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TEACHING WITH TECHNOLOGY
Cardiovascular Physiology and Rehabilitation Laboratory, University of British Columbia, Vancouver, British Columbia, Canada
Address for reprint requests and other correspondence: D. E. R. Warburton, Rm. 205, Unit II Osborne Centre, Cardiovascular Physiology and Rehabilitation Laboratory, 6108 Thunderbird Blvd., Univ. of British Columbia, Vancouver, BC, Canada V6T 1Z3 (e-mail: darren.warburton{at}ubc.ca)
Abstract
Lower body negative pressure (LBNP) is an established and important technique used to physiologically stress the human body, particularly the cardiovascular system. LBNP is most often used to simulate gravitational stress, but it has also been used to simulate hemorrhage, alter preload, and manipulate baroreceptors. During experimentation, the consequences of LBNP and the reflex increases in heart rate and blood pressure can be manipulated and observed in a well-controlled manner, thus making LBNP an important research tool. Numerous laboratories have developed LBNP devices for use in research settings, and a few devices are commercially available. However, it is often difficult for new users to find adequately described design plans. Furthermore, many available plans require sophisticated and expensive materials and/or technical support. Therefore, we have created an affordable design plan for a LBNP chamber. The purpose of this article was to share our design template with others. In particular, we hope that this information will be of use in academic and research settings. Our pressure chamber has been stress tested to 100 mmHg below atmospheric pressure and has been used successfully to test orthostatic tolerance and physiological responses to –50 mmHg.
Key words: orthostatic stress; cardiovascular stress
LOWER BODY NEGATIVE PRESSURE (LBNP) is a well-established research technique with many uses (29). LBNP is most often used as a perturbation to the cardiovascular system and has been applied to simulate gravitational stress (15, 25, 32) and hemorrhage (20, 21, 23), alter preload (16), and manipulate baroreceptors (1, 3, 26). Experiments using LBNP have examined the physiological responses to orthostasis by quantifying a range of parameters such as heart rate, blood pressure, ventilation, muscle sympathetic nervous activity, hormonal responses, and cerebral hemodynamics (5, 6, 13, 19, 21, 23, 31). LBNP has also been used successfully to study a wide variety of populations including patients with heart failure, endurance athletes, astronauts, and the elderly (2, 4, 16, 18).
LBNP may be used alone or in combination with other cardiovascular stressors. Tilting may be used in conjunction with LBNP to reduce the need for high levels of LBNP (which may cause participant anxiety or discomfort) and create a more "natural" gravitational challenge (8). Conversely, LBNP has also been used with the participant seated in an upright position to examine posture-specific (e.g., for aircraft pilots) gravitational effects (22). Combined exercise and LBNP is another well-established use of a LBNP chamber, and this method has important implications for chamber design (7). The many applications and physiological consequences of LBNP make it a useful and innovative technique for both research and academic situations.
During LBNP, participants lie in a supine position with their legs sealed in the LBNP chamber at the level of the iliac crest. Air pressure inside the chamber is reduced by a vacuum pump, making the pressure inside the chamber less than atmospheric pressure. By the laws of fluid dynamics, blood shifts from an area of relatively high pressure (i.e., the upper body, which is outside the chamber) toward an area of relatively low pressure (i.e., the legs inside the chamber). Without physiological compensations, blood is shunted away from the thoracic cavity and ultimately pools in the capacitance vessels of the lower limbs and the lower abdomen (9, 11, 24). Normally, the body compensates by peripheral vasoconstriction and an increase in heart rate, which serve to maintain normal circulation (1, 3, 4). Inadequate physiological compensations in response to increasing negative pressure results in falling arterial blood pressure and, ultimately, syncope (13).
LBNP chambers are commercially available; however, they are often expensive. Furthermore, detailed design plans are rarely available. Several authors have outlined the particular LBNP chamber used in their laboratories; however, we found it difficult to duplicate their chambers based on the design and information provided (12, 14, 22, 27, 30). To combat these limitations, we set out to design and construct a LBNP chamber that is functionally comparable with the commercially available models at an affordable price. The proposed design costs approximately $850 (U.S. dollars). The purpose of this article was to share our design with readers so that they may build their own LBNP chamber for use in both academic and research settings.
