PULMONARY EDEMA (RADIOGRAPHIC FEATURES)

Development of pulmonary edema (increased extravascular lung water) is a common problem . There are three principal varieties:

1.cardiac, commonly resulting from myocardial or valvular heart disease;

2. overhydration, usually caused by excess saline effusion or renal failure with retention of salt and water;

3.capillary permeability, which can be caused by a wide variety of pathologic, traumatic, and infective conditions resulting in injury to the pulmonary microvasculature.

The three principal mechanisms of edema formation are

(1) increased hydrostatic pressure gradient across the capillary wall

(2) diminished osmotic pressure gradient across the wall

 (3) increased capillary permeability (damage to the endothelial cell junctions, which permits both fluid and large molecules to leak out of the vessels). Change in plasma oncotic pressure is usually a contributory rather than a primary cause of pulmonary edema.

(4) A fourth, and often neglected factor, is the ability of the lymphatics to remove excess extravascular lung water (EVLW).

The chest film remains the most frequently used clinical method for the diagnosis of pulmonary edema

The three principal radiologic factors that had the greatest statistical significance in determining  type of edema  are the distribution of pulmonary blood flow,distribution of pulmonary edema, and the width of the vascular pedicle.

Distribution of pulmonary blood flow:

A, Normal; occurs principally in capillary permeability edema.

B, Balanced; occurs principally in overhydration or renal failure.

c. Inverted; occurs principally in cardiac failure.

Distribution of pulmonary edema.

There are three principal patterns, each of which corresponds to specific type of edema:

A. Even: principally basal (gravitational) and homogeneous from chest wall to heart but with perihilar component also. Principally occurs in cardiac edema.

B, Central; occurs principally in overhydration or renal failure.

C, Peripheral; markedly patchy,often spares costophrenic angles. Note air bronchograms.This type of distribution occurs almost exclusively in capillary permeability.

Vascular pedicle width(VPW). There are  three possible variations:

 A, Normal; commonest in capillary permeability or acute cardiac failure.

B, Widened; commonest in overhydration/renal failure and chronic cardiac failure.

C, Narrowed;commonest in capillary permeability edema.

 Normal VPW (for an erect 70-kg patient) is defined as 43-53 mm (48 mm ± 1 SD), diminished is less than 43 mm, and increased is more than 53 mm .

Septal Lines, Peribronchial Cuffing, Air Bronchograms, and Pleural Effusions :None of these factors is of much value in differentiating cardiac from  renal/overhydration edema, but they are value in differentiating capillary permeability edema from the other two varieties.

Radiographic features of pulmonary edema (SUMMARY)

                                                      Cardiac                    renal                         injury

1.Heart size            :                   enlarged             enlarged                     normal

2.Vascular pedicle  :                normal /enlarged    enlarged           normal/reduced

3.Pulmonary blood   :              inverted                  balanced             normal/balanced

   Flow distribution

4.Pulmonary blood    :             normal/increased       increased          normal

Volume

5.Septal line               :               not common              not common        absent

6.Peribronchial cuffs:              very common           very common       not common

7.Air bronchogram :                 not common        not common            very common

8.Lung edema ,regional

distribution(horizontal axis):  even                central                              peripheral

9.Pleural effusion   :                  very common        very common        not common