While writing “Dizzying Heights—Pre-Jump Jitters or Hypoxia” (February 2021 Parachutist), I realized just how little we know about hypoxia in skydivers. The majority of data on hypoxia at altitude comes from studies primarily on young, healthy, military pilots, mostly male. It’s those studies that tell us that less than 5% of hypoxia episodes occur under 14,000 feet. But what percentage of our population are young, healthy males? And does the military data apply to our community of skydivers?
To find out, I did extensive research on portable pulse oximeters, looking for a highly accurate, reliable device that could be worn during a typical skydive without interfering or causing distractions. (Apple watches and other wrist-mounted devices are inherently less accurate.) After purchasing and wearing suitable pulse oximeters and other vital-sign monitoring hardware (see Figure 1), I recruited volunteers to wear them (and wore them myself) on various big-way skydives. Looking at this data, I realized how wrong we may be in our assumptions of when hypoxia begins.
Now, this study was flawed in just the same way as prior studies have been. The first volunteers were all young, healthy individuals, ages 35-39. There was a 50-50 split between females and males. And yes, the sample size was small. However, for this first question of how low our oxygen saturation typically goes at altitude, the “ideal” population of young, healthy jumpers is a good way to start. After all, if that population is seeing low oxygen saturation, it is extremely likely that both age and health issues will only make it worse. That said, this is a preliminary study, and I plan to collect much more data on a wider variety of people. Still, the findings might surprise you.
There are many questions that oxygen saturation data can answer, but for now, let’s look at how low we actually go. First, let’s look at my personal data. The first set of graphs is from a 16-way camp during a hot Arizona summer. The first series is from day one (Figure 2) and shows measurements spanning from boarding the airplane, climbing to 14,500-15,000 feet MSL and touching down on the ground. As you see, the measurements regularly dip into the mid-to-low 80s and even lower.
Looking at the next graph (Figure 3), you can see the duration was about 30 minutes compared to the usual 20, because the pilot had to fly four or five go-arounds for traffic while at 14,500 feet MSL. The lowest my oxygen reached on that jump was 73%, but it stayed in the 70s for approximately 5 minutes. (I’ll tell you, on that jump I definitely felt it.)
Why was the oxygen saturation of a 35-year-old female skydiver in decent shape with no underlying heart or lung issues so low compared to young, healthy military pilots? The reasons may have to do with situations typical for many skydivers. First off, the military guys weren’t lugging around a 25-pound rig and an additional 12-pound weight belt. Secondly, it was 100 degrees Fahrenheit, and I wearing a slick, black jumpsuit and full-face helmet inside a toaster oven of a Twin Otter. Heat takes a toll on the body, which uses a ton of energy trying to cool itself down by creating sweat, making us pant and shunting blood to the skin to release excess heat to the atmosphere. Thirdly, taking a deep breath when crammed rig-to-belly on the plane with the rig itself weighing down the torso is difficult. Lastly, the clincher: Just before exit—when at maximum altitude and at the lowest oxygen level—I get up, put the bench up, squat while waiting for the green light (heart pounding in anticipation), climb out, fight the 120 mph wind and finally use everything I have to push off the plane and present to the relative wind.
So in short: skydivers are already overheated, wear equipment that impairs their ability to let off heat, ride in a tin-can-oven, pack together in a way that causes short and shallow breaths and use maximum exertion just when the environment provides the least oxygen. Since when did any military pilot with a pulse oximeter on go through all that rigmarole?
However, I am just one data point, and how do you know I am not just some weird anomaly? You don’t. So, enter the next set of participants: two males aged 39 and 41, and two females aged 35 and 36.
Unfortunately, the monitors were not working the first days of camp, so the only data is from high-altitude jumps on oxygen (Figures 4-7). The on-oxygen data will be useful for a later study, but what the data shows us now is their drop in oxygen saturation up until the moment the oxygen turns on between 12,500-13,000 feet MSL. (That moment can be clearly seen in the graph as the sharp rise back to normal.) This data demonstrates that it is not uncommon for even young, healthy skydivers to regularly hit fairly hypoxemic levels on normal skydives. Not just that, but there does also seem to be a cumulative effect throughout the day. You might notice that the later jumps in the day tend to include a higher percentage of time under 90% and an overall lower oxygen saturation than the early jumps.
Now, there is one, giant “but” involved in all of this data: No one can say what these numbers really mean. Because the question that really matters is not, “How low is my oxygen when I am about to jump out of the airplane?” but, “Is the low oxygen that I am experiencing actually impairing my thinking or performance?” That is a much more difficult question and one that will take more than just a simple pulse oximeter and vital-signs monitor to answer. So use this information as you want. The more complicated questions will require much more complicated studies.
Laura Galdamez, M.D. | D-41824 and Coach
The Woodlands, Texas