What Are the Causes of Respiratory Perfusion Limitation?

For proper gas exchange to occur in the lungs, air must closely meet the blood that perfuses through the capillary bed surrounding each alveoli, or air sac. Gas can then move back and forth across the alveolar capillary membrane. A perfusion limitation exists wherever the perfusion or flow of capillary blood around the alveolus is insufficient.

Emboli Are the Most Common Cause of Perfusion Limitation

• Different types of substances can become emboli.

  An embolus is any undissolved material in the bloodstream. Emboli can obstruct blood vessels, block blood flow and limit perfusion. Commonly, they are made of clotted blood, fat globules, cellular tissue, gas bubbles or bacteria and other parasites. The most common pulmonary embolus is a blood clot or thrombus; these form from abnormalities in blood vessel walls, from drugs that increase clotting or from coagulation abnormalities. Fat emboli sometimes occur after long-bone fractures if bone marrow enters the blood. Acute pancreatitis, intravenous lipid infusion, liposuction and cardiopulmonary bypass (CPB) also can cause fat embolism. Amniotic fluid emboli made of fluid and fetal cells can sometimes enter the mother's bloodstream during cesarean section or normal delivery.

Pulmonary Artery Pressure

• The pulmonary artery is the only artery that carries deoxygenated blood.

  The pulmonary artery brings low-oxygen blood from the heart to the lungs. Both increased pressure and decreased pressure in the pulmonary artery can cause perfusion limitation in the lungs. Narrowing of the artery, low cardiac output from the heart and low overall blood volume can affect pulmonary artery pressure and lung perfusion.

Acute Respiratory Distress Syndrome (ARDS)

• Fluid in the lungs looks white on xrays.

  Acute respiratory distress syndrome (ARDS) describes a variety of relatively sudden-onset lung problems that can disrupt perfusion to the lungs. One is acute pulmonary vasoconstriction, or pulmonary hypertension, where the lung capillaries constrict in response to low oxygen levels. Another type of ARDS is lung contusion from traumatic accidents that directly damage blood capillaries. Pulmonary edema is another form of ARDS in which fluid leaks into the alveoli from the capillaries, causing lowered perfusion as well as an interruption of gas diffusion.

Chronic Lung Conditions

• Chronic lung disease can be genetic or environmental.

  Long-term, chronic conditions can also limit lung perfusion. In emphysema, loss of tissue elasticity destroys alveolar walls and capillaries. This can also occur in people of advanced age. Lung fibrosis, or thickening of the alveolar capillary membrane, can destroy capillary beds and interfere with perfusion. Some inhalants, drugs and radiation and disorders such as collagen disease can cause fibrosis as well. Rarer conditions that interfere with lung perfusion are bronchogenic carcinoma and histoplasmosis.

Mechanical Ventilation

• Anesthetists control the patient's breathing during surgery.

  Hospital-administered mechanical ventilation can over-distend the alveoli with gas pressures and large air volumes that interrupt capillary blood flow. Direct compression on the base of the lungs during surgery sometimes occurs, as in laparoscopic surgery when the abdominal cavity is purposefully distended with carbon dioxide. This pressure can interfere with lung perfusion.