Hemostasis

Dr. Scariano -  Hemostasis




Secondary Hemostasis
• Goal: generation of massive amounts of fibrin by thrombin
• Activation of (serine protease) zymogens (inactive enzymes) by injury
– Damaged endothelium
– Activated platelets 
– Tissue Factor
• “Thrombin burst” and fibrin generation
• Simultaneous activation of natural anticoagulant and fibrinolytic systems
• Players must assemble and be oriented on a phospholipid membrane
– Importance of vitamin K in post-translational modification of factors 2, 7, 9 & 10 (procoagulent factors)
• Restriction to injured areas

• everything in blood has negative charge (repels other components)
• thrombin circulates in inactive form - activated by factor 10

Fibrinogen: plasma protein synthesized in liver (nl 200-400 mg/dL)

• Soluble fibrinogen is converted to insoluble fibrin by thrombin, an active serine protease 

• The fibrin monomer assembly is covalently cross-linked to form a mature blood clot (13a)

• Prothrombin zymogen circulates in an inactive conformation

• Factor 10a converts prothrombin to its active form

• Factor 10 zymogen circulates in an inactive conformation

• How is factor 10 activated?

Initiation of the clotting cascade

Tissue factor (TF): integral membrane protein cofactor normally expressed on most extravascular cells

• During injury, circulating factor 7 (zymogen and pre-activated form) bind TF

• Factors 7 and 10: reciprocal activation amplification loop

• When factor 7a activity reaches a threshold level, it converts factor 9 zymogen to 9a

Sustained amplification of the clotting cascade

Factor 9a: 50x more catalytically efficient than factor 7a in generating factor 10a 

– Dependency on cofactor (VIIIa)

• 10a converts prothrombin to thrombin – Dependency on cofactor (5a)

• Thrombin cleaves fibrinopeptides from fibrinogen

• Factor 13a transglutaminase crosslinks fibrin monomers

PATHWAYS - 2 OR 1????   MOST SAY ONE.

Vitamin K dependence

• Vutamin K dependent : factors 2, 7, 9, 10, Protein C and Protein S 

• Cofactor for gamma carboxylation of critical glutamate residues

• Allows calcium dependent bridging of factors to phospholipid membranes

• Undercarboxylation results in their inactivity (reduced binding to PL)

• Active form of vitamin K must be regenerated (VKORs)

• Warfarin (Coumadin) inhibits regeneration of the active reduced form of vitamin K and quantitatively reduces K-dependent factors(7a = shortest half-life)

Clotting time tests

1) The prothrombin time (PT) is a common 

clotting time test that uses 

supraphysiological amounts of tissue factor 

(thromboplastin + calcium) as an activator 

of the “extrinsic pathway” , bypassing the 

need for factor 9a.

• The INR is calculated from the prothrombin 

time test: INR = [patient PT/controlPT]

ISI

• Measures the integrity and activity of the 

following factors: 7, 10, 5, prothrombin (2) 

and fibrinogen (1)

• INR is normally 0.8 – 1.2

• Used as a general coagulation screen and 

also to monitor warfarin therapy 

(INR goal 2-4)

• Bleeding risk with elevated INR

2) The activated partial thromboplastin time (aPTT) is another common clotting time test that uses an anionic polymer as an activator of the “intrinsic pathway” 

• The measures the integrity and activity of the following factors: 12, 11, 9, 8, 10, 5, 2 

and 1

• aPTT is normally 25-39 seconds (depends on reagents)

• Used as a general coagulation screen and also as a crude measure of heparin therapy

• Newer heparin activity assay is available

• Bleeding risk with elevated aPTT

Naturally circulating anticoagulants

• On a molar basis, the inhibitors of the clotting cascade are in excess compared with the active procoagulant proteases. Why is this so?

• Important players:

– Antithrombin

– Activated Protein C and Protein S

– Tissue Factor Pathway Inhibitor (TFPI)

– PGI2: (Prostacyclin): Platelet antagonist

– Α-1-antitrypsin, α-2-macroglublin, etc.

• Deficiencies of naturally circulating anticoagulants tip balance toward clotting

FIBRINOLYSIS

• Local Augmentation

• Systemic inhibition

• Fibrin stimulates release of tPA from endothelial cells

• Fibrin is a cofactor for plasminogen activation by tPA

• α-2-antiplasmin & PAI-1 are not effective inhibitors of thrombus-bound plasmin and plasminogen activator

• α-2-antiplasmin & PAI-1 are effective inhibitors of plasmin and plasminogen activator in circulation

D-Dimer

• Specific byproduct of mature crosslinked fibrin degradation

• Quantitative levels available in most labs

• Normal levels < 0.5 μg/mL

• Longer circulating half life than other FSPs

• Increased in patients with acute thromboembolism

• Increased in malignancy and infections

• Sensitive but nonspecific test for thrombi

• Reliable negative predictive value

– if d-dimer is < 0.5 μg/dL, clot (i.e., PE, DVT) is unlikely 

Dr. Ahmed  --Myocardial Oxygen Supply/Demand

simple calculus - supply vs demand

LAD and Circumflex perfuse 80% of the heart


Coronary artery anatomy
 Left main arises from aorta distal to the left coronary cusp of aortic valve
 Bifurcates into left anterior descending (LAD) and circumflex (LCX) arteries
 Can also trifurcate to give off Ramus Intermedius
 LAD runs down anterior portion of the heart and gives off septal (S) and diagonal branches (D)
 LCX runs down lateral and posterior parts of the LV and gives off obtuse marginals (OM) branches



