What are preload and afterload?

From the Guidelines

Preload and afterload are critical determinants of cardiac function. Preload refers to the initial stretching of the cardiac myocytes prior to contraction, which is directly related to the ventricular volume at the end of diastole 1. Afterload, on the other hand, is the resistance that the ventricles must overcome to eject blood during systole, and it is directly proportional to the pressure in the aorta and pulmonary artery 1.

Key Factors Influencing Preload and Afterload

• Preload: influenced by ventricular volume, venous return, and atrial pressure 1
• Afterload: influenced by systemic vascular resistance, pulmonary vascular resistance, and arterial pressure 1

Relationship Between Preload, Afterload, and Cardiac Function

• Increased preload can lead to increased stroke volume and cardiac output, but excessive preload can lead to ventricular dysfunction 1
• Increased afterload can lead to decreased stroke volume and cardiac output, and can also lead to ventricular hypertrophy and dysfunction 1
• The optimal balance between preload and afterload is crucial for maintaining normal cardiac function, and alterations in this balance can lead to various cardiac disorders 1

Clinical Implications

• Understanding the concepts of preload and afterload is essential for the diagnosis and management of various cardiac conditions, such as heart failure, valvular heart disease, and pulmonary embolism 1
• Therapeutic interventions, such as vasodilators and diuretics, can be used to modify preload and afterload and improve cardiac function 1

From the Research

Definitions of Preload and Afterload

• Preload can be defined as all of the factors that contribute to passive ventricular wall stress (or tension) at the end of diastole 2
• Afterload can be defined as all of the factors that contribute to total myocardial wall stress (or tension) during systolic ejection 2

Factors Influencing Preload and Afterload

• Preload is influenced by end-diastolic volume, which can be assessed by ventriculography, echocardiography, or indicator dilution techniques 3
• Afterload is influenced by wall tension during the ejection phase, which can be reasonably monitored by arterial blood pressure 3
• Preload and afterload can also be influenced by contractility, which is difficult to assess under clinical conditions 3

Effects of Preload and Afterload on Cardiac Function

• Increases in preload can lead to increases in stroke volume, while increases in afterload can lead to decreases in stroke volume 4
• The optimal preload for maximal stroke work can vary depending on the level of afterload 4
• Preload and afterload can also influence left ventricular torsion, with increases in preload leading to increases in torsion and increases in afterload leading to decreases in torsion 5

Clinical Implications

• Understanding the effects of preload and afterload on cardiac function is important for the diagnosis and treatment of heart failure 6
• Preload and afterload mismatch can exist in the steady state, leading to operation of the heart on an apparent descending limb of cardiac function 6
• Treatment of heart failure with vasodilators can lead to increased cardiac output by reducing afterload, but the effects of these agents can be influenced by the venous return curve 6