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Cardiovascular Research 1997 36(3):372-376; doi:10.1016/S0008-6363(97)00181-8
© 1997 by European Society of Cardiology
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Copyright © 1997, European Society of Cardiology

Effects of positive pressure on both femoral venous and arterial blood velocities and the cutaneous microcirculation of the forefoot

Bérengère Fromy, Marie-Sophie Legrand, Pierre Abraham, Georges Leftheriotis, Paul Cales and Jean-Louis Saumet*

Laboratoire de physiologie et d'explorations vasculaires, Centre hospitalier universitaire, F-49033 Angers Cedex 01, France

* Corresponding author. Tel.: (+33-2) 41354617; Fax: (+33-2) 41355042; E-mail: jean-louis.saumet@univ-angers.fr

Received 10 April 1997; accepted 10 June 1997


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objective: The balance between the apparent beneficial effect and the risk of arterial ischaemia resulting from an external uniform compression is unclear. The purpose of this study was to determine the effects of a positive uniform compression on the lower limb circulation until a critical threshold was reached. Methods: We used Doppler ultrasound to measure femoral venous and arterial blood velocities. The effects of positive pressure on cutaneous microcirculation were evaluated by laser Doppler flux (LDF), transcutaneous oxygen pressure (tcpO2) and transcutaneous carbon dioxide pressure (tcpCO2) on the forefoot of 17 healthy subjects. Results: The results are expressed as median [lowest observed value–highest observed value]. Whereas the arterial femoral velocity (A.F.V.) decreased from 0.21 [0.08–0.36] to 0.17 [0.08–0.28] m s–1 for an external pressure as low as 10 mmHg (p<0.001), the venous femoral velocity (V.F.V.) decreased from 0.13 [0.06–0.40] to 0.09 [0.05–0.34] m s–1 at 20 mmHg (p<0.001). An increase of tcpCO2 from 39.8 [29.9–53.7] to 40.2 [30.0–55.5] mmHg (p<0.05) and a decrease of LDF from 8.7 [3.1–25.9] to 5.5 [2.3–21.1] A.U. (p<0.001) occurred at 10 mmHg. However, tcpO2 decreased only from 76.7 [40.2–91.2] to 64.6 [18.9–85.2] mmHg when the splint pressure reached 60 mmHg (p<0.05). The observed parameters (LDF, tcpO2, V.F.V. and A.F.V.) decreased further (except for tcpCO2 which increased) up to the end of the study as the applied pressure was increased. Conclusion: Positive pressure on the full leg provided no significant beneficial effect on femoral venous blood velocity. Whereas we showed that for an external uniform pressure as low as 10 mmHg, significant impairments in both arterial inflow of the lower limb and microcirculation of the forefoot appeared in recumbent healthy young subjects.

KEYWORDS Positive pressure; Laser Doppler; Microcirculation; Ischaemia; Femoral blood velocities


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
The external compression on the lower extremities, mostly realised by elastic compression, is used to prevent or to limit oedema formation of various origin: chronic venous insufficiency, cardiac insufficiency, lymphatic alterations are mainly considered. On the other hand, a direct inverse relationship exists between skin blood flow and local applied pressure [1, 2]. Moreover an inverse relationship between transcutaneous oxygen pressure (tcpO2) and external pressure has been found, although controversy exists whether it is linear or parabolic [3, 4]. In attempting to find an explanation for the apparent beneficial effect of compression on venous function suggested as resulting from an improvement in calf muscle pump function with an increase in venous flow velocity or a reduction in venous reflux, studies have focused attention on the flow of blood in the deep or superficial venous system of the lower limbs [5, 6].

A local externally applied pressure as low as 40 mmHg produces a significant decrease of transcutaneous oxygen tension and even a compression of 20 mmHg can significantly decrease the skin blood flow [2]. As the level of applied pressure in both studies varied within the same range, the balance between the beneficial effect on the venous circulation and the risk of tissue ischaemia due to an external uniform compression is still to be evaluated.

