Editing Pathophysiology of Urinary Tract Obstruction (section)

== Hemodynamic changes with obstruction ==

* '''Renal blood flow is autoregulated primarily by afferent arteriolar tone.'''

=== Unilateral ureteral obstruction ===

* '''Triphasic pattern of renal blood flow (RBF) and ureteral pressure changes'''
*# '''First phase'''
*#* '''Initial 1-2 hours'''
*#* '''Increase in pressure within the renal tubules''' '''and a subsequent decrease in GFR'''.
*#* '''To compensate for this decreased GFR, there is an increase in RBF due to afferent arteriolar vasodilation'''
*#** '''Several mechanisms for this increase in RBC have been postulated, including vasodilation from prostaglandins, nitric oxide and tubuloglomerular feedback.'''
*#*** '''Administration of either a prostaglandin synthesis inhibitor (NSAID) or a nitric oxide synthase inhibitor has been shown to block the vasodilatory response and should therefore be avoided in the context of urinary obstruction.'''
*#*** '''ACE-inhibitors or angiotensin receptor blockers reduce constriction at the efferent and affarent arteriole, with a stronger effect on efferent.'''

**# '''Second phase'''
**#* '''From 2-5 hours'''
**#* '''Ureteral[/tubular] pressure remains elevated but RBF begins to decline. As a result, GFR declines'''
**#** '''The decline in GFR is accompanied by an increase in efferent arteriole resistance'''
**#*** '''Activation of the renin-angiotensin system appears to play a role in the efferent arteriole vasoconstriction''' along with endothelin and thromboxane A2.
**#** Note that SASP 2017 suggests that efferent arteriole constriction does not occur in the second phase of UUO, only the second phase of BUO. However, 2015 AUA Update on post-obstructive diuresis suggests that efferent arteriole constriction occurs in the second phase of both UUO and BUO.
**# '''Third phase'''
**#* '''Both ureteral[/tubular] pressure and RBF flow progressively decline, resulting in a gradual loss in renal function'''
**#* '''By about 24 hours after obstruction, the pressure has declined to pre-occlusion levels or even less.''' GFR remains decreased compared to baseline.
**#** '''In addition, there is a shift of blood flow from the outer to inner cortex with UUO that is opposite to that which is seen with BUO.'''
** Renin-mediated hypertension sometimes occurs in acute unilateral renal obstruction and may present a clinical picture suggestive of a renovascular etiology; however, angiographic studies can distinguish if the hypertension is due to a vascular lesion.

=== Bilateral Ureteral Obstruction (BUO) or Obstruction of a Solitary Kidney ===

* Complete obstruction of urinary flow results in anuria.

# '''First phase'''
#* '''Similar to UUO, there is an initial increase in RBF with BUO but this response appears to be less pronounced and of shorter duration (≈90 minutes)'''
# '''Second phase'''
#* '''Large decrease in RBF. As a result, GFR declines'''
#** '''The decline in GFR is accompanied by an increase in efferent arteriole resistance'''
# '''Third phase'''
#* '''The collecting system pressure remains elevated for longer than unilateral obstruction and is often still high at 24 hours'''
#** '''It is believed that this persistent increased pressure is secondary to afferent arteriolar vasodilation and efferent arteriolar vasoconstriction, which may be caused by the accumulation of atrial natriuretic peptide or other factors associated with complete obstruction.'''
#** '''There is a shift of blood flow to the outer renal cortex with BUO, in contrast to the reversed pattern with UUO.'''
#** '''During UUO, preglomerular vasodilation is followed by a more prolonged PREglomerular vasoconstriction''', and this increase in afferent arteriolar resistance causes a reduction in glomerular capillary pressure that in turn results in decreased intratubular pressure. '''In contrast, during BUO preglomerular vasodilation is followed by a prolonged POSTglomerular vasoconstriction'''

=== Partial ureteral obstruction ===

* Effects on renal hemodynamics and GFR are variable, depending on the severity and the duration of obstruction. In general, partial obstruction results in decreased RBF and GFR in the ipsilateral kidney