Clinical and practical knowledge of Intracranial Pressure (ICP) & Cerebral Perfusion Pressure (CPP)

 Quincke in 1891 first reported the measurement of intracranial pressure through lumbar route.

Quickenstedt  established the range of normal ICP and demonstrated the effect of changes in body position and respiration .
Lundberg, in 1960, described the 3 ICP waveforms.
Cranium is like a rigid bony sphere with a constant intracranial volume and it contains three components

1. Brain    1400 mL

2. CSF       150 mL

3. Blood    150 mL

Therefore, any change in the volume of the brain causes a reciprocal change in the volume of other intracranial components,i.e., either blood or CSF.  This is the basis of Monro-Kellie hypothesis introduced in neurosurgery by Cushing.

There is a relationship between  intracarnial volume and intracranial pressure.  Because cranium  is  a rigid and non-distensible structure, any increase in the volume of a component would be accompanied by a reciprocal decrease in the volume of the other two components. Once the volume buffering capacity is exhausted, the ICP would begin to rise.

During gradual expansion of a mass lesion, the volume displaced may be CSF, intravascular blood or brain tissue water. Of the three components, CSF appears to be the main buffer and is the first to be displaced as evident  by compressed ventricles  and obliterarted subarachnoid spaces.

The rate of expansion of an intracranial mass is also important. A rapidly growing intracranial mass lesion may outpace the compensatory shift of CSF and even the smallest increase in mass could produce a life threatening increase in ICP. Thus, a large hematoma could be accommodated within a few hours without dangerous rise in ICP.

Intracranial hypertension can lead to secondary changes by interfering with the cerebral blood flow ( CBF). The normal  cerebral blood flow ( CBF) is about 50 mL/100 g/min.

Cerebral Perfusion Pressure ( CPP) is defined as the difference between mean arterial pressure

( MAP) and intranial pressure (ICP).

CPP= MAP-ICP

Normal range of ICP in an adult is less than 10-15 mmHg.

Cerebral perfusion pressure is normal till the autoregulation mechanism of brain is intact. But there is a range upto which level body is able to maintain CPP.  Between 60 to 160 mmHg of mean arterial pressure brain will be able to receive blood with normal perfusion. But, if MAP falls blow 50 mmHg, features of cerebral ischemia will appear.
Mean Arterial Pressure ( MAP)= Diastolic Pressure+1/3rd of Pulse Pressure 
Pulse pressure= Systolic blood pressure - Diastolic pressure
So, in a normal person MAP = 80 mmHg+ (120mmHg-80mmHg)/3
MAP= 80+40/3
So on average, roughly MAP is about 90-95.

A rise in ICP would lead to a fall in CPP unless buffered by a compensatory rise in blood pressure ( Cushing response). Raised ICP can cause hypertension, bradycardia and respiratory changes. Therefore any patient who is suspected as a case of intracranial space occupying lesion ( ICSOL), like brain tumor or hematoma or granuloma or abscess and complaining of headache, vomiting, blurring of vision then blood pressure and pulse rate should always be monitored. In clinical setting bradycardia is a reliable indicator of rise in ICP in a patient who was otherwise allright sometimes back. Bradycardia is a sign of raised ICP and can precede and ppears before deterioration of conscious level ( Drowsinees, disorientation or poor Glasgow Coma Scale) and papillary asymmetry.

Lundberg described three pressure waves namely A waves, B waves and  C waves .

A waves
A waves are pathological  and indicate rapid rise in ICP  for variable period and then rapid fall to the baseline.
The A waves that persist for longer periods( usually 5-20 minutes) are called plateau waves.
Smaller A waves termed “ atypical” or “ truncated” A waves , that often do not exceed an elevation of 50 mm Hg, are also clinically important early indicators of neurological deterioration.

The A waves are accompanied by clinical features  of raised ICP, like headache, vomiting, decerebrate posturing, papillary changes, bradycardia and hypertension and respond to CSF drainage, hyperventilation and osmotic diuretics.

B waves

Occur at the rate of 0.2-2 per minute and are related to respiration.

B waves may be vasomotor in origin. Lundberg initially described them in patients with intracranial hypertension, though they can occur in normal individuals.
B waves are said to be one of the best predictors of outcome after surgery for normal pressure hydrocephalus.

C waves

C waves are low amplitude with afrequency of 4-8 per minute. These waves are thought to be related to Traube-Hering- Mayer waves.
C waves are of little clinical significance.

There is pressure equilibrium in the skull  but if pressure rises then a part of brain herniates. The herniations are subfalcine, tentorial, and tonsillar. In subfalcine herniation, a part of the frontal lobe herniates below the falx to the opposite side . In tentorial herniation ( Uncal herniation) a part of the medial temporal lobe herniated below through an opening in the tent and compresses over the midbrain. In tonsillar herniation, a part of cerebellum,i.e, Cerebellar Tonsil herniates down through the Foramen Magnum and compresses the medulla oblongata ( Coning). Brain Herniation is life threatening as it causes  brain stem compression which contains vasomotor center. Patient presents with drowsiness, deceerbrate posturing, papillary asymmetry, bradycardia, hypertension and respiratory irregularities.

