Enhanced external counterpulsation

Overview
Enhanced external counterpulsation (EECP) is a procedure performed on individuals with ischemic cardiomyopathy in order to diminish the symptoms of their ischemia. In various studies, EECP has been shown to relieve angina1,2, improve exercise tolerance3, and decrease the degree of ischemia in a cardiac stress test2,3.

Method
While an individual is undergoing EECP, they have pneumatic stockings (also known as cuffs) on their legs and are connected to telemetry monitors that monitor their heart rate and rhythm. The most common type in use involves three cuffs placed on each leg (on the calfs, the lower thighs, and the upper thighs (or buttock)). The cuffs are timed to inflate and deflate based on the individual's electrocardiogram. The cuffs should ideally inflate at the beginning of diastole and deflate at the beginning of systole. During the inflation portion of the cycle, the calf cuffs inflate first, then the lower thigh cuffs and finally the upper thigh cuffs. Inflation is controlled by a pressure monitor, and the cuffs are inflated to about 300 mmHg.

When timed correctly, this will decrease the afterload that the heart has to pump against, and increase the preload that fills the heart, increasing the cardiac output.4 In this way, EECP is similar to the intra-aortic balloon pump (IABP). Since it increases pressure in the aorta while the heart is relaxing (during diastole) EECP also increases blood flow into the coronary arteries, which also occurs during that phase.

Treatment regimen
While the number of EECP treatment sessions vary widely, one widely used regimen in the United States is a total of 35 one hour sessions; One session a day, five days a week, for 7 weeks. This particular regimen is used because it is the regimen studied in the MUST-EECP trial, the first prospective, randomized-control multicenter trial on EECP.2

There are no clear guidelines on how often the treatment can or should be performed. An individual who has shown benefit with the regimen but later begins having complaints attributable to his ischemic cardiomyopathy may benefit from repeated procedures.

Indications for use
EECP is usually reserved as a "last resort" treatment of individuals with symptoms of ischemic cardiomyopathy who are not amenable to percutaneous coronary intervention or coronary artery bypass graft surgery. It is typically only performed on individuals who continue to have symptoms while on the maximum tolerated doses of conventional medications. To meet these criteria, the individual must have coronary artery disease that includes at least one vessel with at least a 70 percent obstruction. In addition, the individual must have evidence of either an infarction or significant ischemia on a stress test with some form of imaging (ie: nuclear or echocardiographic imaging).

In addition, individuals with advanced heart failure due to an ischemic etiology may benefit from EECP. This is the object of the PEECH trial, a large multi-center currently ongoing study.5

Physiological considerations
As mentioned above, the deflation of the cuffs at the beginning of systole will decrease the afterload that the heart has to pump against. Because of this, myocardial oxygen demand (the amount of oxygen required by the heart to function properly) will decrease during the EECP session. This is a relatively short-term improvement, however, and is limited to the session of EECP.

The inflation of the cuffs during diastole (when the aortic valve is closed) increases blood flow to the myocardium (the muscle of the heart). This is because, unlike the tissues of the rest of the body, the myocardium receives the majority of its blood during diastole. The increased flow during diastole caused by EECP is believed to promote the formation of collateral arteries in the coronary circulation.

Presumably, it is these newly opened collateral arteries that produced the sustained benefit that EECP provides to individuals after the EECP sessions are complete. While EECP has been in use since the 1980s, the mechanism in which it provides a lasting clinical benefit is still poorly understood. One theory is that EECP exposes the coronary circulation to increased shear stress, and that this results in the production of a cascade of growth factors that result in angiogenesis.6

Hemodynamic effects
The acute hemodynamic effects of EECP are transitory and only occur during a treatment session. These effects are also related to the number of cuffs that are placed on each leg. It has been shown that if only calf and lower thigh cuffs are used, flow is increased by 19 percent compared to baseline. Addition of the upper thigh (buttock) cuffs increases the flow to 26 percent over the baseline.4

EECP increases cardiac output by a combination of the increased preload and the decreased afterload during the EECP session. Inflation of the cuffs during diastole compresses the venous system in the legs, causing increased venous return to the heart, thereby increasing left ventricular preload. This increased filling of the left ventricle increases cardiac output. Deflation of the cuffs in systole decrease afterload (the pressure the left ventricle has to overcome in order to eject blood), also increasing the cardiac output.

There are long term hemodynamic effects of EECP, which are presumably due to the decreased ischemic burden noted in individuals after completing an EECP regimen. These include a decrease in the left ventricular end diastolic pressure (LVEDP), and subsequently a decreased in brain natriuretic peptide (BNP) levels, and improved diastolic performance of the left ventricle.2

Contraindications
Similar to the intra-aortic balloon pump, contraindications to EECP include severe peripheral vascular disease and significant aortic insufficiency. Other contraindications include:
 * Significant unprotected left main disease
 * Atrial fibrillation
 * Overt congestive heart failure
 * Any severe valvular heart disease
 * Uncontrolled hypertension (blood pressure > 180/100 while on medications)
 * Phlebitis
 * Deep vein thrombosis
 * Lower extremity stasis ulcers
 * Bleeding diathesis (including INR > 2.0)
 * Pregnant or potentially pregnant women

Atrial fibrillation is a relative contraindication, because the varying heart rate makes it impossible to time the inflation and deflation of the cuffs. In the subset of individuals with atrial fibrillation, high degree heart block, and a permanent pacemaker who are pacemaker dependent, it may still be possible to perform EECP.