Cardiac Output and Its Determinants

This set covers the fundamental definitions, calculations, and regulatory factors of cardiac output, including heart rate determinants and stroke volume components like preload, afterload, and contractility.

Cardiac Output (CO)

The amount of blood pumped by the heart in one minute, calculated as the product of heart rate and stroke volume and measured in $mL/min$.

Heart Rate (HR)

The number of heartbeats occurring in one minute.

Stroke Volume (SV)

The volume of blood pumped by each ventricle with each beat of the heart, measured in $mL/beat$.

Normal Cardiac Output

The average volume of blood passing through the heart each minute, typically ranging from $4$ to $6$ liters.

Positive Chronotropic Factors

Factors that increase the heart rate, such as the sympathetic nervous system, adrenaline, and hypercapnia.

Tachycardia

A fast heart rate defined as being over $100\text{ beats per minute}$.

Negative Chronotropic Factors

Factors that decrease the heart rate, such as parasympathetic stimulation and acetylcholine.

Bradycardia

A slow heart rate defined as being below $60\text{ beats per minute}$.

Hypercapnia

A condition of increased carbon dioxide levels which serves as a positive chronotropic factor to increase heart rate.

Preload

The volume of blood entering the ventricles, which determines the amount of stretch in the ventricles before contraction.

Frank-Starling Mechanism

The principle stating that the volume of blood entering the ventricles (end diastolic volume) is proportional to the stroke volume.

End Diastolic Volume (EDV)

The specific volume of blood contained in the ventricles immediately prior to ejection.

Afterload

The resistance the ventricles must overcome to pump blood around the body, often related to aortic pressure.

Contractility

The force with which the myocardium contracts for a given preload; it is a primary determinant of stroke volume.

Positive Inotropic Factors

Factors that increase cardiac contractility, including sympathetic stimulation, caffeine, and hypercalcemia.

Negative Inotropic Factors

Factors that decrease cardiac contractility, such as parasympathetic stimulation, hypocalcemia, increase in potassium, and myocardial hypoxia.

Myocardial Hypoxia

A condition where there is insufficient oxygen supply to the heart muscle cells, leading to a decrease in contractility.

Hypocalcemia

A decrease in calcium levels which can increase heart rate but primarily acts to decrease the contractility of the heart muscle.

Hypercalcemia

An increase in calcium levels which can decrease heart rate but acts as a positive inotropic factor by increasing muscle contractility.