'Wet' and 'dry' lungs: a useful sonographic distinction

Pulmonary edema develops when the movement of liquid from the blood to the interstitium exceed the return of the liquid to the blood. The diagnosis of this interstitial expansion is based on chest X-ray and the basic clinical signs appear when the pulmonary compliance is reduced (aspecific dyspnea), a large mismatch exists between ventilation and perfusion or with the onset of alveolar flooding (moist and fine crepitant rales, wheezes or extreme breathlessness). The detection of an interstitial edema is a crucial step in the diagnostic procedure in a dyspneic patient and the intensivist or the emergency physician must make daily therapeutic decisions on the basis of a bedside clinical examination, often difficult, and chest X-ray, wich is known to be often technically deficient.

Methods and patients

During a 4-month period, 83 patients (49 males and 34 females, mean age 74 years) admitted to our Emergency Room (ER) were included in a prospective study. They showed dyspnea (>25 breaths/min) and/or discomfort and signs of augmented work of breath (inspiratory retraction of the intercostal spaces and supraclavicular fossa) or orthopnea. Immediately after the clinical examination, all patients underwent chest sonography. Longitudinal scans of the anterior, lateral and posterolateral (or posterior in the sitting patient) chest wall were taken using a Toshiba SSA250A portable unit equipped with a 3.75 MHz convex transducer. We particularly studied the respiratory motion of the pleuropulmonary surface (gliding sign) and the comet tail artifacts arising from the lung surface. These artifacts are roughly vertical narrow based projections spreading up to the edge of the screen and appear when there is a marked difference in acoustic impedance between subpleural septa thickened by edema and the alveolar air (alveolar-interstitial syndrome). Chest radiographs of all patients performed during the same period of ER evaluation were interpreted by radiologists unaware the sonography findings and classified on the basis of widely accepted criteria. Once the diagnosis of wet lungs was sonographically confirmed, the patients were considered for hearth failure treatment (diuretics and vasodilators) in absence of other diagnostic possibilities or particulary controindications, while the patients with dry lungs underwent advanced diagnostic work up. Finally we evaluated the effect of the diuretic therapy (in 3 h), the correlations between radiologic and sonographic patterns and the usefulness of ultrasonography in the diagnostic approach to the critically ill patients.

Results

All patients were successfully and quickly (<5 min) analysed using ultrasound (feasibility 100%). Twenty-nine subjects (34%) showed `wet lungs' with diffuse bilateral comet tail artifacts. Of these, 21 (72%) had associated pleural effusions (bilateral in 13 cases), with water levels (WL) between 1 and 8 cm. Chest X ray discovered congestion or edema in 30 pts. (flow inversion, enlarged/iperdense ila: 5 pts. blurred ila, perivascular/peribronchial cuffs, Kerley B lines: 13 pts.; patchy alveolar edema: 7 pts. and confluent alveolar edema: 5 pts.), 29 of them exibiting diffuse artifacts. Pleural effusions were shown radiologically in 13 subjects with eight missed diagnoses (effusions with WL, <2 cm). One discordant case was noted (sonographic false negative) but none of 51 patients with normal X rays had significant comet tail artifacts. Sonographic imaging led to a change in the initial diagnosis (hearth failure) in 11 pts. (13% of the whole group studied), six with COPD, three with pulmonary embolism and one with important anemia, these subjects showed normal chest X rays and sonographic `dry lungs'; one patient had the diagnosis changed from COPD to hearth failure.

Conclusion

We think that echography offers a new method for the diagnosis of alveolar-interstitial syndrome at bedside and may provide vital informations when a radiograph is not readily available or undesiderable. Moreover it may be valuable for differentating cardiogenic pulmonary edema from decompensated COPD or pulmonary embolism showing, in our experience, a sensibility of 96% and a specificity near to 100% for diagnosing `wet lungs'.