Pulmonary edema is the extravascular accumulation of fluid in the lung. This pathologic condition may be caused by one or more physiologic abnormalities, but the result is inevitably impaired gas transfer. As the edema fluid builds up, first in the interstitium and later in alveoli, diffusion of gases – particularly oxygen – decreases.
The capillary endothelium is much more permeable to water and solutes than is the alveolar epithelium. Edema fluid therefore accumulates in the interstitium before it accumulates in the alveoli.
The Starling equation describes the movement of liquid across the capillary endothelium, see the figure above. This equation describes the basic parameters that determine the net fluid movement between the vessel and the interstitium. On the other hand, the lymphatic drainage is another important factor which relieves the interstitial fluid accumulation by simply drainage – removing fluid from the interstitium. Any fluid that makes its way into the pulmonary interstitium must be removed by the lymphatic drainage of the lung. The pulmonary lymphatic vessels are mainly located in the extra-alveolar interstitium. The volume of lymph flow from the human lung is now believed to be as great as that from other organs under normal circumstances, and it is capable of increasing as much as 10-fold under pathologic conditions. It is only when this large safety factor is overwhelmed that pulmonary edema occurs.
Conditions That May Lead to Pulmonary Edema
The Starling equation provides a useful method of categorizing most of the potential causes of pulmonary edema.
Infections, circulating or inhaled toxins, oxygen toxicity, and other factors that destroy the integrity of the capillary endothelium can lead to localized or generalized pulmonary edema.
Capillary Hydrostatic Pressure (Pc)
The capillary hydrostatic pressure is estimated to be about 10 mm Hg under normal conditions. If the capillary hydrostatic pressure increases dramatically, the filtration of fluid across the capillary endothelium will increase greatly, and enough fluid may leave the capillaries to exceed the lymphatic drainage. The pulmonary capillary hydrostatic pressure often increases secondary to problems in the left side of the circulation, such as infarction of the left venticle, left ventricular failure, or mitral stenosis. As left atrial pressure and pulmonary venous pressure rise because of accumulating blood, the pulmonary capillary hydrostatic pressure also increases. Other causes of elevated pulmonary capillary hydrostatic pressure include overzealous administration of intravenous fluids by the physician and disease that occlude the pulmonary veins.
Interstitial Hydrostatic Pressure (Pis)
Some investigators believe the interstitial hydrostatic pressure of the lung to be slightly positive, whereas others have shown evidence that it may be in the range of -5 to -7 mm Hg. Conditions that would decrease the interstitial pressure would increase the tendency for pulmonary edema to develop. These include forced inspiration against an upper airway obstruction and potential actions of the physician, such as rapid evacuation of chest fluids or reduction of pneumothorax. Situations that increase alveolar surface tension, for example, when decreased amounts of pulmonary surfactant are present, could also make the interstitial hydrostatic pressure more negative and increase the tendency for the formation of pulmonary edema. Note that as fluid accumulates in the interstitium, the interstitial hydrostatic pressure increases, which helps limit further fluid extravasation.
The Reflection Coefficient (σ)
Any situation that permits more solute to leave the capillaries will lead to more fluid movement out of the vascular space.
Plasma Colloid Osmotic Pressure (πpl )
Decreased in the colloid osmotic pressure of the plasma, which helps retain fluid in the capillaries, may lead to pulmonary edema. Plasma colloid osmotic pressure, normally in the range of 25 to 28 mm Hg, falls in hypoproteinemia or over administration of intravenous solutions.
Interstitial Colloid Osmotic Pressure (πis)
Increased concentration of solute in the interstitium will pull fluid from the capillaries.
Conditions that block the lymphatic drainage of the lung, such as tumors or scars, may predispose patients to pulmonary edema.
Other Conditions Associated with Pulmonary Edema
Pulmonary edema is often seen associated with head injury, heroin overdose, and high altitude. The causes of the edema formation in these conditions are not known, although high-altitude pulmonary edema may be partly caused by high pulmonary artery pressures secondary to the hypoxic pulmonary vasoconstriction.