Heart Valves and the Cardiac Cycle

Heart Valve Anatomy and the Cardiac Cycle Overview

The cardiac system relies on a precise sequence of mechanical and electrical events to circulate blood throughout the body. The fundamental structural components include the Superior Vena Cava, Inferior Vena Cava, Right Atrium, Tricuspid Valve, Right Ventricle, and Pulmonary Valve on the right side of the heart. On the left side, blood travels through the Left Pulmonary Veins into the Left Atrium, passing through the Mitral Valve into the Left Ventricle, and finally through the Aortic Valve into the Arch of Aorta. Supporting structures such as the Chordae Tendineae and Papillary Muscles are critical for the function of the atrioventricular valves. The movement of blood is driven by pressure differences across these valves, which ensures a one-way flow of blood through the four valves of the heart.

Essential Terminology and Definitions

Understanding the cardiac cycle requires familiarity with clinical and physiological terminology. Systole refers to the period when the ventricles are emptying through contraction, while Diastole is the period when the ventricles are filling. The valves are categorized into Atrioventricular (AV) valves (Tricuspid and Mitral) and Semilunar valves (Aortic and Pulmonic). Structural terms include the Commissure (the site where valve leaflets meet), the Sinus of Valsalva, and Coronary cusps.

Flow dynamics are described using terms such as Upstream (closer to the source) and Downstream (further from the source). Vena contracta refers to the narrowest part of a jet of blood flow. Pathological terms include Stenosis, which is a narrowing or obstruction of the valve area, and Regurgitation, which is the failure of complete coaptation of valve leaflets leading to backflow. Specific timing phases include IVCT (Isovolumic Contraction Time) and IVRT (Isovolumic Relaxation Time). Diastasis is the period of the cardiac cycle where pressures have equalized and blood movement slows before atrial contraction.

Positional and Hemodynamic Principles

Directional terms are used to describe the location of cardiac structures and flow. Proximal refers to a position closer to the start, source, or origin, whereas Distal is further away from the origin. Medial describes a position closer to the center or midline of the body, and Lateral describes a position away from the center or midline.

Hemodynamic principles of flow across an orifice involve the Orifice diameter ($d$), Pipe diameter ($D$), and the Flow itself. As blood moves through a narrowed opening, there is a measurable Pressure drop across the orifice ($h$). This relationship is vital for assessing the severity of valvular conditions.

Electrical Components of the Cardiac Cycle

The cardiac cycle is initiated and coordinated by electrical impulses visualized on an Electrocardiogram (EKG). The EKG trace consists of several segments and intervals. The PR segment and the PR interval precede the QRS complex, which represents ventricular depolarization. The ST segment and T wave represent ventricular repolarization. Other metrics include the RR interval (time between heartbeats), the QT interval, and the TP interval. On standard EKG paper, $1\text{ square} = 0.04\text{ sec/0.1mV}$.

Electrical Systole begins at the R wave and concludes at the end of the T wave. Electrical Diastole starts at the end of the T wave and ends at the middle of the R wave. These electrical events precede and trigger the mechanical actions of the heart.

Detailed Mechanics of Systole

Systole is the phase of the cardiac cycle where the ventricles empty. It begins with Isovolumic Contraction, which generally lasts approximately $0.03\text{s}$ or about $5\text{\%}$ to $6\text{\%}$ of the cardiac cycle. During this phase, volume remains unchanged and is nearly equal in both ventricles, but pressure increases drastically. Muscle tension increases, contributing to the rise in Left Ventricular (LV) pressure.

Mechanical Systole proper begins when the pressure in the LV surpasses the pressure in the aorta, causing the aortic valve to open. The output phase is divided into rapid ejection followed by slow ejection. As blood volume leaves the ventricle, LV pressure eventually drops below the pressure in the aorta, causing the aortic valve to close.

Detailed Mechanics of Diastole

Diastole is the phase focused on ventricular filling. It begins with Isovolumic Relaxation, where the volumes in both ventricles remain unchanged and nearly equal while pressure decreases drastically. Decreasing muscle tension aids in this rapid pressure drop.

When LV pressure falls below Left Atrial (LA) pressure, the Mitral Valve (MV) opens. This is followed by rapid filling, where ventricular pressures are at their lowest and atrial pressures are at their highest. Following this, Diastasis occurs as pressures equalize and volume in the LV increases, leading to a slowing or standstill of blood movement from the LA to the LV. The final stage of diastole is Atrial Systole (the "atrial kick"), where the contraction of the LA forces remaining blood into the LV. This increased volume raises ventricular pressure, leading to the closure of the MV and the beginning of IVCT, which restarts the cycle.

