Azotaemia

Introduction

Azotaemia refers to an increase in the blood concentrations of urea and creatinine. Azotaemia occurs almost exclusively with renal failure that results in an inability to excrete these two metabolic products. Urea and Creatinine vary in the extent of the increase induced by renal pathologies (creatinine can show substantial increases from baseline, whereas smaller increases in urea are significant as levels generally increase by a much smaller factor) and also the timing of the increase - creatinine levels will rise and fall more quickly than changes in urea and can therefore be more useful in monitoring response to treatment.

Clinical Significance

Azotaemia may cause the clinical syndrome of uraemia, affecting multiple organ systems. The causes of azotaemia are usually divided into pre-renal, renal and post-renal:

Pre-renal azotaemia

This occurs when the kidney receives insufficient oxygen via blood flow to maintain its normal metabolic processes. Conditions that can cause this include:

• Dehydration/hypovolaemia, which reduces renal blood flow.
• Heart failure which results in a reduced cardiac output, thereby reducing renal blood flow.
• Anaemia reduces the amount of oxygen delivered to the kidneys.
• Portal hypertension may result in splanchnic pooling of blood and reduced renal blood flow.

Renal azotaemia

This occurs following a direct insult to the kidney from several possible causes including:

• Exogenous toxins including heavy metals, aminoglycosides, tetracyclines, polymixin B, cisplatin, antifreeze (ethylene glycol), lilies in cats and grapes in dogs.
• Endogenous toxins including haemoglobin, myoglobin and calcium.
• Infections such as pyelonephritis, interstitial nephritis (as caused by Leptospira spp. or granulomatous nephritis (as caused by the dry form of Feline Infectious Peritonitis).
• Glomerulonephritis and amyloidosis.
• Neoplasia, notably adenocarcinoma in dogs and lymphoma in cats.

Post-renal azotaemia

This occurs following obstructions within the urinary tract:

• Urolithiasis
• Rupture of the urinary tract, often due to trauma and causing uroabdomen.
• Neoplasia, including transitional cell carcinoma of the bladder and prostatic neoplasia.