Hypovolaemic Acute Kidney Injury

Hypovolaemic acute kidney injury is a fall in glomerular filtration caused by reduced effective circulating volume, which lowers renal perfusion pressure. In the early phase the kidney is structurally intact and the injury is functional, reversible within 24 to 72 hours of volume restoration. If hypoperfusion persists beyond this window, ischaemic tubular necrosis follows.

The kidney defends filtration through afferent arteriolar dilation (prostaglandins) and efferent arteriolar constriction (angiotensin II), and increases proximal reabsorption of sodium, water and urea under the influence of angiotensin II, noradrenaline and vasopressin. This produces concentrated, sodium avid urine with a raised urea:creatinine ratio. Drugs that blunt these defences (NSAIDs, ACE inhibitors, angiotensin receptor blockers, SGLT2 inhibitors, diuretics) convert tolerated volume depletion into acute kidney injury.

Causes:

• Gastrointestinal: vomiting, diarrhoea (including cholera), high output stoma, fistula
• Renal: diuretics, osmotic diuresis in uncontrolled diabetes, post-obstructive diuresis, polyuric recovery phase, salt wasting disorders, adrenal insufficiency
• Skin and insensible: burns, heat exposure, fever, excessive sweating
• Third space sequestration: pancreatitis, bowel obstruction, peritonitis, crush injury, hypoalbuminaemia
• Inadequate intake: dementia, sedation, fasting, poor access to water

Elevated risk groups are older adults, patients with chronic kidney disease, diabetes, heart failure and infants.

• Thirst, reduced urine output (below 0.5 mL/kg/h), dark concentrated urine
• Postural hypotension, tachycardia, dry mucous membranes, reduced skin turgor, low jugular venous pressure, cool peripheries, slow capillary refill (above 3 seconds)
• Weight loss from baseline; sunken eyes and fontanelle in infants
• Confusion and drowsiness in older adults
• History of the underlying loss: vomiting, diarrhoea, bleeding, burns, diuretic use

Red flags: systolic pressure below 90 mmHg, lactate above 2 mmol/L, anuria, potassium above 6.0 mmol/L, bicarbonate below 15 mmol/L, altered consciousness, signs of shock in children (weak pulse, cold extremities, capillary refill above 3 seconds).

• Serial urea, creatinine, electrolytes, bicarbonate, lactate; BUN:creatinine ratio above 20:1 (urea in mmol/L to creatinine in micromol/L ratio above 0.08)
• Urinalysis: concentrated, specific gravity above 1.020, bland sediment, hyaline casts
• Urine indices: urine sodium below 20 mmol/L, fractional excretion of sodium below 1%, fractional excretion of urea below 35%, urine osmolality above 500 mOsm/kg
• Bedside ultrasound: collapsing inferior vena cava (below 1.5 cm with more than 50% inspiratory collapse), small left ventricular cavity
• Dynamic assessment: passive leg raise raising stroke volume by 10% or more
• Response to a fluid challenge (250 to 500 mL over 15 to 30 minutes) with improved urine output and falling creatinine within 24 to 48 hours confirms the diagnosis
• Renal ultrasound to exclude obstruction

Differentials: acute tubular injury (urine sodium above 40 mmol/L, muddy brown casts, urine osmolality below 350 mOsm/kg, no response to fluid), cardiorenal syndrome, hepatorenal syndrome, obstruction, drug induced haemodynamic injury.

Resuscitation

• Balanced crystalloid (Ringer lactate or equivalent) 500 mL over 15 minutes in adults (250 mL in older or frail patients; 10 to 20 mL/kg in children), reassessing perfusion after each bolus; repeat until haemodynamics recover, up to 30 mL/kg, then switch to maintenance.
• Avoid large volumes of 0.9% sodium chloride because of hyperchloraemic acidosis; use it when hypochloraemic alkalosis from vomiting is present.
• Albumin 20% 1 g/kg (maximum 100 g) daily for hypovolaemia in cirrhosis.
• Blood for haemorrhage.
• Vasopressors only after volume is restored or if life threatening hypotension persists.

Cholera and severe diarrhoea

• Moderate dehydration: oral rehydration solution 75 mL/kg over 4 hours; severe dehydration in adults and children over 12 months: Ringer lactate 100 mL/kg, 30 mL/kg within 30 minutes then 70 mL/kg over 2.5 hours, plus oral rehydration solution once tolerated.
• Zinc 20 mg daily for 10 days in children over 6 months.

Remove contributors

• Hold diuretics, NSAIDs, ACE inhibitors, angiotensin receptor blockers, SGLT2 inhibitors and metformin until creatinine is back to baseline and oral intake is normal.
• Treat the cause: antiemetics (ondansetron 4 mg), antidiarrhoeals only in non-infective diarrhoea, insulin for hyperglycaemia, hydrocortisone for adrenal insufficiency.

Maintenance and monitoring

• Maintenance fluid 25 to 30 mL/kg daily plus replacement of ongoing losses.
• Urine output target above 0.5 mL/kg/h; creatinine every 12 to 24 hours; daily weights.
• Potassium: correct with 40 mmol/L added to fluid when below 3.5 mmol/L.

Escalation. Persistent oliguria after 6 hours of adequate resuscitation, rising creatinine at 48 hours, or evidence of tubular injury requires nephrology review; renal replacement therapy for refractory hyperkalaemia, acidosis (pH below 7.15), overload or uraemic complications.

Special groups. Children: calculate fluid by weight, use oral route when possible, and monitor sodium (isotonic maintenance fluid reduces iatrogenic hyponatraemia). Older adults: smaller aliquots and early reassessment of lungs.