Clase 17 Fisiología Cardíaca 2 - Ciclo cardíaco (IG:@doctor.paiva)

Introduction to Cardiac Physiology

Overview of Cardiac Cycle

• The session is the 17th class on cardiac physiology, led by Eduardo Paiva, focusing on the cardiac cycle.

• Key topics include generalities of the cardiac cycle, functions of atria and ventricles, and concepts of preload and afterload.

Phases of Cardiac Cycle

• The cardiac cycle consists of diastole (ventricular relaxation) and systole (ventricular contraction), initiated by spontaneous action potentials from the sinoatrial node.

• Atrial contraction occurs approximately 0.1 seconds before ventricular contraction, highlighting a sequential process in heart function.

Functionality of Atria and Ventricles

Role of Atria

• Atria are referred to as "booster pumps," with 80% of blood flowing into ventricles passively without atrial contraction; only 20% is actively pushed during atrial contraction.

• This indicates that even if atrial contractions are compromised (e.g., in atrial fibrillation), the heart can still function effectively due to its capacity to pump excess blood.

Functionality of Ventricles

• Ventricular filling involves two phases: rapid filling during early diastole and slower filling aided by atrial contraction.

• The volume dynamics include end-diastolic volume (EDV), end-systolic volume (ESV), and stroke volume (SV).

Ventricular Filling Dynamics

Phases of Filling

• Rapid ventricular filling occurs when pressure drops in ventricles post-contraction, allowing AV valves to open for blood inflow.

• The filling phase is divided into three parts: rapid filling, slow filling, and final contribution from atrial contraction.

Ventricular Ejection Process

Ejection Phase Breakdown

• Ventricular ejection consists of two main periods: isovolumetric contraction followed by ejection phase where blood is expelled into arteries.

• During isovolumetric contraction, pressure builds up without a change in volume until it exceeds arterial pressures.

Pressure Requirements for Ejection

Understanding Ventricular Function and Ejection Fraction

Phases of Ventricular Ejection

• The heart's ventricles must overcome pressure to open valves, leading to a rapid ejection of approximately 70% of blood volume, termed the "rapid ejection fraction."

• The ventricular ejection phase is divided into three parts: the first third (rapid ejection) accounts for 70% of blood volume, while the remaining two-thirds (slow ejection) account for 30%.

Relaxation Phase and Valve Closure

• After ventricular contraction, pressures in the ventricles drop, allowing arterial pressures to push blood back into the ventricles.

• This backflow causes closure of semilunar valves (aortic and pulmonary), resulting in the second heart sound.

• Following valve closure, there is a period where ventricular relaxation continues until atrial pressures exceed ventricular pressures.

Volumes Associated with Ventricular Function

• Key volumes include:

• End-Diastolic Volume (EDV): Approximately 110-120 ml when ventricles are filled post-diastole.

• Stroke Volume (SV): About 70 ml expelled during systole into arteries.

• End-Systolic Volume (ESV): Remaining volume after contraction, around 40-50 ml.

Ejection Fraction and Cardiac Output

• The relationship between these volumes shows that EDV equals ESV plus SV. In a healthy heart, about 60% of EDV is expelled per contraction.

• Ejection fraction represents this percentage; however, it’s crucial not to confuse it with stroke volume or cardiac output—cardiac output being total blood expelled per minute.

Clinical Implications of Ejection Fraction

• A patient may have a normal ejection fraction but still present with low cardiac output due to reduced overall volumes—highlighting potential instability despite seemingly normal metrics.

• An example illustrates how concentric hypertrophy can lead to decreased chamber size yet maintain an apparently normal ejection fraction at rest.

Importance of Preload and Afterload

• Preload refers to diastolic filling pressure; increased venous return raises preload.

Understanding Cardiac Cycle Dynamics

Impact of Hypertension on Cardiac Function

• Hypertension, whether systemic or pulmonary, increases the workload on the heart. For instance, systemic hypertension raises the left ventricle's load, while pulmonary hypertension affects the right ventricle.

• The relationship between pressure and post-load is crucial; higher pressures require greater effort from the ventricles to overcome resistance during contraction.

Overview of Cardiac Cycle Events

• The cardiac cycle involves distinct phases characterized by changes in atrial and ventricular pressures. During ventricular contraction, pressure rises leading to closure of atrioventricular valves, producing the first heart sound.

• As ventricular pressure exceeds aortic pressure, ejection begins with opening of semilunar valves. This phase is critical for blood flow from ventricles into arteries.

Phases of Ventricular Relaxation and Filling

• After ejection, as ventricular pressure falls below arterial pressure, semilunar valves close creating the second heart sound. Atrioventricular valves remain closed during this phase known as isovolumetric relaxation.

• When ventricular pressure drops below atrial pressure, atrioventricular valves open allowing rapid filling of ventricles without atrial contraction.

Completion of Ventricular Filling