ICU Primary PrepCast: Systemic Vascular Resistance (SVR)

Introduction

• Discussion on systemic vascular resistance (SVR) and its physiological control.
• Key contributors: Maddy and Swapnil.

Definition of SVR

• SVR = Change in pressure divided by blood flow.
• Blood flow = Volume over time.

Hagen-Poiseuille Law

• Resistance = 8 x Length of vessel x Viscosity of blood / (π x Radius of vessel⁴).
• Radius of the vessel is a significant determinant of SVR.

Vascular System Components

• Comprises elements in series and parallel.

Components in Series

• Total resistance is the sum of individual resistances.
• Small arteries and arterioles contribute ~70% of total resistance.
• Large arteries require significant radius decrease to affect total resistance (only ~1%).

Components in Parallel

• Total resistance = Inverse of the sum of the inverse of individual resistances.
• Capillaries have low resistance despite small diameter, contribute minimally to total vascular resistance.

Control of SVR

• Primarily controlled by changing the radius of arterioles.

Systemic Factors

1. Peripheral and Central Chemoreceptors

   • Respond to hypoxia, hypercapnia, and acidosis.
   • Increase sympathetic activity ➔ Increases SVR.

2. Arterial Baroreflex

   • High-pressure baroreceptors respond to wall stretch ➔ Increase action potentials, inhibit sympathetic vasomotor ➔ Decrease SVR.
   • Low-pressure baroreceptors respond to volume ➔ Increase heart rate, atrial natriuretic peptide release ➔ Vasodilation.

3. Autonomic Central Control

   • Baseline sympathetic tone causes vasoconstriction.
   • Different vascular beds have varying responses (muscle, skin, renal sensitive; cerebral, coronary less so).
   • Increased sympathetic activation ➔ Renin-angiotensin-aldosterone system, adrenal medulla ➔ Increase SVR.

4. Hormonal Control

   • ADH: Triggered by increased osmolarity, decreased volume ➔ Increases ADH, vascular resistance.
   • Renin-Angiotensin-Aldosterone System: Decrease in renal perfusion ➔ Increase renin, aldosterone, ADH ➔ Increases SVR.
   • Catecholamines: Exercise, hypoglycemia ➔ Increase adrenaline/noradrenaline ➔ Vasoconstriction.

5. Temperature

   • Increase in temperature ➔ Vasodilation ➔ Decrease SVR.

Local Control

1. Myogenic Control

   • Vessel pressure increases ➔ Smooth muscle stretches ➔ Vasoconstriction.
   • Varies in sensitivity by organ (e.g., brain).

2. Metabolic Control

   • Increase in metabolic demand releases substances (lactate, ATP) ➔ Vasodilation.

3. Flow Control

   • Local vasodilation ➔ Increases proximal flow ➔ More vasodilatory substances ➔ Further vasodilation.

Conclusion

• SVR is mainly dependent on the radius of small arteries and arterioles.
• Comprehensive understanding of systemic and local control factors is crucial.