From the Guidelines
Preload is the initial stretching of the cardiac myocytes prior to contraction, directly influencing cardiac output through the Frank-Starling mechanism, and is effectively managed through fluid administration, diuretics, or other means to optimize cardiac function in various clinical settings.
Definition and Clinical Significance
Preload refers to the volume of blood in the ventricles at the end of diastole, just before contraction begins 1. It is essentially the stretching of heart muscle fibers before contraction, which directly affects cardiac output according to the Frank-Starling mechanism. In clinical settings, preload is often manipulated through fluid administration or diuretics to optimize cardiac function.
Mechanism and Assessment
When preload increases (through IV fluids), the heart muscle stretches more, resulting in stronger contractions and increased stroke volume. Conversely, reducing preload through diuretics can be beneficial in conditions like heart failure where excessive preload worsens symptoms 1. Preload can be assessed clinically through measures like central venous pressure, pulmonary capillary wedge pressure, or through bedside ultrasound evaluating IVC diameter and collapsibility.
Clinical Management
Understanding preload is crucial for managing hemodynamics in critical care, perioperative, and emergency settings. For instance, in patients with acute respiratory distress syndrome (ARDS), managing preload is critical to avoid overdistension of the right ventricle and to maintain optimal cardiac output 1. The effects of mechanical ventilation on preload, including the impact of positive end-expiratory pressure (PEEP) on venous return and cardiac output, must be carefully considered 1.
Key Considerations
- Fluid Administration: IV fluids like normal saline or lactated Ringer's can be used to increase preload, with rates depending on the clinical situation.
- Diuretics: Medications like furosemide can reduce preload and are beneficial in conditions where excessive preload is harmful.
- Bedside Ultrasound: Evaluating IVC diameter and collapsibility can provide valuable insights into preload status.
- Mechanical Ventilation: Understanding the effects of PEEP and other ventilation strategies on preload and cardiac output is essential in critically ill patients 1.
From the Research
Definition of Preload
- Preload is defined as the initial stretching of the cardiac myocytes prior to contraction, which directly reflects myocardial stretch 2.
- It is the degree to which the cardiac muscle is stretched at the end of diastole, just before contraction begins.
Relationship with Frank-Starling Mechanism
- The Frank-Starling mechanism is activated during any stretch of a cardiac chamber beyond its resting size, which is present immediately before contraction 2.
- Every left ventricle has an intrinsic and limited range of possible volumes at end diastole, and the relationship between left ventricular end-diastolic pressure (LVEDP) and left ventricular end-diastolic volume (LVEDV) is curvilinear 2.
Clinical Relevance
- The difference between the current and the maximal possible LVEDV, or the preload reserve, may be of the most clinical relevance 2.
- A minor or absent left ventricular preload reserve indicates that there will be either minimal or no increase in stroke volume following intravenous fluid administration 2.
- LVEDP, rather than left atrial pressure, provides the most reliable guide to preload reserve in an individual at a specific period in time 2.
Preload in Heart Failure
- Heart failure is characterized by an epidemic disease which affects about 1% to 2% of the population worldwide, with both etiology and phenotype differing largely 3.
- Increased preload can lead to volume overload, tachycardia, dyspnea, and further deterioration of cellular function, creating a vicious circle 3.
- Diuretics, such as furosemide, are commonly used to treat heart failure by reducing preload and alleviating symptoms 4, 5, 6.