Cardiac Cycle Overview

Cardiac Cycle:

• The force of contraction in a cardiomyocyte is influenced by the amount of calcium available to bind to troponin (inotropy) and preload (optimal actin and myosin overlap during diastole).
• Increased inotropy can be stimulated by the sympathetic nervous system, increased heart rate, thyroid hormone, and cortisol.
• Preload is determined by the extent of ventricular filling.
• The Wiggers diagram is a graphical representation of the cardiac cycle, including ventricular pressures, an electrocardiogram (ECG), ventricular volume, and phonocardiogram.
• The Wiggers diagram shows the key landmarks of the cardiac cycle: the a, c, and v waves and x and y descents of the atrial pressure tracing.
• The Wiggers diagram also displays the isovolumetric contraction and relaxation phases of the ventricular cycle, and the changes in ventricular volume during systole and diastole.
• The a wave corresponds to atrial contraction, the c wave to the bulging of the tricuspid valve, the x descent to atrial relaxation, the v wave to passive filling of the atria, and the y descent to the emptying of the atria into the ventricles.
• The right ventricle pumps the same amount of blood per minute as the left ventricle, although it develops lower pressures due to its thinner walls and its connection to the pulmonary circulation.
• The pressure-volume loop is used to assess the workload of the ventricle, contractility, and compliance of the heart.
• The area within the pressure-volume loop represents the total workload of the ventricle.
• The slope of the end-systolic pressure-volume relationship (ESPVR) reflects the inotropic state of the ventricle.
• Increased afterload reduces stroke volume and ejection fraction, and increases myocardial oxygen demand.
• The Ventricular Pressure-Volume Loop is a visual representation of the cardiac cycle, tracking pressure and volume changes in the ventricle throughout systole and diastole.
• Key points of this loop include:
  • Point A: End of diastole, ventricle at low volume, beginning of filling.
  • Point B: End of filling, ventricle at maximum volume (EDV).
  • Point C: Beginning of isovolumetric contraction.
  • Point D: Beginning of ejection, ventricle at maximum pressure.
  • Point A: End of ejection, ventricle at lowest volume (ESV) and beginning of relaxation.
• The length of the loop reflects stroke volume, which is influenced by preload, afterload, and contractility.
• Preload is represented by point B (EDV) on the Pressure-Volume Loop.
• Afterload is represented by the purple line, indicating the pressure that the heart must overcome to eject blood.
• Higher afterload leads to more vertical purple line, indicating a more difficult workloadfor the heart.
• Inotropy (contractility) is represented by the tangent line at point D (ESPVR).
• A steeper tangent line indicates higher inotropic state.
• Negative inotropic agents reduce the force of contraction, leading to reduced sarcomere shortening.

Cardiac Calculations and Parameters:

• End diastolic volume (EDV) is the volume in the ventricle at the end of diastole.
• End systolic volume (ESV) is the volume in the ventricle at the end of systole.
• Stroke volume (SV) is the volume of blood ejected with each heartbeat (SV = EDV - ESV).
• Cardiac Output (CO) is the volume of blood ejected per minute (CO = SV x HR).
• Ejection fraction (EF) is the proportion of EDV that is ejected during systole (EF = SV/EDV = (EDV - ESV)/EDV).
• EDV corresponds to preload. Increasing preload can increase contractility.
• Stroke volume is impacted by preload, contractility (inotropy), and afterload.
• Afterload is the pressure that the ventricle must overcome to eject blood.
• Ejection fraction is a common measure of heart function.

Key Facts & Figures:

• Normal pulmonary artery pressure is ~ 25/7 mm Hg.
• Normal atrial pressures are slightly lower in the right atrium than the left atrium.
• The pressure-volume curve displays systolic and diastolic blood pressure, EDV, ESV, stroke volume, contractility, afterload, and preload.
• The majority of work performed by the ventricle is pressure-volume work.
• Cardiac workload primarily stems from pressure-volume work, with minimal kinetic energy involved in ejecting blood.
• Hypertension increases myocardial oxygen demand.
• Cardiac output is a key factor in delivering oxygen and nutrients to tissues.
• Wall tension can be calculated using Laplace’s law: Wall stress (σ) = (Pressure x Radius) / Wall thickness.
• Cardiac output is influenced by both heart rate and stroke volume.

Heart Failure:

• Hearts that work excessively under strain endure molecular and histological changes.
• Compensation mechanisms in heart failure often compromise the force of contraction.
• Ejection fraction is an important indicator of heart function in heart failure.

Explore the key concepts of the cardiac cycle, including inotropy, preload, and the graphical representation through the Wiggers diagram. Understand how these factors influence heart function, including the role of calcium and the phases of ventricular contraction and relaxation. This quiz will test your knowledge of these fundamental cardiovascular principles.