Venous ulcers commonly occur in the inner lower leg about 9 inches from the floor and, in a 70 inch tall man, 46 inches below the sternal angle or level of the right atrium. Hence, healthy intact venous valves 9 inches above the floor can withstand a water column of 46 inches equivalent to 87 mm Hg. Such a pressure is achieved with quiet standing while lesser pressures are seen with the calf-pump action in walking and with lowering of the sternal angle as with sitting or lying. Occupations requiring quiet standing are associated with the development of venous stasis disease as the veins dilate and the valves become incompetent allowing the pressure to cascade down the veins to the lower leg.

Healthy veins are thinner than the arteries at the same level in the leg and the lymph channels are even thinner. As hypertension and/or arteriosclerosis develops the arteries become more stiff and noncompliant while the compliance of the veins and lymph channels remain unchanged.

Blood pressure as commonly measured represents the force necessary to compress the tissues around the arteries, the wall of the artery and the column of blood within the artery. While the deep veins share the protection of the deeper tissues, the superficial veins are easily compressed with but a fraction of the force needed to compress the arteries.

When a cuff rapidly squeezes the calf, a greater impetus is given to the column of venous blood than the column of arterial blood. Further, as the lumen of the veins exceeds that of the arteries, a greater volume of venous blood is moved. Blood flow through the tissues is impeded by factors increasing venous back pressure. Sequential pumping down the leg obviously greatly increases such venous backflow and, if performed at high pressures has the additional risk of dilating the venous valves below the cuff producing permanent venous insufficiency. We have found the population with combined venous stasis disease and peripheral arteriosclerotic occlusive disease especially disabled; their venous pressure may exceed their arterial in the lower leg. The use of a single compression bag with the Circulator Boot Systems obviates these difficulties. The venous blood and lymph fluids are pushed one way: out of the leg.

The effects of pumping on the leg may be sketched and compared to pressing on a cone. A significant difference is seen between the use of a full bag and leg cuffs around the calf and thigh

When used to support the heart, the Circulator Boot system employs the full leg bag from groin to toes. The inclusion of the upper thigh has been appreciated for years. Soroff et al (Current status of external counterpulsation. Critical Care Clinics 2:277-295, 1986) pointed out that for the "Cardiassist" the inclusion of the large upper thigh muscles was crucial in achieving effective pumping. The arteries are larger in the thigh than the distal leg. The gas-filled intestines greatly dampen the pumping performed over the abdomen. The adjustable bags and straps used with the Circulator Boot provides pumping of the large muscles of the thigh without the dampening effect of expansion of the lower thigh and knee seen with the use of the thigh cuff.

The ECP devices teach that the production of "augmentation" is a desirable goal. By "augmentation" they attempt to produce an elevation in the dicrotic notch to levels equal to or above the height of the systolic pulse wave as noted in the finger PPG curve. The dicrotic notch represents the closure of the aortic valves and usually occurs about 25% down from the peak of the pulse wave at the moment intraventricular pressure is less than aortic pressure. The delay time after the QRS complex before cuff pressurization recommended for and practiced by the ECP firms is shorter than the QT interval (commonly 0.33 seconds and about the time the aortic valves close). In their recent patent literature, Cardiomedics shows in their “Figure 2” of prior art, a delay time of 145 ms placing the onset of their compressions about in the middle of the QT interval. I personally have climbed into their ECP device at Bryn Mawr and found their recommended timing to include a delay time of 145 to 190 (commonly 180). In commencing their pressurization before the closure of the aortic valves, they increase terminal systolic afterload and successfully raise the dicrotic notch. It is no surprise to learn in their recent patent and literature, that Cardiomedics has noted that patients with heart failure do better if they are pumped with lower pressures (a D/S ratio of 0.7) essentially approaching the diastolic pressures utilized by the Circulator Boot. (Always looking for good science, we will be glad to note any objections the ECP firms have to these observations.)

The sheer force created by pneumatic compressions is related to the speed with which the pneumatic device is inflated. A compression stocking at one extreme applies constant pressure and no sheer forces. Like a slowly inflating leg device it may impede blood flow into the leg while it is applied. Liu K et al ( J Orthop Res 17: 415-20, 1999, J Orthop Res 17: 88-95, 1999 and J Appl Physiol 92:559-66, 2002 showed that the rate and duration of inflation does significantly influence the vasodilatory effect of pneumatic compression therapy in skeletal muscle. *The effect can be blocked by NG-monomethyl L-arginine pointing to a nitric oxide mechanism for the vasodilation.response.

Why, people ask, does Circulator Long Boot employ a rigid boot from groin to the toes when other systems use cuffs? As the rigid walls of the boot do not move with air inflation, the force of the compressed air is expended almost totally against the leg. Examination of Boyle's gas Law illustrates the point. In the figures below, it takes the full volume of V1 to inflate the collapsed bag. If the bag were to collapse between beats, it would take the V1 volume just to expand it to a pressure of room air. The Circulator Boot bag is a light plastic which with a few expansions sticks to the plastic walls of the boot. It is seen in the bottom figure that it only takes a volume of 6.8% of the dead space around the leg to pressurize the boot to one PSI above atmospheric pressure. The boot valves can easily deliver such a volume. It cannot deliver 106% of the dead space, the volume needed to pressurize the boot if the compression bag were to collapse with each beat.

The physics and vascular physiology of booting are important to appreciate if the operator is to get the maximum benefits of booting for the patient. The use of canvass or other bags that collapse between beats make it impossible for the boot to pressurize within the designed compression times. Pressure can still be obtained by greatly prolonging the compression times at the expense of losing the desirable sheer forces on the leg and of impeding blood flow into the leg.