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Technical Sciences

The Science of

CBy Daovid Kmeast pression

e know that compression therapy Wworks to reduce and control in extremities but how does it work? Understanding the science of compression

begins by examining how fluid moves FIGURE 1 between the blood vessels, the lymphatics and the tissue space. This movement of movement of larger molecules such In his original work over 100 years water, proteins, nutrients and waste prod - as proteins. Water moves across the ago Starling described an equilibrium in ucts is critical to tissue survival. This is an membrane pulled in the direction of the which there was net filtration of water integrated system and the arteries, veins, higher concentration of proteins. This out of the arterial side of the capillary bed and lymphatics cannot be force is called the oncotic pressure. bed and net re-absorption of fluid on the considered in isolation. The plasma proteins exert a plasma venous side as a result of the balance colloid oncotic pressure pulling water between capillary and tissue hydrostatic Forces moving tissue fluid into the while the proteins pressures as well as plasma and tissue Fluid only moves if it is pushed or pulled. and macromolecules in the interstitial colloid oncotic pressures. Based on calcula - The force pushing fluid is called hydrostatic space exert a tissue oncotic pressure tions it was postulated that approximately pressure. We know that the deeper we pulling water into the issue space. These 80 –90% percent of interstitial fluid was go in a swimming pool the greater the forces pushing and pulling fluid between reabsorbed by the venous side of the force of the water above us. Similarly the the capillaries and the tissue spaces are capillary bed and 10 –20% was handled force of gravity on the column of fluid in sometimes called Starling Forces, named by the lymphatics. Newer research has the blood vessels when a person is standing after a British physiologist who described shown, however, that in the extremities or the leg is dependent exerts a hydrostatic the mechanisms of body fluid shifts. capillary hydrostatic pressure is never less force on the fluid, the capillary hydrostatic The other key component of tissue than the tissue hydrostatic pressure and pressure. Also if more fluid is forced into fluid handling is the lymphatics. Figure 1 a confined space there will be increased shows the relationship between the capil - hydrostatic pressure within that space. lary bed and the lymphatics. The lymphatic Thus as fluid accumulates in the interstitial capillaries have a unique structure where space (the fluid in the spaces between the cells making up the wall of the the cells in the tissue), the interstitial lymphatic capillaries are tethered to the fluid hydrostatic pressure increases. extracellular matrix. As the tissue hydro - Water is also pulled between the static pressure increases, this tethering capillaries and the interstitial space. causes portals to open into the lymphatics The capillary walls are semi-permeable allowing the passage of water, proteins, membranes allowing small molecules such macromolecules and cellular debris into as water to pass through but restricting the the lymphatic capillaries.

David H. Keast MD FCFP is the Medical Director of the Chronic Wound Management Clinic at St. Joseph’s Parkwood Hospital in London, Ontario. He is Clinical Adjunct Professor of Family Medicine, Schulich School of Medicine and Dentistry, Western University (London) and Centre Director, Aging Rehabilitation and Geriatric Care Research Centre, Lawson Health Research Institute. He is Co-director of the Canadian Lymphedema Framework. FIGURE 2

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that under normal circumstances there leaky valves in the veins), obstruction or How compression works is no net re-absorption of fluid into the muscle pump failure. Our understanding of the mechanism of capillary bed. To further complicate the The blind lymphatic capillaries join action of compression therapy on the equation, the movement of proteins to form lymphatic vessels. The initial mobilization of tissue fluid is heavily and other larger molecules across the lymphatic capillaries lead to larger lymph dependent on venous research. However semi-permeable membrane of the collector vessels that have a smooth muscle as previously discussed, the management capillary walls is much more dynamic than layer and valves. The functional unit of of tissue fluid is an integrated system and previously thought. Newer models of tissue the management of lymphedema is closely fluid balance suggest that in fact 100% of related. It has long been believed that tissue fluid is handled by the lymphatics. 1 compression therapy mobilized tissue In other words there is a net filtration of fluid through a pressure gradient with fluid out of the capillary bed with the vast higher pressure at the distal extremity majority of this fluid being reabsorbed by tapering to a lower sub-bandage pressure the lymphatic system. proximally. The Modified Law of Laplace was used to predict sub-bandage pressure. Venous return and lymph propulsion This law predicts that sub-bandage The second key component of an under - pressure will increase as the number standing of the science of compression of layers increases and as more tension is to appreciate how blood is moved back is applied to the bandage. Conversely towards the heart in the veins and how as the limb circumference increases lymph fluid is propelled towards the and as bandage width increases thoracic duct which returns fluid to the according to Laplace’s law venous system at the junction of the left the sub-bandage pressure internal jugular and left subclavian veins. should decrease. Since the Again this is an integrated system and circumference of the limb the vascular system and the lymphatics increases as we move closer cannot be considered in isolation. to the trunk, sub-bandage

