Interface 1: Arterial Vascular Function
Arterial Vascular Function
The arterial circulation functions as a pulsatile hydraulic system, alternating between energy storage during systole and energy release during diastole.
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Systole:
During ventricular ejection, a small part of the stroke volume immediately flows into the small arteries and arterioles, while the majority distends the elastic walls of the large arteries (notably the aorta). This distension stores potential energy within the arterial system, much like the bellows pump below. In the picture below, the bellows pump pumps water from a reservoir into a pressurized tank that temporarily holds the incoming water before releasing it steadily to a hose.
By the same principle, the aorta’s elastic expansion allows blood to be temporarily stored under pressure, converting the intermittent output of the heart into a smoother, continuous flow. The amount of energy stored is determined by the arterial compliance (ΔV/ΔP), or how easily the arteries expand with each ejected volume.
Diastole:
When the heart relaxes, the elastic recoil of these arteries releases the stored energy, pushing blood forward through the peripheral circulation even in the absence of active ventricular contraction. This recoil maintains diastolic flow and tissue perfusion, preventing pressure and flow from rapidly dropping to zero between beats.
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The Windkessel model (from the German “air chamber”) describes this phenomenon by representing the arterial system as a combination of resistance (small arteries and arterioles) and compliance (elastic arteries). Together, these properties convert the pulsatile ejection from the heart into a steady, continuous pressure and flow, ensuring efficient convective delivery of oxygen and nutrients to the tissues.
The Arterial Pressure Waveform
While the Windkessel model provides a conceptual framework for understanding the relationships between pressure and flow in the arterial circulation, it does not fully describe the cardiovascular interactions involved in creating an arterial pressure waveform. An arterial pressure waveform represents the mechanical energy impulse generated by left ventricular (LV) contraction, transmitted through the aortic valve and propagated along the arterial tree as a pressure wave.
Importantly, this pressure (energy) wave travels much faster than the actual flow of blood, as it reflects the transmission of energy through a nearly incompressible fluid rather than the bulk movement of that fluid itself.
The shape and characteristics of the arterial waveform depend on multiple factors, including arterial compliance, peripheral resistance, and wave reflection, each of which modulates how pressure is transmitted and dampened along the vascular pathway.
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Electrical–Mechanical Coupling
The electrocardiogram (ECG) and the arterial pressure waveform are intimately related, reflecting two sides of the same process:
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The ECG records the electrical activation of the myocardium.
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The arterial waveform records the mechanical consequence of that activation; the generation and transmission of pressure by the contracting ventricle.
There is a short but consistent electromechanical delay between the R-wave on the ECG (which marks the onset of ventricular depolarization) and the systolic upstroke of the arterial pressure waveform. This delay, typically around 120–200 milliseconds, represents the time required for:
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The myocardium to develop enough tension to open the aortic valve, and,
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The resulting pressure pulse to travel through the arterial system to reach the transducer site.​
The Arterial Pressure Waveform and the Cardiac Cycle
The arterial pressure waveform is a direct mechanical reflection of left ventricular ejection and its interaction with the arterial system.
This classic Wiggers diagram demonstrates how changes in aortic, ventricular, and atrial pressures are coordinated throughout the cardiac cycle.
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When ventricular pressure rises above atrial pressure, the atrioventricular (AV) valves close, producing the first heart sound (“lub”) and initiating isovolumetric contraction.
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As ventricular pressure exceeds aortic pressure, the semilunar valves open, marking the beginning of ventricular ejection; this corresponds to the upstroke of the arterial waveform.
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When the ventricle relaxes and its pressure falls below aortic pressure, the aortic valve closes, generating the second heart sound (“dub”) and creating the small dicrotic notch on the descending limb of the arterial pressure curve.
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Diastolic pressure then represents the lowest arterial pressure before the next cardiac cycle, while mean arterial pressure (MAP) corresponds to the average perfusion pressure over the entire cycle.
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When isolating a single arterial waveform, the key landmarks (systolic, diastolic, and mean pressure) are highlighted, and the period of ventricular ejection is identified in gray.
Together, these figures emphasize that the arterial waveform is not just a pressure tracing, but a dynamic signature of ventricular-arterial interaction, linking electrical excitation, mechanical contraction, and vascular properties into one continuous physiological sequence.
Systolic Upstroke
The systolic upstroke represents the initial phase of ventricular ejection, beginning as the aortic valve opens and left ventricular pressure rapidly exceeds aortic pressure.
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The slope of the upstroke reflects how quickly pressure rises in the arterial system and is primarily determined by:
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Left ventricular contractility: greater inotropy produces a steeper, sharper upstroke as blood is ejected more forcefully and rapidly.
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Aortic valve function: obstruction (e.g., aortic stenosis) blunts the upstroke and delays its peak.
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Proximal aortic compliance: stiff arteries resist distention, producing a sharper but narrower upstroke, while compliant arteries generate a smoother, rounded ascent.
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In essence, the upstroke encodes both ventricular performance and arterial properties, acting as an early mechanical signature of systolic function and vascular health.