INTRODUCTION
The carotid arteries are two large blood vessels that supply oxygenated blood to the large, front part of the brain.
Carotid artery disease is also called carotid artery stenosis. The term refers to the narrowing of the carotid arteries. This narrowing is usually caused by the buildup of fatty substances and cholesterol deposits, called plaque.
Carotid artery occlusion refers to complete blockage of the artery. A stroke occurs when the blood flow to the brain is interrupted resulting in a loss of nutrients and oxygen. If the blood flow is not restored the brain cells die and there will be permanent brain damage.
Stroke is a major cause of disability and death. Carotid stenosis is one of the many causes of stroke.
EPIDEMIOLOGY
Each year over 2 million new strokes occur in America and the EU, making stroke the third leading cause of death and a principal cause of long-term disability in much of the industrialized world.
Worldwide, 15 million people annually suffer a stroke and of these, 5 million die (equivalent to 10% of worldwide deaths) and another 5 million are left permanently disabled.
Atherosclerotic disease accounts for approximately 25% of ischaemic strokes 8,9% caused mainly by embolic events from carotid artery bifurcation or the aortic arch.
SYMPTOMS
The exact symptoms depend on the area of the brain affected, although many patients are asymptomatic and the stenosis is discovered on clinical exam. Patients may present with a transient ischemic attack.
This cerebrovascular event lasts seconds to hours, but less than 24 hours. This temporary blockage of the blood vessel may cause momentary loss of vision in one eye, weakness/numbness of one side of the body, slurred speech or an inability to get words out.
A TIA is often a warning of an impending cerebrovascular accident (CVA) or stroke. A stroke occurs when the blood supply is interrupted for a longer time, starving cells in the brain of oxygen and causing cell death. This results in a permanent neurological deficit that can severely affect the function associated with the area of the brain that is damaged. Approximately 80% of strokes are ischaemic, i.e. a thrombus (clot) or embolus blocks the blood supply, and 20% are haemorrhagic, caused by the rupture of a blood vessel.
EARLY DIAGNOSIS AND TREATMENT
Detection of a carotid stenosis in patients with peripheral vascular disease will increase the chance of early intervention to reduce the risk of stroke.
Techniques used to diagnose Carotid Artery Disease include a medical history, physical examination, Doppler Ultrasound and Color Doppler.
PHYSICAL PRINCIPLES OF THE DOPPLER SHIFT
Doppler shift is based on the principle that when sound wave are reflected off a moving object, there is a change in the frequency that returns which is dependent on:
- the velocity (speed and direction) of the moving object
- the initial frequency of the sound waves
- the angle at which the waves hit the moving object.
In the case of echocardiography, the ultrasound probe emits a particular frequency, and the moving object is represented by the red blood cells. Reflected ultrasound waves from red blood cells return to the probe with a Doppler shift that is translated by a computer into a velocity.
These elements are represented in the Doppler equation shown below:
where:
- V is the velocity of the moving blood
- c is the speed of ultrasound sound in the body, which is known
- Ft is the ultrasound frequency the transducer emits
- Fr is the backscattered frequency that returns to the transducer
- θ is the "insonification angle" or the angle between the ultrasound beam and the direction of blood flow.
Therefore in ultrasound, the Doppler effect is used to measure blood flow velocity. The reflector in this case is the red blood cell. Ultrasound reflected from red blood cells will change in frequency according to the blood flow velocity. When the direction of blood flow is towards the Doppler transducer, the echoes from blood reflected back to the transducer will have a higher frequency than the one emitted from the transducer. When the direction is away from the transducer, the echoes will have a lower frequency than those emitted. The difference in frequency between transmitted and received echoes is called the Doppler frequency shift, and this shift in frequency is proportional to the blood flow velocity.
In vascular diagnostic, ultrasound used vary between 2 and 10 MHz, and are harmless to biological tissues.
CLINICAL USE OF THE DOPPLER EFFECT
Blood flow through the heart and great vessels has certain characteristics that can be measured using Doppler instruments designed for medical use.
Laminar flow is flow that occurs along smooth parallel lines in a vessel so that all the red cells in an area are moving at approximately the same speed and in the same direction.
In contrast, turbulent or disturbed flow is present when there is some obstruction that results in a disruption of the normal laminar pattern. This causes the orderly movement of red blood cells to become disorganized and produces various whirls and eddies of differing velocities and directions. Obstruction to flow usually also results in some increase in velocity. Thus, turbulent flow is characterized by disordered directions of flow in combination with many different red cell velocities. If the obstruction is significant, some of the red blood cells may be moving at higher velocities than normal and may reach speeds of 7 m/sec. Turbulent flow is usually an abnormal finding and is considered indicative of some underlying cardiovascular pathology.