The purpose of this study was to determine the effects of a positive uniform compression applied to the whole lower limb on both femoral venous and arterial blood velocities and the cutaneous microcirculation of the forefoot until a critical threshold was reached.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
2.1 Population
Seventeen healthy Caucasian subjects participated in the present study. The participants included 8 young males with a median age of 24 [23–28] years, weight of 68 [62–82] kg, and height of 175 [169–182] cm and 9 young females with a median age of 23 [20–26] years, weight of 59 [47–73] kg, and height of 165 [160–180] cm. All volunteers gave their written informed consent to participate in the experimental protocol which was approved by our institutional review committee according to the Helsinki Declaration. After a physical examination, a 12-lead electrocardiogram, the measurements of the ankle to brachial systolic pressure ratio in both limbs, veins and arteries of abdominal and lower limbs examination by colour Duplex scanning were performed in order to exclude subjects with significant cardiovascular disease.

2.2 Measured data
Ultrasound imaging of the femoral vessels was performed with a 7 MHz linear electronic transducer (Acuson 128XP10, Mountain View, CA, USA). Ultrasound coupling between skin and probe was achieved by a large amount of ultrasound transmission gel, avoiding direct contact influence of the probe with the skin. The femoral vessel was scanned in a transversal plane and a regular segment of the common femoral vein was pointed with an indelible marker on the skin, very close to the saphenofemoral junction. The probe was then turned 90° to display a longitudinal view of the vessel and to measure both arterial femoral velocity (A.F.V.) and the venous femoral velocity (V.F.V.) distal to the saphenofemoral junction. Using a maximal true angle between the probe and the skin and an inclined Doppler axis of 20°, the complementary angle to align with the vessel axis was less than 60° in all cases. Both arterial and venous blood flow velocities were the average of the maximal velocities on a 8 second recording as previously reported [7–9](including possible arterial diastolic zero flow), and were not angle corrected.

Transcutaneous oxygen pressure (tcpO2), based upon the electrochemical reduction of oxygen, was measured using a Clark-type oxygen sensing electrode [10, 11]. Transcutaneous carbon dioxide pressure (tcpCO2) was measured using a Severinghaus-type carbon dioxide electrode. A combined transcutaneous oxygen pressure and carbon dioxide pressure electrode comprising a heating element, two temperature sensors and the gas measurement electrodes in a single unit was used (TCM3 Radiometer, Copenhagen, Denmark) and placed distal to the splint. The electrode was attached to the skin on the forefoot with a double-sided adhesive ring tape, the heat generated, automatically stabilised to 44.5°C, was transferred from the heating element via the silver body to the skin surface. This heating produced local vasodilation, increasing the permeability of the skin for oxygen and carbon dioxide, and allowing measurement on the skin surface. All tcpO2 and tcpCO2 measurements were made with the subjects breathing room air, and the data were expressed in mmHg and automatically corrected to 37°C.

Cutaneous blood flow of the foot was measured using a laser Doppler flowmeter (Periflux 4001, Perimed AB, Järfälla, Sweden) applied to the skin of the forefoot with a plastic holder. The helium–neon laser Doppler velocimeter uses a monochromatic light source at 780 nm. Conducted to the body surface via fiber optics, the laser light illuminates and permeates the skin in a diffuse way [12–16]. Doppler-shifted light, proportional to the product of velocity and the concentration of moving red blood cells within the surface capillaries of the skin [14], is processed and expressed in arbitrary units (A.U.). Comparison with non-invasive techniques such as plethysmography or thermal clearance showed that laser Doppler flux (LDF) provides accurate measurement of skin blood flow in human limb [17, 18]and is used widely in the investigation of the skin microcirculation [7, 19, 20]. We have previously shown that cutaneous LDF is not influenced by underlying tissue blood flow [21].

2.3 Data collection
The resulting analogue output laser Doppler signals, tcpO2 and tcpCO2 signals in both feet and the pressure in the full-leg splint were recorded by means of a data acquisition system (MP100, Biopac System Inc., Santa Barbara, CA, USA) and further analysis was performed by specific software (Acknowledge, Biopac System Inc., Santa Barbara, CA, USA). The sampling acquisition was fixed at 25 samples per second.

Lastly, heart rate and arm systolic and diastolic blood pressures were recorded during the whole experiment using an automatic 16 cm large cuff inflation system (Dinamap 1846SX/P, Critikon, Johnson and Johnson, Tampa, FL, USA) in order to check the stability of the general haemodynamic parameters of the participant during the experiment.