In the extremities the venous system pressure is predicted to Pierre-Simon Laplace consists of superficial veins and deep decrease. In the lower extremity calcula - veins connected by perforators. All veins tions using Laplace’s Law predict that have a series of one way valves which correctly applied high compression systems

when functional prevent the backflow of FIGURE 3 should provide sub-bandage pressures blood. The superficial veins are in the graduating from 35 –40 mm Hg at the ankle subcutaneous tissue and the deep veins the collector between the valves is known decreasing to 15 –17 mm Hg at the knee are within the muscle compartment as a lymphangion, and this has intrinsic on the average limb. In 2008 Schuren of the extremity separated by connective pacemaker activity. Intrinsic contractions and Mohr reported on three studies with a tissue called fascia. When the muscles of the muscle occur 6 –10 times total of 744 applications of various contract they push the blood in the deep per minute propelling the lymph compression systems to a lower veins forward towards the heart and fluid away from the extremity. limb model. In only 7.1% of the the valves prevent backflow. When Unidirectional flow is ensured by applications was graduated the muscle relaxes the pressure in the the valves (Figure 3). Skeletal compression observed. 2 superficial veins is greater than in the muscle activity, pulsations of If we consider the limb to deep veins and blood flows along the adjacent arteries, body move - be a fluid filled container pressure gradient from the superficial veins ments and respiration all play a role then sub-bandage pressure into the deep veins, filling them for the in normal lymph transport. Lymphatic Blaise Pascal may be better explained by next contraction of the muscles. This is failure may result from congenital Pascal’s Law. Pascal’s law states that pres - sometimes referred to as the “Second absence or malformation of the lymph sure applied anywhere on a non-flowing Heart” as the function is completely vessels or may be secondary to obstruction, fluid in a closed container will be trans - analogous to that of the heart (Figure 2). trauma or tissue damage, infection, mitted equally in all directions. To test this The venous system may fail due to primary inflammation or from overload due to Schuren and Mohr applied a short stretch or secondary valvular incompetence (or venous dysfunction. bandage system on healthy volunteers with

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sub-bandage pressure sensors at the bottom, middle and top of the leg. 3 They found that when pressure was applied at Working the top of the bandage over the upper pressure sensor by inflating a cuff, Static stiffness the incremental pressures were transmitted equally to both other sensors. If tissue fluid is not mobilized by gradu - ated pressure under the bandage then how does compression work? Again research Resting has largely been done on venous disease pressure but the results can be extrapolated to lymphedema. If we place a sensor just above the ankle on the inside of the leg and measure sub-bandage pressures with activity an explanation becomes apparent.

COMPRESSION 101

Short stretch bandage FIGURE 4 systems have several effects on venous return: When the subject stands up from lying graduated compression of the limb as position the sub-bandage pressure rises. predicted by the modified Law of Laplace but 1. By reducing the cross-section The stiffer the bandage (shorter stretch) the this has not been observed in sub-bandage of the veins blood volume in the more the sub-bandage rises. The difference pressure measurements. Rather the limb legs is reduced in pressures is called the Static Stiffness behaves as fluid filled closed container with 2. Improved valvular function Index (Figure 4). When the calf muscle is sub-bandage pressures transmitted equally reduces reflux and decreases exercised, with contraction the sub-bandage in all directions as predicted by Pascal’s Law. hydrostatic pressure in the veins pressure rises to the Working Pressure Short stretch bandage systems are most 3. Venous flow velocity is increased because the bandage is essentially rigid and effective in mobilizing interstitial tissue fluid. through enhancement of calf resists the contraction. When the muscle They work primarily by enhancing muscle muscle pumping activity. relaxes the sub-bandage pressure drops. pumping function and increasing interstitial 4. Tissue hydrostatic pressure is This rising and falling pressure with activity tissue fluid hydrostatic pressure. This article increased leading to decreased produces a pumping action enhancing the is only a brief review and more detailed infor - net filtration. normal pumping action of the calf muscle. mation can be obtained from the International There are also beneficial effects Elastic systems have higher resting pres - Lymphedema Framework Best Practice Docu - on the lymphatic system: sures and lower working pressures because ment to be found at www.lympho.org. LP 1. Reduction of venous congestion the bandage expands to accommodate the decreases net filtration and fluid contracting calf muscle. For these reasons References load on the lymphatic capillaries elastic systems are less effective and are 1. Levick JR, Michel CC. Microvascular 2. Increased interstitial tissue fluid less well tolerated by patients. fluid exchange and the revised Starling hydrostatic pressure increases principle. Cardiovascular Research . tension on the anchoring filaments Key points 2010; 87:198-210. which will be pulled open In summary, newer research has shown 2. Schuren J, Mohr K. The efficacy of 3. Enhanced muscle pump activity that interstitial tissue fluid is mostly handled Laplace’s equation in calculating will improve lymph propulsion by the lymphatics not the venous side of bandage pressure in venous leg ulcers. though the lymph vessels the capillary bed as previously thought. The Wounds UK . 2008; (4)2:38-47. 4. Down-regulation of inflammatory movement of fluid between the tissue space 3. Schuren J, Mohr K. Pascal’s law and cytokines leads to breakdown of and the capillaries and the lymphatics is the dynamics of compression therapy: fibrosclerotic tissue. much more complex than previously thought. a study on healthy volunteers. Int Angiol . Compression was thought to work through 2010; 29(5):431-435.

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