Introduction
Urine output is often used as a marker for kidney diseases. It also guides in fluid resuscitation in critically ill patients. An increase or decrease in urine output is associated with pathological abnormalities. Variations in urine output also depend on fluid intake. It is different for every individual. Healthy individuals empty their bladder between four and ten times a day.
What Is Normal Urine Output?
The storage capacity of the bladder is up to 500 milliliters (ml) of urine in women and 700 milliliters of urine in men. The sensation of urinating occurs when the bladder is filled with 200 to 300 ml of urine. Normal urine output is 800 to 2000 ml per day, with a fluid intake of about 2 liters. Normal urine output shows that the kidneys are well-perfused and oxygenated. Changes in urine output indicate improper fluid intake or the presence of a pathological entity. An increase in urine output is called polyuria, and a decrease in urine output is called oliguria. Anuria is the absence of urine output. Urine output is a common parameter of renal function in acute kidney injury. It helps in identifying the changes in renal hemodynamics. Dynamic changes in urine output indicate renal failure, renal injury, or end-stage kidney disease.
What Does Increased Urine Output Mean?
Polyuria is excessive urine output that is more than 2.5 liters or 3 liters over 24 hours. It is also called diuresis. It may be due to increased thirst and increased fluid intake. It is usually due to uncontrolled diabetes. Uncontrolled diabetes causes osmotic diuresis due to high glucose levels, and this glucose is excreted in the urine, thereby increasing urine output.
A decrease in aldosterone secretion due to adrenal cortical tumor, excessive fluid intake, central diabetes insipidus, and nephrological diabetes insipidus also causes polyuria. It can also be caused by the intake of caffeine, alcohol, and diuretics. Exposure to cold, childbirth, high altitude, removal of obstruction in the urinary tract, supraventricular tachycardia, and atrial fibrillation also causes excessive urine formation.
The following causes are associated with polyuria:
General Causes
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Polydipsia.
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Psychogenic polydipsia.
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Diuretic drugs.
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Osmotic diuresis.
Changes in Urinary System
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Interstitial cystitis.
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Renal tubular acidosis.
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Urinary tract infections.
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Fanconi syndrome.
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Nephronophthisis.
Hormonal Changes
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Hypokalemia.
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Diabetes mellitus.
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Pheochromocytoma.
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Corticosteroid use.
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Diabetes insipidus.
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Hyperparathyroidism.
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Hypercalcemia.
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Hyperthyroidism.
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Hypopituitarism.
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Conn’s disease.
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Hyperglycemia.
Circulatory Changes
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Congestive heart failure.
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Postural orthostatic tachycardia syndrome.
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Cardiorespiratory disease.
Neurological changes
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Migraine
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Neurological damage.
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Cerebral salt-wasting syndrome.
Other Causes
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Altitude diuresis.
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High doses of vitamin D.
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High doses of riboflavin.
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A side effect of lithium.
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Hemochromatosis.
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Ochratoxicosis.
The treatment for excessive urination is treating the underlying cause since polyuria is not a disease by itself and is usually a sign or symptom of another disease. Excessive urine output becomes normal once the underlying cause is resolved.
What Does Decreased Urine Output Mean?
A decrease in urine output of more than 80 ml but less than 400 ml per day is called oliguria or hypouresis. It is called anuria if the urine output is less than 80 or 100 ml per day.
Causes of Oliguria
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Dehydration due to poor fluid intake, cardiogenic shock, massive bleeding, sepsis, or diarrhea
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Kidney failure due to hypoperfusion or rhabdomyolysis.
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Hyperosmolar hyperglycemic nonketotic syndrome (HHNS).
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Hypovolemic shock.
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Multiple organ dysfunction syndromes.
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Urinary obstruction.
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Urinary retention.
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Diabetic ketoacidosis.
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Pre-eclampsia.
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Urinary tract infections.
Treatment for oliguria is based on the treatment for the underlying cause. If fluid intake is insufficient, it can be managed by increasing the fluid intake with plain water or rehydration solutions containing electrolytes.
Causes of Anuria
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Acute kidney failure.
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Diabetes.
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High blood pressure.
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Obstruction due to stones or tumors in the urinary tract.
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Enlarged prostate gland.
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Cardiac failure.
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Mercury poisoning.
Drugs like mannitol are given to improve the amount of water removed from the blood, thereby increasing urine output. Treating the underlying causes, like relieving the obstruction, improves urine output.
How Is Urine Output Measured?
A 24-hour volume test is performed to measure urine output in a day. A bag or a container is used to collect the urine every time for a 24-hour period. The volume of urine collected in the bag gives the measurement for urine output. It is a normal test, and it does not cause any discomfort. It is used to measure urine output and assess renal function. The amount of protein, albumin, creatinine and other substances are also analyzed in this test.
What Is the Significance of Urine Output in Critically ill Patients?
Urine output indicates the state of kidney function in critically ill patients. It is used to assess the reaction to treatment. Measuring the urine output and documenting the recordings is essential in the management of critically ill patients. The urine output is monitored manually or with a device that is capable of monitoring the urine output automatically. It helps in the resuscitation of septic shock patients and the resuscitation and management of burn patients. Monitoring the urine output helps in the earlier recognition of acute kidney injury in critically ill patients and also helps in fluid management.
What Is the Association Between Urine Output and Acute Kidney Injury?
Urine output shows end-organ perfusion in the kidneys, which is essential for the early diagnosis of acute kidney injury. Acute kidney injury causes a rapid reduction in the ability to filter waste from the blood. This disturbs the fluid, electrolyte, and acid-base balance. It is common in critically ill patients, and it develops over a few hours or days. Acute kidney injury reduces urine output, causes a build-up of wastes, and affects homeostasis.
Conclusion
Urine output depends on fluid intake, diet, and medications. It is said to be pathological if the fluid intake is normal. Adequate fluid intake is essential for normal urine output and to prevent dehydration. It is important to consult the doctor if any changes in the urine output are observed. Urine output is an important criterion for acute kidney injury, and it is an essential diagnostic marker. Medical attention is required to identify the cause of the increase or decrease in urine output and to treat the underlying cause.
