Effect of compression on blood flow in lower limb wounds

Abstract

Blood flow is believed to be a key parameter in the formation and management of lower limb wounds. Patients with venous leg ulcers (VLUs) have high venous pressures, due to the partial or complete failure of calf muscle pump, which in turn disturbs the local blood flow within the lower limb. Compression has currently been the mainstay for treatment of VLUs and is thought to restore valvular competence and reduce or suppress superficial and deep venous reflux. Efficacy and assessment of compression therapy can be understood in a better way by measuring blood flow in lower limbs. Publications applicable to the effects of compression on lower limb blood flow parameters are summarised. However, they have shown varying results due to the different methodology and assessment techniques used. This article seeks to explore the methods of assessment of blood flow in the lower limb associated with wound management and compression in particular and provides suggestions for future explorations.

Introduction

Blood flow is believed to be a key parameter in the formation and management of lower limb wounds. In venous leg ulcer (VLU) patients, the presence of high ambulatory venous pressure in the veins of the lower leg may be caused by valvular incompetence 1, 2, 3) which in turn results in the partial or complete failure of the calf muscle pump (4). The high venous pressures associated with this failure disturb the local blood circulation within the lower limb, affecting both macro‐ and microcirculation (1, 5, 6).

Several studies on venous haemodynamics have shown that the venous reflux is the primary cause of venous ulceration 7, 8, 9). Single or in combination dysfunction of superficial, deep or perforating venous systems may contribute to the venous reflux (8, 10). It has been suggested that the venous hypertension damages the capillary regulatory function resulting in increased blood flow and an associated increase in capillary filtration (11).

An increase in microcirculation in the skin surrounding VLUs has been observed (12). Belcaro et al. (13) have indicated that, compared with healthy subjects, the skin blood flow was significantly higher in the lower limbs with venous hypertension, both in the supine and standing positions. Mayrovitz and Larsen (14) measured elevated skin blood perfusion in the ulcerated regions of patients with VLU. Blood capillary circulation is also impaired in limbs with chronic venous insufficiency (CVI) (15).

A number of proposals have been put forward to assess ulcer healing by measuring venous reflux (16), skin microcirculation (5, 14, 17) and leg pulsatile blood flow (18) in response to compression therapy.

Compression therapy is thought to restore valvular competence and reduce or suppress superficial and deep venous reflux. There are various methods of compression therapy including specialised compression stockings, elastic and inelastic compression bandages and intermittent pneumatic compression systems. Blood flow will vary greatly according to the compression system applied and types of materials used. Compression therapy is the principal mode of treatment for VLUs (19). The precise mode of action of compression is not fully understood, but numerous perceived benefits have been put forward (9, 19, 20, 21). These include decrease in oedema, softening of lipodermatosclerosis, acceleration of venous flow back towards the heart, decrease in venous volume, reduction in venous reflux, increase in arterial flow, improvement in microcirculation and improvement in lymph drainage.

There have been numerous reports on the effects of compression on lower limb blood flow using different assessment techniques. These include plethysmography (22, 23), Doppler ultrasonography (24, 25) and laser Doppler fluxmetry (5, 13, 26, 27), capillary microscopy (15, 28), radioisotope clearance methods (29, 30) and magnetic resonance flowmetry (18, 31). However, the literature is vague and has yielded conflicting results on the effect of compression on lower limb blood flow parameters. These differences may probably be due to the assessment technique used, type of compression therapy, level of compression and the medical history of subjects used for each of these studies.

This article seeks to explore the methods of assessment of blood flow in the lower limb associated with wound management and compression in particular and provides suggestions for future explorations.

Blood flow and pressure assessment techniques in common use

The assessment of arterial and venous blood pressure and blood flow in the lower limbs is considered to be of prime importance in wound healing.

There are a number of techniques in common use to assess blood flow and pressure in lower limbs. These can be grouped as: continuous wave and colour Doppler ultrasonography; air‐, strain gauge‐ and photo‐plethysmography; phlebography (venography); laser Doppler fluxmetry and capillaroscopy; and magnetic resonance flowmetry. They are summarised in Table 1.

Continuous wave Doppler and colour Doppler ultrasonography

Continuous wave Doppler ultrasonography is used to assess arterial and venous insufficiency and to ascertain the presence of venous reflux. Although it is inexpensive and now being used routinely in outpatient clinics, it cannot easily differentiate between superficial and deep veins and is therefore only used for screening. Venous imaging using colour Doppler ultrasonography is widely used for assessing all lower limb vessels, providing both anatomical as well as functional information with high sensitivity (25).

