Editing Pathophysiology of Urinary Tract Obstruction (section)

=== Post-obstructive diuresis (POD) ===

==== Definition ====
'''Post-obstructive diuresis: a period of significant polyuria (>200 ml/hr) occurring after the relief of urinary tract obstruction'''
* Note that definition varies based on source

==== Classification (2) ====

* '''Physiological vs. Pathological'''

* '''Physiological'''
** '''More common'''
** '''An appropriate diuresis in response to the volume and solute overload'''
** '''Self-limiting and stops when homeostasis is restored'''
* '''Pathological'''
** '''Less common'''
** '''An inappropriate diuresis in response to the volume and solute overload'''
** '''Can result in leading to derangements in blood chemistry and/or volume status.'''
*** '''For example, a pathological water diuresis can cause hypernatremia and a pathological sodium loss can cause hypovolemia.'''

==== Causes ====

*'''Will often accompany relief of BUO''' in the presence of a normal contralateral kidney 
**The accumulation of extracellular volume stimulates the synthesis and release of ANP, which promotes increased GFR and sodium excretion. Decreases in the aquaporin water channels in the kidney further promote the diuresis.
**'''COX-2 activity may be increased in the post-obstructive phase and contributes to polyuria, and impaired urine-concentrating ability'''
**One study found that predictors of POD were higher serum creatinine, higher serum bicarbonate and urinary retention.
*'''Not typically observed with relief of UUO''', secondary to the presence of a functional contralateral kidney that can maintain fluid balance
** '''Fractional excretion of sodium after relief of obstruction is greater in BUO than UUO'''
*** The natriuresis following relief of BUO is typically greater than that after UUO
* '''<span style="color:#ff0000">Most patients do not demonstrate a clinically significant POD after relief of urinary tract obstruction</span>'''
**'''<span style="color:#ff0000">Patients at risk for clinically significant POD typically exhibit signs of fluid overload, including (3):</span>'''
**#'''<span style="color:#ff0000">Edema</span>'''
**#'''<span style="color:#ff0000">Congestive heart failure</span>'''
**#'''<span style="color:#ff0000">Hypertension</span>'''
**'''Special consideration should also be given to patients with high urine outputs (>5 to 6 L daily).'''

==== Diagnosis and Evaluation ====
* '''<span style="color:#ff0000">History and Physical Exam</span>'''
** '''<span style="color:#ff0000">Physical Exam</span>'''
***'''<span style="color:#ff0000">Vital signs and volume status</span>'''
* '''<span style="color:#ff0000">Labs (3):</span>'''
*# '''<span style="color:#ff0000">Serum electrolytes</span>'''
*# '''<span style="color:#ff0000">Creatinine/Blood urea nitrogen</span>'''
*# '''<span style="color:#ff0000">Urinalysis (including osmolality)</span>'''
* '''Urine osmolality'''
** '''Can indicate if the diuresis is more of a water or solute/osmotic diuresis.'''
*** Water diuresis is defined by a lower urine osmolality (often <150 mOsm/kg).
*** Urine osmolality of 150-300 mOsm/kg indicates mixed diuresis
*** Urine osmolality of 300-500 mOsm/kg is usually a solute/osmotic diuresis
** During the obstruction, water retention can lead to expansion of the extracellular volume that leads to water diuresis as the extracellular volume is restored.

==== Management ====
* '''<span style="color:#ff0000">Continuous urinary drainage</span>'''
**'''Intermittent or gradual drainage is no longer recommended'''
*** Intermittent or gradual drainage of an obstructed bladder was thought to potentially decrease the rates of hematuria and hypotension. However, with quick drainage the rates of hematuria are generally low and not severe, while the decrease in blood pressure represents a normalization of the pressure without cardiovascular compromise.
* '''<span style="color:#ff0000">In patients with normal mental status, no evidence of fluid overload, normal electrolytes, and normal renal function</span>'''
** '''<span style="color:#ff0000">Regular monitoring of (3):</span>'''
**#'''<span style="color:#ff0000">Orthostatic vital signs</span>'''
**#'''<span style="color:#ff0000">Electrolytes</span>'''
**#'''<span style="color:#ff0000">Urine output</span>'''
** '''<span style="color:#ff0000">Free access to oral fluids</span>'''
*** '''Generally, patients with a normal mental status should not be given IV fluids because this may prolong the period of diuresis'''
* '''<span style="color:#ff0000">In patients with evidence of a pathological post-obstructive diuresis</span>'''
** '''<span style="color:#ff0000">More frequent monitoring of vital signs, urine output, electrolytes (every 12 hours or more often if necessary), and urine osmolality</span>'''
*** '''Patient can become hypovolemic as a result of excess water loss, and electrolyte abnormalities may develop as result of salt or potassium wasting'''
**'''<span style="color:#ff0000">Patients should continue to have free access to oral fluids.</span>'''
* '''<span style="color:#ff0000">Patients with poor cognitive function [or signs of dehydration] should be given IV fluids, although at a rate below maintenance.</span>'''
** '''<span style="color:#ff0000">The type of fluid used depends on the volume and sodium status</span>''' (from AUA update)''':'''
*** '''<span style="color:#ff0000">Normal volume status</span>''' (normal blood pressure):
**** '''<span style="color:#ff0000">Fluid replacement with free water (oral or IV) should be considered if there is a water diuresis</span> (suggested by hypernatremia and a low urine osmolality).'''
***** Caution should always be taken to avoid correcting sodium alterations too rapidly.
*** '''<span style="color:#ff0000">Hypovolemia (low blood pressure)</span>'''
**** '''<span style="color:#ff0000">0.9% saline is usually the fluid of choice</span>'''
**** '''If the volume deficit is not severe, 0.45% saline can also be used (particularly if hypernatremia is present)''' to provide sodium and water to help the kidneys maintain homeostasis.
*** Sometimes it is recommended that urine output be replaced with 0.5 ml fluid per 1 ml urine. While this may help prevent hypovolemia, it can also propagate the diuresis if the fluid replacement is inappropriately high for the patient.
*** '''Hypokalemia can occur and lead to life threatening arrhythmias if severe.''' As a result, potassium should be replaced aggressively, particularly if renal function is improving. Other electrolyte abnormalities should be corrected as well.
** '''The urine is usually isosthenuric initially, and IV fluid replacement with 0.45% saline administration at a rate lower (50-75%) than the hourly urine output is recommended; 0.9% normal saline and Lactated Ringer have no role in post-obstructive diuresis and should not be given''' '''[SASP contradicts AUA Update]'''