Progress In U.S. Navy Diving Biomedical Research

In the US Navy diving community there is an essential interdependence between the biomedical researcher, the engineer and the operator. Before operational capabilities can advance, biomedical research must define the nature of the biological events that accompany exposure to extreme environmental conditions. Based on the results of this research, the biomedical researcher and the engineer work together to develop specifications for diving equipment and systems, and the biomedical research scientist and operator work to develop procedures for use during diving operations. The engineer and operator often encounter problems in the course of their work for which they refer to the diving biomedical research community for elucidation, and hopefully, solution. Thus, the biomedical researcher, engineer and operator comprise a triad that is necessary for the advancement of operational capability. By their contributions, Navy biomedical researchers have enhanced substantially US Navy operational capabilities in the past 15 years. This paper will highlight some of these contributions and show the impact that they have had on the diving capability of the United States Navy.


I n t h e US Navy d i v i n g community t h e r e is an
have been established for decompression sickness that occurs during saturation decompression. These guidelines specify the amount of recompression required, the necessary time to spend at treatment depth, and the concentration of oxygen in treatment gas.
Considerable progress has occurred in the elucidation of the pathophysiological processes underlying cerebral air embolism and decompression sickness involving the central nervous system. This work, done in animal models, indicated that potentially fatal cardiac arrhythmias can result from cerebral air embolism, and that disturbances of cardiac rhythm are mediated by the brain and its nervous control of the heart. Additionally, Navy medical researchers have shown that after cerebral air embolisin there is a massive outpouring of substances such as vasopressin, epinephrine and norepinephrine, which are potent vasoconstrictive agents and cause huge increases in arterial blood pressure. Pharmacological means to ameliorate the cardiac arrythmias and rise in blood pressure have been developed. Therapeutic algorithms have been devised to treat the various disorders resulting from cerebral air embolism.
Our understanding of the mechanism operant in decompression sickness that affect the central nervous system has progressed substantially in the last decade. We now know that the initial events in decompression sickness affecting the spinal cord are initiated by physicochenical interactions between bubbles of gas and blood, and that these interactions start a complex series of biochemical events wherein many pharmacologically potent substances are released. These substances, which are liberated in blood, then interact with damaged tissue and make it difficult for adequate blood flow to the tissue to be re-established. Navy researchers have shown that these events ultimately produce a situation in which the venous system draining blood away from the spinal cord is occluded, thereby preventing nutrient blood flow from reaching the spinal cord.
Research is underway to define the best depths and inspired oxygen concentrations for recompression therapy. Because recompression is not always curative, experimental work has sought to develop adjunctive pharmacological methods to enhance recovery and restore function. This work, which has been performed in animal models until recently, has now progressed to the point where clinical human trials are being conducted at the Bethesda Naval Hospital (Bethesda, MD) in patients with acute stroke.
Considerable research has been directed to the study of respiratory function in the diving environment. Diving operations and experimental work have demonstrated that breathing resistance in many types of underwater breating equipment is excessive. This interacts with other factors such as immersion, dense gas and static pressures to hinder che diver's ventilatory performance and work capacity, and to hazard his safety. Navy investigators' research in the last 10 years has provided a better understanding of the response of the lungs and the respiratory muscles to these environmental factors. 4s part of this research, a noninvasive, nonresistant method was developed to accurately measure the pulmonary ventilation of working divers. This technique permits the fractionation of each breath into those contributions that the rib cage and the diaphragm/ abdomen make, and is the pulmonary equivalent to an electrocardiogram.
This technique, coupled with other methodologies developed by Navy scientists, now allows researchers to measure the efficiency of the respiratory muscles under various conditions of loading. The results of this work have permitted us to better define the limits of resistance permissible in a divers' breathing equipment and to serve as the basis for the improved design and development of underwater breathing gear. To assist ventilation and gas exchange in divers, methods that enhance ventilation of the working diver have been examined. Navy investigators have shown that the respiratory muscles can be trained for both strength and endurance. This work serves as a basis for inproving a diver's respiratory status, and is now also widely used for training the respiratory muscles of patients with various forms of chronic lung disease. Another promising technique that has been examined is the use of a high-frequency, low-volume ventilator that can be incorporated into a diver's breathing equipment to augment ventilation and gas exchange. These methods hopefully provide a means of circumventing the respiratory limitations of working at depths greater than 1000 ft.
Cold is pervasive in the diving environment. !here thermal drain occurs, the diver's performance is affected first and subsequently his safety may be jeopardized to the point where mere survival is at stake. The Navy's study of cold stress in diving has included several objectives. Firstly, this work has been deternining the physiological events that occur in cold stress, and secondly, it has been deternining the amount and distribution of heat needed to maintain a diver in an adequate, functional and safe condition.
To obtain this information an assay of 13 pairs of heat flux sensors and thermistors are placed at various locations on one side of the diver's body. This system, which has been validated against whole body and regional calorimetry, provides measurements of whole body and regional heat l o s s in dry, highpressure environments and during immersion. Information derived from this system permits one to cite how much insulation and supplemental heat is required for all areas of the diver.
Navy biomedical researchers have shown that working divers breathing cold helium-oxygen in a euthermic environment at depth incur large losses of heat via the respiratory system. Based on these findings, the US Navy has required that the diver's breathing gas be heated during dives deeper than 500 ft. Research has determined the changes in lung function that occur while breathing either hot or cold gases. This research has provided criteria for safe minimal and maximal temperatures of breathing gases in the diving environment.
Navy biomedical researchers have achieved considerable progress in other areas in the last decade and a half. Examples are: (1) The demonstration that in the case of a set amount of pulmonary oxygen toxicity a greater amount of time must be spent in decompression.
(2) The demonstration that corneal edema can result from diving with hard contact lenses. Subsequent Navy research showed that this problem can be circumvented by the use of soft lenses or fenestrated hard lenses.
( 3 ) The demonstration of the operational advantages and the physiological and performance limitations of one-atmosphere diving systems for Navy use.
( 4 ) The evaluation of the virulence and pathogenicity of common infectious disease agents of man u n d e r h y p e r b a r i c c o n d i t i o n s t o i n s u r e t h a t t h e r e was no untoward hazard.
(5) The development of a schema f o r s a f e l y a d m i n i s t e r i n g a n e s t h e s i a a n d d r u g s t o a n i n j u r e d d i v e r a t d e p t h . (8) The d e m o n s t r a t i o n t h a t i n p r e g n a n t a n i m a l s t h a t developed decompression sickness, fetal development i s thereafter normal if the decompression sickness is treated promptly and adequately. Mrs. E l l e n Hughes and Ms. Janet Gaines.