M04.03.01 Control of stroke volume

Cardiac output refers to the volume of blood the heart pumps per minute and is calculated as the product of stroke volume (SV) and heart rate (HR). This means that any changes in stroke volume directly influence cardiac output. This article discusses stroke volume, its regulation, Starling’s Law, and its clinical relevance in shock conditions.

Stroke Volume

Stroke volume is defined as the amount of blood expelled by the heart during each heartbeat. It is calculated as the difference between the end-diastolic volume (EDV), the volume of blood in the heart at the end of diastole, and the end-systolic volume (ESV), the volume remaining in the heart at the end of systole:

SV=EDV−ESV

Control of Stroke Volume

Stroke volume is controlled by the amount the heart fills during diastole (preload) and how efficiently the heart expels blood during systole. Several factors influence stroke volume, including central venous pressure (CVP) and total peripheral resistance (TPR).

Factors Affecting Stroke Volume

Central Venous Pressure (CVP)

CVP represents the blood pressure in the vena cava and is a measure of the blood returning to the heart. As CVP increases, stroke volume increases due to:

1. Increased blood entering the heart during diastole, raising EDV.
2. Stronger ventricular contraction, decreasing ESV (as per Starling’s Law).

Increased preload (increased CVP) enhances the stretching of heart muscle fibers, allowing a more forceful contraction and a higher stroke volume.

Total Peripheral Resistance (TPR)

TPR is the resistance that blood faces as it passes through the arteries. A higher TPR (also called afterload) makes it more difficult for the heart to pump blood, thereby decreasing stroke volume.

Starling’s Law of the Heart

Starling’s Law states that the more the heart fills (higher EDV), the stronger the contraction, and hence the greater the stroke volume. This occurs because increased filling stretches the muscle fibers, creating more sites for actin-myosin cross-bridges, resulting in stronger contractions. However, if the fibers are overstretched, stroke volume decreases due to diminished contractile efficiency.

Starling Curve

The Starling Curve demonstrates the relationship between stroke volume and venous pressure, with contractility defining the slope of the curve.

Autonomic Regulation of Stroke Volume

Stroke volume is also influenced by the autonomic nervous system:

• The sympathetic nervous system increases heart contractility through β1-adrenoceptors.
• The parasympathetic nervous system (via muscarinic receptors) decreases contractility.

Autonomic regulation is essential for adjusting heart rate and contractility in response to blood pressure changes, monitored by baroreceptors and chemoreceptors.

Clinical Relevance: Shock

Shock is a medical condition characterized by inadequate blood flow and oxygenation of tissues, leading to ischemia. It can result from various causes:

1. Cardiogenic shock – The heart cannot pump blood effectively, leading to low arterial pressure.
2. Mechanical/obstructive shock – Conditions like cardiac tamponade impedes heart filling, resulting in high CVP and low arterial pressure.
3. Hypovolemic shock – Blood loss reduces circulating volume and arterial pressure, impairing organ perfusion.
4. Distributive shock – In sepsis or anaphylaxis, vasodilation reduces blood pressure and tissue perfusion.

Key Symptoms of Shock:

• Tachycardia (rapid heart rate)
• Hypotension (low blood pressure)
• Rapid breathing

Treatment often includes fluid resuscitation, vasopressors, and management of the underlying cause.

Points to Remember

1. Cardiac output is the product of stroke volume and heart rate.
2. Stroke volume is influenced by EDV, ESV, preload, and afterload.
3. Starling’s Law shows that increased heart filling results in increased stroke volume.
4. Autonomic regulation controls heart contractility via sympathetic and parasympathetic systems.
5. Shock leads to inadequate tissue perfusion and may result from cardiogenic, obstructive, hypovolemic, or distributive causes.

Bibliography

• OpenStax College. “Cardiovascular System: Blood Flow, Heart Sounds, and Cardiac Cycle.” CC BY 3.0
• Bitzblitz. “Frank-Starling Law.” CC BY 3.0