A redout is a temporary reddening of vision caused by extreme acceleration forces acting on the body. This condition is the inverse of the more commonly known blackout. It occurs when forces push fluids toward the head, an occurrence almost exclusively associated with high-performance aviation maneuvers. The redout is a distinct warning sign that the body is under severe physiological stress.
Defining the Redout Phenomenon
A redout is caused by exposure to negative G-forces (-Gz), which are forces directed from the feet toward the head. The visual field appears uniformly red or pink because immense pressure causes blood to engorge the delicate vessels of the eyelids and conjunctiva. Light passing through this blood-laden tissue creates the perceived “red curtain” effect across the field of view.
Negative G-forces are most often encountered in specific aerobatic maneuvers, such as a sharp push-over into a dive or the execution of an outside loop. These maneuvers subject the body to an upward force that pulls internal organs against the seat restraints. Initial symptoms of visual reddening can begin at relatively low forces, sometimes starting around -2 to -3 Gz. While less common than a blackout, the redout is a serious indicator of extreme physiological strain.
The Physiological Mechanism
The mechanism behind a redout is the abrupt shift of blood toward the upper body and head. Negative G-forces overwhelm cardiovascular mechanisms, causing a rapid increase in venous pressure in the cranial vessels. This cephalad fluid shift forces a greater volume of blood into the head, leading to headward hyperemia. The surge in blood volume dramatically elevates the hydrostatic pressure within the arteries and veins of the brain and eyes.
This intense pressure causes the fine capillaries and blood vessels in the face, eyelids, and retinas to become severely congested. The blood vessels of the eye’s conjunctiva and sclera dilate and engorge under the strain, contributing to the visual red hue. Individuals frequently experience intense headache, facial flushing, and a sensation of the eyes bulging outward. Sustained exposure risks rupturing small blood vessels, which can lead to temporary hemorrhages, retinal damage, or hemorrhagic stroke.
Redout Versus Blackout
The redout and the blackout represent opposite sides of the acceleration spectrum, defined by the direction of the G-force and the resulting displacement of blood. A redout is caused by negative G-forces, pushing blood toward the head, while a blackout is caused by positive G-forces (+Gz), pulling blood toward the lower extremities. Blackouts, or G-induced Loss of Consciousness (G-LOC), are preceded by a greyout where vision fades and peripheral sight narrows into tunnel vision. This visual degradation is caused by a lack of oxygenated blood reaching the brain.
In contrast, the redout is characterized by too much blood in the head, resulting in red-tinged vision rather than vision loss from cerebral hypoxia. Positive G-forces are the greater concern for immediate loss of consciousness, with G-LOC occurring around +4 to +6 Gz for an untrained individual. Negative G-forces are less likely to result in immediate unconsciousness, though redout symptoms start at smaller magnitudes. However, the risk of physical damage, such as burst capillaries and stroke from extreme intracranial pressure, makes the redout a significant danger.