The Arterial SystemThe Arterial System

The Windkessel EffectThe Windkessel Effect

Arterial Pressure DeterminantsArterial Pressure Determinants– COCO– TPRTPR

Changes in the Pulse WaveformChanges in the Pulse Waveform

The hydraulic filter converts The hydraulic filter converts pulsatile flow to steady flowpulsatile flow to steady flow

Hydraulic filtering converts the intermittent output of the heart to a steady flow through the capillaries.

Part of the energy of cardiac contraction is dissipated as forward capillary flow.

The remaining energy is stored as potential energy in the distensible arteries.

During diastole, the elastic recoil of the arterial walls converts this potential energy into blood flow.

Mean Arterial PressureMean Arterial Pressure

The mean arterial pressure is the The mean arterial pressure is the pressure in the large arteries, pressure in the large arteries, averaged over time.averaged over time.MAP can be measured from the MAP can be measured from the arterial pressure tracing by measuring arterial pressure tracing by measuring the area under the pressure curve.the area under the pressure curve.It can also be estimated by the It can also be estimated by the formula: MAP = Pd + (Ps-Pd)/3formula: MAP = Pd + (Ps-Pd)/3

The arterial blood pressure is The arterial blood pressure is determined by physical and determined by physical and

physiological factorsphysiological factors

The physiological (alterable) factors are the cardiac output and the peripheral resistance.

The two physical (dependent) factors are the blood volume within the arterial system and the compliance of the system..

Cardiac OutputCardiac Output

The change in pressure in response to a change in cardiac output can be appreciated by considering:Let the cardiac output = 5 L/min and MAP = 100 mmHg.The total peripheral resistance is; R = P/Q = 100/5 = 20 mmHg/L/min (Omh’s Law: R=V/I)

Cardiac OutputCardiac Output

If cardiac output (Qh) suddenly increased to 10 L/min, P will initially remain unchanged.Outflow (Qr) remains unchanged at 5 L/min because it depends on depends on P and R.When Qh > Qr, mean arterial blood volume increases.Because P depends on the arterial blood Because P depends on the arterial blood volume (V) and on the arterial compliance volume (V) and on the arterial compliance (C), an increase in V will increase P.(C), an increase in V will increase P.

Peripheral resistance

Similar reasoning can explain the changes in P that accompany alterations in peripheral resistance.Increase R suddenly to 40 mmHg/L/min.Qr = P/R = 2.5 L/min.Thus, the peripheral runoff would only be 2.5 L/min, even though cardiac output was 5 L/min.If Qh remained constant at 5 L/min, Qh would If Qh remained constant at 5 L/min, Qh would exceed Qr and V would increase; therefore, P exceed Qr and V would increase; therefore, P would rise.would rise.

The pressure curves change in The pressure curves change in arteries at different distances from arteries at different distances from

the heartthe heart

The radial stretch of descending aorta The radial stretch of descending aorta from ejection initiates a pressure wave that from ejection initiates a pressure wave that is propagated down the aorta and its is propagated down the aorta and its branches.branches.

The pressure wave travels much faster The pressure wave travels much faster than does the blood itself.than does the blood itself.

The velocity of the pressure wave varies The velocity of the pressure wave varies inversely with the vascular compliance.inversely with the vascular compliance.

The pulse velocity increases progressively The pulse velocity increases progressively as the wave travels from aorta towards the as the wave travels from aorta towards the periphery. periphery. The arterial pressure contour becomes The arterial pressure contour becomes distorted as the wave is transmitted down distorted as the wave is transmitted down the arterial system.the arterial system.

– Three major changes occur in the arterial pulse Three major changes occur in the arterial pulse contour.contour.

First, the incisura (a notch that appears at the end of First, the incisura (a notch that appears at the end of ventricular ejection), is damped and disappears.ventricular ejection), is damped and disappears.

Second, the systolic portions of the pressure wave become Second, the systolic portions of the pressure wave become narrow and elevated.narrow and elevated.

Third, a hump may appear on the diastolic portion of the Third, a hump may appear on the diastolic portion of the pressure wave.pressure wave.

– The dampening of the high-frequency components of The dampening of the high-frequency components of the arterial pulse is caused largely by the viscoelastic the arterial pulse is caused largely by the viscoelastic properties of the arterial walls.properties of the arterial walls.

– The precise mechanism for the peaking of the The precise mechanism for the peaking of the pressure wave is controversial.pressure wave is controversial.

– Several factors contribute to these changes, including Several factors contribute to these changes, including reflection, tapering of the arteries, resonance, and reflection, tapering of the arteries, resonance, and velocity of transmission.velocity of transmission.

The Windkessel EffectThe Windkessel Effect

Arterial Pressure DeterminantsArterial Pressure Determinants– COCO– TPRTPR

Changes in the Pulse WaveformChanges in the Pulse Waveform

Questions?Questions?