THE LOWER BODY NEGATIVE PRESSURE (LBNP)

TECHNICAL NOTE Design of a Chamber for Lower Body Negative Pressure with Controlled Onset Rate Jonny Hisdal, Karin Toska, and Lars Walløe HISDAL J, TOSKA K, WALLØE L. Design of a chamber for lower body negative pressure with controlled onset rate. Aviat Space Environ Med 2003; 74:874 8. We have designed a lower body negative pressure (LBNP) chamber and control system that makes it possible to apply LBNP either very rapidly ( 300 ms), or more gradually, according to predefined protocols. The capability of the new, high-performance agile aircraft to reach a high-g onset rate makes it highly desirable to be able to study immediate, rapid, and transient cardiovascular responses to simulated gravitational stress. Our new LBNP chamber has been used to study the main cardiovascular variables during onset and release of mild LBNP ( 20 mmhg). We have revealed large transient physiological responses during the onset and release of mild LBNP. This new finding was largely made possible by precise control of the onset and release of LBNP during the experiments. The purpose of this paper is therefore to describe some of the technical solutions which made rapid and controlled changes in LBNP possible, focusing on the importance of precise control of the LBNP chamber. Keywords: lower body negative pressure, methods, cardiopulmonary baroreceptors, arterial baroreceptors. THE LOWER BODY NEGATIVE PRESSURE (LBNP) technique was introduced to study circulatory responses to simulated gravitational shifts of blood in humans. The LBNP technique was first described in 1965 by Stevens and Lamb (3). Despite frequent use of the technique in physiological investigations during the last few decades, the time course of the onset and release of LBNP and the transient cardiovascular responses to this stress have seldom been discussed. New high-performance agile aircraft can reach high-g levels in less than a second. Since aircrew regularly experience the rapid onset of acceleration exposure and there is a possibility of rapid transitions between zero and negative or positive G forces, research tools such as human centrifuges and LBNP chambers must also meet more stringent technical requirements. More knowledge of the time course, dynamics, and mechanisms of cardiovascular responses is now needed. This must be obtained through studies of cardiovascular responses and mechanisms, using continuous recordings of the cardiovascular variables. The sampling frequency must be high enough to observe transient changes (i.e., within a heartbeat) in the cardiovascular variables recorded. Both cardiopulmonary and arterial baroreceptors must be studied. We have designed a LBNP chamber and control system that makes it possible to apply LBNP either very rapidly ( 300 ms) or more gradually, according to predefined protocols. Studies using the new chamber have revealed large transient physiological responses during the onset and release of mild LBNP (1,2). This new finding was largely made possible by precise control of the onset and release of LBNP during the experiments. Since little has been published about the design of LBNP chambers, we spent a great deal of time and money on designing and developing optimal solutions for the LBNP chamber. The present paper therefore has two aims. The first is to share information on the design of our LBNP chamber. We hope this will be helpful for colleagues who intend to build or improve LBNP chambers. The second aim is to focus on the importance of precise control of the LBNP chamber, not only during the steady state parts of experiments but also during the onset and release of LBNP. METHODS Fig. 1 shows the LBNP chamber and the bench. The main parts of the device are a tube of sufficient size to accommodate the lower body, a reservoir tank (117 L) to allow rapid release of pressure, and a mobile bench with an airtight waist seal and a regulated vacuum source. The tube is made of transparent polymethylmethacrylate (PMMA), length 1250 mm, inner diameter 480 mm, outer diameter 500 mm. A vacuum cleaner (Mièle S251 i, Mièle, Inc., Princeton, NJ) with adjustable effect from 300 1400 W is used to generate sub-atmospheric pressure in the reservoir tank. Vacuum cleaners generate greater air flow and are far cheaper than comparable commercial vacuum pumps. From the Department of Physiology, Institute of Basic Medical Sciences, University of Oslo, Norway (J. Hisdal, L. Walløe); and the Royal Norwegian Air Force, Institute of Aviation Medicine, Oslo, Norway (K. Toska). This manuscript was received for review in January 2002. It was revised in January 2003. It was accepted for publication in February 2003. Address reprint requests to: Jonny Hisdal, M.Sc., who is a researcher, Department of Physiology, Institute of Basic Medical Sciences, University of Oslo, P.O Box 1103 Blindern, N-0317 Oslo, Norway; jonny.hisdal@basalmed.uio.no. Reprint & Copyright by Aerospace Medical Association, Alexandria, VA. 874 Aviation, Space, and Environmental Medicine Vol. 74, No. 8 August 2003

