The reversal of blood flow can be observed at color and spectral Doppler,
which is a hemodynamically significant signal of stenosis or occlusion in proximal arterial bed.
The arterial refilling occurs by collateral circulation,
and the antegrade flow to distal bed,
since the arrival of collateral artery and retrograde flow to proximal segments from collateral artery.
The degree of stenosis of the carotid and vertebral arteries is directly estimated based on color Doppler measurements of flow velocities in the vascular lumen.
However,
the signal which characterizes the establishment of collateral circulation involving the cervical region is the inversion of the direction of flow.
There is no speed criteria to discriminate if a flow is related to a collateral circulation.
The tardus-parvus pattern is associated with post-stenotic flow,
but not pathognomonic of collateral circulation.
In this paper we will present the main collateral circulation pathways that can be observed in the carotid-vertebral system studies using high frequency linear transducer (9-15 MHz).
The following collateral circulation pathways are known:
1-BRANCHES OF EXTERNAL CAROTID ARTERY: The external carotid artery runs ascending path after the bifurcation of the common carotid artery and has an average of eight branches that supply the face and neck structures: superior thyroid artery,
lingual artery,
facial artery,
occipital artery,
maxillary artery and the superficial temporal artery.
The branches of the external carotid artery establish collateral circulation especially with branches of thyroid-cervical trunk,
vertebral arteries and arteries of the brain circulation (not visible on ultrasound).
All of these arteries can participate in collateral circulation in cases of occlusion of the common carotid artery and the internal carotid artery.
Occurs refilling the external carotid artery through its branches,
especially:
- Reversing the direction of flow of the external carotid artery by its branches,
refilling the internal carotid artery by stenosis or occlusion of the common carotid artery.
2- BRANCHES OF INTERNAL CAROTID ARTERY: the ICA does not emit branches up into the skull,
where its branches supplying the brain.
These branches are hardly studied by ultrasound,
due to sound access,
except for the ophthalmic artery.
In case of occlusion of the proximal internal carotid artery,
ophthalmic artery inversion can submit its flow direction in an attempt to refill the distal internal carotid artery,
through the filler present angular artery,
temporal branch,
branch of the external carotid artery.
It may also occur reversal of the distal internal carotid artery through the circle of Willis to replenish the ophthalmic artery in occlusions of the proximal internal carotid artery,
but this phenomenon can not be observed with high frequency linear transducers.
3- BRANCHES OF SUBCLAVIAN ARTERY: The arteries of the thyroid-cervical trunk can refill,
retrograde,
external carotid arteries,
in attempting to establish collateral circulation to the system of internal carotid artery.
The vertebral arteries may also reverse its direction of flow,
to refill the subclavian artery or brachiocephalic trunk,
when there is a proximal occlusion or stenosis.
This condition is known as "subclavian steal",
and their progressive establishment determines changes in wave morphology,
according to the procedure.
ARTERIES MEASUREMENTS:
Can be observed by ultrasonography,
with high frequency linear transducer and present reversal of flow direction,
the following arteries:
1.
External Carotid Artery
2.
Facial Artery
3.
Temporal Artery
4.
Superior Thyroid Artery
5.
Inferior Thyroid Artery
6.
Maxillary Artery
7.
Vertebral Artery
8.
Ophthalmic Artery