Design
The LBNP chamber we designed is a rectangular prism with exterior walls constructed out of 1.9-cm-thick (
-in.) plywood. The plywood was fastened to an inner framework made of 3.8 x 6-cm (1
x 2
-in.) wood. We designed a strong inner framework to ensure the integrity of the chamber and the safety of our participants during high levels of negative pressure. The 3.8 x 6-cm wood of the inner chamber was cut into pieces and fastened together with common carpentry screws. Six rectangles of plywood were cut to fit over the inner skeleton and form the outer walls of the pressure chamber (see Fig. 1). The top and bottom plywood panels had dimensions of 156.5 x 85 cm (61
x 33 in.), and the two side panels had a length of 152.5 cm (60 in.) and a height of 76 cm (30 in.). The back plate (feet face this side) and front plate (iliac crest is sealed at this end) had a height and width of 78 cm (30 in.) and 85 cm (33 in.), respectively. The resultant combined dimensions were as follows: a length of 156.5 cm (61 in.), a height of 78 cm (30 in.), and a width of 85 cm (33 in.) (see Fig. 2). The total inner volume of the pressure chamber was 
1.03 m3. Note that the smaller the inner volume of the pressure chamber, the quicker the pressure can be reduced. Our chamber is longer and taller than may normally be required because it was specially designed to fit a stand-alone cycle ergometer inside for experiments requiring lower body cycling exercise during LBNP. Thus, our chamber is suitable for standard LBNP experiments as well as investigations involving LBNP and concurrent exercise.
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-in.) piece of pipe foam was attached to the top of the inner framework with two-sided tape. The pipe foam is required to form a tight seal between the removable top and the rest of the LBNP chamber. During negative pressure, the lid is drawn down onto the pipe foam, compressing it and forming an airtight seal (see Fig. 3). The remaining seams between two adjoining pieces of wood were sealed with commercially available clear silicon. Particular attention must be paid to applying the silicon to ensure an airtight seal. We recommend testing the airtight nature of the structure before and after cutting the opening in the chamber (as described below). This can be accomplished by cutting a small hole for the vacuum pump (as described below) and connecting the vacuum pump to the chamber. Turning on the vacuum pump with the removable top in place will allow for the evaluation of any potential leaks. If leaks are present, they can be sealed using additional silicon.
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in.) wide x 41 cm (16
in.) long (see Fig. 6). To attach the kayak skirt to the chamber, a square lip of wood surrounding the half-oval opening was created, and the kayak skirt was custom built to secure snuggly (elastic sewn into the neoprene) to the lip. The lip was made by attaching four pieces of 3 x 3-cm (1 x 1-in.) wood around the opening for the waist. Next, four additional pieces of 3.35 x 1.75-cm (1 x -in.) wood were screwed on top of the 3 x 3-cm wood. The second layer of wood formed the lip for the elastic rim of the kayak skirt to stretch over. All together, the kayak skirt creates a tight seal around the waist of the individual and the neoprene skirt attaches securely to the pressure chamber, forming a seal that is enhanced once the negative pressure is created within the chamber.
