The cranial vena cava is responsible for draining the unoxygenated blood from the head, thorax and forelimbs into the right atrium. Meanwhile, the caudal vena cava drains unoxygenated blood from the abdomen and the hindlimbs into the right atrium. Therefore, when pressures increase in the right side of the heart, specifically the right atrium, the cranial and caudal vena cava experience an increase in hydrostatic pressure. This increase in hydrostatic pressure in the cranial vena cava or the caudal vena cava causes fluid flux out of the vessels and into the pleural cavity or abdominal cavity respectively.
During the evaluation of thoracic radiographs, fluid in the pleural cavity can be detected by observing certain radiographic features such as: fissure lines and the leafing of lung lobes.
Fissures are the normal divides between the lobes of the lungs; they are not normally visible radiographically, but become radiographically visible in the presence of fluid in the pleural space or under conditions that cause the pleural lining of the lung to become thickened. It should be noted that the most common cause of fissure lines is pleural thickening. When there is pleural effusion however, the fluid collects within the fissure, wedges the lobes apart (with an agent [fluid] that contrasts with the radiopaque lungs) making the fissure radiographically visible. The fissure will generally appear as a discrete radiopaque line. Fissure lines created by pleural fluid are wider at the lung lobe margins and become narrower centrally. If a linear structure is observed in an area that doesn't normally have a lung fissure, then other differentials must be investigated.
A - Fissures of the lateral aspect of the left lung (looking medial to lateral), usually seen when patient is in left lateral recumbency.
B - Fissures located on the lateral aspect of the right lung (looking medial to lateral), usually seen when patient is in right lateral recumbency
C - Fissures located on the dorsal aspect of the lungs and are seen when the patient is in dorsal recumbency.
D - Fissures located on the ventral aspect of the lungs and are seen when the patient is in sternal recumbency.
Fissure Line - example 1
Fissure Line - example 2
Both fissure lines and lobar signs appear as lines along the lobes of the lung, they both occur at the same location (along a lung lobe fissure) but they are easily differentiated. Radiographically, a fissure line is observed as a single white line due to the wicking of fluid between the two lung lobes (image: left below). A lobar sign is represented by a white margin adjacent to an area of radiolucency. This white margin represents the edge of a lung lobe that is consolidated often due to the presence of exudates, edema and/or hemorrhage. These are important to differentiate because they represent different disease processes. One represents fluid collecting in the pleural space (fissure line due to fluid) and the other represents flooding of the alveoli with fluid (note that exudates and hemorrhage are essentially fluid).
The same basic principles behind lung fissure lines apply to the leafing of the lung lobes that is sometimes visible in a patient's radiograph. Instead of involving one lung fissure, leafing of the lung lobes occurs when there is enough fluid to surround multiple lung lobes, the lobes are displaced from the chest wall by fluid and fissure lines are very wide. Leafing of lung lobes and fissure lines represent two ends of a spectrum associated with fluid accumulation in the pleural space; leafing represents marked fluid accumulation and fissure line represent mild fluid accumulation. In DV or VD views, the outlines of the lung lobes and fissures can be clearly seen and was originally described as looking like staggered, layered leaves. Again, leafing of lung lobes will appear identically on radiographs regardless of the fluid type found in the pleural space around the lungs. Therefore, evidence of leafing doesn't automatically specify the lung's condition as cardiogenic and other causes and types of pleural fluid should be investigated
Leafing of Lung Lobes - example 1