Toe blood pressure and leg muscle oxygenation with body posture

RESEARCH ARTICLE

Toe Blood Pressure and Leg Muscle Oxygenation with Body Posture Armando Rosales-Velderrain, Michael Cardno, Jaime Mateus, Ravindra Kumar, Thomas Schlabs, and Alan R. Hargens ROSALES-VELDERRAIN A, CARDNO M, MATEUS J, KUMAR R, SCHLABS T, HARGENS AR. Toe blood pressure and leg muscle oxygenation with body posture. Aviat Space Environ Med 2011; 82:531–4.

Introduction: In 1980 Katkov and Chestukhin measured blood pressures and oxygenation invasively at various body tilt angles at different locations on the body, including the foot. To our knowledge, such measurements have not been performed noninvasively. Therefore, the purpose of this study was to measure toe blood pressure (TBP) and lower limb muscle oxygenation noninvasively at various body tilt angles, and to assess the use of a Finometer for noninvasive TBP measurements. Our noninvasive results are compared with those performed by Katkov and Chestukhin. We hypothesized that: 1) the Finometer provides a noninvasive measurement of TBP at different tilt angles; and 2) muscle oxygenation is highest with 0 and 26°, and decreases with increased head-up tilt (HUT). Methods: There were 10 subjects who were exposed to different body tilt angles (26, 0, 10, 30, 70, and 90°). At each angle we measured TBP noninvasively with a Finometer and muscle tissue oxygenation by near infrared spectroscopy. Results: We found a strong correlation between TBP using the Finometer and TBP predicted by adding the hydrostatic component due to body tilt to the standard arm blood pressure measurement. At 10, 30, 70, and 90° both TBP and tissue oxygenation were significantly different from the 0° (supine) level. Oxygenation decreased and TBP increased with higher HUT angles. No differences were observed in TBP or oxygenation between 26 and 0°. Conclusions: The Finometer accurately measures TBP noninvasively with body tilt. Also, muscle oxygenation is highest at small HUT angles and decreases with increased HUT. Keywords: tilt table, blood pressure, noninvasive, Finometer, NIRS.

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N 1980 KATKOV and Chestukhin measured blood pressure and oxygenation invasively at various tilt angles and at different locations on the body, including the foot (11). They compared their blood pressure measurements with theoretical calculations based on hydrostatic contributions to baseline values (11). To our knowledge, such measurements have not been performed using noninvasive techniques.

Tilt table studies are frequently used to investigate the effect of different gravitational environments on the cardiovascular system by altering the hydrostatic pressure gradient in vessels (11,12,20). The increase in the hydrostatic component of blood pressure when going from a supine to upright position in Earth’s gravitational environment is approximately 100 mmHg at the foot level (6). The hydrostatic pressure component is an important factor affecting cutaneous blood perfusion in the foot (3) and lower limbs (2), and is important for maintaining vascular structure and function (5,21).

New devices are now available to measure local blood pressure and tissue oxygenation noninvasively. The Fi-

nometer uses arterial pressure waveforms at the finger to measure beat-by-beat blood pressure and it has been established as a reliable noninvasive approach to taking blood pressure measurements (10,14,15). Near infrared spectroscopy (NIRS) is a noninvasive method of measuring muscle hemoglobin and myoglobin oxygenation (4). NIRS is used to determine the relative changes in oxygenated and deoxygenated hemoglobin (7). NIRS has been used to estimate oxygen uptake at rest and during exercise (8), and during different tilt angles (1).

The purpose of this study was to: 1) determine that the Finometer can be used to measure blood pressure noninvasively at the foot level over various tilt angles; and 2) to measure muscle oxygenation in the lower limbs at various tilt angles. We compare our data to a previous study where blood pressure and blood oxygenation data were acquired using invasive techniques (12). We hypothesized that the Finometer can accurately measure toe blood pressures at different tilt angles and that muscle oxygenation increases with head-up tilt (HUT).

