A patient under your care was treated for hypovolemic shock following severe haemorrhage two days ago. There is no further blood loss, however despite the patient taking on a significant amount of IV fluid they show signs of dehydration. The patient is also hyponatraemic and hypokalaemic.

Despite all of this, the patient is passing very high volumes of urine (polyuria).

1) Diagnose what condition is causing this high urine output despite dehydration and low electrolyte levels and explain your reasoning for how this condition is leading to these symptoms. (There are several reasons why this condition results in these symptoms. Describe 3.)

2) How would treating the patient with noradrenaline help to reverse this issue?

Question

A patient under your care was treated for hypovolemic shock following severe haemorrhage two days ago. There is no further blood loss, however despite the patient taking on a significant amount of IV fluid they show signs of dehydration. The patient is also hyponatraemic and hypokalaemic.

Despite all of this, the patient is passing very high volumes of urine (polyuria).

1) Diagnose what condition is causing this high urine output despite dehydration and low electrolyte levels and explain your reasoning for how this condition is leading to these symptoms. (There are several reasons why this condition results in these symptoms. Describe 3.)

2) How would treating the patient with noradrenaline help to reverse this issue?

BlackTom AI · Accepted Answer

The question presents a patient who recently recovered from hypovolemic shock due to severe haemorrhage. Now, despite adequate IV fluids, the patient shows signs of dehydration, hyponatraemia, hypokalaemia, and currently has very high urine output (polyuria). This pattern is most consistent with an acute kidney injury (AKI) in the diuretic (recovery) phase rather than ongoing pre-renal failure.

1) Diagnosis and rationale (three supporting mechanisms):
- Primary diagnosis: Diuretic phase of acute kidney injury (AKI) due to acute tubular necrosis (ATN) from previous hypoperfusion. In the recovery phase of ATN, the tubules regain some function but cannot reabsorb water and solutes efficiently, leading to large volumes of dilute urine.
- Mechanism 1: Impaired tubular reabsorption. Following ATN, tubular cells are damaged and their ability to reclaim solutes (Na+, K+, water) is reduced. This results in a high fractional excretion of sodium and water, manifesting as polyuria with ongoing electrolyte losses (hence hyponatraemia and hypokalaemia).
- Mechanism 2: Loss of concentrating ability. The damaged tubules cannot generate or maintain the medullary osmotic gradient necessary for urine concentration. As a result, even with dehydration, the kidneys excrete a large volume of dilute urine, perpetuating volume loss and hyperosmolar challenge.
- Mechanism 3: Delayed or incomplete recovery of renal perfusion and function. After initial hypoperfusion, renal perfusion pressure and GFR may improve gradually, but the nephrons remain dysfunctional in their handling of solutes and water. This protracted recovery phase leads to continued polyuria and electrolyte disturbances until tubular integrity and function are restored.
- Additional contributing factors to the observed hyponatraemia and hypokalaemia: ongoing renal solute losses (Na+, K+), potential dilutional effects if fluids are given in a way that doesn’t match losses, and possible intrinsic tubular dysfunction affecting electrolyte reabsorption, all of which align with AKI diuretic phase physiology.

2) How treating with noradrenaline (norepinephrine) would help reverse this issue: think about the hemodynamics and renal perfusion in AKI.
- Mechanism A: Restores mean arterial pressure and renal perfusion. Norepinephrine is a potent vasoconstrictor that raises systemic vascular resistance, increases blood pressure, and improves effective circulating volume. This can help restore renal blood flow in a recovering kidney, supporting glomerular filtration rate (GFR) and aiding the transition out of the diuretic phase as tubular function begins to recover.
- Mechanism B: Optimizes renal autoregulation during recovery. By maintaining adequate perfusion pressure, norepinephrine helps ensure that the kidneys receive sufficient blood flow to support filtration without excessive renal vasoconstriction that could worsen ischemia. This balancing effect can help reduce further tubular injury and support clearance of accumulated metabolites while the tubular epithelium recovers.
- Mechanism C: Reduces ongoing prerenal influence if present. If any residual suboptimal circulating volume or subtle prerenal component persists, improving perfusion with norepinephrine can mitigate ischemic stress on the tubules, potentially shortening the duration of the diuretic phase and aiding in stabilization of urine output and electrolyte handling.
- Mechanism D: Indirectly supports fluid and electrolyte management. By stabilizing hemodynamics, clinicians have a better platform to judiciously manage IV fluids and electrolytes (Na+, K+) to correct hyponatraemia and hypokalaemia as tubular reabsorption begins to recover.

In summary, the presentation is most consistent with the diuretic phase of ATN after hypovolemic shock, characterized by polyuria with concomitant dehydration and electrolyte losses due to impaired tubular reabsorption and reduced concentrating ability. Noradrenaline can help reverse this issue by improving renal perfusion and GFR during recovery, thereby supporting tubular healing and stabilizing the patient’s fluid and electrolyte status.

PHYS30010_2025_SM1 Challenge Question 3: Renal Failure

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