II. Background
- Not affected by systemic Blood Pressure
- Sensitive to blood pCO2
- Hyperventilation lowers Intracranial Pressure
III. Technique
- Counsel patient to try to relax
- Avoid Hyperventilation (lowers pressure)
- Avoid straining (raises pressure)
- Patient lies in lateral decubitus position
- Legs and neck should be in neutral position
- Opening pressure not accurate in sitting position
IV. Interpretation
- Unit Conversions
- cmH2O = 1.3 x mmHg
- cmH2O = mmH2O / 10
- Normal
- Adults and older children: 10 to 15 mmHg (13 to 20 cm H2O)
- Younger children: 3 to 7 mmHg (4 to 9 cm H2O)
- Term Infants: 1 to 6 mmHg (2 to 8 cm H2O)
- Rangel-Castillo (2008) Neurol Clin 26(2): 521-41 [PubMed]
- Modifying Factors that increase Intracranial Pressure
- With pulse: 1 to 4 mmHg (2 to 5 cm H2O)
- With respirations: 3 to 8 mmHg (4 to 10 cm H2O)
- With sitting upright (compared with lateral decubitus): 11.4 mmHg (14.8 cm H2O)
- Decreased: Intracranial Hypotension (Opening Pressure <60 mmH2O or <6 cmH2O, uncommon)
- Trauma with secondary Cerebrospinal Fluid Leak
- Recent Lumbar Puncture (Spinal Headache)
- Spontaneous, Idiopathic Intracranial Hypotension (associated with positional Headache)
- Increased: Intracranial Hypertension (Opening Pressure >250 mmH2O, >25 cm H2O)
- Mild if 20 to 30 mmHg (26 to 39 cmH2O)
- See Increased Intracranial Pressure Causes
V. Calculation: Cerebral Perfusion Pressure (CPP)
- Cerebral Perfusion Pressure (CPP) = Mean Arterial Pressure (MAP) - Intracranial Pressure (ICP)
- Critical to maintain in Increased Intracranial Pressure in Trauma
- If continuous ICP monitoring, Arterial Line transducer should zeroed at the ear (kept at same height of head)
- Decreased Cerebral Perfusion Pressure risks brain ischemia
- Monroe-Kellie Hypothesis
- CSF volume (blood, brain, csf) remains constant
- When one volume increases (e.g. blood in Intracranial Hemorrhage), then another decreases (e.g. csf, brain)
- Typical volumes (total 1450 ml)
- Brain: 1300 ml
- CSF: 65 ml
- Blood: 110 ml
- CSF volume (blood, brain, csf) remains constant
VI. References
- Kooiker in Roberts (1998) Procedures in ER, p. 1067-75
- Ravel (1995) Lab Medicine, Mosby, p. 294-9
- Tunkel in Mandell (2000) Infectious Disease, p. 974-8
- Seehusen (2003) Am Fam Physician 68:1103-8 [PubMed]