Design of the Waist Seal For precise regulation of the pressure in the chamber, it is very important to avoid air leaks between the waist of the test subject and the waist seal. The design of the waist seal appears to have caused problems for many researchers. In 1971, Wolthuis et al. (5) published one solution to this problem. For the chamber descriped in our paper, a rigid PMMA board (thickness 20.0 mm) was cut to fit the test subject s waist (Fig. 2). Pipe foam (the type used to insulate water pipes) was split lengthwise and laid on the rigid board to cushion the subject s back and make the waist seal airtight. This type of foam is available in a range of thicknesses and can be adjusted to the size of the subject. In addition to pipe foam, test subjects wore a neoprene skirt that fit smoothly around the hips. The skirt may be custommade, but commercial skirts used for kayaking are also suitable and are available in various sizes. We used commercial neoprene kayaking skirts in our study (sizes S, M, and L; Wild Water, Bolton, UK). Fig. 1. The LBNP chamber, with the foot end of the mobile bench on the left side of the photograph. The bench has an adjustable footboard (1). (A saddle could also be used so that the subject s legs are suspended and serve as a fluid reservoir.) To guide the LBNP chamber into the right position, a small wheel (2) that fits the rail at the bottom of the tube is mounted on the end of the bench. A reservoir tank (3) is connected to the transparent PMMA tube (4). A large-diameter hose connects the reservoir tank and the tube (5). Sub-atmospheric pressure is generated in the tank and the tube by a vacuum cleaner that is insulated and enclosed in the box beside the tank (6). The chamber may be operated from the control box (7) or by digital signals generated from pre-programmed profiles stored in a computer. The PMAA tube lies in a soft rubber cradle (8). Getting In and Out of the Chamber Both the bench and the LBNP chamber are wheeled. Once the bench has been placed in the predetermined position, the wheels are locked, and the test subject can lie down on the bench. The chamber then slides along the bench so that it encloses the test subject s lower body (see Fig. 2). A small wheel is mounted on the end of the bench (see Fig. 1). This wheel fits into a rail that is glued to the bottom of the tube to guide the LBNP chamber easily in and out in the correct position. The end wall that closes the tube is divided into two (see Fig. 2). This construction makes it possible first to place the test subject on the bench and then to move the LBNP chamber to surround the lower body. Rapid Onset and Release of LBNP Large volumes of air must be removed from the LBNP chamber to reduce the pressure to given levels below room pressure. At least for pressures lower than around 10 mmhg below room pressure, it is difficult or impossible to reduce the pressure very rapidly and precisely using only a vacuum pump. A reservoir tank must be connected to the LBNP chamber via large diameter hoses (Ø 75 mm), so that large volumes of air can be removed from the chamber in a short time period. Rapid-action valves, controlled by electrical solenoids, are the best way of opening and closing the connection between the reservoir tank and the tube. Full opening of the valves must be achieved as quickly as possible to ensure rapid air flow between the reservoir tank and the tube. We used custom-built valves. The electrical solenoids control the valves are from Isliker Magnete (Andelfingen, Switzerland). We used GE-50 to control the valve at the tank, GE-70 to control the valve between the tank and the tube, and GE-60 in the valve on the back plate of the LBNP chamber (Fig. 1). For slower profiles, pressure in the tube may be regulated by changes in the power of the vacuum pump, or by adjusting the airflow into the LBNP chamber by means of a spring valve controlled by an electrical step engine instead of rapid opening and closure of solenoid-controlled valves. To ensure that the valve has a short response time, it is important to use a valve flap Fig. 2. Details of the waist seal. The end wall that closes the tube is divided into two. The lower part is permanently mounted on the bench (1). The upper part (2) is added after the LBNP chamber is placed around the lower body, and is held in the right position by two rubber straps (3). Pipe foam is split lengthwise and laid on the rigid board to cushion the subject s back and make the waist seal airtight (4). This type of foam is available in different thicknesses and can be adjusted to the size of the subject. Test subjects also wore a neoprene skirt that fit smoothly around the hips. Both the test subject and the test leader can operate emergency controls that open valves and shut down the vacuum pump. The test subject is holding the emergency control in her right hand (5). Aviation, Space, and Environmental Medicine Vol. 74, No. 8 August 2003 875

