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Heparin was invented accidentally by a 26 year old  , Jay McLean, a  pre clinical  medical student  in 1916 .It was one of the greatest discovery  in  medicine .It helped us prevent blood from clotting.Frozen blood inside human circulatory system constituted one of important mechanisms  of  human  death.This ranged from acute myocardial infarction to cerebral thrombosis  .

heparin3

As we decoded the mechanism of action of heparin , it was clear it bound to the  naturally occurring molecule antithrombin 3 and effectively blocks the intrinsic coagulation mechanism and thus behaves as an important anticoagulation agent.

How heparin acts as a thrombolytic agent ?

We know , our hematological system has a powerful  natural  fibrinolytic mechanisms  to protect against unwarranted( pathological ) intravascular coagulation. This is mediated by  anti thrombin, protein C , protein S  ,  plasminogen  system etc  . Natural concentrations of tissue plasminogen activator (Tpa)  also  help in lysing intravascular clots.

There is a constant  , delicate balance between procoagulant , anticoagulant and antifibrinolytic molecules .Intra vascular  clots occur when a vascular  injury triggers  a clot formation and the clinical event occurs.

But,   once insulted ,   the  circulating blood   does not remain a silent spectator . It is  constantly  on the look out for a foe to attack the thrombus that is interfering  with its natural flow  . Antithrombin 3 is one such molecule. Success  of lysis depends on the power of natural forces. There are hundreds of episodes of microlysis that take place every day  (Which happen without our knowledge ) .In  patients with vascular  disease these episodes are likely to be further more.

What does  Intravenous heparin in high doses  do ?

Heparin immediately  blocks of powerful procaogualtion activity .One of the important heamatological principle  is “Thrombus begets thrombus “. It is  a vicious cycle. This is immediately  tackled by heparin .The powerful trigger of thrombus induced thrombus propogation is shut off .

This makes a  2 cm sized clot to remain  in  2cm . After  making sure of this , the blood in the immediate vicinity   start percolating the clot.  The heparinised blood   switches to  a pro- fibrinolytic mode as the balance of forces  is fully tilted in favor of fibrinolysis or thrombolysis.

Is there clinical evidence to call heparin as thrombolytic agent ?

Yes . Contrary to the popular scientific  principle we have only clinical evidence  . laboratory evidence is not convincing as heaprin lyses clot only in vivo . Since ,  evidnece based medicine requires  laboratory evidence  we hesitate to call this as  thrombolytic agent !

It has been a strong clinical observation ,   many  major intracardiac or  intravascular  clots  regress in size

(or totally dissolve )  with intensive heparin  regimen .The effect is seen in 48-72 hours.Some times in first 24 hours.

What are the clinical situations where heparin has successfully lysed the clots*?

  • Pulmonary embolism
  • LV clot
  • LA clot
  • Cortical venous thrombus
  • Deep vein thrombosis
  • Coronary thrombosis**
  • Portal vien thrombois
  • Renal vein thrombois

* Plenty of case reports available for each condition

** Sustained micro  thrombolysis  is the major mechanism of benefit in NSTEMI

If it is true ,  heparin dissolves thrombus , why  it is not called as thrombolytic agent ?

Why not ?  You decide yourself !

How does heparin compares with  the great thrombolytic agents*  like  Strepotiknase, Urokinase,Altepase, Retepalse , Teneckteplase (TNK TPA) ?

Many (Rather most . . .)  would consider it ,  as  foolish , to compare heparin with these agents .But the fact of the matter is except for streptokinase there is no comparison studies available. Attempting such a study  in humans will  be considered unethical. Without   a proper scientific  data  heparin  can not be ignored either.

But ,  some of the control groups in major  studies of thrombolysis  through some light !

In pulmonary embolism thrombolytic agents and heparin have similar effects on intrapulmonary thrombus

An important point to remember here is   , the powerful thrombolyic agents are administered  in as short duration (Bolus / 1  hour infusion ) .This is invariably  followed by heparin infusion . Why do we  do that ? because we know it is important . One may never know , how much of lysis is done  by the trhombolytic agent and how much by heparin .

if you analyse the  data  success rate of thrombolytic agents are infact attributable  to the follow up heparin

Thrombolytic agents  piggy packs on heparin and claims the  credit for thrombolysis *

In thrombolytic  therapy  , heparin  is considered  as an adjunct to streptokinsae but in reality  streptokinase  may an  adjunct to heparin

Importance of  heparin In Acute MI (HEAP Trial)

It should be realized  there is a time window for heparin too . . .  early administration  can have  great benefit

Early heparin prevents formation of  core  of the clot .The   importance of acute administration of  aspirin  in suspected STEMI  is well recognized  by paramedics  .  A bolus of heparin (10000 u)  immediately  could have great impact on the outcome as well  .Paradoxically we talk more  about emergency PCI,  on  transit TPA  etc . . . We have seen  number of patients  referred  with  STEMI   from   suburban areas traveling for hours with out any anticoagulants but promptly getting sorbitarate tablets ! Unfortunately prehospital heparin is rarely stressed in literature .

Watch the video : Heparin : The forgotten hero

Final message

  • Heparin is   an  under rated drug  as a thrombolytic agent.
  • Just because it has no direct action  on thrombus it is considered an inferior agent.( One other reason  for it to be  considered  inferior ,   it  is  very cheap  !)
  • Heparin too ,  has a time window effect in acute MI (Class 3 evidence ie   wide clinical experience)
  • It’s  usage should be early  and  liberal , especially  in out of hospital setting in vascular  emergency.
    Note of caution : This article is not meant  to  defame  the thrombolytic agents.It only stresses a point that , heparin has also a role , as a thrombolytic agent. *Whenever rapid thrombolysis is required in life threatening situations specific thrombolysis is indicated as per guidelines.

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LV clot formation is one of the important complications of acute myocardial infarction. Preventing this is difficult and managing this problem is still more difficult.Some of these clots are linear and laminar along the shape of LV apex and carry less risk of dislodging.

 While mobile LV clots , even if it is small can cause a embolic episode. Most of these patients have a significant LV dysfunction and they are candidates for early CAG and revascularisation. Even If the coronary anatomy is very ideal for a PCI these patients are often sent for CABG and physical removal of LV clot . If  only ,we have an option to remove these LV clots by a catheter based modality, we can offer them a totally non surgical cure.

This is not impossible,  considering  we are in the era of percutaneous implantation of prosthetic valve in Aorta ! The only issue is potential embolism into carotids and periphery .A temporary distal protection at the level of aortic root will prevent that .

Device companies shall produce one such exclusive catheter system to remove LV clot.

Dr .S.Venkatesan, Madras medical college, Chennai,India

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