Inventing the Apollo Spaceflight Biomedical Sensors

During the Mercury, Gemini, and Apollo missions, one of NASA’s concerns was the safety of its crews, something it monitored rigorously through the use of biomedical instrumentation. As initial flight planning commenced in 1959, biomedical equipment capable of transmitting from space did not exist. NASA quickly brought together medical staff and hardware engineers to develop biomedical technology. As they blasted off from Earth, the first American astronauts were wearing electrodes to collect electrocardiograms (ECGs, measuring the classic heartbeat waveform); a heated thermistor that detected breathing by cooling due to air movement in and out of the mouth; and, most unfortunately for them, a rectal probe that captured highly accurate body temperature readings. No wonder that astronauts, accustomed to self-sufficiency and relative isolation during their test pilot days, chafed at this literal and metaphorical intrusion.

Throughout the 1960s, NASA continued to tinker with its bioinstrumentation to find an ideal balance between obtaining accurate, important information and astronaut comfort. The assembly in the picture below is one of their early test models for the Apollo program. This object was featured in an earlier blog post on conservation, which you can find here. This time, I’d like to explore the function of these components.

The most interesting variable to NASA’s medical division was cardiovascular function. Did the heart’s ventricles, normally ready to pump blood to the body every second, have trouble filling with blood in the weightlessness of space? Did fluids reach the lower extremities sufficiently without gravity? What about adaptations to space—werethey detrimental to an astronaut upon his return to Earth?

ECG electrodes were the first tool to gauge heart health. Readings from Mercury flights were often thwarted by movement, vibrations, and bumps. For Apollo, NASA contracted Spacelabs, Inc. to develop more reliable and accurate readings by use of a signal conditioner. Electrodes transmitted their raw signal via the orange wires to the two black conditioners on the left in this picture, which consisted of complex circuitry to identify and reject unwanted noise so the output was more representative of the astronaut’s state of being.

In addition to heart rhythm, NASA wanted to measure blood pressure. They initially introduced a semiautomatic sphygmomanometer (blood pressure cuff with pressure transducer and microphone) during Mercury and, for the most part, it remained similar for the Gemini and Apollo missions. The pressure cuff would slowly deflate, and the microphone would record pulse sounds to pinpoint the systolic (during heartbeats) and diastolic (in between heartbeats) blood pressures. This information was transferred to a signal conditioner, shown in the picture and diagram below. For this signal conditioner, NASA’s contractors designed a tiny pressure transducer (converting pressure to voltage), built a filter to precisely pick up noise at the systolic and diastolic blood pressures, and managed to make the entire signal conditioner small, lightweight, and low on power usage.

Beyond the heart, NASA wanted to keep tabs on astronaut breathing. Because the thermistor of Mercury days (a resistor that changed its resistance at different temperatures) did not reliably track respiration, NASA moved to an impedance pneumograph technique. At the time, impedance pneumography was a little known technique, yet through NASA’s research and development it was eventually found to be a very successful tool, even in a demanding flight environment. When a constant electric current is introduced into a human’s chest tissues, the fat, muscle, lungs, air, and fluid all create a natural impedance (or opposition to current flow), which can be measured via voltage. As the subject draws in air and stretches the body tissues, the impedance changes which subsequently changes the voltage drop. NASA stuck an electrode close to the astronauts’ sixth ribs (the optimal spot) and used results from a Baylor University study to correlate impedance to the amount of air in lungs. Spacelabs, Inc. built another signal conditioner for the impedance sensor shown below. The end result was so good that flight surgeons in Houston watched every deep breath as astronauts hopped onto the surface of the Moon or headed out the hatch for a deep space extravehicular activity or EVA.

Finally, NASA came to understand that temperature monitoring via the rectum was not optimal for astronauts on long journeys to the Moon and back. They replaced it with an oral sensor for the Gemini and Apollo flights shown below, built with a Velcro patch for attachment inside the astronaut’s helmet. The black neoprene sleeve on the probe was simply for traction, as the probe itself, coated in Teflon, proved to be slippery and difficult to hold in the mouth. Intermittently, astronauts would place the sensor beneath their tongue for up to five minutes to produce a reading.

While far more complex biomedical information has been collected on astronauts since the 1960s, the instruments seen here were the pioneering designs, compiling information about the influence of weightlessness on essential bodily functions. The astronauts themselves may have been irritated with NASA’s meticulous cataloging of their body’s performance, but it came in handy in many instances, like when Gene Cernan physically struggled to perform his EVA tasks during Gemini IX-A, and when Dave Scott and Jim Irwin had heart irregularities on the surface of the Moon during Apollo 15. Biomedical monitoring allowed NASA to identify problems in real time and devise solutions for current and future astronauts, and today we continue to probe human health in space so that one day we might prepare for the challenges of sending astronauts to Mars and other celestial targets.