METHODS Subjects There were 10 healthy volunteers, 5 men and 5 women [26 6 3.7 yr, height 1.70 6 0.09 m, weight 72.1 6 13.6 kg, BMI 24.8 6 2.4 kg z m22, and heart-toe distance 1.23 6 0.07 m (means 6 SD)], who consented to participate in this study. None of the subjects had a history of syncope or cardiovascular disease and none were taking any medication. This study received IRB approval from UCSD and all subjects gave informed, written consent prior to participating.

From the Department of Orthopaedic Surgery, University of California, San Diego, CA; and the Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA. This manuscript was received for review in September 2010. It was accepted for publication in January 2011.

Address correspondence and reprint requests to: Armando RosalesVelderrain, M.D., Department of Orthopaedic Surgery, University of California, San Diego, UCSD Medical Center, 350 Dickinson St., #121, MC 8894, San Diego, CA 92103-8894; rosalesarmando@gmail.com.

Reprint & Copyright © by the Aerospace Medical Association, Alexandria, VA. DOI: 10.3357/ASEM.2939.2011

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TOE BP WITH BODY POSTURE—ROSALES-VELDERRAIN ET AL. Equipment Toe blood pressure was measured with the Finometer device (Finometer Model 1, Finapres Medical System, Amsterdam, Netherlands) and arm blood pressure was measured with a standard blood pressure cuff (HEM775, Omron, Schaumburg, IL). Tissue oxygen saturation was measured with a NIRS device Invos system (Somanetics, Troy, MI). Lastly, temperature over the toe was measured with a thermometer (Van Waters and Rogers Inc., Portland, OR).

Procedure and Design The NIRS probe was placed on the tibialis anterior muscle of the leg at two cross fingerbreadths from the tibial tuberosity and two cross fingerbreadths external to the shin. A Velcro strap that holds the Finometer’s air pump was loosely wrapped around the lateral aspect of the foot so as to not obstruct the dorsalis pedis artery, the main blood supply to the foot (17,18). The finger cuff of the Finometer was attached to the second toe of the right foot. To decrease the possibility of toe vasoconstriction due to temperature changes and to ensure proper toe blood flow, we used an infrared heat lamp positioned at a 50-cm distance from the toe of all subjects and maintained a constant air temperature at the toe of 31 6 1°C; the temperature was measured using a thermometer during each angle. A standard arm blood pressure cuff was placed on the subjects’ right arm and measured blood pressure at heart level.

Each subject was tilted to six different body-tilt angles (26, 0, 10, 30, 70 and 90°) in a randomized order, and kept at each angle for 5 min, by which time a steady state was obtained. An electrically actuated tilt table was used to tilt the subjects. Subjects were positioned such that their feet were in contact with the footplate at the end of the table, thus allowing for part of their weight to be supported by their legs at the positive tilt angles. At all tilt angles indicated above, tissue oxygenation as well as arm and toe blood pressures were recorded. Within 2-3 min, blood pressure and NIRS values reached steady states, and only the last 2 min at each angle were used to obtain the blood pressure and oxygenation values used in the analysis.

The Finometer blood pressure data were compared to the theoretically expected blood pressure values at toe level and to those measured invasively by Katkov and Chestukhin (11). The theoretical values were calculated by adding the hydrostatic pressure component, added to the standard arm blood pressure measurement. The hydrostatic component of pressure was calculated as P 5 rgh•sin a, where P 5 the hydrostatic pressure, r 5 density of blood, g 5 gravitational acceleration constant, h 5 distance from the heart to point of measurement, and a is the tilt angle.

the Finometer blood pressure and the theoretical values.

Paired t-tests were used to compare the Finometer blood pressures at all non-zero angles to the blood pressure at the 0° (supine) level. Paired t-tests were also used to compare NIRS tissue oxygenation data at each non-zero angle to those for supine posture. An experienced statistician from the UCSD General Clinic Research Center reviewed our methods.

