G-force induced Loss Of Consciousness (G-LOC) is a temporary blackout resulting from exposure to acceleration forces. This occurs when the force, measured in Gs, is directed from the head toward the feet, a vector known as positive Gz. The condition is a primary concern in high-performance environments, such as military aviation, where rapid changes in speed and direction generate high G-loads.
The Physiological Cause of G-LOC
G-LOC is caused by the effect of force on the cardiovascular system, specifically in the head-to-foot direction (positive Gz). When a pilot pulls up sharply, the Gz force increases the weight of the blood mass. This causes blood to pool rapidly in the lower extremities, overcoming the heart’s ability to pump blood against the acceleration.
This pooling leads to a drop in blood pressure at the brain, a condition called cerebral hypotension. The body’s natural compensatory reflex, which takes six to nine seconds to activate, is too slow to counteract the rapid onset of high G-forces. Consequently, the brain is deprived of oxygenated blood, leading to cerebral hypoxia.
The brain is highly sensitive to this lack of oxygen, having a reserve time of only about four to six seconds before function is lost. If the high G-load is sustained beyond this brief period, the neurons begin to fail, resulting in unconsciousness. An untrained individual may experience G-LOC at a sustained force between 4 and 6 Gz, depending on the person and the rate at which the G-force is applied.
The Progression of Symptoms
Before G-LOC occurs, reduced blood flow to the brain creates a predictable sequence of visual symptoms. Because the blood vessels supplying the retina are sensitive to pressure changes, visual disturbances appear first. The initial sign is a loss of peripheral vision, often described as tunnel vision, as blood flow ceases to the outer parts of the retina.
Following tunnel vision, the pilot may experience a greyout, which is a partial or complete loss of color vision. If the G-force continues to increase, the visual symptoms progress to a complete loss of sight, known as a blackout. G-LOC, the actual loss of consciousness, occurs after the blackout, usually within a few seconds, as the lack of blood flow reaches the brain tissue itself.
Upon the reduction of the G-force, recovery is usually prompt, but a period of disorientation follows. The unconscious period, termed absolute incapacitation, typically lasts around 12 seconds. This is succeeded by a period of relative incapacitation, lasting about 16 seconds, during which the individual is conscious but confused, disoriented, and unable to perform complex tasks.
Preventing G-LOC
The primary piece of equipment used to prevent G-LOC is the Anti-G Suit, or G-suit. This garment has inflatable bladders covering the lower abdomen and legs. When G-forces increase, the suit automatically inflates to apply pressure, mechanically compressing the blood vessels in the lower body.
This compression restricts the pooling of blood in the extremities, helping to keep blood pressure elevated and forcing blood back toward the heart and head. Modern G-suits can increase a pilot’s G-tolerance by approximately 1 to 1.5 G. The Anti-G Straining Maneuver (AGSM) is a physical technique used with the G-suit to further increase G-tolerance.
The AGSM involves two main components: forceful, rhythmic muscle contractions and controlled breathing. The pilot tenses the muscles in the abdomen, legs, and chest to raise internal thoracic pressure. This helps to maintain blood pressure and cerebral perfusion.
The breathing component, sometimes called the “Hook” maneuver, consists of short, forced exhalations followed by brief, rapid inhalations to stabilize pressure. Effective execution of the AGSM can increase G-tolerance by an additional 2 to 3 Gs. Training is often conducted in a human centrifuge, which safely exposes pilots to high G-loads to practice the AGSM and recognize personal tolerance limits.