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Magnitude of Cardiovascular System Response is Dependent on the Dose of Applied External Pressure in Lower Body Negative and Positive Pressure Devices

Elizabeth Bird1*, Alan R. Hargens1 and Lonnie G. Petersen1, 2
  • 1 University of California, San Diego, United States
  • 2 University of Copenhagen, Denmark

Background: Environmental and tissue pressures can have large effects on arterial and venous blood flows. For example, application of positive pressures in the range of 20 to 40 mmHg to a small area of the lower leg can increase local muscle, skin and bone microvascular flows, while negative pressures between -20 and -40 mmHg have been shown to decrease microvascular blood flow.[5-7, 9, 10] Additionally, large LBNP devices create foot-ward venous fluid shifts and decrease stroke volume and blood pressure at -20 and -40 mmHg.[2,3] Diverse patient populations benefit from these phenomena, including astronauts and individuals with chronic wounds. [2,3,8,13] Astronauts experience cardiovascular and skeletal muscle deconditioning, worsening vision, cerebral changes, and bone loss during long space flight due to the head-ward distribution of the venous blood volume and musculoskeletal unloading. [1-4,11,12] Large, Lower Body Negative Pressure (LBNP) devices can mitigate these changes by altering the macrovascular hemodynamics and returning more venous volume to the lower limbs, and creating a ground reaction force under the feet that loads the musculoskeletal system. [2,3] Patients with chronic wounds, such as venous stasis ulcers or diabetic ulcers, have underlying microvascular changes that increase the difficulty of wound healing. Compression bandages and negative pressure wound therapies both apply external pressures over small areas, leading to decreased wound healing time and increase microvascular blood flow. [8,13] But in one population that requires an external pressure driven macrovascular effect and another that requires a microvascular effect, how do you ensure that adverse microvascular macrovascular effects are limited, respectively? Additionally, what is the dose threshold needed to achieve a negative pressure driven venous fluid shift in astronauts or the dose threshold needed to achieve microvascular flow increase in patients with chronic wounds? Despite extensive literature on the many effects of external pressure on microvascular or macrovascular blood flow, no study exists to examine the relationship between the blood flow in the cardiovascular system, bone microvasculature, skin microvasculature and muscle microvasculature and the dose of applied external pressure. The purpose of this trial is to examine the effect of different doses of external pressure exposure, defined in terms of pressure exposure magnitude and area, on the bone, skin and muscle microvasculature and systemic blood flow. This abstract deals with the preliminary cardiovascular data of this trial. Hypothesis: Altering the exposure area and magnitude of external, pneumatic pressure will affect stroke volume, mean arterial pressure, heart rate and total peripheral resistance of healthy individuals. It is expected the largest exposure area and most extreme pressure magnitudes will lead to the largest change in systemic cardiovascular parameters relative to baseline. Methods: Healthy adults (n=3, all female, ages 21 to 26) had different areas (unilateral below the ankle/“Ankle Chamber”, unilateral above the knee/“Leg Chamber” , and bilateral below the iliac crest/”LBNP chamber”) exposed to varying magnitudes of external pressure (+ 40, + 20, and 0 mmHg). Pressure exposure was maintained for 5 minutes, while heart rate, blood pressure, stroke volume and systemic vascular resistance were measured during the last minute of exposure using a finger sphygmomanometer and finger arterial pulse contour analysis system (Nexfin Blood Pressure Monitor, BMEYE,USA). Skin, muscle and bone microvascular blood flow were measured with a photoplethysmography device over the tibialis anterior muscle and tibia bone (Data not discussed here). There was a 5 minute rest period between pressure exposures of different magnitudes to allow subjects to return to their baseline hemodynamics. Only one area exposure was tested in a given day. Subjects were tested on 3 different occasions to obtain cardiovascular and microvascular measures for each exposure area and magnitude. A 2-way ANOVA was used to analyze the presented cardiovascular data. Results: Data for n = 3 subjects suggest that stroke volume (figure 1) is significantly affected by the magnitude of the applied pressure (p = 0.009) with the lowest stroke volumes seen with negative pressure application, especially in the whole, lower-body negative pressure chamber. There is also a significant interaction effect between pressure exposure area and magnitude on heart rate (p = 0.034, figure 2), with the largest deviation in heart rate from baseline control (ambient or zero pressure) seen at -40 mmHg applied external pressure in the LBNP (large area of pressure application). Mean arterial pressure (MAP) increases with increasing area of pressure exposure, but this trend was not significant. For this preliminary data set, total peripheral resistance (TPR) and MAP do not appear to be significantly affected by the magnitude or total area of pressure exposure. Discussion: Preliminary results suggest that both area and magnitude of external pressure application significantly affect the cardiovascular system. Completion of the entire study population (n = 20) is expected to confirm these initial findings. These results are important in establishing the ideal external pressure chamber size and applied pressure magnitude for safe, effective, and practical microgravity countermeasure development, as well as adverse effect avoidance in chronic wound patients, who have many cardiovascular comorbidities. Interpretation of these results with the bone, skin and muscle microvascular flow data will further support the development of effective therapies with minimal adverse events for astronauts and chronic wound patients.

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Acknowledgements

Supported by NASA grant NNX13AJ12G. We thank the individuals who participated in this study, as well as colleagues and lab members.

References

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Keywords: Lower body negative pressure (LBNP), Space Flight Countermeasure, Dose - response, cardiovascular, Stroke Volume

Conference: 39th ISGP Meeting & ESA Life Sciences Meeting, Noordwijk, Netherlands, 18 Jun - 22 Jun, 2018.

Presentation Type: Extended abstract

Topic: Cardiovascular, Fluid Shift and Respiration

Citation: Bird E, Hargens AR and Petersen LG (2019). Magnitude of Cardiovascular System Response is Dependent on the Dose of Applied External Pressure in Lower Body Negative and Positive Pressure Devices. Front. Physiol. Conference Abstract: 39th ISGP Meeting & ESA Life Sciences Meeting. doi: 10.3389/conf.fphys.2018.26.00031

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Received: 02 Dec 2018; Published Online: 16 Jan 2019.

* Correspondence: Ms. Elizabeth Bird, University of California, San Diego, San Diego, United States, embird@ucsd.edu