Now, all of this talk of blood flowing towards and away from the head, should make us suspicious of hypoxia, and if it's going to play a role this week. Not so shockingly, it is very relevant. In particular, hypoxia caused by inadequate blood flow is called shock. Low blood flow can be caused by too low blood pressure, which can be caused by either excessive vasodilation, or loss of blood in a bleeding event. Additionally, shock may be triggered by issues with the heart or blockages of the circulatory system. Together, these form the different types of shock; distributive, from excessive vasodilation, hypovolemic, from loss of blood, cardiogenic, from issues with the heart, and obstructive, from blockages in the circulatory system. In all cases, the body will compensate for the low blood flowing using vasoconstriction of the arms and legs, to direct blood away from the hands and feet, and vasodilation of the large central arteries, to allow the limited blood to remain near the internal organs. Compensated shock will slowly drain a person's energy stores until they can no longer fight the low blood flow, leading to decompensated shock. In decompensated shock, energy stores are running low. Certain functions start to shut down. Eventually, irreversible shock sets in, when energy stores are completely gone. While pilots rarely suffer from shock, it provides a useful tool for understanding how suddenly and strongly a person reacts to troubled blood flow. In aerospace medicine, we take the principles that are true in our day to day lives, and take them to the extreme. In this case, due to high g-forces. Rather than the orthostatic intolerance of standing up too quickly, and having blurry vision, jet craft will provide sustained periods of intense g-forces,, pulling the blood away from the head and towards the feet. If done without care, this can instantly cause tunnel vision, confusion, and anxiety, or worse, complete loss of consciousness. If a high g-force maneuver, produces a diminished but not total loss of consciousness, this is labeled ALOC, or almost loss of consciousness. If the accelerations are too strong and provided for too long, a person will fall completely unconscious, from a sustained g-force. This is deemed G-LOC, or g-force-induced loss of consciousness. G-LOC not only requires high g-forces, but also enough time to drain the blood away from the head, which results in the rapid progression and symptoms of hypoxia. Starved of oxygen, the brain, and consciousness with it, shuts down. Generally, positive 5G_z is enough to overcome the body's natural abilities to compensate, and causes blood flow to pool in the legs. We will see G-LOC occur in most people after a few seconds. Aircraft pilots are trained to resist G-LOC of up to 9G_z, and are taught specifically an anti-G strain maneuver, where in a pilot closes their throat, tenses their leg muscles, and pushes against their seat belt and chair with their chest muscles. This increases blood pressure around their heart, to push blood towards their head. Additionally, the seated posture of pilots, minimizes the vertical distance, so that the blood isn't drawn into the legs as far. Pressure applied onto pilot's legs by anti-G suits, also ensures blood doesn't pool there. Finally, flight maneuvers, such as the split S, will turn the pilot upside down, and ensure that vertical g-forces remain mostly positive along the z-axis. The body actually has a higher tolerance in the positive z direction, than the negative one. If we instead consider negative g-forces experienced when a person turns upside down, or accelerate towards the Earth very quickly, blood pools in the head. Well, this person doesn't experience hypoxia. The brainstem and brain are not designed to sustain the high pressure that our legs easily resist. Here, negative 2G_z becomes difficult, and negative 4G_z, less than half of the maximum tolerance in the opposite direction, is considered deadly. This is because the higher pressure crashes the sensitive brainstem, and can trigger internal bleeding in the eyes and brain. Thinking back to turning the cup upside down, under small negative g-forces, the arteries in the head and neck will vasodilate, as to lower pressure. This vasodilation will lead to push-pull effects. Or a pilot moving from negative G_z to positive G_z, will have a much lower tolerance against G-LOC, because the sudden move from high pressure, catches the body off guard. Push-pull effects contribute to 30 percent of all cases of G-LOC, which affects anywhere from eight to 25 percent of military air crews. Major Stephan Debug was suspected to have suffered from push-pull effects, moving between extreme aerial maneuvers. Starting at negative 2G_z and rapidly entering the split S at just under 9G_z, meant that his tolerance for this already difficult maneuver was lowered, leading to G-LOC. For space medicine, these physiological considerations limit how fast we can design our rocket ships to accelerate, and our astronauts' posture during takeoff. The human body can sustain more g-forces in the x-direction, or forward and backwards, because acceleration in these directions, doesn't drain blood away from, or cause crushing pressure in the head. Accordingly, we see astronauts lying on their backs with their feet up, and in a pressurized suit to avoid blood pooling in their legs. When the spacecraft accelerates, 3GXs are applied. All this is much lower than nine. This is sustained for minutes, rather than seconds.
