Introduction

When pilots fly high-performance aircraft, they conduct maneuvers in the air that expose them to high acceleration in the +GZ axis. These +GZ forces acting cephalad-to-foot direction induces cerebral hypotension that may impair cerebral blood flow. G-induced loss of consciousness (G-LOC ) occurs when acceleration forces produce a situation in which the cardiovascular system is unable to supply oxygenated blood to the regions of the nervous system that support consciousness. When aircrew of high-performance aircraft are exposed to +Gz acceleration, symptoms ranging from peripheral visual loss, blackout, and loss of consciousness may occur. A condition known as almost loss of consciousness (A-LOC) first described by the US Navy in the 1980s may occur when a pilot is exposed to +Gz stress at levels that are insufficient to cause G-LOC. It is a syndrome that encompasses a wide variety of cognitive, physical, emotional, and physiological symptoms. The most prevalent symptom is reported to be a disconnection between cognition and ability to act. In centrifuge studies of 500 healthy individuals, individuals who experience G-LOC were observed to have a blank stare and show signs of unconsciousness. The majority of persons who experience G-LOC have myoclonic jerks and which they do not remember after recovery from the episode. The convulsions last for about 4 seconds and occur after the return of cerebral blood flow. Memorable dreams follow the myoclonic jerks.[1][2][3]

A study conducted by Green and Steven showed that 20.1% of United Kingdom Royal Air Force (RAF) aircrew had suffered one episode of G-LOC and 82% had suffered two episodes. G-LOC is reported to occur most commonly in training aircrew. Prevalence of G-LOC in front-line aircraft is low at 11.3%, and 6% in fast jet aircraft. It is difficult to predict pilot characteristics that may predispose to G-LOC. The most recent survey done in United Kingdom RAF showed a decrease in the incidence of G-LOC from 20% IN 2005 to about 15% in 2012. Sevilla and Gardner found that time on aircraft type less than 600 hours, pilotage younger than 30 years, and poor anti-G straining maneuver is associated with 72% of G-LOC accidents.

Issues of Concern

Physiology of G-LOC

The baroreceptor reflex is the compensatory response to hypotension induced by +Gz acceleration. It takes about six to nine seconds to respond to increased blood pressure. The brain is very sensitive to cellular hypoxia with the cerebral hypoxia reserve time being only four to six seconds. At increasing +Gz acceleration, the sympathetic response is inadequate and cerebral hypoxia occurs. Furthermore, intraocular pressure is typically higher than intracranial pressure, making vision even more sensitive to decreased perfusion than consciousness. This may manifest as a peripheral visual loss at about 3.4 +Gz to 4.8 +Gz, blackout at 4-5.6+Gz, and unconsciousness at 4.5+Gz to 6.3 +Gz. Thus, visual signs and symptoms often precede G-LOC. However, a rapid onset of +Gz may cause G-LOC to occur without the individual experiencing visual symptoms, making this situation particularly dangerous.[4][5][6]

G-LOC is divided into a period of absolute incapacitation and a period of relative incapacitation. In centrifuge studies of 500 healthy human subjects, the period of absolute incapacitation manifested by unconsciousness lasted about 11.9 seconds (range two to 38) and was followed by a period of relative incapacitation which presented as confusion or disorientation and lasted about 16 seconds (range 2 to 97). The total incapacitation time is about 28 seconds. During these periods the pilot is incapacitated and not able to control the aircraft; failure to recover may lead to loss of aircraft and pilot.

In another study of 888 centrifuged induced G-LOC episodes in healthy pilots, Whinnery and Forster determined that G-LOC will occur in a mean time of 9.10 seconds for a +GZ onset rate greater than 1.0 G per second, and it did not depend on G onset rate. For a rapid onset G rate, greater than +7GZ, G-LOC occurred in a mean time of 9.65 seconds. The minimum +GZ threshold tolerance level was 4.7 G in the 888 individuals. For any acceleration exposure, G-LOC did not occur earlier than five seconds.

G tolerance curves have been formulated for predicting the threshold of +GZ levels that when exceeded will lead to neurologic signs and symptoms in healthy humans exposed to +GZ.  The widely known G tolerance curve is the stoll curve that was modeled in 1956. Since then, several other models have been formulated by researchers. Whinnery and Forster formulated one that encompassed a larger sample size than the stoll curve.

Whinnery et al. also analyzed return of consciousness in 760 G-LOC induced episodes and determined that for healthy humans the return of consciousness time after G-LOC is 10.4 seconds (range 1 sec to 38 sec) and the higher the +GZ exposure the higher the duration of unconsciousness; however, return to consciousness is not dependent on only the +GZ exposure level but also on other factors. The minimum time range for recovery of consciousness after a complete loss of consciousness is 4.49 sec to 12.09 sec. There is a hyperbolic relationship between +GZ offset rate(y-axis) and absolute incapacitation period(x-axis). These return of consciousness variables are very important and applicable when reconstructing aircraft accident profiles of suspected G-LOC related aircraft accidents during aircraft accident investigations.[7][8][9]

Prevention

Prevention of G-LOC in aircrew is necessary to prevent loss of aircraft and aircrew in G-LOC-related accidents. Aircrew needs to be educated on G-LOC awareness and factors that reduce G tolerance. Centrifuge-based training of aircrew is necessary to recognize the warning signs of impending G-LOC, and learning correctly to perform the anti-G straining maneuver will help to mitigate G-LOC. structured conditional training of to increase muscle mass will aid aircrew perform the anti-G straining manuevre properly. Other important factors include the use of appropriately fitted anti-G trousers and physiologic G tolerance, which is improved through repetitive exposure and overall fitness. Protective measures against the ominous symptoms of +GZ is achieved through shortening the heart-to-head distance, raising the blood pressure at the aortic valve, making sure the duration of pulled G is less than 6 seconds, and avoiding the push-pull effect.

Clinical Significance

This issue highlights the risk of aircraft accidents, a delay in progress through training, and reduced skills at the end of training for student pilots, as well, as poor decision making and poor performance in operational aircrew. About 20 fatalities in the US Air Force attributed to G-LOC has been reported; a Canadian CF-18 also was lost due to G-LOC, and an RAF Red Arrows Hawk aircraft was lost due to A-LOC.