Cardiac Output (CO) is a measure of the volume of blood the heart pumps per minute. It is a key indicator of the heart's ability to supply oxygen and nutrients to tissues and organs. The formula for calculating cardiac output is:
\[ CO = Heart Rate (HR) \times Stroke Volume (SV) \]
Where:
- Heart Rate (HR) is the number of times the heart beats per minute.
- Stroke Volume (SV) is the volume of blood pumped from the left ventricle of the heart with each beat.
Thus, cardiac output can be modified by changing either the heart rate or the stroke volume. For example:
- An increase in heart rate (e.g., during exercise) will typically increase cardiac output if stroke volume remains stable.
- An increase in stroke volume (e.g., due to increased venous return) will also increase cardiac output, assuming heart rate is constant.
Preload
Preload refers to the degree of stretch of the heart muscle fibers (specifically, the ventricles) at the end of diastole, just before contraction. It is determined by the volume of blood returning to the heart (venous return) and filling the ventricles. Essentially, preload is a measure of the capacity of the ventricles to fill with blood.
Relationship Between Preload, Stroke Volume, and Cardiac Output
According to the Frank-Starling law of the heart, an increase in preload leads to an increase in stroke volume, up to a certain point. This occurs because:
- As preload increases, the heart muscle fibers are stretched more. Up to an optimal point, increased stretching enhances the force of contraction during systole.
- Greater force of contraction leads to more blood being ejected from the heart with each beat (increased stroke volume).
Consequently, since stroke volume is a component of cardiac output, an increase in preload (and the resulting increase in stroke volume) will lead to an increase in cardiac output, assuming heart rate remains constant.
Changes in Preload
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Increased Preload: This can occur with increased venous return (e.g., during exercise or fluid overload). It generally enhances stroke volume and cardiac output.
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Decreased Preload: This can happen in conditions such as dehydration, hemorrhage, or when there is significant blood pooling in the extremities (e.g., septic shock). A reduction in preload reduces stroke volume and cardiac output.
In summary, cardiac output is influenced by heart rate and stroke volume, and preload plays a crucial role in determining stroke volume. Changes in preload can either enhance or diminish cardiac output, depending on the direction of the change.
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