Blood flow redistribution, also referred to as cephalization of pulmonary blood flow, is a radiological finding on a chest X-ray where blood flow in the upper lobes of the lungs becomes more prominent than in the lower lobes. This occurs due to increased pulmonary venous pressure, most commonly in the context of left-sided heart failure.

Pathophysiology
Normally, pulmonary blood flow is gravity-dependent, with greater perfusion in the lower lobes when a patient is upright.
In conditions causing elevated left atrial pressure (e.g., heart failure, mitral valve disease):
Pulmonary venous pressure increases.
Vessels in the upper lobes, which are normally less distended, become engorged due to backpressure.
This results in a redistribution of blood flow, with upper lobe vessels appearing larger or more prominent than lower lobe vessels.
Radiological Features
Chest X-Ray (Upright):

Upper Lobe Vessel Enlargement:
Upper lobe pulmonary veins become as large as or larger than the lower lobe veins.
Lower Lobe Vessel Attenuation:
Decreased visibility or narrowing of lower lobe vessels compared to the upper lobes.
Overall Appearance:
Gives the impression of increased vascular markings in the upper zones.
Cardiomegaly:
Often present in the setting of heart failure.
Other Findings Suggesting Pulmonary Venous Hypertension:

Interstitial Edema:
Kerley B lines (short, horizontal lines at the lung bases).
Alveolar Edema:
“Bat-wing” or perihilar opacities in severe cases.
Pleural Effusion:
Often seen bilaterally.
Clinical Context
Blood flow redistribution is a hallmark of Stage 1 pulmonary congestion due to elevated left atrial pressure:
Commonly seen in acute decompensated heart failure.
May also occur in conditions like volume overload or mitral valve stenosis.
Diagnostic Considerations
Upright vs. Supine Imaging:
Redistribution is best evaluated on an upright chest X-ray; a supine film may mask the finding.
Corroborative Testing:
Echocardiography to assess left ventricular function.
Brain natriuretic peptide (BNP) to confirm heart failure.
Clinical Relevance
Blood flow redistribution is an early indicator of pulmonary venous hypertension and left-sided heart failure. Recognizing this finding on chest X-ray can prompt timely evaluation and management of the underlying cardiac dysfunction.