Skinner Robert B., Rawal Amit R.
North Florida Regional Medical Center
University of Central Florida
Physiologic parameters at high altitudes vary from those at sea level. An understanding of flight and altitude physiology is essential to prevent pre-hospital fight-induced barotrauma. Boyle’s law explains that “the volume of a gas is inversely proportional to the pressure to which it is subjected.” Based on this law, pressure decreases with increased altitude, thereby causing an increase in the volume of gas. These changes are demonstrated by the fact that atmospheric pressure at 10000 feet is 10.1 pounds per square inch (psi) (68 kPa), compared to 14.7 psi (101 kPa) at ground level. These physiological factors affect both helicopter pre-hospital transport and aeromedical airplane transport. Federal regulations, such as those promulgated by the FAA, require cabin pressure to be below atmospheric pressure equal to the pressure at 8000 feet above sea level. This setting is possible through in-cabin pressurization, thereby decreasing barotraumatic risks that would be in effect at higher altitudes. Fortunately, acceptable cabin altitude levels, or the atmospheric height experienced inside a flight cabin, have been safely increased over time due to aircraft and technologic improvements. Another physiological factor that changes with altitude is the decrease in the partial pressure of oxygen as height above sea level increases; this leads to a reduction in FiO2 (fraction of inspired oxygen) at a higher altitude compared to sea level. One report demonstrated a decrease of 32 mm Hg, from 159 at sea level to 127 at the height of 6200 feet. In-flight hypoxia as altitude increases can have a marked clinical significance in the transport of the critically ill. Additional physiological factors of high altitude transport, the details of which are beyond the scope of this discussion, include decreasing temperatures, dehydration, and gravitational forces. A typical helicopter pre-hospital transport reaches altitudes of 1000 to 3500 feet above ground level, while airplane transport typically transports at altitudes of 10000 to 40000 ft above sea level. An understanding of flight physiology is essential, as even an increase of 1000 to 1500 feet above sea level can cause gas expansion leading to clinical significance in the critically ill.
高海拔地区的生理参数与海平面地区不同。了解飞行和高空生理学对于预防院前飞行引起的气压伤至关重要。玻意耳定律解释说：“气体的体积与其所承受的压力成反比。” 根据该定律，压力随着海拔升高而降低，从而导致气体体积增加。这些变化体现在以下事实中：10000英尺处的大气压力为每平方英寸10.1磅（psi）（68千帕），而地面水平为14.7 psi（101千帕）。这些生理因素会影响直升机院前运输和航空医疗飞机运输。联邦法规，如美国联邦航空管理局颁布的法规，要求机舱压力低于等于海平面以上8000英尺处压力的大气压力。通过机舱增压可以实现这种设置，从而降低在更高海拔处会产生的气压伤风险。幸运的是，由于飞机和技术的改进，随着时间的推移，可接受的机舱海拔水平，即飞行机舱内所经历的大气高度，已安全提高。另一个随海拔变化的生理因素是，随着海平面以上高度增加，氧气分压降低；这导致与海平面相比，在更高海拔处吸入氧分数（FiO2）降低。一份报告显示，氧气分压从海平面的159毫米汞柱降至6200英尺高度的127毫米汞柱，降低了32毫米汞柱。随着海拔升高，飞行中的缺氧在危重症患者运输中可能具有显著的临床意义。高空运输的其他生理因素，其细节超出了本讨论范围，包括温度降低、脱水和重力。典型的直升机院前运输到达地面以上1000至3500英尺的高度，而飞机运输通常在海平面以上10000至40000英尺的高度飞行。了解飞行生理学至关重要，因为即使海平面以上增加1000至1500英尺也可能导致气体膨胀，对危重症患者产生临床影响。