In the present context, the colour Doppler ultrasonographic scanning is widely accepted as the gold standard for venous assessment but is dependent on an experienced operator. Various authors have used colour Doppler ultrasonography techniques for the study of the effect of compression on lower limb blood flow. For instance, Sarin et al. (24) studied the effect of external compression on venous reflux in the deep and superficial veins of patients with CVI, using colour Doppler ultrasound imaging, and suggested an improvement in calf‐muscle‐pump function and a reduction in venous reflux associated with a decreased vein diameter.

Labropoulos et al. (32) investigated the immediate effects of intermittent pneumatic compression (IPC) on popliteal artery blood flow in normal volunteers. During application of the IPC, the popliteal artery blood flow increased significantly due to a decrease in the peripheral vascular resistance, as the peak systolic and end diastolic flow velocities increase and the reverse‐flow component diminishes.

Benko et al. (25) assessed the physiological effect of low‐pressure graded compression stockings on the blood flow of the lower limb in healthy volunteers with the use of colour Doppler ultrasound. They observed significant increase in blood flow velocity in the femoral vein and significant decrease in the vein diameter and cross‐sectional area of the popliteal vein.

Fromy et al. (33) studied the effects of whole lower limb compression applied to the healthy subjects, using Doppler ultrasound to measure femoral vein and arterial blood velocities. They showed that an external pressure as low as 10 mmHg decreased the arterial blood flow velocity causing significant impairment in arterial inflow whereas it provided no significant beneficial effect on venous blood flow velocity.

Plethysmography

Plethysmography is a non invasive volume‐measuring method and has been used to measure changes in volume of blood flow in the extremities. Air‐plethysmography can measure venous reflux, obstruction and poor calf‐muscle‐pump function. Strain gauge‐plethysmography is mainly used to measure ambulatory calf volume changes. Photo‐plethysmography is used to measure the venous refilling time. It is also used for screening to detect superficial and deep venous incompetence and assessing the overall physiological function of the lower limb veins.

Air‐plethysmography, comprising air chambers surrounding the lower limb, was used by Christopoulos et al. (22) to conduct studies on the effect of external compression on venous haemodynamics of the leg. Firstly, they measured leg volume changes during exercise to investigate calf muscle pump in normal and diseased extremities. The venous volume is increased in limbs with venous disease. Secondly, they studied the effect of elastic compression on the calf muscle pump by measuring ambulatory venous pressure using foot vein cannulation in patients with superficial venous insufficiency and with deep venous disease. Elastic compression produced a significant reduction in ambulatory venous pressure and venous reflux in both groups. The venous refilling times after the application of stockings did not reduce. The lower ambulatory venous pressure produced by elastic compression was the result of a reduction in reflux and an improvement in the calf‐muscle‐pump‐ejecting ability. They found this method clinically useful to determine the optimal amount of compression for a particular patient. In another study, Christopoulos et al. (34) investigated the effect of elastic compression on the increased arterial inflow in limbs with primary varicose veins, skin changes and deep venous disease, using air‐plethysmography. After the application of elastic compression, the arterial inflow was decreased in all three groups.

Partsch et al. (23) used air‐plethysmography to measure venous volume and venous filling index (as quantitative parameters of venous reflux) in order to compare the efficacy of compression bandages of varying pressure and material. They concluded that using the same bandage pressure, inelastic material is more effective at reducing deep venous reflux than elastic bandages in patients with VLU. Air‐plethysmography technique was, however, unable to distinguish between deep and superficial insufficiency (35).

Strain‐gauge plethysmography relies on the impedance change of silicone rubber tubes placed around the circumference of the leg. Cooke et al. (36) studied the effect of graduated compression stockings on lower limb venous haemodynamics, using strain‐gauge plethysmography. Venous flow parameters, venous capacitance and venous outflow from the lower limb were measured and found to be significantly increased in patients wearing the stockings. However, this volume‐measuring technique also has poor specificity and suffers from thermal drift.

Photo‐plethysmography measures variation in light absorption of skin by haemoglobin in dermal venous plexuses. Photo‐plethysmography has been used by Samson et al. (37) and Nicolaides and Miles (38) to study blood flow and blood volume changes in the skin under compression. Using this technique, venous reflux due to short refilling times was shown in patients with CVI. Both studies illustrated that with the application of compression, venous reflux is decreased. Nevertheless, this non invasive technique only measures regional blood flow and therefore related to superficial venous insufficiency. Moreover, the nature of output is not certain and may not relate to changes in blood volume or red cell orientation.

