Читать книгу Fulcrum and Torsion of the Helical Myocardium - Jorge C. Trainini - Страница 7

The function of the heart is of an anisotropic mechanical complexity that must be addressed in terms of its structure. In the study of myocardial anatomy, we postulate the principle that its organization is strictly related to its functional capacity. This led our investigations to explain its morphological and mechanical integrity. If we stop in classical descriptions of the heart we realize that ventricular anatomical attention was focused on its external and internal surfaces, granting scant importance to the intimate muscle conformation. It was determined that its arrangement forms two contiguous ventricular chambers, limited by a homogeneous, solid, compact muscle thickness and with a global uniform contraction. It was not considered that cardiac functional capacity required a reinterpretation of the spatial myocardial fiber organization and motion, introducing us in other topics of its functioning that were practically disregarded by cardiology.

An explanation for this apparent ventricular muscular homogeneity with an intricate anatomical arrangement that hides its helical conformation, implies considering that its structural solidity is required in birds and mammals to eject blood at high velocity in a limited time span by an organ that must serve two circulations (systemic and pulmonary). Currently, the helical myocardium can be confirmed by the anatomical study of the heart via an adequate dissection that manages to uncoil it in its entirety and by other procedures, namely: histological exploration, magnetic resonance diffusion tensor imaging studies, speckle-tracking echocardiography analysis, electrophysiological studies with three-dimensional electroanatomic mapping and “animal villi” investigations. All of these procedures were used for this research.

Dissection leads to differentiate the real internal myocardial anatomy, contrary to the classical concept, finding a helical structure with defined planes that allows the successive physiological motions of narrowing, shortening-torsion, lengthening-detorsion and expansion depending on the propagation of the electrical stimulus along its muscle paths.

This pathway leading from structure to function induced the understanding of topics poorly explained by their mechanical organization, but which should be considered complementary among them and essential for physiology, namely:

1 Anatomical and histological research on the myocardial segmental continuity. Can the myocardium be considered a continuous, single and integral muscle?

2 The unavoidable emerging question is that in order for the muscle segments that make up the ventricular chambers to twist they should have a supporting point, similarly to what a skeletal muscle does in a rigid insertion: Do they exist in the heart? If this support is real, how does the myocardium insert into this structure? This aspect about a myocardial support is not the only argument to consider since cardiac power generates a force capable of ejecting the ventricular content at a speed of 200 cm/s at low energy expenditure. Undoubtedly, in view of this developed capacity, it is necessary to attach the myocardium to a point of support to achieve its motions.

3 Myocardial torsion constitutes the functional solution to eject the ventricular blood content with the necessary energy to supply the whole organism. In this way, the phylogenesis cardiac morphology is similar to ventricular mechanics, but lacks the comprehension of an electrical stimulus along its muscle pathways to correctly explain its motions. The studies undertaken on this topic aim to demonstrate the integrity of an essential cardiac structure-function. The analysis of left ventricular endo and epicardial electrical activation by means of three-dimensional electroanatomic mapping carried out in a series of patients, allowed us to address a transcendental question to analyze: How does myocardial torsion occur? Before answering this question, it is important to underline that throughout the whole book, when we speak of “myocardial torsion” it should be understood that it refers to an elastic and non-compressible material such as the myocardium. This means that this torsion, determined by the cardiac base and apex rotation in opposite directions implies a simultaneous longitudinal shortening. Although in the literature the terms twist, torsion and rotation have been used interchangeably, they do not mean the same thing, as will be seen later. “Torsion” is the term most frequently used to refer to cardiac motion, but we must not forget that the rotational motion described for the myocardium is accompanied by a simultaneous longitudinal shortening between base and apex.

4 The sliding motion between the myocardial segments during ventricular torsion-detorsion, implies that there must be an anti-friction mechanism to avoid the dissipation of the energy employed by the heart. Is there a specific histological explanation for this fact? Do Thebesian and Langer venous conduits play a role in this mechanism? Is there an organic lubricating source?

5 The development of the interventricular vortex studied by echocardiography is the consequence of torsion and of the necessary energy impulse the blood fluid requires to be ejected. Its flow dynamics can be understood through the physical theory of dissipative structures, which explains the organization of this intraventricular turbulence.

6 A passive cardiac full phase would be unfeasible due to the small pressure difference with the periphery. Ventricular filling was investigated as secondary to a generation of negative intraventricular pressure with energy expenditure during early diastole. The sudden lengthening of the left ventricular base-apex distance, after the ejective phase, produces a suction effect by an action similar to that of a “suction cup”: Could this mechanism be explained by the ascending segment persisting contraction during the first 100 ms of diastole? Does this mechanics allow us to consider suction during the early protodiastolic phase as an essential element in the physiology of the circulatory system, by being the continuity link between the pulmonary and systemic circulations?

7 According to the aforementioned bases, should a coupling phase be considered between systole and diastole where cardiac suction takes place?

8 In this three-stage heart (systole, suction, diastole): How does the energetic mechanism in the active suction phase act? Could the cardiac energy of suction and ejection be estimated? Should the left ventricular ejection fraction be considered a poorly reliable index? According to these considerations, would it not be more logical to speak of ejective cardiac energy as a parameter that summarizes the cardiac potential and to which all non-independent variables would concur?

9 Through the cardiac resynchronization procedure, Is it possible, to restore negative pressure to generate left ventricular suction with stimulation in the correct site, according to the path of the stimulus along the myocardial segments?

10 Could the understanding of this cardiac structure-function of the heart be of importance for surgical procedures of ventricular reduction and containment?

The methods used in this research to explain the hypothesis of the anatomo-functional integrity of the heart consisted of:

1 Cardiac dissection in bovine and human specimens.

2 Histological and histochemical analysis of the anatomical samples.

3 Left ventricular endocardial and epicardial electrical activation in humans by means of three-dimensional electroanatomic mapping.

4 The study of left ventricular suction physiology in dogs after removal of the right ventricle.

5 Measurement of left intraventricular pressure in ventricular resynchronization therapy.

6 Pathophysiological reinterpretation in experimental and clinical studies of heart failure (right ventricular bypass surgery, cardiomyoplasty, ventricular containment techniques, cardiac resynchronization, univentricular mechanical assistance, left ventricular repair).

7 Echocardiographic analysis to corroborate the research and usefulness of this knowledge in clinical practice since this technique has the ability to provide non-invasive knowledge in the complex mechanism of myocardial contraction.

8 Diffusion tensor sequences using cardiac magnetic resonance imaging to identify the orientation and deformation of myocardial fibers.

The findings and clinical data here presented are the result of experimental tests performed under the approval of all required regulatory authorities and under the informed consent of patients following the principles described in the current World Medical Association Declaration of Helsinki (2013). The experiments were conducted in accordance with the UK Animal Scientific Procedures Act 1996, the EU Directive 2010/63/EU for experimental animals and the “National Institutes of Health” guide for the care and use of laboratory animals (NIH Publication No. 8023, updated in 1978).