Cause of spontaneous echocardiographic contrast as assessed by scanning electron microscopy.

Spontaneous echo contrast ("echocardiographic smoke") is known to occur in low-flow states and to require the presence of red cells and plasma proteins. Limited morphologic information regarding the microanatomic structure of red cells exhibiting spontaneous echo contrast is known. The study was designed to evaluate the microanatomic features of red cells exhibiting spontaneous echocardiographic contrast with scanning electron microscopy. With human blood, a beaker, and a stirring bar, a simple model for demonstration of spontaneous echo contrast and its reversal was devised. Blood elements were "sampled" from within this model at times of high and low spontaneous echogenicity by adherence of blood elements to poly-L-lysine-coated slides that were subsequently fixed and examined with scanning electron microscopy. Spontaneous echo contrast was maximal at complete stasis or low-flow states and could be abolished by agitation of blood with continuous stirring. Sampling during low-flow states with high echogenicity showed a preponderance of clumped red cells, whereas at high-flow and low echogenicity states red cells were dispersed and usually solitary. No morphologic features suggestive of activation of the coagulation system were noted. Spontaneous echo contrast is caused by reversible red blood cell clumping, which occurs in fresh human blood at low-flow (low shear rate) states and can be abolished by increasing flow. This phenomenon is independent of activation of the clotting system.