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-in.) plywood and covered with foam padding and vinyl upholstery. The seat was mounted via screws onto a 10.15 x 10.15-cm (4 x 4-in.) wooden post before it was upholstered. A hole was drilled completely through the seat post to allow for a bolt to secure the seat in place. Next, an additional piece of plywood with holes drilled every 2.5 cm (1 in.) was attached to the bottom (inside) of the chamber. This arrangement allows for the seat post to be adjusted at 2.5-cm (1-in.) increments to fit the individual needs of each participant. A similar setup was created with a 10-cm-tall (4-in.) and 70-cm-wide (27-in.) piece of plywood to act as a foot rest further down the chamber (see Figs. 5 and 7). The foot rest only needs to partially support the participants weight; therefore, the height of the foot rest can be designed accordingly. It is important to note that participants should contract their leg muscles minimally against the foot rest during experiments; the foot rest is there merely to prevent excessive movement (suction) into the chamber and to reduce pressure from the seat. We chose to use a seat and footrest in conjunction to reduce the pressure on the seat if it is used alone and to minimize muscular contraction, which may occur if the footrest is used alone. The use of a seat and/or footrest depends on the goals of the experiment and are not always necessary components, particularly during exercise (7, 28). Alternatively, it may be possible to increase the friction of the surface the individual is laying on to reduce movement into the chamber.
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6-in.) steel pipe [diameter of 2 cm ( in.)] with 90° open-close manual lever action. Two holes were drilled into the head of the pressure chamber into which the release valves were snuggly fit and sealed with clear silicon. The purpose of the safety valves was to allow the participant the opportunity to reduce the pressure inside the chamber within a matter of seconds if they became overly anxious or uncomfortable. For example, at operating pressures of –50 mmHg, opening of the valves will increase the chamber pressure by 30 mmHg within 5 s, leaving the pressure at –20 mmHg. When returning the pressure toward atmospheric pressure, it is advisable to reduce the pressure slowly so as not to induce bradycardia and asystole. Returning pressure back to atmospheric levels is often done over a 90-s period to allow for physiological compensations.
Generating and Monitoring of the Negative Pressure
A common 4,476-W (6 horsepower), 60-l wet-dry vacuum (Ridgid) was used to create the negative pressure in the chamber. A circular hole, large enough (6.5 cm) to fit the suction end of the vacuum hose, was cut into the back plate of the pressure chamber. The vacuum was plugged into a variac autotransformer (Ningbo, China) with output voltages of 0–140 V. To generate negative pressure, the vacuum and regulating transformer were both turned on, with the transformer output voltage set to 0 V. The output voltage on the transformer was then increased manually until the desired pressure was achieved. To maintain a constant chamber pressure, the vacuum pump was left on continuously during the test.
The pressure inside the chamber was continuously monitored with a digital differential manometer [model 840080, SPER Scientific (range: 259 mmHg, resolution: 0.2 mmHg)]. The manometer had two inlet ports; one was exposed to room air (atmospheric pressure), and the other was connected to a piece of rubber tubing that had the opposite end inside the pressure chamber. A small hole (5 mm) was drilled in one of the side panels to allow for placement of the tubing inside the chamber. The digital manometer was verified simultaneously with a Validyne differential pressure transducer (MP45). Both manometers were calibrated with a mercury manometer prior to each experiment.
Chamber Performance
Our pressure chamber has been tested for pressures ranging from atmospheric pressure to –100 mmHg (relative to atmospheric pressure). This degree of negative pressure goes beyond what is normally reported in the literature and is the maximum to which the chamber has been tested. For safety purposes, we only operated our chamber down to pressures of –50 mmHg. Chamber pressures of –50 mmHg can be generated from atmospheric pressure within 3 s of vacuum pump activation. Once the pressure has been generated in the chamber, the vacuum is left on and the pressure remains stable.
To date, we have successfully used our pressure chamber to test orthostatic responsiveness and tolerance in both healthy and endurance-trained males. Our laboratory has future plans to test females, older adults, and patients with cardiovascular disease.