In vessel patency with normal blood, flow is laminar. The laminar flow becomes turbulent when the flow velocity of the liquid exceeds a certain value, called the critical speed.
It is defined as follows:
NR = Reynold number (dimensionless)
r = radius of the conduit
v = average speed of flowing
ρ = density of the liquid
η = viscosity of the liquid
If we consider a cylindrical conduit, there is laminar flow for a value of NR < 1000, while the turbulent motion becomes increasingly likely for values of NR > 1000, to generally always observable values of NR > 1500. The critical speed is the one that gives a Reynolds number ≈ 1200.
Duplex Doppler ultrasound uses standard ultrasound methods to produce a picture of a blood vessel and the surrounding organs. Also, a computer converts the Doppler sounds into a graph that gives information about the speed and direction of blood flow through the blood vessel being evaluated.
Graphical representation of the Doppler signal with the acceleration of the flow during each heartbeat.
Mild stenosis (left) and severe (right figure) determine changes in different speeds and flow profile, which can be used to quantify the degree of stenosis itself.
CAROTID COLOR DOPPLER
The blood flow through these arteries is evaluated with Doppler and Color Doppler.
Color Doppler involves the use of standard ultrasound methods to produce a picture of a blood vessel. In addition, a computer converts the Doppler sounds into colors (typically red and blue) that are overlaid on the image of the blood vessel. These colors represent the speed and direction of blood flow through the vessel. Color Doppler assigns color values which depend on whether blood is moving towards or away from the transducer. In addition to showing the direction of flow, the colors also vary in intensity depending on the velocity of the flow, allowing people to see how quickly the blood is moving.
In color flow imaging, the colors red and blue represent direction of a given jet; when turbulence is present, a mosaic of many colors results.
Red is flow toward, and blue is flow away from the transducer.
The above image above shows the region of highest velocity, which corresponds to the narrowest segment of the ICA.
Picture of a narrowing of the carotid artery in the neck. The blood flow is represented in color. In the middle you recognize an atherosclerotic plaque (arrow), which reduces the flow and being calcified due to signal attenuation (shadow under the plate).
CLINICAL EVALUATIONS
The intima-media thickness of the extracranial carotid arteries is a measurable index of the presence of atherosclerosis. The intima-media thickness of the CCA is thought to be associated with risk factors for stroke. The bifurcation intima-media thickness and the presence of plaque are more directly associated with risk factors for ischemic heart disease. Intima-media thickness measurements must be obtained from a gray-scale image, not from a color Doppler image. Only the intima and the media are included in the measurement. Increased intima-media thickness has also been reported as a physiologic effect of aging. An intima-media thickness of less than 1 mm is normal.
Procedure: The room is usually darkened for the exam. A gel is applied to the neck area to provide good contact for the handheld transducer. The transducer is placed on the neck and sound is sent into the body and is reflected off arteries and returned to the transducer. The echoes are converted electronically into images of the arteries that can be seen on a monitor. These images are recorded on paper or film. With Doppler and Color Doppler the sound waves reflected from the blood cells are converted to audible sounds that can be heard during the exam. Color is used to represent the blood flow in the artery and the speed and direction of the flow are assessed by the physician. This procedure takes approximately 45 minutes.
STENOSIS
Calculation of the ratio of the peak systolic velocity (Peak Systolic Velocity, PSV).
We evaluate the PSV in the narrowing of the vessel (right) and in an area near normal (left). The relationship is given by the value of PSV in the area of narrowing divided by that in the normal zone.
The Peak Systolic Velocity from an ultrasound is used to determine the % of a carotid artery blockage. Higher PSV means more blockages.
The highest PSV in the diseased ICA will be seen at the point of tightest stenosis.
A stenosis produces no reduction of flow rate (capacity) if it does not exceed 90%, in this case is defined as "hemodynamically significant". However, it should also be considered significant when a stenosis > 70% (with a lumen diameter reduced by at least 50%).
Classification of stenosis according to hemodynamic repercussions:
- Lower: 0-60%
- Moderate: 61 - 80%
- Severe: 81 - 95%
- Preocclusive: 96 - 98%
- Occlusive: 99-100%
In the following image, the blue areas represent areas where the flow is reversed due to the abrupt expansion of the carotid bulb.
In the picture above a narrowing of the common carotid artery, caused by a circumferential plaque, is shown in the prebulbar segment.
REFERENCES
| Books |
|---|
| Luigi F. Agnati, Cardiovascular physiology |
