Extreme Physiology of the Human Body and Deep Sea Diving

The human body can adapt its physiology to a number of environments in order to maintain homeostasis or cope with these different environments in order to survive. It does this by regulating different systems in the body such as the respiratory, circulatory, nervous, cardiovascular, digestive, or endocrine systems, to name a few of many. For instance, when the body and face is immersed in water, it stimulates the vagus nerve, a parasympathetic branch of the autonomic nervous system. This stimulation leads to a drop in heart rate, decreasing blood flow to the extremities and therefore, reducing the need for oxygen intake (Yerworth, Rebb. “Physiology of Deep Sea Diving”).

This essay will cover how the physiology of the human body can adapt to stressful environmental factors of deep sea diving with scuba equipment, as well as review and critique the article Effect of Shallow and Deep SCUBA Dives on Heart Rate Variability.

 Deep sea diving has existed for thousands of years, dating as far back as the 4^th century BCE when it was used by people of Ancient Greece and the diving bell was first described by Aristotle (Emley, Bryce. “How the Diving Bell Opened the Ocean’s Depths.”). Today, it is mainly characterized as a recreational activity, although it can be used for commercial reasons such as acquiring lost items or exploring ocean depths (only to the extent the human body can withstand) ((Bosco, Gerardo, et al. “Environmental Physiology and Diving Medicine.”). SCUBA, or Self-Contained Underwater Breathing Apparatus, diving involves the use of closed circuit or open circuit system equipment carried with the diver throughout the dive (Walker, J R., and Heather M. Murphy-Lavoie. “Diving Rebreathers.”). Open circuit SCUBA diving utilizes equipment consisting of a container with compressed gas, and the diver exhales the gas into the surrounding environment. This is the more commonly used method of diving with equipment, however it is categorized as inefficient because a portion of oxygen is unused. Closed circuit diving, more commonly known as diving with rebreathing equipment, “recycled” the gas that is exhaled by the diver and removes the carbon dioxide. Once this gas is removed, the remainder is enriched with oxygen and via a breathing loop returned to the diver to be reused.

 In Effect of Shallow and Deep SCUBA Dives on Heart Rate Variability, the researchers investigate the effect shallow and deep dives have on the cardiac autonomic nervous system. Monitoring SCUBA divers at various depths for different time intervals, they were able to collect electrocardiogram (ECG) data while the divers experienced different gas mixtures consisting of oxygen, nitrox and trimix. Research regarding the physiological adaptations to deep dives within SCUBA divers using ECG data is scarce, and the authors emphasized the importance of their study as one of the firsts of its kind in this area.

 After obtaining approval of the study by Worcester Polytechnic Institute’s Institutional Review Board and security for the involvement of human subjects, the researchers began their study with 24 experienced divers. Two thirds (16) of these subjects utilized open circuit SCUBA equipment, while the remaining eight utilized closed circuit SCUBA diving equipment (rebreathers) (Schipke, J. D., and M. Pelzer. “EVect of Immersion, Submersion, and Scuba Diving on Heart Rate Variability.”). Over a span of five days, each diver engaged in a number of dives at depths of 33, 66, 99, 150, and 200 feet, with the use of three different breathing gases of oxygen, nitrox, and trimix. The first dives at the first two depths took place in the morning and afternoon of the first day, and one dive at the remaining depths per the next three days (one depth per day). The final day was used to obtain additional data for dives that required it. Each diver wore a five lead digital Holter ECG monitor under a thermal undergarment and a dry diving suit. To record and set baseline ECG data, divers immersed in the water and remained at surface level for approximately 10 minutes. After recording this baseline, divers descended to each previously stated depth and maintained a horizontal position for a set amount of time (Schipke, J. D., and M. Pelzer. “EVect of Immersion, Submersion, and Scuba Diving on Heart Rate Variability.”). Again, as previously mentioned, this protocol was carried out in order to obtain data to analyze how different time, depth, and breathing gases induce and effect the response of the autonomic nervous system, more specifically heart rate variability (HRV). A table summary of dive information of various depths and time durations can be found below.

 Along with electrocardiogram usage to detect and obtain data for heart rate variability measurements, the researchers in this study also used non-linear methods including power spectral density (PSD) and approximate entropy (Schipke, J. D., and M. Pelzer. “EVect of Immersion, Submersion, and Scuba Diving on Heart Rate Variability.”). Power spectral density measures different frequency bands, low and high, that characterize sympathetic and parasympathetic activity as well as parasympathetic tone. Approximate entropy (ApEn) quantifies a time series signal to allow for the understanding of regularity and predictability or unpredictability within a system over that series of time. Included in the study is the procedure (equations) for obtaining the approximate entropy of heart rate. Principle dynamic modes (PDM) is another non-linear method used by the authors to report non-linear dynamics of heart rate control, as well as sympathetic or parasympathetic systems (Schipke, J. D., and M. Pelzer. “EVect of Immersion, Submersion, and Scuba Diving on Heart Rate Variability.”).

