What is Hypertrophy?

Hearts, like any muscle, get bigger and thicker when it has a lot of resistance/workload to frequently push against.  It’s like lifting hand weights to increase the size of the bicep.  The cells in heart tissue get individually larger – not more numerous. This enlargement of muscle cells – whether it’s cardiac or skeletal muscle – is known as hypertrophy.  The muscle cell hypertrophies because it is adding more contractile proteins to enable greater contractile strength.  Going back to the bicep example – the larger bicep is because each individual bicep muscle cell undergoes hypertrophy even though it has not changed in muscle cell number.

Thus, the walls of  muscle hypertrophy when there is chronically a lot of resistance/workload.  That means each cardiomyocyte is undergoing hypertrophy, rather than dividing into more cells (hyperplasia), as cardiomyocytes can’t undergo cell division.

During hypertrophy, cells of the heart wall can:

a) Become bigger in circumference.  These plumper cardiomyocytes make the heart wall grow thicker from the outside in, resulting in  the heart becoming heavier.  The larger in circumference the cells get, the harder it is for oxygen and nutrients to diffuse inside the cell which will eventually cause metabolic problems. This is the basis of concentric hypertrophy

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b) Become longer in length and more narrow in circumference. there more narrow cardiomyocytes are ‘stretched out’ because the heart wall is undergoing more tension with over-filling with blood volume.  To adapt to this stretching, the heart remodels replacing contracting cardiomyocytes with non-contractile cardiac cells. As a result, the heart wall get thinner with weaker contractile strength. This is the basis of eccentric hypertrophy.

c) Remodel (i.e. change their cellular make up in tissue).  When the cardiomyocytes do not get enough oxygen and/or nutrients (e.g.  heart attack, extreme hypertrophy) or receive increased inflammatory signals (e.g. inflammation, infection), the cardiomyocytes will die. As a result, the cardiomyocytes are then replaced with connective tissue – principally collagen – which is NOT contractile.  As a result, the heart tissue will be less contractile and thus harder to empty the chamber.  In addition, the collagen- filled heart wall becomes stiffer thus not as stretchy to fill easily.

Concentric Hypertrophy

Concentric hypertrophy occurs when the left ventricular wall is remodelling due to a chronically increased resistance to push against.  The LV wall gets thicker (due to plumper cardiomyocytes) resulting in  the space within the ventricle (i.e. lumen) shrinking. This shrinkage of lumen means less volume of blood available to fill during diastole.   At first, ejection fraction is preserved despite less diastolic filling, thus giving rise to the alternate terms of “diastolic heart failure ” or “heart failure with preserved ejection fraction”.  Because ejection fraction is preserved, the body is getting sufficient supply of blood thus there would be few signs/symptoms.

However, as the wall gets thicker, it becomes stiffer and not as “stretchy” to fill easily.  Eventually, there will be a point where the diastolic filling volume will not be enough to maintain sufficient ejection fraction, regardless of how well the LV wall can pump. This is when signs and symptoms will begin to be noticed.

Eccentric Hypertrophy

When the heart chambers get overloaded with volume, the cells of the heart wall will compensate by becoming thinner & longer.  This overfilling will cause the heart to remodel into a larger lumen with thinner walls and less contractile strength. If this eccentric hypertrophy occurs in the ventricle, this would be systolic failure or “heart failure with reduced ejection fraction” as the volume to pump is sufficient but the pumping ability is too weak.  Unfortunately, as ejection fraction is reduced, there will be even more fluid left in the chamber – thus complicating the already chronic volume overload.

Section Review

• You can have both concentric and eccentric cardiomyopathy if the heart has to deal with both increased pressure and increased volume.
• Both systolic and diastolic failure have the same end result: the heart can’t pump enough blood out to feed the body.

Review Questions