A new paper titled, Changes in Heart Rate and Rhythm During a Crossover Study of Simulated Commercial Flight in Older and Vulnerable Participants, has just been published Frontiers of Physiology. The authors found that among passengers that are typical of elderly flyers (an increasingly common demographic of both flyers and flight crew - flight attendants and pilots) and who would have been cleared to fly without supplemental oxygen or any other particular support based on current medical guidelines experirenced changes in heart rate and rhythm.
What this means is that we are seeing changes that are potentially worrisome on a systemic level, and could be related to an increased risk of cardiac events occurring both in-flight and post-flight. The implications of this for longer-haul flights are unknown, and something that needs to be studied, along with many other flight-related exposures. For example, the monitoring in the recently published studies extended to approximately 4 hours, however, any transcontinental trip from the U.S., and well as many domestic flights, will automatically be longer than this.
The authors also saw that, within the four hour window, the longer the flight continued, the greater effect on heart rate and rhythm. These effects were pretty linear, and how that may change with long-haul or increasingly common ultra-long-haul flights (whether the association would level off, remain linear, become exponential, any other option) is a total unknown and necessary to investigate given (1) the billions of passengers and crew flying each year, (2) the increasing amount of flight by those who are elderly and have diagnosed and undiagnosed cardiopulmonary disease, (3) increasing capabilities of extended flight, and (4) the ability of airlines to pressurize their cabins to lower levels, which has currently been implemented on a small minority of aircraft.