RESULTS Blood pressure values obtained with the Finometer correlated well with those obtained by adding the hydrostatic blood pressure component to the blood pressure measured at heart level. There was a significant positive correlation between the two blood pressure measurements (r 5 0.87, P 5 0.01), as can be seen in Fig. 1.

The systolic and diastolic Finometer blood pressures at the various tilt angles are shown in Fig. 2. Systolic blood pressures increased with HUT and were significantly higher than supine at all the positive HUT angles of 10° [t(9) 5 24.00, P 5 0.003], 30° [t(9) 5 25.41, P , 0.001], 70° [t(6) 5 217.03, P , 0.001], and 90° [t(4) 5 210.83, P , 0.001]. Diastolic blood pressures also increased with HUT and were significantly higher than supine at all the positive HUT angles of 10° [t(9) 5 –7.24, P , 0.001], 30° [t(9) 5 –8.44, P , 0.001], 70° [t(6) 5 –15.61, P , 0.001], and 90° [t(4) 5 29.81, P , 0.001]. Blood pressures at 6° head-down tilt (HDT) were not significantly different from baseline (supine).

The tissue oxygenation saturation data expressed as relative O2 saturation (rSO2) at the various tilt angles are shown in Fig. 3. The rSO2 data followed a very similar pattern on both legs, with no significant differences between the right and left legs. Since the right and left leg

Statistical Analysis All statistical analyses were performed with Statistical Package for the Social Sciences (SPPS, Chicago, IL). We computed the Pearson correlation coefficient between 532

Fig. 1. Correlation between noninvasive and theoretical blood pressures at toe level (r 5 0.87, P 5 0.01).

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TOE BP WITH BODY POSTURE—ROSALES-VELDERRAIN ET AL. in the magnitudes of the error bars. It is also possible that there is more inherent variability in using our noninvasive measurement techniques.

In five cases the Finometer did not function correctly at 90° and we also had problems measuring toe blood pressures at 70° HUT. We believe that this is probably due to an increase in contraction of muscles in the lower extremity and perhaps also due to changes in the pulse waves, thus resulting in missed data and a lower sample size for data at the 70 and 90° HUT angles. The Finometer is designed to be used in the finger and is thus likely optimized to work at a pressure range lower than the large pressures that are experienced at the foot level at the high HUT angles.

DISCUSSION Fig. 2. Systolic (top lines) and diastolic (bottom lines) blood pressures at various tilt angles. Data from our experiment using the Finometer (solid lines) and from Katkov’s invasive measurements (dashed lines). All data expressed as means 6 SD. * P , 0.05 Finometer blood pressure data at given angle compared to baseline.

rSO2 values were very similar, we pooled this data together when comparing rSO2 at various tilt angles to the supine baseline. Tissue rSO2 was significantly lower than baseline at all positive HUT angles of 10° [t(19) 5 3.49, P 5 0.002], 30° [t(19) 5 8.409, P , 0.001], 70° [t(19) 5 9.78, P , 0.001], and 90° [t(19) 5 10.365, P , 0.001].

Tissue oxygenation at 6° HDT was not significantly different from baseline.

We can also see that our blood pressure data follows the same pattern as observed by Katkov and Chestukhin (Fig. 2). Different angles were used in each study, though both experiments used angles of 0, 10, and 30° of HUT. The variance in our data is much larger than what was reported by Katkov and Chestukhin. We have assumed that the data reported by Katkov and Chestukhin represents means 6 SD; however, this is never explicitly mentioned in their paper. If their data were expressed as means 6 SEM, this could explain the large discrepancy

Fig. 3. Tissue rSO2 of both legs at various tilt angles. All data expressed as means 6 SD. * P , 0.05 rSO2 at given angle compared to baseline.

We were able to measure toe blood pressure using the Finometer and tissue oxygenation in the tibialis anterior muscle of both legs using NIRS. As far as we know, this is the first time that a noninvasive finger blood pressure device has been applied for successful blood pressure measurements at the toe level at various tilt angles. Our results show that an increase in blood pressure at foot level is proportional to an increase in the HUT angle and we also observed a pressure decrease when the subjects move to a HDT angle, as shown in Fig. 2.

