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Tuesday, June 13, 2017

Is subendocardial fibrosis a reason for the age associated decrease in diastolic function?

It seems to be all about diastole.Simply put- the aging heart has more functional loss in ventricular filling than in  ejection of blood ( as least as depicted by the usual measures of systole namely the ejection fraction (EF) at rest and stroke volume at rest).  Similarly the  largest  difference between the exercise capacity of the average jogger and the elite endurance athlete is found in diastole-the elite athlete has markedly superior diastolic function likely on a genetic basis.

Hollingworth and co-workers from Newcastle on Tyne (1) studied left ventricular torsion and diastolic function in presumably healthy adults in various age ranges using  cardiac magnetic imaging with tissue tagging.

First some background;
The architecture of the left ventricle  has left handed subepicardial fibers and right handed subendocardial fibers which leads to a rotational deformation during cardiac contraction referred to as torsion. The subendocardial fibers are activated first leading to a  brief clockwise rotation ( as viewed from the apex of the heart as if you were on the spleen looking up) and then a counterclockwise rotation  while the base rotates clockwise. This action has been compared  to the two handed movement of wringing out a washrag.

The twisting of the left ventricle during systole is followed by a recoiling  or untwisting releasing the energy stored during systole. The untwisting largely occurs mainly in the isovolumic relaxation time . (IVRT). The IVRT is the  time after the aortic valve closes and before  the mitral valve has not yet opened-a period when no blood is either entering or leaving the ventricle.

 The rotation action of the heart can be explained by the orientation of the muscle fibers. The subendocardial  fibers are oriented in a right handed direction while the subepicardial fibers run in in a left handed direction and a midlayer is oriented circumferentially .The contraction of all three sets of fibers account for all contractile actions  of the heart and the rotational movement.

What at first seem counter-intuitive is the observation that  subendocardial disease is associated with hyper-rotations. (2) This is possibly explained by subendocardial fibrosis decreasing the rotational counterbalance to the mechanical advantaged ( longer radius) epicardial rotational direction.

Hollinsworth's study showed the expected decreased early diastolic filling and a " torsion to shortening ratio" that was consistent with lessened subendocardial shortening possibly  due to subendocardial fibrosis. So possibly some of the age related diastolic dysfunction may be related to altered untwisting mechanics.

1) Hollingsworth, KG et al Left ventricular torsion energetic and diastolic function in normal human aging. Am j Physiol heart circ physiology 302, H 885-H892,2012  (full text is available on line)

2.Nakatani, S. Left Ventricular rotation and twist:Why should we learn? J. Cardiovas Ultrasound 2011;19 (1): 1-6. Full text is available on line)

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