Right coronary artery (RCA)
 Comes off the right coronary cusp of the aorta
 Will first give rise to SA node artery to supply SA Node
 As it courses in right AV groove, gives off acute marginal (RV) branches to RV
 Continues posteriorly to give off posterior left ventricular branches (PLV) and posterior descending artery (PDA) in most individuals (70-85%)


Coronary Dominance
 Determined by which artery supplies the posterior descending artery 
– 85% RCA (right dominant)
– 7% Circumflex (left dominant)
– 8% both (co-dominant)

Blood flow x arterial O2 = O2 delivery

Flow most important  (occurs mainly during diastole)

• Q = P/R
• R related to r^4

Flow Control

•  Most potent stimulus is myocardial hypoxia
•  Works through release of mediators such as adenosine and nitric oxide, as well as activation of the ATP-sensitive K+ channels.
• Other Determinants and Factors influencing flow:
• Vasodilation
• Neural Input
•  Alpha stimulation leads to vasoconstriction
•  Beta and vagal stimulation lead to vasodilatation
• Autoregulation
•  Ability to regulate constant flow at varying coronary pressures under basal conditions
•  Multiple factors such as paracrine factors, neurohormonal agonists, neural tone and shear stress modulate coronary vasomotor tone autoregulate coronary flow.

Oxygen Content


Arterial oxygen content (CaO2) (ml/O2/dl) = (Hgb x 1.36 x SaO2 

) + (0.0031 x 

PaO2

)

SaO2 = % of hemoglobin saturated with oxygen
(Normal range: 93-100%)
Hgb = hemoglobin
Normal range(Adults): Male: 13-18 g/dl Female: 12-16 g/dl
PaO2= Arterial oxygen partial pressure 

(Normal range: 80-100 mm hg)

Concepts:
 CaO2

: Directly reflects the total number of oxygen molecules in 

arterial blood (both bound and unbound to hemoglobin)

 Supply can be reduced by anemia, carbon monoxide poisoning, 

hypoxia

 Oxygen extraction is the difference between what comes in and 

what goes out (i.e. CaO2 –CvO2)

Oxygen Demand

1. Wall Tension

Related to intracavitary pressure and volume by the 

Law of Laplace

 Law of Laplace: 

– Wall tension = pressure X radius/

wall thickness

 Increased cavitary pressure and increased
ventricular size both increase wall tension, while 

increased thickness decreases wall tension.

Concept: Law of Laplace determines wall tension and is directly related
to intra-cavitary pressure and size and inversely related to wall
thickness.

dioesn't talk about overall mass - wall thicker = more myocytes consuming oxygen
2. Contractility

• shifts of Starling Curves
  
  

3. Heart rate

Supply Demand Problems

Supply problems

– Nonocclusive thrombus on preexisting plaque

– Dynamic obstruction (spasm)

– Progressive mechanical 

obstruction

– Inflammation and/or infection

- Acute coronary syndrome

Demand problems

– Secondary angina

 Fever, tachycardia, anemia

Medications - Acute Coronary Syndrome = supply problem

"MONA"

• morphine
• oxygen
• nitroglycerine
• aspirin - morbity & mortality decreases

 Increase supply

– Nitrates – coronary vasodilators

– Calcium channel blockers - vasodilators

 Decrease demand

– Beta blockers – decrease HR, 

contractility, BP

– Calcium channel blockers – decrease HR, 

contractility, BP (limited role)

Dr. Spaulding 

- Hypertension

Dr. Valenzuela - Pharmacology of RAAS

ACE = endothelial enzyme - abundant in lungs
ACE inhibitors

• vasodilator effects

• cardiac benefits = decrease in remodelling
• Indications
• Congestive heart failure (CHF)
• Hypertension monotherapy (young caucasians)
• Hypertension associated with CHF or diabetes
• Cardioprotective after acute myocardial infarction - block remodel pathways
• Delay the progression of kidney disease, including diabetic nephropathy
• side effects

• cough
• angioedema
• hyperkalemia
• hypopolarization
• faster repolarization - peaked T wave
• slows upstroke - wide QRS
• bradycardia - inc K conductance = keeps sa node cells polarized = low slope of phase 4.
• renal failure in pts. with renal artery stenosis - AT2 constricts efferent arteriole

• Drug Interactions
• NSAIDS

• Diuretics
• Thiazides - cause K loss= good interaction since ACE inhibitors can cause hyperkalemia
• e.g., lisinopril/hydrochlorothiazide for HTN

Angiotensin 2 receptor blockers (ARBs)

Aldosterone Blockers

Spironolactone, Eplerenone

Antagonize effect of high aldosterone levels in heart failure, renal disease, 

and post-myocardial infarction:

1. Antagonize fibrotic and inflammatory effects of aldosterone

2. Prevent aldosterone escape (or aldosterone breakthrough) phenomenon:

• ACE inhibitors and ARBs initially cause a decrease aldosterone levels but 

these then gradually increase in some patients, decreasing effectiveness 

of these medications

• Incidence can be as high as 10-50% over 12 months

• Should test for it in refractory cases (i.e., measure aldosterone levels)

• 

Mechanisms: non-ACE enzymes can cleave angiotensin-I into 

angiotensin II? Other factors can increase aldosterone (corticotropin, 

vasopressin) 

Toxicity:

1. Hyperkalemia in patients with renal disease or

those taking ACE inhibitors, angiotensin II receptor 

antagonists or b-blockers

2. Gynecomastia, impotence and menstrual abnormalities (not 

reported with eplerenone). (structure similar to cholesterol-inhibits testosterone production; increases estrogen)