An ultrasonographic sign of intrapulmonary shunt

Excerpt

Lung ultrasound is a valuable bedside tool for the differential diagnosis of hypoxemia, a frequent issue in critically ill patients. In particular, a tissue-like pattern visualized above the diaphragm corresponds to complete loss of aeration, defined as lung consolidation (Fig. 1a). A consolidated lobe may have a variable impact on oxygenation, depending on hypoxic vasoconstriction and degree of intrapulmonary shunt. Color Doppler ultrasound may be used to identify vessels within consolidations: in some patients, intrapulmonary arteries can be visualized (Fig. 1b), identified by a pulsing pattern synchronous with heartbeats. In these patients, the non-aerated lobe appears to be well perfused, which corresponds to the definition of intrapulmonary shunt. Although no quantification is possible, the visualization of a main vessel within a consolidation is strongly suggestive of significant shunt and therefore of an important impact of consolidation on oxygenation. This pattern appeared in a mechanically ventilated patient affected by diffuse alveolar hemorrhage (Fig. 1c), in which PaO₂/FiO₂ suddenly worsened from 130 to 65. Once an intrapulmonary shunt is identified, hypoxemia treatment should focus on improving lung aeration and/or reorienting blood flow (fiberbronchoscopy, recruitment maneuver, prone/lateral position, nitric oxide). In our patient, reaeration of lower lobes was obtained by fiberbronchoscopic suction of obstructing clots, with subsequent dramatic improvement of oxygenation.

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