What is peripheral vascular resistance?

Peripheral Vascular Resistance: Definition and Clinical Significance

Peripheral vascular resistance (PVR) is the resistance to blood flow offered primarily by the arterioles (vessels <400 μm in diameter), which represent the principal site of resistance adjustment in the cardiovascular system. 1

Physiological Basis

Peripheral vascular resistance is a fundamental hemodynamic parameter that plays a crucial role in cardiovascular function. It consists of three major resistance components:

• R1: Epicardial vessels
• R2: Small arteries and arterioles (primary site of resistance adjustment)
• R3: Intramyocardial capillary system 1

The arterioles are particularly important as they can actively change their diameter through vasoconstriction and vasodilation, thereby regulating blood flow to tissues and organs throughout the body.

Mathematical Definition

PVR is mathematically defined by the formula:

SVR = (Mean Arterial Pressure - Right Atrial Pressure) / Cardiac Output

The units are typically expressed as dynes·sec·cm⁻⁵ 1

This relationship demonstrates that PVR is:

• Inversely related to cardiac output when other factors remain constant
• Directly related to blood pressure (MAP = CO × SVR + RAP) 1

Physiological Control Mechanisms

Several mechanisms regulate peripheral vascular resistance:

1. Myogenic tone: A hallmark feature of resistance vessels that maintains arteriolar and resistance artery internal diameter at 50-80% of their maximum passive diameter, allowing these vessels to dilate or constrict as needed 2

2. Neural control: Sympathetic nervous system activity can cause vasoconstriction, increasing PVR

3. Local metabolic factors: Tissues can regulate their own blood flow through release of vasodilators (e.g., nitric oxide, adenosine) during increased metabolic activity

4. Hormonal influences: Substances like angiotensin II, vasopressin, and endothelin cause vasoconstriction, while atrial natriuretic peptide promotes vasodilation

Clinical Significance

Peripheral vascular resistance has important clinical implications:

• Blood pressure regulation: PVR is a major determinant of arterial blood pressure 3

• Shock states: In septic shock, patients with fatal outcomes show less capability to augment vascular resistance when cardiac index decreases 4

• Heart failure management: PVR guides vasodilator therapy and helps differentiate types of shock 1

• Hypertension: Hypertensive patients show impaired vasodilator reserve in both coronary and peripheral microcirculation 5

• Diabetes: Young type 1 diabetic patients demonstrate reduced decrease of peripheral vascular resistance during exercise 6

• Orthostatic regulation: Baroreceptor-induced adjustments of vascular resistance help maintain cerebral perfusion when changing positions 3

Pathophysiological Considerations

In various disease states, alterations in PVR can contribute to clinical manifestations:

• In autonomic failure, cardiovascular sympathetic fibers are unable to increase total peripheral resistance in the upright posture, leading to orthostatic hypotension 3

• In peripheral arterial disease, the term "peripheral vascular disease" encompasses arterial diseases other than the aorta and coronaries, with altered structure and function affecting blood flow 3

• In microvascular disease, flow resistance in small vessels (e.g., 10-micron microvessels) can exceed that of corresponding glass tubes by a factor of approximately 4, highlighting the complex nature of resistance in vivo 7

Understanding peripheral vascular resistance is essential for comprehending cardiovascular physiology and pathophysiology, and for guiding therapeutic interventions in various clinical scenarios.