Dr.S.Venkatesan MD

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  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.