Describe the regulation of cerebral blood flow? and what the factors which affect the intracranial pressure peri -operatively

Regulation of cerebral blood flow

Total cerebral blood flow in adults is 750ml/min (15 to 20% of cardiac output) 

Average around 50ml/100gm/min

Cerebral Perfusion Pressure 

CPP is the difference between Arterial Pressure and Intracranial Pressure or central venous pressure whichever is higher

CPP = MAP -ICP(CVP)

CPP is 80 -100 mm of Hg 

Increases in ICP over 30 mm of Hg can  compromise CPP with values below 25 mm of Hg can cause irreversible brain damage

Autoregulation

The cerebral vasculature is CPP dependent which in turn is regulated by MAP
Decrease in CBF causes vasodilation and increases in CBF causes vasoconstriction
CBF remains normal between 60 to 160 mm of Hg
The cerebral auto regulation curve is shifted to right in chronic hypertension
Both Myogenic and Metabolic mechanisms control cerebral auto regulation
Myogenic mechanisms involve the intrinsic smooth muscle in cerebral arterioles to changes in MAP
Cerebral metabolic demands determine the CBF when the tissue demands exceeds the blood flow then there s release of tissue metabolites which cause vasodilatation.
EXTRINSIC MECHANISMS
Respiratory Gas Tensions
The significant extrinsic factor is partial pressure of  Co2
⇧PaCO2 ⟶⇧CBF
Temperature
Changes of 5-7% in CPP per 1 degree centigrade change in temperature
Hypothermia decreases both CMR/CBF
Hyperthermia increases CMR/CBF
Viscosity 
Changes in blood viscosity doesnt cause much changes in CBF
The most imp determinant of blood viscosity is haematocrit
⇧Haematocrit  ⟶⇩viscosity ⟶⇧CBF
           ↓
⇩Oxygen Carrying capacity
Autonomic Influences
Intra cranial vessels are innervated by sympathetic ,parasympathetic and non cholinergic non adrenergic fibres
⇧Sympathetic stimulation ⟶⇧ Vasoconstriction ⟶⇩CBF
Autonomic innervation plays an imp role in cerebral vasospasm
Perioperative Factors Affecting Intracranial Pressure

Point of Care Coagulation Tests and perioperative implications

Point of care coagulation are the tests performed near the patient which are non lab tests with rapid results reducing the turnaround time,  requires minimal amount of whole blood and reduces inappropriate blood transfusion.

POC tests in the peri operative period can be broadly classified as

• Functional assay to monitor heparin anticoagulation 
• viscoelastic measures of coagulation
• platelet function monitoring
• clotting factor tests

Functional assays to monitor heparin Anticoagulation

Activated Clotting Time         

     The ACT is used to measure heparin therapy as in cardiac surgery, hemofiltration, extracorporeal     oxygenation etc with normal range between 90-150 secs and values of 480 secs are considered safe in  setting of CPB

Advantage  low cost and linear response with high heparin conc.

Disadvantages lack of sensitivity at low heparin conc
false prolongation at hypothermia, coagulation factor deficiency, warfarin
 
 2.   High Dose Thrombin Time

Viscoelastic measures of coagulation

These tests measure the whole spectrum clot formation and the coagulation parameters are assessed in real time on whole blood
Devices based on viscoelastic principles are

1) Thromboelastography

 2) Rotational thromboelastography

  3) Sonoclot

Thromboelastography
Analyses and graphically displays the changes in viscoelastography across all stages of clot formation and resolution
TEG is more sensitive to fibrinolysis
The shape of the TEG helps in assessment of different coagulation states
                             
                                     

R/CT indicate conc of soluble clotting factors in plasma / K-time indicates clot kinetics / alpha angle  indicate rapidity of fibrin build up/ MA indicate number and function of platelets and fibrinogen conc./CL30 and 60 indicate the clot stability and fibrinogen conc

• Viscoelastic test are rapid and overall coagulation status of patient is Assessed with derived parameters helping in administration of blood and blood products
• TEG analyses all stages of coagulation like initiation, amplification and propagation reflecting the interaction of plasma and cellular components of coagulation and fibrinolysis
• TEG and ROTEM based transfusion algorithms reduce the rates of transfusion of blood and blood components 
• viscoelastic measures have been shown of significance in hyper coagulable states in post operative period
• TEG cartridges are extremely sensitive to residual heparin which may benefit in inadequate heparin reversal

PLATELET FUNCTION TESTS

Whole blood point of care tests of platelet function are significant to assess the effectiveness of anti platelet drugs and the recovery of platelet function when they are stopped in the setting of widespread usage of such drugs
1) PFA 100  measures the adhesion and aggregation of platelets under high shear stress
the CT is 5-8 secs which is prolonged in Von Willebrandt Disease both (Epi CT and ADP CT)
 while only Epi CT is prolonged in Aspirin therapy but ADP CT remains normal
2) Platelet works cheap rapid assessment of all classes of antiplatelet  agents
3) Verify Now
4) TEG Platelet mapping

COAGULATION TESTS

Point of care coagulation tests of coagulation were useful in patients taking Coumarin Derivatives are Prothrombin Time(PT), International Normalised Ratio(INR), aPTT

Choosing the correct point of care coagulation test

the selection of POCT devices should be tailored to clinical situation like in hyperfibrinolysis and plasma coagulation defects in liver Transplant,Trauma

Limitations of POCT

• Differences with Lab based tests
• Experience of operator
• Regent sensitivity
• Thorough familiarity with device functioning.