These findings are in agreement with the results from the experiments done by Katkov and Chestukhin. As hypothesized, the Finometer was able to measure toe blood pressure noninvasively and the toe blood pressure data showed a strong correlation with theoretical values. Even though it is not commonly used at toe level, in this paper we have validated this finger device for measuring beat-by-beat blood pressure noninvasively and accurately in the toe. In comparison to established invasive techniques for toe blood pressure, such as cannulation of the dorsalis pedis artery (17), this device provides a noninvasive technique to measure systolic and diastolic blood pressures at the toe. This technique could potentially be used in patients with severe burns in the upper extremities or in amputees in which arm blood pressure cannot be measured noninvasively. Another group to consider is the obese population, in which measuring blood pressure with a standard arm blood pressure cuff is not possible; therefore, monitoring blood pressure at the toe could be beneficial.

Although the Finometer may be used as a noninvasive method to measure blood pressure at toe level, we encountered some operational limitations. In some subjects we had difficulty obtaining readings at 70 and 90° HUT, which may be due to the fact that the sensors are designed for the finger; therefore, we believe that this could be solved by designing a toe cuff. The blood pressures at the toe and the high HUT angles are likely higher than the blood pressure range over which the Finometer is designed for. Additionally, the changes and decreases in the pulse waves associated with the high levels of peripheral vasoconstriction at high HUT angles may also impair the proper functionality of the Finometer.

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TOE BP WITH BODY POSTURE—ROSALES-VELDERRAIN ET AL. Nevertheless, we were able to use the Finometer in the toe at moderate HUT angles and get results that closely matched those measured invasively by Katkov and Chestukhin, and our data were also strongly correlated to the theoretically calculated blood pressures. Another problem that we faced during our pilot studies was that the Finometer was unable to take measurements with exposed toes at room temperature due to the resulting vasoconstriction. This required us to use a heat lamp to control the air temperature at the toe level, which was kept at 31 6 1°C. As hypothesized, rSO2 decreased at higher angles of HUT as shown in Fig. 3. This may be due to an increase in the hydrostatic component that increases the transmural pressure gradient and produces a vasoconstriction response. It has been shown that the myogenic response is of great importance in the lower limbs and it has been suggested that in an upright posture approximately 55% of the cutaneous vasoconstriction is due to the myogenic response (16). The arteries and arterioles are the main vessels that respond to transmural pressure (13), and this response causes an increase in the diameter of the arterioles in skin and muscle in response to decreased transmural pressure (1,13). However, an increase in the hydrostatic component produces vasoconstriction in the lower limbs at high angles of HUT through both the myogenic and sympathetic mechanisms (9,19).

We conclude that the Finometer can be used to noninvasively take blood pressure measurements at the foot level at supine and moderate HUT angles. These blood pressure measurements are in agreement with previous data published by Katkov and Chestukhin as well as theoretical blood pressure measurements as calculated by adding the hydrostatic pressure component to a standard arm cuff blood pressure measurement. Additionally, rSO2 in the limbs decreased at higher HUT angles, as hypothesized.

ACKNOWLEDGMENTS We thank Somanetics Corporation for their support with the Invos System. We also thank the General Clinic Research Center, University of California San Diego, particularly Kimberley Duke for her statistical assistance.

Authors and affiliations: Armando Rosales-Velderrain, M.D., Michael Cardno, M.D., Ravindra Kumar, M.B.B.S., M.S., Thomas Schlabs, and Alan R. Hargens, Ph.D., B.S., Department of Orthopaedic Surgery, University of California, San Diego, CA, and Jaime Mateus, M.Eng., S.M., Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA. REFERENCES 1. Binzoni T, Quaresima V, Ferrari M, Hiltbrand E, Cerretelli P. Human calf microvascular compliance measured by nearinfrared spectroscopy. J Appl Physiol 2000; 88:369–72.

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