Cardiac Output

Cardiac Output (CO) definition

The volume of blood pumped by each ventricle per minute

Cardiac Output Components (3)

• Average Heart Rate: ~70 beats per minute (bpm)

• Average Stroke Volume: ~70–80 mL per beat

• Average Cardiac Output: ~5,500 mL per minute

Cardiac Output Table

Factors Affecting Stroke Volume (SV) (10)

• Venous return

• Filling time

• Autonomic innervation

• Hormones

• Vasodilation/Vasoconstriction

• Preload

• Contractility

• Afterload

• End diastolic volume (EDV)

• End systolic volume (ESV)

Venous Return Influencers (7)

• Sympathetic vasoconstrictor tone

• Respiratory pump

• Increased MSFP

• Blood volume

• Skeletal muscle pump

• Gravity

• Cardiac suction

MSFP

Mean Systemic Filling Pressure

Sympathetic Vasoconstrictor Tone (3+3) - Venous Return

• Arterioles

  • Vasoconstriction increases venous return by opening arterioles and arterio-venous shunts

• Capillaries

  • Opening pre-capillary sphincters decreases venous return

• Veins

  • Decreased venous tone results in decreased venous return

Respiratory Pump - Venous Return

Increased negative pressure during inspiration aids venous return

↑ MSFP - Venous Return

Pressure in the vascular system during circulatory arrest, affecting venous return

Blood Volume - Venous Return

Increased blood volume leads to increased venous return

Skeletal Pump - Venous Return

Contraction of skeletal muscles and the presence of venous valves increases venous return

Gravity - Venous Return

Supine position enhances blood flow to the right atrium, increasing venous return

Cardiac Suction - Venous Return

Atrial chambers act as suction, assisting venous return to the heart

Stroke Volume (SV) Formula

SV = EDV - ESV

Filling Time

Heart rate >180/min decreases diastolic filling time

Autonomic Innervation

Sympathetic stimulation increases contraction strength

Hormones

Adrenaline enhances contraction

Vasodilation/Vasoconstriction

Increased aortic impedance → increased afterload

Impedance

Resistance to blood flow from the left ventricle

Preload (2)

• Initial stretch of cardiac muscle cells before contraction

• Related to ventricular filling

Afterload (2)

• Force against which the heart contracts to eject blood

• Primarily due to arterial resistance

Contractility (3)

• Intrinsic cardiac muscle strength independent of preload

• Influenced by preload via Starling's Law

  • will affect the force of contraction

Factors Affecting Heart Rate (HR) (3)

• Atrial Reflex

• Autonomic Innervation

• Hormones*

Atrial Reflex - Affecting Heart Rate

Increased venous return stretches the atrial walls, triggering a reflex that increases heart rate

Autonomic Innervation - Affecting Heart Rate (2)

• Sympathetic Stimulation: Increases heart rate

• Parasympathetic Stimulation: Decreases heart rate

Hormones - Affecting Heart Rate (2)

• Adrenaline: Increases heart rate

• Thyroid hormones: Can also increase heart rate

Cardiac Output (CO) Formula

CO = HR × SV