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Monitoring of physiologic parameters intracranial and extracranial.
Intracranial
monitoring
Pressure and oxygenation two intracranial physiologic parameters most commonly monitored today
Intracranial pressure (ICP) can be monitored by measuring the pressure in any intracranial space because the pressure intracranial is the same throughout.
Because the intracranial compartments communicate without valves there are no pressure gradients and ICP can be measured by putting a catheter with a hydrodynamic pressure interface with intracranial extravascular fluid in the intraventricular, subarachnoid, or even the interstitial space.
The most common site for placement of a monitoring device outside of the ventricles is the subdural space.
Oxygen saturation of brain tissue
(btpO2) is monitored with the Licox (Integra).
Following head injury the intracranial space is accessed to measure intracranial pressure (ICP) and, if necessary for ICP reduction, to drain off cerebrospinal fluid.
Intracranial access
A craniostomy is a small diameter passageway through the skull.
Pressure responsive devices can be placed into the intracranial compartment and connected through a variety of transducer systems, to extracranial monitoring equipment.
Devices that measure ICP by a hydrostatic interface allow for CSF drainage as well as pressure measurement.
Ventriculostomy
Intracranial
pressure monitoring
Brain tissue
oxygen monitoring
Extracranial
monitoring
Extracranial monitoring includes that of arterial and jugular venous oxygen saturation as well as end-tidal CO2.
Arterial oxygen saturation
Oxygen saturation is accurately and non-invasively monitored with a simple device that measures oxygen saturation of small arteriolar and capillary blood optically scanned at the patient's earlobe or finger.
Jugular venous O2 saturation
Cerebral utilization of oxygenation can be calculated and monitored by addition of a jugular venous access catheter allowing sampling for measurement of jugular venous oxygen saturation.
End-tidal CO2
End tidal CO2 is a good indicator of CO2 in the alveoli of the lungs and can be used to follow the effects of managed ventilation.
Central venous pressure
Central venous pressure is the pressure of the blood in the capacitance portion of the circulation. This parameter best reflects the fluid volume status.
Mean arterial pressure
The product of stroke volume and vascular resistance, mean arterial pressure increases geometrically with an increase in either.
Cardiac output
Cardiac output is an indicator of cardiac contractility.
Pulmonary capillary wedge pressure
In addition to volume status, pulmonary capillary wedge pressure is an indicator of the ability of the left ventricle's forward pumping to keep up with venous return. Increases in vascular volume may cause fluid to return to the heart faster than the heart can pump it through the lungs and out back into the arterial circulation.
1. Oxygen
Oxygenation of stressed cerebral tissue depends on adequate perfusion with adequately saturated blood. Supplementary oxygen is a mainstay of the management of severe head injury both in the resuscitation and acute phases. Arterial oxygen saturation should be maintained at 95% or greater.
The most common causes of inability to adequately oxygenate a comatose intubated head trauma victim are: 1. Pulmonary edema (neurogenic or due to fluid overload - hemodynamic monitoring may be necessary to determine which), 2. Misplaced endotracheal tube, 3. Insufficient FiO2.
2. Fluids
Fluids should be iso- or hypertonic. If ICP is a consideration patients are best maintained euvolemic. Diuresis risks making patients hypovolemic (as well as hypokalemic) …
3. Pressors
Pressors can be used judiciously for optimization of cerebral perfusion pressure in the face of elevated ICP in hypotensive patients whose response to fluids is suboptimal or in whom administration of fluids is contraindicated (that is, euvolemic).
4. Intracranial pressure
CSF should be drained to maintain ICP such that cerebral perfusion pressure (CPP = MAP - ICP) is above 70 mmHg.
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