COGNITIVE DISSONANCE THEORY

The optimal performance of the LV depends on its ability to cycle between two states, a compliant chamber in diastole that allows the LV to fill from low-pressure left atrium and a contractile chamber in systole that effectively pumps blood at normal arterial pressures.

LV diastole is the period between mitral valve opening when there is early passive and late active ventricular filling till just before opening of the aortic valve which heralds onset of LV systole. This is the period during which the myocardium loses its ability to generate force and shorten, and thereby returns to an unstressed length and force i.e a relaxed state, which is also the time for majority of cardiac perfusion. By extension, diastolic dysfunction occurs when these processes are prolonged, slowed or incomplete¹⁰⁰.

The LV diastole is composed of active relaxation and passive compliance, the stored energy which is released as relaxation starts contributes to left ventricular pressure fall and is referred to as elastic recoil or suction. Left ventricular compliance is a passive function which indicates the distensibility of the chamber during filling of the left ventricle³⁵.

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A normal diastolic function is clinically defined as the capacity of the left ventricle to receive a filling volume and its ability to guarantee an adequate stroke volume, operating at a low pressure regimen both at rest and during the stress of a variable heart rate, stroke volume, end-diastolic volume and blood pressure¹⁰¹.This is determined by its passive viscoelastic compliance and active relaxation properties both of which contribute to the process that returns the myocardium to its resting force andlength⁴⁹.

Left ventricle (LV) diastole has four identifiable phases:

1. Isovolumetric relaxation: This is the period from the aortic valve closure and opening of the mitral valve. In this phase LV pressure continues its rapid fall due to myocardial relaxation and elastic recoil, while LV volume remains constant¹⁰². 2. Early LV rapid filling phase: This begins when LV pressure falls below LA

pressure and the mitral valve opens. This period represents a complex interaction between LV suction (active relaxation), visco-elastic properties of them myocardium (passive compliance) and LA pressure. About 80% of LV filling occurs during this phase¹⁰².

3. Diastasis: This occurs when LA and LV pressures are almost equal. LV filling is essentially maintained by the flow coming from pulmonary veins and momentum of blood during the preceding rapid filling phase. Approximately 5% of LV filling occurs during this phase¹⁰².

4 Atrial systole: This phase occur following LA contraction and ends at the mitral valve closure. With atrial contraction, there is a small increase in LA-LV pressure gradient which has a booster effect on LV filling, contributing approximately

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15%of the LV filling. This phase depends on LV compliance, pericardial resistance, atrial force, and atrio-ventricular synchronicity¹⁰².

The phases of diastole of the pulmonary venous inflow is divided into three periods:

1. Forward flow during ventricular systole: this period is composed of mono- or biphasic forward flow. The early and late systolic flow is determined mainly by the extent of left atrial relaxation and left ventricular contraction respectively. Atrial relaxation decreases the left atrial pressure, which allows the pulmonary veins to fill the left atrium. During LV contraction, the downward displacement of the mitral valve annulus occurs and left atrial pressure further decreases, resulting in late systolic flow¹⁰³.

2. Forward flow during early diastole: This period begins after the onset of the early ventricular filling leading to a decrease in the LAP. The LA acts as an open conduit between the pulmonary veins and the left ventricle, during this phase flow is affected not only by the gradient from the pulmonary vein to the LA but also by the LV relaxation and compliance¹⁰³.

Young and healthy individual can therefore exhibit large D wave indicating forceful elastic recoil of the LV rather than high left atrial pressure¹⁰⁴.

3. Reverse flow during atrial contraction: Atrial contraction produces a small reverse flow into the pulmonary veins in addition to the forward transmitral flow into the LV. Both forward and reverse atrial flow is influenced by left ventricular pressure, LV compliance and contractility of LA¹⁰³.

In general, many factors determine LV filling with a varying relative importance in all phases.

These factors overlap in time and are influenced by each other, by LV systolic function, heart

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rate and by cardiac conduction system. Their final combined effect is on the transmitral pressure gradient, which actually determines LV filling. Below is the list of factors affecting left ventricular filling.

Factors affecting Left Ventricular filling¹⁰³ (1) Myocardial relaxation

(2) Atrial contraction (3) Viscoelastic properties (4) Diastolic restoring forces (5) Pericardial restraint (6) Ventricular interaction (7) Coronary artery turgor (8) Loading conditions

(9) Atrial and ventricular uniformity (10) Cardiac conduction system

Echocardiographic parameters routinely measured when assessing diastolic function are listed below:

Indices of LV Diastolic Function¹⁰³ a. Mitral valve inflow

(1) Peak velocity of early rapid filling (E)

(2) Peak velocity of late filling by atrial contraction (A) (3) Deceleration time (DT) from peak early velocity (4) Isovolumic relaxation time (IVRT)

(5) Duration of A-wave (Adur)

xli (6) E/A ratio

b. Pulmonary venous flow

(1) Peak forward systolic (S) velocity (2) Peak forward diastolic (D) velocity (3) Peak atrial reverse (Ar) velocity (4) Ar duration (Ar dur)

(5) S/D ratio (6) Ar duration – A dur

2.8 LEFT VENTRICULAR DIASTOLIC DYSFUNCTION IN DIABETES MELLITUS