Arterial Pressure Pulsations

Introduction to Arterial Pressure Pulsations

• Pressure pulses are waves traveling through blood, distinct from blood flow itself. Pressure pulses are waves traveling through blood, whereas blood flow is the actual movement of blood. It's crucial to understand that while related, they are not the same.

• Analogy: Think of sound waves. Air molecules mostly stay in place, vibrating to transmit the sound, while the sound wave itself propagates. Similarly, blood serves as the medium through which pressure waves travel.

• In the arterial tree, blood flows and pressure waves travel simultaneously. These pressure waves interact with blood flow, influencing arterial dynamics.

Generation and Transmission of Pressure Waves

• The heart generates pressure during systole, the contraction phase.

• Blood entering the aorta pushes existing blood forward, but inertia prevents sudden, instantaneous movement. The blood's inertia resists immediate acceleration.

• Incoming blood increases pressure in the aorta, which then spreads as a wave throughout the arterial tree. This pressure wave propagates much faster than the actual blood flow.

• This pressure wave causes fluctuations in blood pressure that are detectable and clinically significant.

Types of Pressure

• Systolic Pressure: The highest pressure during the pulse, approximately 120 mmHg in a healthy aorta. This represents the peak pressure exerted against arterial walls.

• Diastolic Pressure: The lowest pressure during the pulse, approximately 80 mmHg in a healthy aorta. This is the pressure in the arteries when the heart is at rest between beats.

• Pulse Pressure: The difference between systolic and diastolic pressure.

  • Calculation: Pulse\ Pressure = Systolic\ Pressure - Diastolic\ Pressure. In a healthy aorta, 120 - 80 = 40 mmHg.

  • Depends on stroke volume and arterial distensibility.

    • Increased stroke volume $\implies$ increased pulse pressure: A larger stroke volume leads to a greater difference between systolic and diastolic pressures.

    • Increased distensibility $\implies$ decreased pulse pressure (due to dampening): More distensible arteries can absorb more of the pressure wave, reducing the pulse pressure.

Mean Arterial Pressure (MAP)

• Definition: The average pressure experienced by the arteries over a cardiac cycle, representing the effective pressure driving blood to the tissues.

• Not simply the average of systolic and diastolic pressures because the heart spends more time in diastole. Diastole's longer duration means it contributes more to the average.

• Approximation: MAP is closer to diastolic pressure. This is because the heart spends more time in diastole.

• Normal MAP: Approximately 95 mmHg. This ensures adequate perfusion of organs and tissues.

• During exercise: Diastole duration decreases, so MAP shifts toward systolic pressure. The increased heart rate shortens diastole, influencing the MAP.

• MAP \approx Diastolic\ Pressure + \frac{1}{3} Pulse\ Pressure

Arterial Distension and Pulse Examination

• Pressure waves cause arteries to distend, a physical expansion of the arterial walls.

• Palpating radial pulses at the wrist detects these distensions. This allows clinicians to assess heart rate, rhythm, and arterial elasticity.

Velocity and Damping of Pulses

• Velocity

  • Varies along the arterial tree: Low near the aorta, high in smaller arteries. The speed of the pressure wave changes as it moves distally.

  • Depends on vessel wall stiffness (compliance).

    • Less stiff/more compliant vessels: Energy lost in distension, slowing the wave. Compliant vessels absorb more energy.

    • Stiffer vessels: Less energy loss, increasing wave speed. Stiffer vessels transmit the wave more efficiently.

  • Stiffness increases from the aorta to smaller arteries. This increase in stiffness affects pulse wave velocity.

  • Pulse waves travel much faster than blood flow.

    • In the aorta, pulse waves are ~15 times faster than blood flow. This disparity highlights the difference between pressure wave propagation and fluid movement.

• Damping

  • Pulsation fades from the aorta to the capillaries. The intensity of the pressure wave decreases as it moves peripherally.

  • Pulsation nearly disappears in capillaries. This ensures smooth, continuous blood flow at the microcirculation level.

  • Reasons for damping:

    • Distensibility: Energy loss due to vessel distension. The more the vessels expand, the more energy is absorbed.

    • Resistance: Blood movement to transmit pressure is opposed by resistance, indirectly dampening the wave. Resistance to blood flow reduces the pressure wave's amplitude.

  • Pressure waves are strongest in the aorta and weaken towards the capillaries. This gradient is essential for circulatory function.

Summary

• Pressure waves travel like sound waves, faster than blood flow. This allows for rapid communication of pressure changes throughout the arterial system.

• Systolic pressure: Peak pressure during pulsation, indicating the force of ventricular contraction.

• Diastolic pressure: Minimum pressure during pulsation, reflecting arterial pressure during ventricular relaxation.

• Pulse pressure: Difference between systolic and diastolic pressures, an indicator of arterial stiffness and stroke volume.

• Mean arterial pressure: Average pressure over a cardiac cycle, crucial for organ perfusion.

• Pressure pulses cause vessel distension, felt as a pulse. This distension is a physical