 For their results, the authors initially presented the diving profiles of the 33 and 66 foot dives, however, only 11 of the 16 divers that utilized the open circuit SCUBA diving equipment at these depths were able to be used to study the effect of time and depth on heart rate variability due to complications maintaining the dive for a set time interval. For the 33 foot dive, they found that as the time interval increased, heart rate continued to decrease below baseline.  For the 66 foot dive they found  similar results, with heart rate continuing to decrease below baseline as time increased at the specified depth. Parasympathetic parameters depicted an increase within the first 20-25 minutes at the each depth, with a decrease in the last five minute stretch of time. However, the authors deemed this change an insignificant trend.

 For depth comparison, the authors found and stated that HF (high frequency) results for PSD that depicted parasympathetic activity showed an increase at all depths, however, they emphasized increase at 66 and 105 feet as significant. The authors developed an algorithm to estimate breathing rates from ECG signals, and for respiratory rate results they found that there was not a difference significant enough for these rates at 99, 150, and 200 feet. (Schipke, J. D., and M. Pelzer. “EVect of Immersion, Submersion, and Scuba Diving on Heart Rate Variability.”). Results for the effect of the different breathing gases displayed a decrease in heart rate and increase in  parasympathetic activity according to PDM values when air and nitrox were used at 99 feet.

Overall, the authors concluded that there was a consistent observation of decrease in heart rate and the majority of responses were regulated by parasympathetic activity throughout the dives and with the use of different breathing gases. However, as the depth pressure increased, sympathetic activity was more present because of the increase in stress on the divers’ bodies. This response follows normal “fight-or-flight” responses by the autonomic nervous system. The authors also reported in their discussion that results were taken in waters that measured around approximately 60F-70F, and this is also a factor that could have contributed to additional stress on the divers, increasing sympathetic activity (specifically heart rate) as time duration increased at previously mentioned depths.

For depth comparison, overall the authors found a significant increase in parasympathetic activity at increasing depths for each set time. Alongside this, they also found that cardiac parasympathetic tone of heart rate variability at these depths were also heightened. The authors also attributed this increase in parasympathetic activity and decreased heart rate to an increase in gas density and water pressure (Noh, Yeonsik, et al. “Effect of Shallow and Deep SCUBA Dives on Heart Rate Variability.”).

The approach to this study was done well, with approval obtained by the Worcester Polytechnic Institute’s Institutional Review Board, and thorough screening of individuals completed to ensure they were in proper health to participate in this study. Because this study was one of the first to evaluate the physiological effects of time, depth, and different breathing gases on divers’ bodies, this warranted  the need for a thorough methodical approach in each area. With the use of electrocardiogram, principal dynamic modes (PDM), power spectral density (PSD), approximate entropy (ApEn), and a number of statistical analysis tests, the data obtained from the study, in regards to critique, appeared to give sufficient evidence for a significant change in heart rate variability during deep sea SCUBA dives. One weakness of the study, however, was the sample size. The authors explained this flaw mainly arose from a limited financial budget, as well as an inability to acquire significantly experienced divers. They also correlated this issue with an inability to conclude exactly how different breathing gases may effect HRV and the other autonomic nervous system responses, mainly due to insufficient data.

Works Cited

• Bosco, Gerardo, et al. “Environmental Physiology and Diving Medicine.” Frontiers in Psychology, 2 Feb. 2018, www.ncbi.nlm.nih.gov/pmc/articles/PMC5801574/pdf/fpsyg-09-00072.pdf.
• Emley, Bryce. “How the Diving Bell Opened the Ocean’s Depths.” The Atlantic, Atlantic Media Company, 23 Mar. 2017, www.theatlantic.com/technology/archive/2017/03/diving-bell/520536/.
• Noh, Yeonsik, et al. “Effect of Shallow and Deep SCUBA Dives on Heart Rate Variability.” Frontiers in Physiology, vol. 9, 27 Feb. 2018, doi:10.3389/fphys.2018.00110.
• Schipke, J. D., and M. Pelzer. “EVect of Immersion, Submersion, and Scuba Diving on Heart Rate Variability.” British Journal  of Sports Medicine, 29 Jan. 2001, www.ncbi.nlm.nih.gov/pmc/articles/PMC1724326/pdf/v035p00174.pdf.
• Tetzlaff, Kay. “Short- and Long-Term Effects of Diving on Pulmonary Function.” CrossMark, 14 Sept. 2016, err.ersjournals.com/content/errev/26/143/160097.full.pdf.
• Walker, J R., and Heather M. Murphy-Lavoie. “Diving Rebreathers.” StatPearls [Internet]., U.S. National Library of Medicine, 24 Feb. 2019, www.ncbi.nlm.nih.gov/books/NBK482469/.
• Yerworth, Rebb. “UCL WIKI.” Physiology of Deep Sea Diving – Biomedical Engineering Case Studies – UCL Wiki, 11 Dec. 2015, wiki.ucl.ac.uk/display/BECS/Physiology+of+deep+sea+diving.