Haemodynamics Detailed Notes

Haemodynamics

The study of the physical and physiological principles governing the movement of blood through the circulatory system.

Intended Learning Objectives

• Define pulse pressure and mean blood pressure, state normal values for a healthy young adult:

  • Systolic Blood Pressure: Generally around 120 mmHg (16kPa)

  • Diastolic Blood Pressure: Generally around 80 mmHg (10.7kPa)

  • Pulse Pressure: Difference between systolic and diastolic pressures (normal is around 40 mmHg).

• Explain arterial compliance and how it relates to:

  • Pulse Pressure

  • Stroke Volume

• Discuss Poiseuille's Law and its importance in understanding:

  • The relationship between vessel radius and resistance to flow.

  • Significance of changes in pressure within the circulatory system.

• Explain relationships between:

  • Cardiac Output, Peripheral Resistance, and Blood Pressure.

• Discuss the structure of capillaries and implications of blood viscosity.

• Comment on Laplace’s Law:

  • Relationship between vessel radius and pressure.

Blood Circulation Anatomy

Pathway of Blood Flow:

• Left Ventricle (contraction) → Aorta → Elastic Arteries → Muscular Arteries → Arterioles → Capillaries → Venules → Large Veins → Vena Cava → Right Atrium → Pulmonary Circulation.

Blood Vessel Composition

Blood Pressure Definitions

• Blood Pressure: Pulsatile

• Systole: Contraction of the heart

• Systolic Blood Pressure: Maximum pressure in arteries (normal: 120 mmHg).

• Diastole: Relaxation of the heart

• Diastolic Blood Pressure: Minimum pressure in arteries (normal: 80 mmHg).

• Pulse Pressure: Difference between systolic and diastolic.

Mean Arterial Pressure (MAP)

• Calculation of MAP:

  • MAP = Diastolic Pressure + (1/3) × Pulse Pressure

  • For example: For 120 mmHg systolic and 80 mmHg diastolic, MAP = 80 + (1/3) × 40 = 100 mmHg.

Hypertension

• Definition:

  • Abnormally high blood pressure and a major risk factor for vascular disease.

• Categories:

  • Prehypertension: Systolic 120-139 mmHg or Diastolic 80-89 mmHg.

  • Hypertension: Often goes unnoticed and untreated.

• Statistics:

  • 30% unaware of hypertension.

  • 40% not treated, and 67% of treated don’t achieve controlled BP < 140/90 mmHg.

Blood Flow Dynamics

• Laminar Flow: Smooth blood flow in parallel layers.

• Turbulent Flow: Chaotic flow that occurs due to:

  • Increased velocity

  • Narrowing of vessel radius

  • Decreased viscosity.

Korotkoff Sounds and Blood Pressure Measurement

• Phase 1: First sound, systolic reading.

• Phase 2: Swishing sound during deflation.

• Phase 3: Softer thumping.

• Phase 4: Muffled sound, first diastolic reading.

• Phase 5: Silence, second diastolic reading.

Compliance and the Windkessel Effect

• Compliance: The elasticity of blood vessels;

  • C = ΔV/ΔP (volume change over pressure change).

• Compliance decreases with age.

• Windkessel Effect: Large arteries store and release blood; similar to a buffer in water pumps.

Poiseuille’s Law

• Blood flow Q through a tube is determined by:

  • Pressure P

  • Radius r (to the power of 4)

  • Length l

  • Viscosity η.

• Resistance to flow is influenced by these factors.

Cardiac Output

• Stroke Volume (SV): Blood volume per heartbeat (typically 70 mL).

• Cardiac Output (CO): Total blood flow out of the heart (liters/min) calculated using:

  • CO = Heart Rate × SV.

• Example:

  • Heart rate: 75 beats/min

  • CO: 5.25 L/min.

Factors Affecting Cardiac Output

• Heart rate and stroke volume influence CO:

  • Stroke Volume = End-Diastolic Volume - End-Systolic Volume.

• Exercise increases both heart rate (up to 2.5 times) and stroke volume (up to 1.5 times).

Capillary Structure and Blood Viscosity

• Capillaries function without smooth muscle, facilitating gas and fluid exchange.

• High viscosity affects heart work; influenced primarily by haematocrit (approx. 45% red cells).

• Conditions like polycythaemia and sickle cell disease can increase viscosity.

Laplace’s Law

• In cylindrical vessels, the tension limit withstands pressure:

  • Tension = Pressure × Radius.

• The law states that pressure capability depends on tension, radius, and wall thickness.

Aneurysms and Hypertension

• Aneurysms occur when artery walls weaken; radius increases, reducing effective tension.

• Hypertension exacerbates the risk, especially in atherosclerosis prone patients.

Summary of Key Concepts

• Pulse Pressure (PP): PP = Systolic - Diastolic

• Mean Arterial Pressure (MAP): MAP = Diastolic + (1/3) PP

• Compliance: Measures elasticity; decreases with age, facilitating Windkessel effect.

• Poiseuille’s Law: Flow determination by pressure, radius, length, and viscosity.

• Cardiac Output: CO = BP / PR or CO = HR × SV

• Viscosity: Primary determinants include haematocrit and red cell deformability.

• Laplace’s Law: Pressure withstands = tension / radius.