A-lines and B-lines: lung ultrasound as a bedside tool for predicting pulmonary artery occlusion pressure in the critically ill.

BACKGROUND: The risk of pulmonary edema is the main limiting factor in fluid therapy in the critically ill. Interstitial edema is a subclinical step that precedes alveolar edema. This study assesses a bedside tool for detecting interstitial edema, lung ultrasound. The A-line is a horizontal artifact indicating a normal lung surface. The B-line is a kind of comet-tail artifact indicating subpleural interstitial edema. The relationship between anterior interstitial edema detected by lung ultrasound and the pulmonary artery occlusion pressure (PAOP) value was investigated.

METHOD: We performed a prospective study in medicosurgical ICUs of university-affiliated teaching hospitals. We enrolled 102 consecutive mechanically ventilated patients who all underwent pulmonary artery catheterization. We defined A-predominance as a majority of anterior A-lines and B-predominance as a majority of anterior B-lines. These patterns were correlated with PAOP.

RESULTS: For diagnosing PAOP <or= 13 mm Hg, A-predominance had 90% specificity, 67% sensitivity, 91% positive predictive value, and 65% negative predictive value. For diagnosing PAOP <or= 18 mm Hg, A-predominance had 93% specificity, 50% sensitivity, 97% positive predictive value, and 24% negative predictive value, respectively.

CONCLUSIONS: A-predominance indicates dry interlobular septa. Specific to predicting a low PAOP value, A-predominance suggests that fluid may be given without initial concern for the development of hydrostatic pulmonary edema. B-predominance indicates interstitial syndrome, which is usually related to interstitial edema. B-predominance is observed in a wide range of PAOP values, precluding conclusions about the need for fluid therapy. This bedside potential will be appreciated by those intensivists who envision fluid therapy based on low PAOP values and who consider that using the concept of a safety factor provided by lung ultrasound is logical.