Objective: The behavior of the human microcirculation in the setting of cardiac arrest is largely unknown. Animal experiments have consistently revealed that global hemodynamics do not necessarily reflect microvascular perfusion. In addition, the time it takes for capillary blood flow to stop after the heart arrests is debated. Estimations range from 50 seconds to 5 mins, but data in humans are lacking. Aortic arch surgery frequently necessitates deep hypothermic circulatory arrest and subsequent selective antegrade cerebral perfusion. To elucidate microvascular behavior surrounding cessation of human circulation, we used sublingual microvascular imaging in this setting. DESIGN: Prospective, observational study. Setting: Operating room of a large tertiary referral center for cardiac surgery. Patients: Seven patients undergoing elective aortic arch repair. Interventions: We used sidestream dark field imaging to study the sublingual microcirculation immediately before circulatory arrest, during circulatory arrest, and immediately after selective antegrade cerebral perfusion. Measurements and main results: Results are reported as mean (sd) unless indicated otherwise. Before circulatory arrest, perfused vessel density was 6.41 (1.18) for small (<20 μm) and 1.57 (0.88) mm for large (>20 μm) microvessels. Microvascular flow index was a median of 3.0 (interquartile range 3.0-3.0) for both vessel sizes. After circulatory arrest, there was no equilibration of arterial and venous blood pressure before onset of selective antegrade cerebral perfusion after 59 (17) secs (range, 40-80 secs). Flow in small microvessels came to a complete stop after 45 (9) secs (range, 34-57 secs) after transition to circulatory arrest. However, flow in larger microvessels did not completely stop before selective antegrade cerebral perfusion started. Selective antegrade cerebral perfusion restored microvascular flow, reaching precirculatory arrest levels after 45 (27) secs (range, 20-85 secs). Conclusions: In a controlled surgical setting, circulatory arrest in humans induces a complete sublingual small microvessel shutdown within 1 min. However, flow in larger microvessels persists. Selective antegrade cerebral perfusion was able to restore microvascular flow to precirculatory arrest levels within a similar timeframe.