Recognition and Management of Intracranial Hypertension

Intracranial pressure and intracranial compliance When it is monitored, the ICP tracing is a ballistic waveform similar to the systemic arterial pressure curve (Fig. 1); it has a narrow “pulse pressure” and is expressed, by convention, as its mean measured in millimeter of mercury (mmHg). The normal mean ICP is generally between 5-10 mmHg, and it fluctuates to higher levels depending on physiological factors such as Valsalva maneuver and body position. The ICP wave contains three consecutive peaks in descending order, Peak 1 to 3 which are called percussion wave, tidal wave, and dicrotic wave, respectively. In addition, fluctuations in the baseline of the ICP waveform with breathing usually reflect a decrease in intracranial compliance, and it can occur and accentuate even before an elevation in the actual ICP value. This knowledge can be helpful in selected situations. For example, when a patient has fulminant hepatic failure observation of these subtle changes in the ICP waveform can signal the evolution of brain swelling before it is clinically apparent or the ICP has increased. In normal adults, the skull is a rigid container and contains brain parenchyma, fluid (interstitial and cerebrospinal), and blood (arterial and venous). In the average adult male the intracranial volumes approximate to 1,450 ml for the brain, 110 ml blood, and 65 ml of CSF. Expansion of any of these “compartments” or the addition of a space occupying process can lead to ICP elevation. The extent of the ICP elevation depends on many factors, most importantly the individual patient’s intracranial compliance, that is, the unit change of intracranial volume per unit change of intracranial pressure (ΔV/ΔP). A high compliance denotes a system that will accommodate significant changes in volume (i.e., by diverting blood and CSF out of the cranium) without little increases in pressure and vice versa, a reduced compliance is found when the addition of small volumes to the intracranial space induces marked changes in ICP. Compliance or more technically correct described as its inverse, the brain elastance (change of pressure per change of volume (ΔP/ΔV), can graphically be depicted in the intracranial INVITED REVIEW