2.4 Experimental protocol
A standard inflatable splint extending from the ankle to the upper thigh (Mast III-AT anti shock trousers, David Clark company, Worcester UK) was placed around the right leg. The skin of each subject was cleansed using alcohol tabs at the selected measuring sites. The subject, fully equipped, was placed supine upon an examination table with a pillow under the head, in a quiet room for 20 minutes to allow stable conditions at rest. The room temperature was maintained at 25 [23–27]°C and the humidity was 58 [52–68]%. Once the measurements started, each subject was asked to lie immobile until the end of the experiment.

After measurements under basal conditions were finished, the splint was inflated to 10 mmHg and maintained for three minutes. During the last two minutes, sonographic measurements were performed by a well-trained physician. The duration of each step was 3 minutes in total. After this first step, the inflation of the splint was increased to 20 mmHg, then successively increased by steps of 20 mmHg. The experiment was ended when no decrease in LDF and tcpO2 was observed between two successive levels of splint pressure. The minimum values were defined as the physiological zero of the participant, observed at the individual maximum reached splint pressure.

2.5 Statistical analysis
The results are expressed as median [lowest observed value–highest observed value]. Analyses of differences between paired values were carried out using the non-parametric Wilcoxon matched-pairs signed-ranks test (for within subjects differences) with the basal condition (splint not inflated) as the reference. The p values were calculated taking into account the number of participants at each pressure. A p value of less than 0.05 was considered significant. Statistical computation was carried out with use of the Statistical Package for Social Sciences (SPSS Inc. Chicago, IL, USA).


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
Every subject had stable heart rate and arm systolic and diastolic blood pressures during the full duration of the experiment. These measured parameters did not change significantly: from 62 [45–80] min–1, 108 [90–123] mmHg, 58 [54–74] mmHg to 66 [44–86] min–1, 111 [95–144] mmHg, 63 [50–81] mmHg over the first three minutes and the last three minutes of the experiment, respectively. The results of various splint pressures on laser Doppler flow, tcpO2, tcpCO2, venous and arterial femoral velocities are summarised in Table 1 for all subjects (n = 17). The minimum values were reached at 80 mmHg in 11 subjects. Therefore only 6 subjects were observed at a pressure of 100 mmHg. The mean pressure in the splint to reach the physiological zero was 80 [80–100] mmHg for the overall of the participants.


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  		Table 1 Effects of uniform positive compression on both venous and arterial femoral velocities and forefoot cutaneous microcirculation on young healthy subjects

 
There was no significant change in the V.F.V. at a splint pressure of 10 mmHg. A 20 mmHg compression reduced (p<0.001) the venous femoral velocity significantly: an average of 30.8% decrease compared to basal conditions. The venous velocity further decreased as the applied pressure was increased. At the individual maximum splint pressure, the maximal decrease in venous velocity compared to the resting value was 84.6% (p<0.05).

The A.F.V. decreased for an external pressure as low as 10 mmHg (p<0.001). The minimum arterial femoral velocity was 0.04 [0.01–0.11] m s–1 at the individual maximum splint pressure, representing a 81.0% decrease compared to the basal velocity (p<0.05).

Although it tended to decrease, no significant change was found for tcpO2 before the splint pressure reached 60 mmHg. However, tcpCO2 values increased significantly (p<0.05) at 10 mmHg and further increased up to the end of the study.

A significant decrease in LDF (36.8% compared to the mean resting value) measured on the forefoot occurred when the external pressure was 10 mmHg (p<0.001). The LDF continued to decrease until a minimum skin blood flow level of 3.0 [1.2–11.1] A.U. (65.5% decrease compared to the basal conditions), although this minimum value was reached at different splint pressures (80 or 100 mmHg) for each participant.


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
The findings of the present investigation demonstrate a significant impairment of A.F.V., LDF and tcpCO2 with a compression value as low as 10 mmHg, although V.F.V. and tcpO2 did not decrease. On the other hand, under our experimental conditions, tcpO2 appears less relevant than LDF and tcpCO2 to evaluate local micro-circulatory impairment.

Very little information is available regarding the effect of external compression on venous blood flow in the total extremities [22, 23]. On the other hand, few results are available of the effect of uniform compression on arterial blood flow [24], since most recent studies have been performed on the effects of locally applied pressure on the microcirculation [25, 26]. Even less is known regarding the effects of externally applied pressure on both venous and arterial blood flow, and distal microcirculation.

