shunt: one construct, many systems

In respiratory and cardiovascular physiology, a shunt is blood that reaches the arterial side without being oxygenated, because it bypasses ventilated lung or crosses an abnormal communication between the heart's chambers. Riley and Cournand's 1949 analysis of ventilation–perfusion relationships set out how this venous admixture is inferred from the gap between ideal and measured arterial oxygenation. The referent is a portion of the circulation taking an alternate route around the normal gas-exchanging path, reported as a fraction of cardiac output, the same bypass construct that appears in a drained ventricle and in a parallel circuit branch.[1]

Unit

The shunt fraction: the proportion of cardiac output that bypasses gas exchange

Recovery means

Not applicable

Measured by

Estimated from the alveolar–arterial oxygen difference, often via the shunt equation while breathing high inspired oxygen

In neurosurgery, a shunt is an implanted catheter-and-valve system that drains surplus cerebrospinal fluid from the brain's ventricles to another body cavity, relieving the raised pressure of hydrocephalus. Nulsen and Becker's 1967 report on valve-regulated shunting is part of the work that turned chronic CSF diversion into a durable treatment. Here the construct is an engineered bypass that reroutes fluid around an obstructed or overfilled pathway, judged by whether it holds pressure steady without over- or under-draining.[2]

Unit

The opening pressure and flow the valve maintains between ventricle and drainage site

Recovery means

Not applicable

Measured by

Assessed by control of intracranial pressure and ventricle size, and by the valve's opening-pressure and flow characteristics

In electrical engineering, a shunt is a deliberate low-resistance path placed in parallel with a component so that a known, large share of the current bypasses it; the classic case is the ammeter shunt that lets a sensitive meter read heavy currents. Horowitz and Hill's The art of electronics treats such parallel current-dividing elements. The referent is a parallel branch diverting a defined fraction of a flow around the main element, fixed by the ratio of the resistances, the engineering meaning from which medicine and physiology borrowed the word for a bypass of flow.[3]

Unit

The fraction of current diverted, set by the parallel resistance ratio

Recovery means

Not applicable

Measured by

Determined from the shunt and load resistances by the current-divider relationship