Phlebography

Phlebography or venography involving injection of contrasting medium (isotopes) into the veins is used to define the anatomy and to assess venous insufficiency (39). Partsch (40) showed a narrowing of veins due to compression using phlebography. The phlebography can image all lower limb veins at the expense of an exposure to ionising radiation and a considerable pain to the patient. Therefore, use of this technique is limited to special situations due its invasive nature.

Laser Doppler fluxmetry

Laser Doppler fluxmetry (LDF) is a continuous, non invasive method for assessing skin blood flow. It is now an accepted method for studying blood flow to the skin and other tissues. LDF has been used to assess the effect of compression therapy on local skin blood flow; however, it has shown varying results.

The effect of compression on microcirculatory blood flow characteristics of the skin of patients with CVI was studied using laser Doppler fluxmetry by Abuown et al. (26). They applied pressures from 10 to 100 mmHg using a blood pressure cuff. The laser Doppler probe was inserted in a polyethylene chamber underneath the cuff and both inflated with air so as the pressure in the cuff was equal to the compression applied to the skin. Pressure of 20 mmHg caused an increase in laser Doppler flux and in blood flow velocity, but higher pressures resulted in a decrease in both of the parameters while the patients were in horizontal position. When the patients were in the sitting position, higher compression levels up to 60 mmHg resulted in increase in laser Doppler flux and blood flow velocity. The increased laser Doppler flux was attributed to the increase in the volume of moving blood cells in the skin. Compression in the range of 20–30 mmHg, commonly applied by the use of compression stockings, was sufficient to increase the laser Doppler flux; moreover, the pressures reaching 40–60 mmHg may be effective in patients with CVI. Therefore, their findings indicating increased microcirculatory flow velocity on compressing the skin were attributed to the mechanisms of the action of the effect of compression on venous insufficiency.

Pekanmaki et al. (27) used laser Doppler fluxmetry to measure skin microcirculation before and after IPC in patients with post‐thrombotic venous insufficiency. They reported that the skin blood flow was increased after a single IPC session while the subjects were in supine position.

Contrary to this, Belcaro and Nicolaides (6) established that there was a decrease in the microcirculation on the application of intermittent compression to patients with venous hypertension. This is also supported by Mayrovitz et al. (41) who showed significant reductions in toe blood perfusion as a consequence of compression bandages applied to the lower limb of healthy subjects. Creutzig et al. (5) also found that during post occlusive reactive hyperaemia, the LDF value was significantly decreased in comparison to healthy subjects.

The reasons for the different results from these studies on the microcirculation after the application of compression may be due to number of reasons, namely the methodology used, movement of the subject and physical positioning of the laser Doppler probe and the disruption of the skin–bandage interface by placing the probe directly under the bandage (41) or inside a chamber beneath the bandage (26) or through a window cut in the bandages (42).

Melhuish et al. recently proposed a new method for measuring cutaneous blood flow through intact compression bandages, using single‐point LDF (43). Light‐transmissive gel applied to the bandage and the probe was placed on the outer surface of the bandage. A vascular challenge showed that the probe was measuring the microcirculation through the bandage as it was clearly reduced on inflation of a blood pressure cuff. A decrease in LDF signal was demonstrated on the bandaged limbs with minimum disruption to the skin–bandage interface.

Capillaroscopy

Capillaroscopy has been used to study the skin capillary circulation in the areas of patients with CVI susceptible to nutritional disturbances (15). Associated with the high skin blood flow in the limbs of patients with VLUs, an interference to nutritional circulation of skin occurred manifested by the microvessels being dilated and coiled. It was concluded that compression bandaging causes a reduction in oedema, facilitating the transport of nutrients from the capillaries to the skin cells. Although this technique directly measures capillary red cell flux, it may bear no relationship to total skin blood flow.

Magnetic resonance flowmetry

There are also reports on other general flow‐measuring techniques which have been applied for assessing pulsatile blood flow in lower extremities such as nuclear magnetic resonance flowmetry (18, 31, 44). In the studies of Mayrovitz and Larsen (18, 44) using this technique, compression bandaging applied to healthy subjects was shown to augment the leg arterial pulsatile blood flow significantly, but after 7 hours it was reduced. They suggested that the subbandage pressure should be sustained over a longer period of time as it was closely linked to increased pulsatile blood flow.

Discussion

To understand the precise mechanism of benefit of compression therapy for treatment of venous ulceration, many investigations focused on venous haemodynamics. Leg compression is associated with an increase in the flow velocity in deep veins of the lower limbs when legs are subjected to compression. Compression also causes significant increase in leg pulsatile blood flow. There exists a direct inverse relationship between skin blood flow and local applied pressure.