of low mass. We regulated the chamber pressure by regulating the force of the valve spring by an electrical step engine (STP-42ND48SVE-50, Epson Electronics, San Jose, CA). A micro controller (PIC16C84, Microchip Technology, Westford, MA) was used to control the step engine. These solutions make it possible to induce linear changes in pressure. Results from a study using gradual onset and release of LBNP are presented in Hisdal et al. (1). Control of the Chamber Pressure inside the chamber is controlled either by regulating the power of the pump and thereby the amount of air removed from the tube, or by reducing the controlled air flow into the tube through the valves connected to the tube. Pressure sensors continuously monitor pressure inside the tube and the tank. We used pressure sensors from Lucas Novasensor (NPC-1210-005-D, Fremont, CA). The pressure sensors must give an analog output signal that can be sampled during the experiments, and it must also be possible to control and calibrate them before use. During the tests, analog electrical signals are sent to a computer, where the pressure is recorded. Air leakage during the experiments is not normally a problem if the waist seal described above is used. If it is difficult to maintain a stable pressure in the chamber, one solution is to add a micro controller that compares the measured pressure with the set point. If there is a difference, the controller can be programmed to send signals to the valves or the vacuum pump to modulate chamber pressure and reduce the difference. Onset and Release of LBNP The onset and release of LBNP are controlled by a computer program, which sends digital signals from the computer to the control system (PIC16C73A, Microchip Technology). We used a DOS-based program (REGIST) developed by Morten Eriksen in our group. Any commercial program that has the ability to send out digital signals can be used. The onset and release of LBNP may be programmed to occur at specified times during an experiment, e.g., 120 s after the start of the experiment. Alternatively, if the aim is to apply LBNP within a diastole of a defined heartbeat, its onset and release may be triggered by defined R-waves in the ECG signal (Fig. 3). Noise during Operation of the Chamber Acoustic noise is produced during operation of the LBNP chamber, since large volumes of air are moved through small openings or valves. This noise may have physiological effects on the test subjects and must therefore be reduced as much as possible. One solution is to equip the test subjects with ear protectors. Another is to install the air intake of the pump in an adjacent room. The acoustic noise from the pump engine may be sufficiently reduced by proper insulation of the engine (Fig. 1). Another option is to install the pump in a Fig. 3. The upper panels (A) of the figure show ECG and LBNP during one experiment. Sample frequency is 50 Hz for both variables. Onset and release are triggered by R-waves in defined heartbeats. The black arrows in the upper and lower panels indicate the R-wave that triggers the onset of LBNP. The lower panels (B) show details of the onset of LBNP. In the recording shown, the control system is programmed to activate the onset of LBNP during the first heart beat detected 120.00 s after the start of the recording. The onset of LBNP is programmed to be delayed for 300 ms after detection of the R-wave, and is then applied within 200 300 ms. This ensures that LBNP is applied during the diastole of the first heart beat detected after t 120.00 s. soundproof enclosure and move it away from the LBNP stand to reduce noise and vibration. (The pump or vacuum cleaner produces a lot of heat during operation.) Thermal Effects on the Test Subjects The evacuation of air from the LBNP chamber increases the cooling of the test subject s lower body. This happens even at a relatively high ambient temperature. The additional cooling may have an influence on the variables recorded. It is especially important to be aware of the cooling problem in studies where acral skin blood flow or skin temperature is recorded. This is a difficult problem to deal with, since some air flow through the chamber is needed to maintain a stable pressure. The best solution is probably to insulate the test subject s lower body sufficiently to prevent cooling. In our lab we have used custom-made thermal boots. See Vissing et al. (4) for one example of how additional cooling may affect results obtained in studies using the LBNP technique. To reduce the problem, the tube can be constructed so the airflow is routed under the table. 876 Aviation, Space, and Environmental Medicine Vol. 74, No. 8 August 2003

Safety Precautions The chamber is designed and tested to operate at pressures from 0 100 mmhg below room pressure. The PMAA tube lies in a soft rubber cradle, and is held in the right position by its own weight. No holes are drilled in the PMMA tube, since this would introduce unpredictable weak points. The weakest point in our construction is at the ends of the tube. With an LBNP of 100 mmhg, the safety factor is calculated to be approximately 100. However, every laboratory should carry out individual calculations based on the dimensions and material of the tube used. Both the test subject (see Fig. 2) and the test leader can operate emergency controls that open the valves and shut down the vacuum pump. A manually operated emergency valve ( spring controlled ) is adjusted to open if pressure drops below the set level. All valves are designed to open if electrical power is lost. The design of the waist seal makes it easy to evacuate a test subject from the LBNP chamber if an emergency should occur. RESULTS Studies using our LBNP chamber