28.3: Acute Kidney Injury III: Clinical Manifestations

Acute Kidney Injury (AKI) progresses through distinct clinical phases: the oliguric, diuretic, and recovery phases, each marked by unique manifestations and challenges.

Oliguric Phase:

The oliguric phase is the initial stage of AKI, typically lasting 10 to 14 days. This phase is marked by a significant reduction in urine output, usually less than 400 mL per day, indicating decreased kidney function. Fluid retention is a prominent feature, leading to symptoms such as edema, hypertension, and distended neck veins. Pulmonary edema and heart failure may occur as a result of the fluid overload. Metabolic acidosis develops due to the kidney’s inability to excrete hydrogen ions, leading to compensatory deep, rapid breathing known as Kussmaul respirations​.

Elevated concentrations of urea nitrogen and creatinine in the bloodstream is called azotemia, are key markers of kidney dysfunction. These metabolic wastes, if not properly excreted, can cause neurological symptoms such as confusion, lethargy, and even seizures​. Hyperkalemia, or elevated serum potassium levels, increases the risk of serious cardiac arrhythmias, which may present as peaked T waves on an electrocardiogram (ECG). Common symptoms during this phase also include nausea and flank pain.

Diuretic Phase:

The diuretic phase signals the beginning of kidney recovery, typically lasting 1 to 3 weeks. During this time, urine output increases significantly, often reaching 1 to 3 liters daily and sometimes exceeding 5 liters. While this increased output reflects the kidneys’ ability to remove excess fluid, the kidneys can still not effectively concentrate urine. This leads to a significant loss of electrolytes, especially sodium and potassium, which can lead to hyponatremia and hypokalemia. The patient is also at risk of dehydration and hypotension due to the excessive fluid loss​.

Although urine output increases, kidney function remains impaired, and the high urine volume results from osmotic diuresis caused by elevated urea levels. Careful monitoring of electrolyte levels, fluid balance, and blood pressure is essential to avoid complications. Towards the end of this phase, electrolytes and waste product levels such as blood urea nitrogen (BUN) and creatinine begin to stabilize.

Recovery Phase:

The recovery phase can last from 2 weeks to 12 months, depending on the severity of the injury and the patient’s overall health. During this phase, the glomerular filtration rate (GFR) gradually improves, allowing blood urea nitrogen and creatinine levels to return to normal as kidney function is restored. Most patients experience significant recovery of kidney function within the first few weeks, but residual impairment may persist ​for some. Older patients or those with pre-existing kidney disease may be at greater risk for incomplete recovery, which could progress to chronic kidney disease (CKD).

Complete recovery may take up to 12 months, and in some cases, permanent reductions in GFR, though minor, may occur. Regular follow-up and monitoring are crucial during this phase to ensure long-term kidney health.