Blood flow measurements have shown different results when the subject is in supine or in standing position. In healthy or diseased extremities, the resting venous blood flow is different for each subject and it changes over time because of natural fluctuations in the arterial inflow to the limb. In ambulation, compression is believed to lead to increased venous flow, decreased venous reflux and increased ejection volume with calf‐muscle‐pump function. External compression increases the tissue pressure which favours resorption of oedema fluid (40).

It should be noted that the lower limb pressures are falsely elevated in the presence of calcified, non compressible vessels, which is common in patients with diabetes or renal failure. Therefore a toe pressure or toe brachial index is a more reliable indicator of blood flow because the digital arteries are usually less affected by calcification (45). The effect of compression therapy on patients with diabetic foot ulceration was specifically studied by Bowering (46). Multi‐layered compression therapy was an effective and safe treatment in diabetic patients with adequate arterial circulation. Reduced compression was also reported to be helpful in some patients with arterial compromise.

External compression decreases the overall cross‐section of the venous system of the lower limb and therefore increases the deep venous blood velocity (47). The beneficial effects of compression have been perceived as reduction of venous reflux, increase in the ejecting capacity of the calf xmuscle pump and reduction of arterial inflow towards normal (21).

With the application of external compression to the lower limbs of patients with CVI, a reduction in venous reflux has been measured with photo‐plethysmography, air‐pletyhysmography and strain gauge‐plethysmography. However, these techniques only measure regional blood flow and therefore related to superficial venous insufficiency.

From the results of blood flow studies performed with colour Doppler ultrasound, it can be seen that this technique can detect the venous reflux in the deep and superficial systems with high sensitivity. Therefore this technique has been regarded as the gold standard for the purpose of investigating venous insufficiency. However, blood flow studies with compression have shown contradictory results.

Tissue perfusion, a relative measurement of blood flow may indicate the healing status of a wound. It is generally accepted that tissue pressure brought about by limb compression tends to reduce blood flow in vascular bed (30).

The influence of compression on the skin microcirculation has been shown by laser Doppler fluxmetry. Assessment of microcirculation under compression bandages with laser Doppler probes were realised by placing the probe through a window cut out in the bandage to gain local access (42) or inside a chamber beneath the bandage (26) or directly beneath the bandage (41). All of these methods may disturb the bandage–skin interface, change the bandage pressure and have an effect on the haemodynamics at the site of measurement due to pressure applied by the probe. Therefore, we recently have proposed a new method for measuring cutaneous blood flow through intact compression bandages, using single‐point LDF (43). Light‐transmissive gel applied to the bandage and the probe was placed on the outer surface of the bandage. A vascular challenge showed that the probe was measuring the microcirculation through the bandage, as it was clearly reduced on inflation of a blood pressure cuff. A decrease in LDF signal was demonstrated on the bandaged limbs. Further studies are necessary to assess the bandage thickness in relation to laser Doppler signal.

The LDF is a single‐point measurement technique limited to measuring only a small region of the tissue microcirculation. Therefore, LDF measurements are sensitive to the location of the probe on the tissue, which may not represent the status of the tissue in and around the wound. This may result in subjective measurements which cannot be quantified. Laser Doppler Imaging (LDI) and Laser Speckle Imaging (LSI) are the two recent advances in the field of laser Doppler flowmetry, both of which may overcome this problem (48). LDI is a quantitative, non contact technique which uses a laser scan to provide a two‐dimensional colour image of tissue perfusion without disrupting the tissue of interest. In LSI, the entire tissue is illuminated with laser light and a CCD camera captures an image containing the superimposed speckle pattern. It is also a quantitative method which has a higher resolution than the LDI. Therefore, use of these two techniques in the study of effect of compression on the skin microcirculation in patients with CVI may shed light on the contradictory results of the LDF measurements reported above and may be of great value in understanding the skin blood flow under compression.

Conclusion

Compression is currently the mainstay for treatment of VLUs. Wound management with the assistance of blood flow assessment techniques should take into account whether CVI is caused by the patency in deep, superficial or perforating veins. Efficacy and assessment of compression therapy can be understood in a better way by measuring blood flow in lower limbs.

Currently there is no single satisfactory method for measuring blood flow. Each technique has its advantages and disadvantages. Many blood flow studies on the compressed limb have shown varying results due to the methodological problems. Ideally, a measuring system, which should be capable of assessing the blood flow in the whole anatomy of the lower limb and measuring bandage pressure, while the compressed limb is in ambulation would be of value.