2 and 4 hours later
-if USG increases, animal has CDI
