Cyanosis is the bluish or purplish discoloration of the mucous membranes or skin due to excessive amounts of desaturated (poorly oxygenated) hemoglobin in the blood stream. Oxygenated blood is red. Poorly oxygenated blood is dark blue. The more deoxygenated hemoglobin in the bloodstream the more bluish coloration will be imparted to the tissues.
There are two general "types" of cyanosis: central and peripheral. Central cyanosis is a result of the entire systemic blood supply being desaturated. Central cyanosis is due to a decrease in oxygenated blood throughout the systemic circulation. All tissues are affected.
Peripheral cyanosis is due to desaturated hemoglobin that may be confined to a specific region of the body, for example, if a blood clot has obstructed blood flow to a particular body part or if a tourniquet has been applied. Peripheral cyanosis implies a purplish coloration in the peripheral tissues (oral mucous membranes, vaginal or penile mucous membranes, paw pads or nail beds, etc). All animals with central cyanosis also have peripheral cyanosis, because the entire bloodstream is desaturated. However, it is possible to have peripheral cyanosis without having central cyanosis, if the cause of the decreased oxygenation is localized to a specific region, such as a blood clot that interrupts the blood supply to a specific limb.
In young animals, the most likely cause is a congenital heart disease where poorly oxygenated blood that is returning to the heart erroneously bypasses the lungs and is sent back out into the systemic circulation without picking up more oxygen. This is called "right-to-left shunting" because poorly oxygenated blood from the right side of the heart is shunted to the left side of the heart where it is pumped out into the general circulation.
Any age animal can develop cyanosis secondary to severe pulmonary (lung) disease, such as severe pneumonia, or to diseases that prevent the lungs from expanding properly, such as fluid or air in the chest cavity.
Genetic defects in hemoglobin can alter its ability to carry oxygen, however, these defects are rare in companion animals. Hemoglobin defects are more likely to develop secondary to ingestion of, or exposure to, chemicals and oxidants.
Causes of peripheral cyanosis include:
Anything that would cause central cyanosis, with resultant bluish coloration in all peripheral tissues.
Hypothermia. The low body temperature constricts the vessels in the skin.
Thromboembolism, or a blood clot. The most common situation is a "saddle thrombus" which occurs in cats with heart disease. A blood clot develops in the heart and travels down the aorta where it obstructs the blood supply to both rear legs.)
Application of a tourniquet (accidental, deliberate, or malicious)
Shock (inadequate blood flow to the tissues)
Causes of central cyanosis would include:
Congenital heart disease
Tetralogy of Fallot, which is a genetic defect involving four abnormalities of the heart and great vessels
Atrial septal defect (the proverbial "hole in the heart"), with subsequent right-to-left shunting
Ventricular septal defect ("hole in the heart"), with subsequent right-to-left shunting
Reversed patent ductus arteriosus (this congenital defect does not initially cause cyanosis. Only if it goes undetected or untreated so that blood flow in the heart reverses, can changes occur that lead to cyanosis.)
Hypoventilation (inability to breathe properly)
Pleural effusion (blood, pus, lymphatic fluid, cancer-induced fluid, etc. that gathers in the chest cavity and prevents the lungs from expanding properly).
Respiratory muscle failure
Due to muscle disorder (like a diaphragmatic hernia)
Due to neurologic disease
Tumor, abscess, granuloma, foreign body obstructing a large airway
Inadequate oxygen due to improperly administered anesthesia
Ventilation-perfusion mismatch (improper blood supply to the lung, combined with improper lung function, or both)
Pulmonary thromboembolism (blood clot in the lungs)
Infiltration of the lung tissue with fluid (edema)
Inflammatory cells (infection, inflammation)
Acute respiratory distress syndrome (ARDS)
Pulmonary fibrosis (pulmonary scar tissue)
Abnormal hemoglobin (methemoglobin) can result in cyanosis due to chemicals that render the hemoglobin nonfunctional.