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  • Etymology:
    • The term “redistribution” derives from the Latin roots “re-” meaning again or back, and “distribuere,” meaning to distribute or allocate. In the context of pulmonary blood flow, it refers to the reallocation of blood flow patterns in response to altered pressure gradients or vascular resistance.
  • AKA:
    • Cephalization of blood flow (in upright chest X-rays).
    • Pulmonary vascular redistribution.
  • What is it?
    • Redistribution of blood flow in the lungs occurs when blood is redirected from the lower zones to the upper zones due to increased pulmonary venous pressure or other factors altering normal flow dynamics.
  • Characterized by:
    • Prominence of pulmonary vessels in the upper lung zones relative to the lower zones.
    • A reversal of the usual gravity-dependent pattern of pulmonary perfusion.
  • Anatomically affecting:
    • Pulmonary vasculature, including the pulmonary veins and arteries.
    • Primarily involves the upper and lower lobes of the lungs.
  • Pathophysiology:
    • Increased left atrial pressure, commonly due to left heart failure or mitral valve disease, causes elevated pulmonary venous pressure.
      • Normal left atrial pressures: Typically range from 4-12 mmHg.
      • Pressures leading to redistribution: Redistribution is generally observed when left atrial pressure exceeds 12-20 mmHg, reflecting significant pulmonary venous hypertension.
    • Redistribution most commonly reflects left ventricular end-diastolic pressure (LVEDP) and therefore is a direct indicator of left ventricular pathophysiology.
    • Less commonly, it can also be seen in conditions like mitral regurgitation and mitral stenosis.
    • When the left atrial pressure rises, it reflect by back pressure on the pulmonary veins which in turn reflects on the pulmonary arteries resulting in distention of all the upper lobe vessels- ie redistribution.
  • How does it appear on each relevant imaging modality?
    • Principles:
      • Parts: Pulmonary arteries and veins.
      • Size: Enlargement of upper lobe vessels compared to the lower lobe vessels.
      • Shape: Prominent, engorged upper lobe vessels.
      • Position: In the supine position, blood flow is more evenly distributed throughout the lungs due to the loss of gravitational effects, with lower and upper lobe vessels appearing similarly prominent. In contrast, abnormal redistribution in the supine position still shows enlarged upper lobe vessels, reflecting elevated pulmonary venous pressure or pathology, even without the influence of gravity.
        • Unilateral redistribution: This occurs when blood flow is disproportionately redirected to one lung due to conditions such as:
          • Acute mitral regurgitation, such as in papillary muscle rupture, can contribute to redistribution when the regurgitant jet is directed toward the right pulmonary veins. This mechanism imposes a localized volume overload, causing distention of the pulmonary veins in the affected lung regions and leading to redistribution.
          • Unilateral pulmonary venous obstruction.
          • Large unilateral pleural effusion compressing one lung.
          • Unilateral pulmonary artery stenosis or obstruction.
          • Post-pneumonectomy, where blood flow is redistributed to the remaining lung.
          •  
      • Character: Symmetrical or asymmetrical prominence, depending on underlying pathology.
      • Time: Develops over time with increasing pulmonary venous pressures and resolves with treatment of the underlying cause.
    • CXR:
      • Enlarged upper lobe pulmonary vessels. Assessment of redistribution is best evaluated by comparing the size of the bronchus and the accompanying artery.
        • Normally, the bronchus-to-artery ratio is:
          • Lower lobes: 1.2:1.
          • Mid lung fields: 1:1.
          • Upper lobes: 0.8:1. In redistribution:
        • In redistribution the bronchus-to-artery ratio is:
          • Lower lobes: <1:1.
          • Mid lung fields: >1:1.
          • Upper lobes: 1:1 or greater.
        •  
      • Associated findings: Kerley B lines, cardiomegaly, or interstitial edema in advanced cases.
    • CT:
      • On CT imaging, normal supine positioning shows relatively uniform distribution of blood flow between upper and lower lung zones, as gravity has less influence.
      • Abnormal redistribution in the supine position manifests as engorged upper lobe vessels even in the absence of gravitational effects.
      • Additional findings on CT include signs of elevated pulmonary venous pressure, such as interstitial thickening, ground-glass opacities, or pleural effusion.
    • Other:
      • Swan-Ganz catheter findings reflect left atrial pressure by measuring pulmonary capillary wedge pressure (PCWP) when the balloon catheter is wedged in a pulmonary arteriole.

        Normal PCWP ranges from 4-12 mmHg.

      • Ultrasound (echocardiography) may demonstrate elevated left atrial pressures or valvular disease causing redistribution.

 

  • Differential Diagnosis:
    • Left heart failure (most common).
    • Mitral valve disease.
    • Hypervolemia.
    • Pulmonary venous obstruction.
    • Chronic lung disease with pulmonary hypertension.
  • Recommendations:
    • Further evaluation with echocardiography or cardiac MRI to identify underlying cardiac pathology.
    • Consider treatment directed at reducing pulmonary venous pressure, such as diuretics or management of heart failure.
    • Correlation with clinical and laboratory findings, including BNP levels and cardiac enzymes.
  • Key Points and Pearls:
    • Redistribution of blood flow is a hallmark radiological sign of elevated pulmonary venous pressure.
    • Prominent upper lobe vessels on an upright chest X-ray are suggestive of cephalization.
    • Redistribution is easier to assess on a chest X-ray (CXR) compared to CT, as vascular prominence and gravity-dependent changes are more distinctly visible on upright CXR imaging.
    • Early recognition can guide timely management of underlying cardiac or pulmonary pathology.