Can you explain the importance that timing and technique have on the heart and legs when using the Circulator Boot?
It
has been long appreciated that occlusive arterial disease in the legs
effectively increases cardiac afterload as does clamping the aorta or
iliac arteries during vascular surgery (Henein et al 1966). Pneumatic
compression boots have long been considered risky for patients with
heart disease and peripheral edema lest excessive volumes of fluid are
returned to the lungs. The effects of pneumatic boots on afterload
however have not commonly been considered.
Leg
compressions during either systole or diastole increase venous return
to the heart and, according to the work of Frank (the isolated frog's
heart, 1895) and Starling, might be expected to increase cardiac
output. Starling utilized a canine heart-lung preparation in
which the lungs were artificially ventilated (potentially preventing
pulmonary congestion as right atrial filling pressures were increased).
While allowing aortic pressure to increase about 5%, he found perhaps a
70% increase in stroke volume after a 53% increase in right atrial
filling pressure and provided us with the now classic concept that
cardiac output increases to a point as the heart fibers are stretched
with increases in ventricular filling (Patterson SW, Piper H, Starling
EH: J Physiol 48:465, 1914). Again, he noted increases in stroke
volume, albeit less prominent, when ventricular volume was increased by
increasing peripheral vascular resistance and aortic blood pressure.
Two potentially detrimental effects of his experiment are to be noted,
however. First, ventricular dilatation significantly increases the
force each heart fiber must exert to obtain a given intraventricular
pressure; heart work increases and heart efficiency decreases. Second,
coronary artery perfusion occurs predominantly in diastole and
increased diastolic ventricular pressures decrease coronary perfusion.
Therapy
with the Circulator Boot does increase preload by returning the venous
and lymphatic volumes to the heart and in doing so relieves the heart
of this work in the resting patient. However, the increase in preload
is shown not to be crucial. In contrast to Frank and Starling, as seen
below, Circulator Boot therapy increases stroke volume by decreasing
afterload while at the same time decreasing heart work and maintaining
or increasing coronary perfusion.
In Figure 1, we have
re-examined Starling's Law with the use of the Circulator Boot and an
electrical impedance apparatus allowing measurement of stroke volume
and cardiac output on a beat-by-beat basis.
Figure
1: Both the stroke volume and heart rate were divided by ten to keep
their values on the graph. "Baseline" or "B" represents the time the
Circulator Boot was inactive. During the "2 Legs 1:1" time interval
both legs were pumped in end-diastole from the toes to the groins; the
combination of an increase in venous return and the sudden decrease in
after-load, that accompanies the explosive decompression of the boot in
presystole, resulted in an immediate marked increase in stroke volume
and cardiac output. During the "Td/0" time interval, the delay time
after the QRS complex was set to zero and both legs were pumped during
systole thus increasing both pre-load and after-load; cardiac output
and stroke volume immediately decreased below baseline levels. During
the "2 Legs 2:1" time interval, both legs were pumped in end-diastole
on alternate beats. During the "1 Leg 1:1" time interval, one leg was
pumped in end-diastole on every beat. Both intervals increased cardiac
output and stroke volume but less effectively than pumping both legs
1:1 in end-diastole. During the "Clock" time interval, the boot pumped
both legs for 0.42 seconds at a 70 compressions a minute independent of
the heart rate. Sometimes the heart was assisted and sometimes opposed.
The boot did not capture the heart which continued to beat at a rate
independent of the boot.
A failing heart has commonly been
compromised by years of hypertension, hyperlipidemia, diabetes
(resulting in glycosylated myocardium) and arteriosclerosis. A "stiff"
heart and diastolic dysfunction are common. The results of Circulator
Boot therapy on such a heart are shown in Figure 2:
Figure
2: This diabetic man was referred for peripheral vascular disease. Note
that both cardiac output and stroke volume increase rapidly with the
onset of boot therapy, but the increase is progressive as his heart "loosens up".
The
timing of compressions and the volume of tissue pumped are important in
producing optimal effects in the extremities also. Rapid inflation
produces sheer forces on the vascular endothelium which elaborate
fibrinolysins, nitric oxide, prostacyclin and endothelial growth
factors. Liu et al found that inflation in 0.5 second, 5 seconds and 10
seconds produced vasodilations in distant skeletal muscle of 4+, 2+ and
0 magnitude respectively and that the effect could be blocked by
inhibiting nitric oxide production with NG-monomethyl L-arginine (J
Orthop Res 17(3):415-20, 1999). Tarnay et al found that systemic
fibrinolytic activity increased more with bilateral full leg
compressions than with bilateral calf compressions which in turn had
greater effect than arm compressions (Surgery 88:489-96, 1980).
Nikolovska et al found the healing rate of venous ulcers increased with
the frequency of boot compressions: rapid boot 0.09cm2/day vs slow
boot 0.04cm2/day (Med Sci Monit 11:CR337-43. 2005).
The
Circulator Boots inflate rapidly (0.340in42 seconds), frequently (with
every heartbeat or every other heartbeat) and over a large volume of
the legs. Unlike other devices which employ cuffs to squeeze the legs,
the Circulator Boots employ a single bag enclosing the leg from the
toes distally to the proximal margin (chosen by the physician) and do
not send a high pressure pulse wave down the veins against their valves
potentially damaging them. With the use of vascular testing equipment,
one can demonstrate the adverse effects on blood flow in the legs by
blocking inflow with compressions lasting several heartbeats, with
compressions during systole or early diastole, or with compressions
using inflation pressures well above systolic blood pressure. Thus,
Werner et al showed a decrease of 67% in pedal blood flow during EECP
therapy which employs cuffs with pressures well above systolic pressure
and timed compressions beginning just before the dicrotic notch of the
T-wave (late systole) (Angiology 58: 185-190, 2007). In Figure 3, we
show how the photoelectricplethysmographic (PPG) curves on the dorsum
of the foot can be altered with variations in boot timing in a
volunteer diabetic with a blood pressure above the knee of 160 mm Hg. A
blood pressure cuff was applied above the knee and inflated to various
pressures to simulate an obstructive lesion.
PPG Curves Illustrating Peripheral Circulation as Affected By Boot Therapy Under Various Conditions
Sensor over distal 2nd metatarsal - Sleeve ending at ankle - Old heart monitor
As
demonstrated above, the Circulator Boot is a powerful therapy that can
provide maximum benefits to your patients when you appreciate the
impact to the heart and the legs that cardio-based compression therapy
(consistent with the Operator's Manual) has to offer.