have revealed transient physiological responses during the onset and release of mild LBNP. Our most important findings have been made in studies designed to test the hypothesis that there may be transient effects on mean arterial pressure (MAP) during the onset and release of mild LBNP that are not observed in the steady state parts of protocols using mild LBNP (1,2). Fig. 4 shows the responses of stroke volume (SV), heart rate (HR), cardiac output (CO), and mean arterial pressure (MAP) to the rapid onset and release of mild ( 20 mmhg) LBNP maintained for 300 s. This demonstrates that there are significant transient effects on SV, HR, CO, and MAP during the onset and release of LBNP that are not observed during the steady state period. DISCUSSION Mild LBNP is often defined as LBNP between 0 and 20 mmhg. It is defined as mild since it has been found not to have any significant effect on MAP. Mild LBNP has therefore been used as a method of inducing a significant reduction in central venous pressure (CVP) without affecting MAP. The idea has been that only cardiopulmonary baroreceptors are affected by the fall in CVP, while arterial baroreceptors are not stimulated, since MAP is found to be unchanged. Furthermore, any physiological response or lack of response observed has been related to isolated stimulation of the cardiopulmonary baroreceptors. We have demonstrated that this is incorrect, and that MAP is affected during both rapid (2) and gradual onset of mild LBNP (1). Fig. 4 shows that MAP is at the same level during the steady state part of the protocol as before and after the application of mild LBNP. Precise and uniform application and release of LBNP, in addition to continuous recording of the cardiovascular variables in all tests, made it possible to observe that there are physiologically very important Fig. 4. Averaged stroke volume (SV), heart rate (HR), cardiac output (CO), and mean arterial pressure (MAP) for 16 test subjects before, after, and during 20 mmhg LBNP. Several transient effects on the recorded variables are observed. There is a marked asymmetry in the response of SV and CO to the onset and release of LBNP. SV takes about 50 60 s to stabilize after application of LBNP, whereas it is normalized in 10 12 s after release of LBNP. There is also a significant drop in SV during the first heartbeats after release of LBNP. The most important finding illustrated in this figure is that there are significant changes in MAP both during onset and release of LBNP. This indicates that not only cardiopulmonary but also arterial baroreceptors are stimulated. More details of the study from which these data are taken have been published in Hisdal et al. (2). transient effects on MAP during the onset and release of mild LBNP. To allow proper comparisons of the results of studies using this technique, descriptions of the LBNP chambers used need to be more detailed than has generally been the case. During the steady state, information on how precisely the chamber pressure is controlled should be given, e.g., 1 mmhg. Finally, it is particularly important to specify the rate of onset and release of LBNP. This is probably also of importance when studying steady state and transient responses to higher levels of LBNP. Our studies and the example above show how important it is to control pressure in the LBNP chamber precisely when using the technique. In projects where the aim is to study the influence of baroreceptor responses on various cardiovascular variables, precise control of the pressure is essential. If the onset and release of LBNP is imprecise or variable, important transient physiological responses may be impossible to Aviation, Space, and Environmental Medicine Vol. 74, No. 8 August 2003 877

detect, and in the worst case the results may be interpreted incorrectly and the wrong conclusions drawn. Additional construction details and more pictures of our LBNP chamber can be provided by the authors on request. ACKNOWLEDGMENTS The Norwegian Council on Cardiovascular Diseases funded the construction of the LBNP chamber. We are grateful to Morten Eriksen for writing the computer program used to sample the cardiovascular parameters. We would also like to thank A. Sira, Ø. Løkeberg, and E. Salberg for technical assistance with the design and construction of the LBNP chamber. REFERENCES 1. Hisdal J, Toska K, Flatebø T, Walløe L. Onset of mild lower body negative pressure (LBNP) induces transient change in mean arterial pressure (MAP) in humans. Eur J Appl Physiol 2002; 87(3):251 6. 2. Hisdal J, Toska K, Walloe L. Beat-to-beat cardiovascular responses to rapid, low-level LBNP in humans. Am J Physiol Regul Integr Comp Physiol 200l; 281:R213 21. 3. Stevens PM, Lamb LE. Effects of lower body negative pressure on the cardiovascular system. Am J Cardiol 1965; 16:506 15. 4. Vissing SF, Scherrer U, Victor RG. Increase of sympathetic discharge to skeletal muscle but not to skin during mild lower body negative pressure in humans. J Physiol 1994; 481(Pt 1): 233 41. 5. Wolthuis RA, Hoffler GW, Baker JT. Improved waist seal design for use with lower body negative pressure (LBNP) devices. Aerosp Med 1971; 42:461 2. 878 Aviation, Space, and Environmental Medicine Vol. 74, No. 8 August 2003