failure

Chapter 40 Heart failure




Questions


image What are the most common causes of heart failure?

image What are the clinical sequelae of heart failure?

Heart failure is the clinical syndrome that results when the heart is unable to pump enough blood to supply the metabolic needs of the body. Cardiogenic shock (Ch. 13) is a type of heart failure. However, in clinical practice, the term heart failure tends to be used for chronic conditions, while cardiogenic shock implies acute pump failure. Heart failure can be thought of conceptually in a number of different ways that help to explain the mechanism of failure.




Backward failure and forward failure


Heart failure can be considered as either backward or forward in type. Backward heart failure describes the condition in which the ventricle is unable to discharge its contents properly during systole, resulting in an increase in end-diastolic volume. The volume and pressure in the atrium behind the failing ventricle rise, and the pressure in the venous and capillary beds behind the ventricle increases. In a sense, the blood ‘dams up’ behind the failing ventricle. The congestive features of heart failure (see below) can be explained by this hypothesis.


The concept of forward heart failure describes inadequate delivery of blood to the arterial system because of reduced cardiac output, so organs are inadequately perfused: reduced perfusion of skeletal muscles causes fatigue; reduced perfusion of the extremities causes them to be cold and pale; and underperfusion of the kidneys causes fluid retention in part by a reduction in glomerular filtration rate and in part by activation of the renin–angiotensin–aldosterone system. The fluid retention contributes to the heart failure by increasing the plasma volume, thus increasing the metabolic demands on the myocardium.



Diastolic and systolic failure


Concepts of diastolic and systolic failure are useful in thinking about what is happening in the heart itself. In diastolic failure, the ability of the heart to accept blood is impaired because of failure of normal ventricular relaxation (e.g. in hypertrophic cardiomyopathy, amyloidosis and restrictive cardiomyopathy). Insufficient blood enters the ventricle during diastole so end-diastolic volume is low. Even if most of this blood is expelled by a strongly contracting myocardium, the cardiac output will be low. In systolic failure, blood enters the ventricle normally but the ejection fraction is low because the muscle does not contract forcefully. Again, the result is a low cardiac output.


Although pure examples of systolic and diastolic dysfunction occur, in most patients both types coexist. For example, consider the pathophysiology in a patient with a previous large myocardial infarction that has been replaced by a scar. Diastolic dysfunction may occur because the scarred myocardium is not distensible and so the capacity of the myocardium to accept blood is reduced. However, there may also be systolic failure caused by loss of contractile myocardium in the scarred area.

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Jul 3, 2016 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on failure

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