Wigger’s Diagram and Correlation of Events

The Wigger's Diagram provides a comprehensive view of pressure and volume changes. Aortic pressure typically fluctuates between $80\text{ mm Hg}$ and $120\text{ mm Hg}$. Ventricular volume ranges from approximately $50\text{ ml}$ at the end of systole to $130\text{ ml}$ at the end of diastole. The phonocardiogram records the heart sounds. The first heart sound (S1) is associated with the closure of the A-V valves, and the second heart sound (S2) is associated with the closure of the aortic valve. A third heart sound (S3) may be noted during the rapid inflow phase of diastole.

Valvular Flow Dynamics

Normal valvular flow is Laminar. It is characterized by blood traveling in one direction at the same velocity with no swirling. This produces uniform color and steady velocities on spectral and color Doppler.

In contrast, Turbulent flow involves increased pressures and velocities with swirling blood moving in numerous directions. On color Doppler, this appears as an increased mosaic of colors, and spectral Doppler shows increased velocities. Turbulence typically occurs downstream of a stenotic valve or upstream during regurgitation.

Pathological Conditions: Stenosis and Regurgitation

Valvular stenosis is the narrowing or obstruction of the valve area. This causes a restriction of flow, an increase in the pressure gradient across the valve, and turbulent flow in the downstream chamber.

Valvular regurgitation is the failure of complete coaptation (closure) of valve leaflets or cusps. It can be caused by a breakdown of the valve apparatus, enlargement of the associated chamber, poor cardiac function, trauma, or infection. Regurgitation results in retrograde (backward) turbulent flow into the upstream chamber.

Anatomy of the Atrioventricular (AV) Valves

AV valves are located between the atria and ventricles. They open when atrial pressure exceeds ventricular pressure, which occurs during diastole. When the ventricles contract, the resulting pressure drives the leaflets upward until their edges meet and close. Papillary muscles contract and Chordae Tendineae tighten to prevent the leaflets from inverting or prolapsing into the atria.

The Tricuspid Valve resides between the Right Atrium (RA) and Right Ventricle (RV). Its orifice is larger than the Mitral Valve. It consists of three leaflets: Anterior (largest), Septal (medial), and Posterior. Its apparatus includes an annulus, leaflets, chordae tendineae, and three papillary muscles (anterior, posterior, and medial/septal). The posterior leaflet of the tricuspid valve may have $1$ to $3$ sections called scallops.

The Mitral Valve (Bicuspid Valve) is located between the LA and LV. It consists of two leaflets: Anterior (largest) and Posterior. These are joined at two commissures: the Lateral Commissure and the Medial Commissure. The mitral apparatus includes the annulus, leaflets, chordae tendineae, two papillary muscles (posterior and anterior), and the ventricular myocardium. According to Carpentier’s nomenclature, the leaflets are divided into scallops. The Anterior leaflet is termed "A" ($A1$ lateral third, $A2$ middle third, $A3$ medial third). The Posterior leaflet is termed "P" ($P1$ lateral third, $P2$ middle third, $P3$ medial third).

Anatomy of the Semilunar Valves

Semilunar valves include the Aortic and Pulmonic valves. They comprise three crescent moon-shaped cusps attached to the artery wall by a convex outer margin. These valves open during systole when ventricular pressure exceeds arterial pressure, allowing ejection. They prevent backflow into the ventricles during diastole.

The Aortic Valve allows blood to flow from the LV to the Aorta. It has three cusps: Right coronary, Left coronary, and Non-coronary (sometimes referred to as posterior). Each cusp has a Commissure, a lunule, a Nodule of Arantii (or Nodule of Arantius), and a Sinus of Valsalva. Portions of the Non-coronary cusp (NCC) are continuous with the Anterior Mitral Valve Leaflet (AMVL), a region known as the aortic mitral curtain. The most common congenital heart defect in the adult population is a Bicuspid Aortic Valve.

The Pulmonary Valve is located between the RV and the pulmonary artery. It consists of three cusps: Right, Left, and Anterior. Each cusp features a small nodule at the tip called the nodule of Arantius. The mid-portion of each cusp contains an indentation called the Lunule, and behind each cusp is a pouch-like dilation called the Sinus.

Questions & Discussion

Test yourself:

• This is the time that all the valves are closed and the pressures are increasing.     Response: Isovolumic contraction (IVCT).

• This is the period of time that the ventricles are filling.     Response: Diastole.

• The ventricles fill ______% during rapid filling.     Response: While specific numbers vary by source, the transcript focuses on this as the primary filling phase.

• Atrial systole is also known as __________.     Response: Atrial kick.

• The semilunar valves are open during _________.     Response: Systole.

• The atrioventricular valves are open during _______.     Response: Diastole.