The applied pressure produced by elastic compression or bandages can be classified according to the amount of compression [27]. The compression bandages range from 14 to 17 mmHg for the light compression bandages, from 18 to 24 mmHg for moderate compression bandages, from 25 to 35 mmHg for high compression bandages, and is up to 60 mmHg for extra high-performance compression bandages [28]. Nevertheless, as already underlined, the pressure values used in the patient management are in the same range as those discriminated for an arterial ischaemia.

Sabri et al. found that a 5 mmHg inflatable splint compression produces a non-significant increase in the femoral arterial and venous flows [29]. Unfortunately, the arterial flow was measured in dogs, whereas the venous flow was measured in humans. They mentioned that a higher compression pressure is required before any significant decrease of the femoral venous blood flow can be seen. In contrast, Spiro et al. mentioned an increase of 13% of the femoral vein flow in both limbs at 5 mmHg by inflatable splint [30], but no statistical analysis was made in this study. However, they concluded that the optimum pressure usually lies between 5 and 12 mmHg but may well vary from patient to patient depending on such factors as limb adiposity, limb circumference, the presence of lymphoedema, or previous episodes of thrombosis. Our results are consistent with the investigation of Sabri et al., as we showed that there was no significant increase of the femoral venous velocity at a 10 mmHg compression and that the venous velocity decreased significantly at 20 mmHg splint pressure.

Furthermore, we found that a significant decrease of 19.0% compared to the resting value in maximal arterial inflow occurred for a splint pressure as low as 10 mmHg. In agreement with our investigation, Sabri et al. showed that "a compression pressure exceeding 5 mmHg produces a progressive diminution of the femoral arterial blood flow" [29]Halperin et al. reported also that an externally applied pressure of 10 mmHg was sufficient to reduce the arterial circulation in normal limbs [24]. They attributed the resultant decrease of arterial inflow and venous outflow to two possible factors: first, a reduction of arteriovenous pressure gradients, and, secondly, a decrease in the calibre of small vessels in the compressed area that caused an increase in resistance to flow. Consistently with arterial flow decrease at 10 mmHg, the skin blood flow showed a significant decrease at 10 mmHg.

The tcpO2 decrease with increasing pressure shown in recumbent subjects found in the present study is in agreement with Gaylarde et al., who mentioned that a compression of the lower limb whilst the subject is recumbent leads to a fall in tcpO2. These authors suggested that when the patient is confined to bed, only lightweight stockings (the applied pressure lies between 5 and 10 mmHg) are safe in the prophylaxis of post-operative deep-vein thrombosis [31]. However, our results suggest that tcpO2 alone does not interpret the arterial haemodynamics properly under our experimental conditions. Indeed, a significant decrease was observed only at 60 mmHg with tcpO2 readings. Maybe the skin blood flow impairment due to a splint pressure up to 60 mmHg was not high enough. On the other hand, for normal arteriolar pressure values, tcpO2 mainly reflects arterial pO2. TcpO2 becomes flow dependent only when arterial pressure is reduced below a certain threshold [2]. The late decrease of the tcpO2 values found in the present work may result from relative uncompromised distal arteriolar pressures. Since arteriolar pressure in the foot was not recorded in the present study, this hypothesis cannot be proved. Nevertheless, for a pressure as low as 10 mmHg, tcpCO2 increased significantly, while LDF decreased significantly. We hypothesise that pH increased and that a distal microcirculation impairment was already observed on the forefoot. Andreozzi et al. already showed that a discrepancy between tcpO2 and tcpCO2 exists [32]. In a further study, they mentioned that "in some cases, the tcpO2 cannot provide correct assessment of the risk of skin necrosis, while tcpCO2 measurement could" [33]. On the other hand, some authors already mentioned that a significant difference exists between tcpO2 and LDF changes at the forefoot to different physiological stresses [34]. Therefore, tcpO2 measurement on its own may be not a sufficient tool under our experimental conditions to control the distal microcirculation.

From this study it can be seen that positive pressure on the full leg provided no significant beneficial effect on femoral venous blood velocity. Whereas we showed that for an external uniform pressure as low as 10 mmHg, significant impairments in both arterial inflow of the lower limb and microcirculation of the forefoot appeared in recumbent healthy young subjects. Although such a result was found in healthy volunteers, the technique used in the present work is an interesting approach in the understanding of the compression effects. A study of patients with vascular disease, both arterial and venous, would be relevant. It might allow a better understanding of potential beneficial and harmful effects of compression in such patients groups. Further work and additional data will be necessary to sort out these issues.

Time for primary review 18 days.


   References
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 

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