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