Vascular Function

Arterial Elasticity as a Hydraulic Filter

functions like a Windkessel of antique fire engine

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

Important consequences: An intermittent pump (piston) must do 2X the work to create the same flow if it is pumping into a rigid tube than if it is pumping into a compliant (elastic) tube

The increased work means a compromised heart is more likely to fail if it faces a non-compliant resistance than one that is compliant. Ý W Þ Ý O₂ demand and Ý likelihood of infarction. Therefore heart and Vasculature determine heart performance

Vascular Compliance = dV/dP This is inversely proportional to age. Compliance decreases with age

Arterial: determined by arteries not arterioles Venous >>> Arterial

Static Pressure-Volume Relationship: Human aortic compliance (V/P) is hyperbolic. (Slope of this = compliance)

Note: Veno = Veins Vaso = Arteries

Determinants of Arterial Blood Pressure

PA = SV X HR X SVR

CO = PA / SVR

PA = Mean Arterial Pressure

HR = Heart Rate (bpm) -modulated by parasympathetic and sympathetic control

SV = Stroke Volume -determined by preload, afterload, and inotropic state

SVR = Systemic Vascular Resistance: net static resistance of all vascular beds

Incicera = abnormal – valves do not close at same time (pulmonic or aortic) : not the same as Dicrotic Notch

Dicrotic Notch = flaps of valve invert slightly for slight transient decrease in pressure – normal

Central driving pressure allows individual organ systems to balance supply and demand by modulating pre and post capillary demand, thus blood flow

Peripheral Resistance: primarily determined by the arteriolar resistance.

Sympathetic activity Þ (1) vasoconstriction (resistance) and (2) veno-constriction (capacitance)

Neural influence of Sympathetic (vasoconstriction = ß arteriolar Xsectional area) >>> parasympathetic (vasodilatation = Ý arteriolar cross-sectional area)

Total vascular resistance is dictated by the sum of the individual organ systems

The degree of vasodilatation is often dictated by the accumulation of local vasodilator metabolites overriding autonomic influences

Venoconstricion = ß venous cross-sectional area

Autoregulation controls the balance of constriction and dilation of respective pre-capillary bed arterioles

Systemic Resistance > Pulmonary resistance

Arterial Pulse Pressure (PP) Capacitance and Compliance dependent

PP = Systolic – Diastolic Pressure

Inversely proportional to vascular compliance (given constant Stroke Volume)

Directly proportional to Stroke Volume (given constant compliance)

Ý PP: due to ß compliance and/or Ý SV (as in arteriosclerosis-old age)

ß PP: due to Ý compliance and/or ß SV

widened PP is common in older people: diastole is same, systole is increased

Mean Pressure Resistance dependent

Diastolic Pressure + 1/3 Pulse Pressure

Ý MP: due to ß compliance and/or Ý SV

ß MP: due to Ý compliance and /or ß SV

MP increases as resistance increases

as compliance ß , pressure Ý and velocity Ý

Physiological pulse wave distortion

inspiration Þ Ý SV

early heartbeat Þ ß SV ; next beat Þ Ý SV b/c Ý filling time between beats

Palpitation felt at SV