Alternate Design Suggestions
The details presented above are specific to the chamber used in our laboratory. Alternative design options are plentiful and may be found useful by individual users, a few of which are given in this section. Many LBNP chambers have a "window" on one of the side panels of the pressure chamber to allow the entrance of measurement devices (e.g., strain gauge wires) or to view the increased volume of the lower abdomen during LBNP (22). In addition, the chamber described in this article used a custom-made neoprene waist seal attached to a rectangular opening. It is also possible to use an "over-the-counter" kayak skirt attached to an elliptical lip. The lip may be harder to create, but the kayak skirt may be easier to acquire over the lifespan of the pressure chamber. Alternatives to the manually operated pressure release safety valves described above are available, including spring-loaded valves, which can be set to release above a certain pressure. A push-button release valve may also be used as a safety release, which may be easier to activate for some participants. Automated electronic pressure control may be desirable for some laboratories to accurately maintain a constant chamber pressure. Using the vacuum mechanism outlined in this article, we have not encountered vacuum noise issues with our participants. However, ear protection may be worn if noise increases participant anxiety or if it has an anticipated effect on physiological measurements (e.g., cerebral hemodynamics). We have also had no apparent difficulties with increases in chamber temperature during the testing (due to the participant’s legs giving off heat in an enclosed space). This may be due to the fact that there is a small amount of air circulation in the chamber, which is why the vacuum pump needs to be constantly left on. However, the potential for increasing chamber temperature is something for users to be aware of.
Chamber Safety Considerations
Before the entry point was cut in the chamber, the structure was tested at pressures down to 100 mmHg below atmospheric pressure. This was near the limits of the vacuum and transformer; therefore, the chamber was not tested beyond –100 mmHg. The current design has only been used up to –50 mmHg during human experimentation, and it is recommended that your chamber should be tested to twice the pressure that it will be operated at to ensure safety. It is imperative that the pressure chamber is sufficiently tested prior to experimental use and that the chamber can withstand pressures higher than those that will be used during experimentation.
Before the chamber was built, several decisions were made to prepare the chamber for high levels of negative pressure and to ensure participant safety. First, wood was chosen as the material for construction. Metal, plastic, or fiberglass are all possible alternative materials. However, we chose wood for several reasons: relative weight (light compared with some metals), inexpensive, easy to manipulate, and adequate strength required for our purposes. We chose to use a thick (1.9-cm) piece of plywood of high quality (no cracks, holes, etc.). In addition to careful choice of the material, it was important to design the structure with care. A rectangular prism with a strong inner framework is more than adequate for the purposes of providing a safe human LBNP chamber.
There have been reports in the literature (10) of vasodepressor syncope during LBNP (even at low levels), although they are very rare. This highlights the necessity of heart rate and blood pressure monitoring (beat by beat if possible) during LBNP, particularly during presyncopal trials. During all LBNP testing, it is advisable to have multiple testers present, including trained medical personnel. Due to the possibility of vasodepressor syncope, it is recommended to have a defibrillator, oxygen, and appropriate medications nearby. During all LBNP testing, it is important for the testers to be attentive to the physiological measurements as well as chamber pressure and participant comfort. Participants should also be screened for a history of syncope, hernia, high blood pressure, and other cardiovascular pathologies. The potential risk of vasodepressor syncope, although small, may make LBNP too risky for undergraduate teaching settings. Qualitative symptoms of nausea, lightheadedness, or discomfort can all be reasons for the termination of a presyncopal (or any) LBNP test, in addition to set reductions in heart rate and/or blood pressure.
Conclusions
LBNP is a useful perturbation to examine the many compensatory mechanisms that occur within the human body. The ability of LBNP to manipulate the cardiovascular system in a very controlled manner, allowing for the measurement of numerous physiological parameters, makes it an important research and teaching tool. This design represents a simple to construct, affordable, and highly effective LBNP chamber for both research and academic purposes.
GRANTS
J. M. Scott and B. T. A. Esch were supported by the Natural Sciences and Engineering Research Council of Canada, the Michael Smith Foundation for Health Research, and the Canadian Space Agency during the completion of this project. D. E. R. Warburton is funded through the Michael Smith Foundation for Health Research, the Canadian Foundation for Innovation, the Natural Sciences and Engineering Research Council of Canada, Canadian Institutes of Health Research, the British Columbia Neurotrauma Fund, and the British Columbia Knowledge Development Fund.
Received for publication February 9, 2006. Accepted for publication December 13, 2006.
REFERENCES
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