Hemodynamic Factors: Steal/Breakthrough Bleeding

Pearls

Abnormal vessels in the iAVM nidus allow direct transmission of high-pressure flow into venous low-pressure vessels, thereby increasing bleeding risk.
Decreased resistance in abnormal iAVM vessels allows blood to flow into them more easily and “steal” blood flow from the surrounding brain, causing neurologic deficits.
Venous variations secondary to iAVMs—dilation, varices, or stenoses—lead to increased risk for bleeding.
Bleeding after successful iAVM resection may be due to abnormal blood flow regulation or decreased venous outflow.
The iAVM nidus results in altered blood flow on both the arterial and the venous side with resultant changes in vascular anatomy—dilation, aneurysms—increasing bleeding risk.

Introduction

Hemodynamics is the study of blood flow distribution and biophysical changes in the vascular network. Normally, blood under high pressure is transported to tissues from the heart through the arteries and carried back through the veins. The connection between arteries and veins is facilitated by capillary beds, which serve to mitigate arterial pressure and allow for adequate oxygenation and supply to tissues. This normal vascular architecture is also found in the brain, where capillary beds serve to oxygenate brain tissue, supply nutrients, and maintain appropriate perfusion pressure. Intracranial arteriovenous malformations (iAVMs) are abnormal arterial-to-venous connections that precipitate pathological changes in blood flow, perfusion, and vascular architecture. In this chapter, we will closely look at the known hemodynamic changes associated with iAVMs and their secondary effects on the brain.

Hemodynamic Principles

Overview

To understand what happens in AVM hemodynamics, it is essential to know some basic concepts about how the normal cardiovascular system works. The cardiovascular system consists of the heart and an extensive branched system of vessels—arteries, veins, and capillaries. Hemodynamics studies the distribution of pressures and flows in the cardiovascular system. The pressure is the force applied to an area, and flow is the action of moving along a stream. When we refer to blood pressure, we are talking about the pressure of the circulating blood against the blood vessel wall. This is generated by the pumping heart and helps move blood throughout the circulatory system. Blood flow refers to the velocity and the amount of blood passing at a given point along the vessel’s length. The circulatory system’s essential function is to maintain adequate blood flow distribution to the body’s tissues and organs. The distribution of blood flow can be controlled by the active contraction or dilation of blood vessels, a mechanism referred to as autoregulation. Together, these mechanisms regulate the distribution of blood flow across tissues, including the brain.

Types of flow

The movement of blood through a blood vessel is characterized by the principle of laminar flow. This type of flow is characterized by concentric layers of blood moving parallel to the vessel’s long axis. The highest velocity of blood is found at the center point of the vessel. With each concentric layer further removed from the center, the velocity decreases such that the velocity of blood moving along the vessel wall approaches zero (standstill).

Laminar flow is disrupted by turbulence across these imaginary concentric layers. Turbulent blood flow can occur where a vessel lumen becomes narrowed, at a bifurcation (fork), or where high flow or pressure is found. This type of movement has several consequences. It can promote platelet aggregation resulting in thrombus development and vessel obstruction. It can also increase shear stress, which is the force that the blood flow applies to the vessel wall’s inner layer, the endothelium. Shear stress impacts vessel wall composition and will be discussed in more detail later in the chapter.

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