Coronary arterial obstruction is considered, dangerous because it obstructs the coronary blood flow . Is it possible , for an obstruction to have little impact on the blood flow ? Fortunately ,”yes” , the physics of fluid dynamics is patient friendly .It is well known , coronary blood flow goes on smoothly, uninterrupted until very late stages of obstruction .* This has created the concept of flow limiting lesions and non flow limiting lesion .
The most popular form of reporting coronary blood flow across a stenosis is TIMI grading. Originally used following thrombolysis , now universally used for all angiogram (Is it appropriate ?)
TIMI Grading
Grade 0 (No perfusion): There is no antegrade flow beyond the point of occlusion.
Grade 1 (Penetration without perfusion): The contrast material passes beyond the area of obstruction but “hangs up” and fails to opacify the entire coronary bed distal to the obstruction.
Grade 2 (Partial perfusion): The contrast material passes across the obstruction and opacifies the coronary bed distal to the obstruction. However, the rate of entry of contrast material into the vessel distal to the obstruction or its rate of clearance from the distal bed (or both) is perceptibly slow.
Grade 3 (Complete perfusion): Antegrade flow and clearance of the dye distal to the obstruction occurs as promptly as antegrade flow .
When does a coronary blood flow gets impeded following obstruction ?
Contrary to the popular belief , the distal blood flow in a coronary artery is less dependent on the degree obstruction than the status of the distal microvasculature.Classical teaching tells us if a coronary artery narrows >70% diameter stenosis (90%area) the blood flow gets impeded on exertion . For resting blood flow to get blocked it needs still further narrowing .
These rules are written in the era before we knew the concept of coronary vascular reserve . We , have since understood ( or confused !) more about coronary microcirculation.The major misconception could be what we interpret as epicardial blood flow is actually reflect the status of coronary micro vascular integrity.
How else you explain a patient with a same degree of coronary obstruction has vastly different distal blood flow profile ! There are innumerable examples of patients with 50% obstruction having TIMI one flow and a 99% obstruction with TIMI 3 flow !
Have a look at this angiogram,
Click here or on the image to view the video
What are the factors other than the degree obstruction that determine the distal flow?
- Acuteness of obstruction.
- Status of coronary microvascular bed
- Interstitial (Myocardial) edema
- Coronary microvascular reactivity
- Recruitment of collaterals
- Coronary perfusion pressure (Aortic diastolic pressure – RVEDP/Coronary sinus pressure)
Final message
The most important determinant of blood flow distal to obstruction is the vascular reactivity , tone and the integrity microvascular reserve .This rule applies both in in acute and chronic coronary syndrome . In CTOs it may not apply as distal flow is near zero. Here also the inherent intraluminal resistance of collapsed distal vessel will determine the distal flow.
Note:TIMI 3 flow ,does not represent a homogeneous class of coronary blood flow .They can have variable myocarial blush and frame counts.It may need further analysis.
Coronary arterial obstruction is considered, dangerous because it obstructs the coronary blood flow . Is it possible , for an obstruction to have little impact on the blood flow ? Fortunately ,”yes” , the physics of fluid dynamics is patient friendly .It is well known , coronary blood flow goes on smoothly, uninterrupted until very late stages of obstruction .* This has created the concept of flow limiting lesions and non flow limiting lesion .
The most popular form of reporting coronary blood flow across a stenosis is TIMI grading. Originally used following thrombolysis , now universally used for all angiogram (Is it appropriate ?)
TIMI Grading
Grade 0 (No perfusion): There is no antegrade flow beyond the point of occlusion.
Grade 1 (Penetration without perfusion): The contrast material passes beyond the area of obstruction but “hangs up” and fails to opacify the entire coronary bed distal to the obstruction.
Grade 2 (Partial perfusion): The contrast material passes across the obstruction and opacifies the coronary bed distal to the obstruction. However, the rate of entry of contrast material into the vessel distal to the obstruction or its rate of clearance from the distal bed (or both) is perceptibly slow.
Grade 3 (Complete perfusion): Antegrade flow and clearance of the dye distal to the obstruction occurs as promptly as antegrade flow .
When does a coronary blood flow gets impeded following obstruction ?
Contrary to the popular belief , the distal blood flow in a coronary artery is less dependent on the degree obstruction than the status of the distal microvasculature.Classical teaching tells us if a coronary artery narrows >70% diameter stenosis (90%area) the blood flow gets impeded on exertion . For resting blood flow to get blocked it needs still further narrowing .
These rules are written in the era before we knew the concept of coronary vascular reserve . We , have since understood ( or confused !) more about coronary microcirculation.The major misconception could be what we interpret as epicardial blood flow is actually reflect the status of coronary micro vascular integrity.
How else you explain a patient with a same degree of coronary obstruction has vastly different distal blood flow profile ! There are innumerable examples of patients with 50% obstruction having TIMI one flow and a 99% obstruction with TIMI 3 flow !
Have a look at this angiogram,
Click here or on the image to view the video
What are the factors other than the degree obstruction that determine the distal flow?
Final message
The most important determinant of blood flow distal to obstruction is the vascular reactivity , tone and the integrity microvascular reserve .This rule applies both in in acute and chronic coronary syndrome . In CTOs it may not apply as distal flow is near zero. Here also the inherent intraluminal resistance of collapsed distal vessel will determine the distal flow.
Note:TIMI 3 flow ,does not represent a homogeneous class of coronary blood flow .They can have variable myocarial blush and frame counts.It may need further analysis.