Increased ICP is indicated by a sustained elevation in pressure above 15 mmHg or when intermittent A or B waves are recorded.

The normal CSF pressure measured through the lumbar route ranges from 50 to 200 mm H₂O in the lateral decubitus position.

ICP and CPP monitoring are important in the management of head injury patients, especially in whom the decision to operate is equivocal. Surgery may be required if ICP is progressively rising and not responding to conservative treatment with cerebral decongestants. ICP monitoring may also be required in patients of spontaneous subarachnoid hemorrhage (SAH) to assess the effect of cerebral vasospasm and in patients of arrested hydrocephalus and  normal pressure hydrocephalus to take decision about CSF diversion procedure.

Various methods of monitoring the ICP

1.       Intraventricular catheters like External Ventricular Drainage ( EVD): Most accurate, lower cost, also allows therapeutic drainage of CSF

2.       Intraparenchymal catheters (eg. Camino labsor Honeywell/Phillips)

3.       Epidural catheters ( e.g. Fibreoptic tipped catheter: Ladd fireoptic)

4.       Subarachnoid bolt (screw)

5.       Subdural ( eg. Cordis Cup catheter)

Monitoring Systems can broadly be divided into Fluid coupled system and Non-fluid cupled system

In fluid-coupled system a fluid filled catheter or a hollow bolt placed in the ventricle, subarachnoid space or the subdural space connected to a pressure transducer through a fluid-filled line. The transducer converts the hydraulic pressure into an electrical signal which can be displayed  digitally or an oscilloscope.

In Non-fluid coupled systems, the transducer is mounted on the monitoring device itself.

In infants and in children below 18 months of age , the anterior fontanelle is open. Tense anterior fontanelle indicates raised ICP and intraventricular pressure. CSF drainage can be done from the right side lateral angle of the diamond shaped anterior fontanelle.

In clinical setting  cerebral edema is one of the important causes of raised ICP.  if a patient presents with clinical features of raised intracranial pressure, then following steps may be helpful:

Bed rest  reduces the cerebral  metaboloic  rate of oxygen consumption and decreased blood supply

Oxygenation

Elevation of head end of the bed to 30^o

Acetazolamide ( Diamox tablet) is a carbonic anhydrase inhibitor and is available in tablet form . In an adult 250 mg tablet can be given orally three times a day( tds)

Frusemide or Furusemide ( Lasix) is a loop diuretic and is available in both oral and injectable form. A dose of 40 mg twice a day reduces the cerebral edema ICP. But Frusemide use may cause  potassium  loss leading to hypokalemia so serum electrolyte monitoring should also be done. To avoid hypokalemia , potassium supplement is advised for example syp Potklor  1 TSF twice a day or Injection KCL  in Intravenous  infusion may be given. Another drug can be prescribed is Spiroolactone( Lasilactone), a potassium sparing diuretic and then potassium supplementation is not required.

Injection Mannitol 100 ml stat or 100 ml 8 hourly ( 1 -1.5 Gm/ kg body weight in divided doses in an adult) for three days and then Syp Glycerol 6 TSF three times a day for about 2 weeks.

Dexamethasone 4 mg 6 hourly in injectabe or oral form. Ranitidine or other antacid should be prescribed alog with steroid to avoid gastritis. Dexamethasone is diabetogenic and raises blood sugar level. Prolong  use is associated with fluid retention and swelling over face and body.

CSF drainage is another way to reduce ICP. Ventricular tap is done usually through the point just anterior to the coronal suture on right side , about 3 cm lateral to the sagittal suture . This is a ethod of reducing ICP in a patient with post meningitic hydrocephalus and at the time of surgery. And if CSF pressure is persistently high then External ventricular drainage system can be used.

Elective hyperventilation is a mode or reducing ICP. Hyperventilation leads to CO₂ wash out which  causes vasoconstriction and decreased blood supply to the brain leading to decreased ICP. In this procedure patient is intubated after giving muscle relaxant and put on ventilation for about 48 hours. The ventilator mode is Controlled Mechanical Ventilation ( CMV) the respiratory rate is low,i.e., about 16/minute and monitoring of the patient is done with arterial blood gases(ABG) in which the pCO₂ is about 25mm Hg ( Normal range of arterial partial pressure of Carbon Dioxide ranges from 25mm Hg to 42 mmHg). Elective hyperventilation is often advise in patients with severe head injury, diffuse axonal injury, in a patient of spontaneous subarachnoid hemorhhage ( SAH) presenting with features of vasospasm, after a prolonged surgery with brain swelling during surgery.

Some surgical ways of reducing ICP are CSF diversion procedures ,  decompressive craniectomy or excision of the intracranial space occupying lesion( ICSOL) like hematoma, tumor or abscess.

Sources: 

Chapter 6. Intra-operative monitoring written by Babu KS, Rajsekhar VRamamurthy & Tandon’s manual of Neurosurgery ,  Editors:  PN Tandon, Ravi Ramamurthy, Pradeep Kumar Jain N, first edition: 2014 ISBN 978-93-5152-192-1

Handbook of Neurosurgery, Mark S Greenberg, 7^th edition ( Thieme Publishers)