fig 1.

Principles of the technique.

A, After predicting the appearance of a periodic comet, Sir Edmund Halley, in 1716, proposed that a diving bell could provide a pressurized compartment for deep-sea exploration. Air could not escape because the bell was closed at the top and sides. Water could not enter at the open bottom because buoyant air was trapped inside the bell, forming a stable chamber.

B, Our device traps a small column of CO₂ in an artery by blocking proximal blood flow with a balloon and by positioning the target artery subhorizontally. The injected gas evacuates blood and buoys itself against the balloon.

C, The column of CO₂, a negative contrast medium, is visible at fluoroscopy (arrows).