The function of the kidney is to rid the body of waste produced during metabolism. Removal of waste takes place within the kidneys through a system of nephrons. Contained in each nephron is a glomerulus (a sieve-like filtering structure). As a cat ages, the kidneys lose the ability to filter the blood due to the destruction of the glomeruli. When 80 per cent of these filters are clogged or destroyed, waste levels start to increase in the bloodstream. Outward symptoms such as rapid, shallow breathing (tachypnea) can arise.
Metabolic acidosis can cause rapid, shallow breathing in cats. Metabolic acidosis occurs in 65 per cent to 75 per cent of cats with kidney disease. Excessive urine flow, which commonly accompanies kidney failure, flushes away bicarbonate ions that are needed to prevent acid build up. This causes acid levels to increase and fosters muscle breakdown and weight loss. Vomiting, twitching, weakness, mouth ulcers and breathlessness can all occur with acidosis. Breathing problems caused by metabolic acidosis is referred to as kussmaul breathing.
Anemia is a result of decreased red blood cell production. Red blood cells are manufactured within bone marrow. Cats suffering from kidney disease are unable to produce enough erythropoietin, which is a hormone controlling the production of red blood cells. This can occur in cats of any age but is most common in middle-aged to older cats. Rapid breathing is a symptom of anaemia caused by kidney disease.
Cats can develop fluid build-up during kidney failure. This can be a sign of a deteriorating kidney or heart problems. Fluid retention can also occur from overhydration when manually administering therapy fluids. Fluid collecting in the lungs (pulmonary oedema), around the lungs (pleural effusion) or in the abdomen (ascites) can all cause difficulty breathing. The cat can appear uncomfortable, change positions or refuse certain positions due to breathing difficulties. If the cat is breathing with its mouth open, seek medical attention immediately as this indicates heart problems.
Rapid breathing and an elevated heart rate is a symptom of congestive heart failure. At rest respiratory rate of a cat is about 20 to 30 breaths per minute. Fluid retention is another sign of congestive heart failure. Difficulty breathing, coughing or open-mouth breathing occur with congestive heart failure deriving from kidney failure. In addition, to assist with breathing, the cat might refuse to lie down. Congestive heart failure requires urgent veterinary care.
During renal failure, the cat's body attempts to conserve water. By reabsorbing water from the stool through the intestinal wall, the body can retain liquid needed to help flush an already crippled filtration system. However, the stool is left quite dry and makes passing the stool very difficult. With severe constipation, elevated heart rate and fast breathing might ensue.