The human body was not made to live at 18,000 feet. Altitude sickness occurs when people go to a higher altitude too quickly and do not get enough oxygen in the air they breathe. Common symptoms include having a headache and not feeling like Oxygen is so scarce at that altitude that few plants or animals can live. For visiting humans the thin air fogs the mind, makes small actions arduous and keeps cuts and infections from healing. Even the most sedentary activity of all, sleeping, is severely disturbed in this alien environment. At altitude normal people often develop periodic breathing in sleep - regularly recurring periods ofhyperpnea and apnea.

This phenomenon is probably explained by instability of the negative feedback system for controlling ventilation. Such systems can be modeled by sets of differential equations that describe behavior of key components of the system and how they interact. Mathematical models of the breathing control system have increased in complexity and the accuracy with which they simulate human physiology. Recent papers by Zbigniew Topor et al. (5,6) describe a model with two separate feedback loops, one simulating peripheral and the other central chemoreceptor reflexes, as well as accurate representations of blood components, circulatory loops and brain blood flow. This model shows unstable breathing when one chemoreceptor loop has high gain while the other has low gain, but not when both have high gain. It also behaves in counter-intuitive way by becoming more stable when brain blood flow is reduced and unresponsive to blood. gas changes. Insights from such models may bring us closer to understanding high altitude periodic breathing.