Acute cardiac unloading is any maneuver, therapy, or intervention that decreases the power expenditure of the ventricle and limits the hemodynamic forces that lead to

ventricular remodeling In cardiology, ventricular remodeling (or cardiac remodeling) refers to changes in the size, shape, structure, and function of the heart. This can happen as a result of exercise (physiological remodeling) or after injury to the heart muscle (patho ...

after insult or injury to the

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

. This technique is being investigated as a therapeutic to aid after damage has occurred to the heart, such as after a

heart attack A myocardial infarction (MI), commonly known as a heart attack, occurs when Ischemia, blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) to the heart muscle. The most common symptom ...

. The theory behind this approach is that by simultaneously limiting the oxygen demand and maximizing oxygen delivery to the heart after damage has occurred, the heart is more fully able to recover. This is primarily achieved by using temporary minimally invasive mechanical circulatory support to supplant the pumping of blood by the heart. Using mechanical support decreases the workload of the heart, or unloads it. Cardiac traumas such as

myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when Ischemia, blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) to the heart muscle. The most common symptom ...

(commonly called a heart attack),

myocarditis Myocarditis is inflammation of the cardiac muscle. Myocarditis can progress to inflammatory cardiomyopathy when there is associated ventricular remodeling and cardiac dysfunction due to chronic inflammation. Symptoms can include shortness of bre ...

peripartum cardiomyopathy Peripartum cardiomyopathy (PPCM) is a form of dilated cardiomyopathy that is defined as a deterioration in cardiac function presenting typically between the last month of pregnancy and up to six months postpartum. As with other forms of dilated ca ...

cardiogenic shock Cardiogenic shock is a medical emergency resulting from inadequate blood flow to the body's organs due to the dysfunction of the heart. Signs of inadequate blood flow include low urine production (<30 mL/hour), cool arms and legs, and decreased ...

, and takotsubo cardiomyopathy result in an impaired ability of the heart to pump blood. Without proper blood flow the person will ultimately die. Maintaining sufficient

cardiac output In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: tha ...

is the primary objective of therapeutic approaches treating these cardiac conditions. However, many therapies aimed at increasing cardiac output place further stress on the heart. In this way a well-document vicious cycle begins in which increased cardiac output is required, but in order to achieve this the heart must work harder. This exacerbated stress leads to poorer outcomes. With the exception of

cardiopulmonary bypass Cardiopulmonary bypass (CPB) or heart-lung machine, also called the pump or CPB pump, is a machine that temporarily takes over the function of the heart and lungs during open-heart surgery by maintaining the circulation of blood and oxygen throug ...

, current therapeutic approaches do not allow the heart to rest and recover. The workload of the heart (pumping blood) is never uncoupled from heart function. Acute cardiac unloading is able to functionally uncoupleD. S. Burkhoff, G.; Doshi, D., Uriel, N., Hemodynamics of Mechanical Circulatory Support. ''Journal of the American College of Cardiology'' 66, 2663-2674 (2015). the heart from cardiac output, allowing the heart to rest and recover from damage.

Power expenditure

End Systolic Pressure Volume Relationship

The pumping of blood is considered the workload of the heart and requires power expenditure. Acute cardiac unloading is any maneuver, therapy, or intervention that decreases the power expenditure of the ventricle while maintaining cardiac output. Oxygen consumption (MVO2) is a direct measure of the total energy requirements of the heart, including the energy needed to pump blood.Suga, H. Total mechanical energy of a ventricle model and cardiac oxygen consumption. '' The American Journal of Physiology'' 236, H498-505

MVO2

Pressure-volume (PV) loop analysis provides a framework for understanding how acute cardiac unloading reduces MVO2 in the heart.Burkhoff D. HARVI: Cardiovascular Physiology & Hemodynamics. Part II. Advanced Physiological Concepts (Version 2.0.0) obile application software Available at: https://itunes.apple.com/gb/ app/harvi/id568196279?mt¼8. Accessed October 2, 2015. The PV loop characterizes the events occurring during a single cardiac cycle (a single heartbeat). The total area bound by the PV loop is the mechanical energy ( pressure-volume work) used to actively pump blood every beat, measured in mmHg·mL (aka, a

joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...

). This is known as the stroke work (SW). The remaining area bound by the ESPVR and EDPVR that is outside of the loop is the potential energy (PE) that resides in the

myofilament Myofilaments are the three protein filaments of myofibrils in muscle cells. The main proteins involved are myosin, actin, and titin. Myosin and actin are the ''contractile proteins'' and titin is an elastic protein. The myofilaments act togeth ...

s that was not transduced into the work of pumping blood. The sum of these two areas (PE + SW) is known as the pressure-volume area (PVA). PVA is a first order approximation of MVO2. Acute cardiac unloading decreases the workload of and the oxygen demand of the heart. This can be visualized as an overall decrease in the PVA of the PV loop. Mechanical unloading of the heart by a percutaneous ventricular support device such as the Impella device can achieve this in two ways. First, the device is a continuous flow device. It directly aspirates blood from the ventricle into the

aorta The aorta ( ; : aortas or aortae) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart, branching upwards immediately after, and extending down to the abdomen, where it splits at ...

. This decreases the preload and results in a left-shift and loss of the normal isovolumic contraction line. Under conditions of mechanical support, mean aortic pressure (MAP) is maintained independent of native ventricular function, and ventricular and aortic pressures become uncoupled.

Myocardial infarction

When the heart is damaged by a

a portion of muscle is permanently lost. The heart has a limited innate ability to replace dead muscle with new, functional muscle.Murry, C. E. & Lee, R. T. Development biology. Turnover after the fallout. ''Science'' 324, 47-48, (2009). The dead heart muscle is replaced by non-contractile fibrotic tissue, forming the myocardial scar. Scar tissue does not contract, and it does not help the heart pump blood. This persistently stresses the heart and increases the workload of the lasting myocardium as measured by MVO2. Clinical research indicates that as the size of the myocardial scar increases, so does the likelihood of the patient to develop heart failure.Stone, G. W. ''et al.'' Relationship Between Infarct Size and Outcomes Following Primary PCI: Patient-Level Analysis From 10 Randomized Trials. ''

Journal of the American College of Cardiology The ''Journal of the American College of Cardiology'' is a peer-reviewed medical journal covering all aspects of cardiovascular disease, including original clinical studies, translational investigations with clear clinical relevance, state-of-the-a ...

'' 67, 1674-1683, (2016). Acute cardiac unloading decreases cardiac MVO2 and has been demonstrated to limit the amount of scar tissue that forms, thus preserving heart function after a heart attack.Kapur, N. K. ''et al.'' Mechanically unloading the left ventricle before coronary reperfusion reduces left ventricular wall stress and myocardial infarct size. ''Circulation'' 128, 328-336, Sun, X. ''et al.'' Early Assistance With Left Ventricular Assist Device Limits Left Ventricular Remodeling After Acute Myocardial Infarction in a Swine Model. ''Artificial organs'', (2015).

References

{{reflist Cardiac procedures