Brain monitoring: do we need a hole? An update on invasive and noninvasive brain monitoring modalities

Abstract

The ability to measure reliably the changes in the physical and biochemical environment after a brain injury is of great value in the prevention, treatment, and understanding of the secondary injuries. Three categories of multimodal brain monitoring exist: direct signals which are monitored invasively; variables which may be monitored noninvasively; and variables describing brain pathophysiology which are not monitored directly but are calculated at the bedside by dedicated computer software. Intracranial pressure (ICP) monitoring, either as stand-alone value or study of a dynamic trend, has become an important diagnostic tool in the diagnosis and management of multiple neurological conditions. Attempts have been made to measure ICP non-invasively, but this is not a clinical reality yet. There is contrasting evidence that monitoring of ICP is associated with better outcome, and further RCTs based on management protocol are warranted. Computer bedside calculation of "secondary parameters" has shown to be potentially helpful, particularly in helping to optimize "CPP-guided therapy." In this paper we describe the most popular invasive and non invasive monitoring modalities, with great attention to their clinical interpretation based on the current published evidence.

Figure 1

Case of a head injury patient with a highly unstable ICP, due to “plateau” waves. Details are in the text. ICP: intracranial pressure. ABP: arterial blood pressure. CPP: cerebral perfusion pressure.

Figure 2

Correlation between mortality rate and ICP. General threshold between normal and elevated ICP between 20 and 30 mm Hg. ICP: intracranial pressure.

Figure 3

(a) Cortical cerebral blood flow (CBF) can be monitored with laser Doppler flowmetry, and CBF plotted against CPP shows the autoregulatory curve, called Lassen's curve, with clear lower limit of autoregulation (LLA). ICP: intracranial pressure. ABP: arterial blood pressure. CPP: cerebral perfusion pressure. LDF: laser Doppler flowmetry. (b) Lassen's curve. CPP: cerebral perfusion pressure. LDF: laser Doppler flowmetry.

Figure 4

Example of multiple days monitoring of PRx along with ICP, ABP, and CPP where a patient attained good outcome after severe head injury. Image extrapolated from ICM+ software. ICP: intracranial pressure. ABP: arterial blood pressure. CPP: cerebral perfusion pressure. PRx: pressure reactivity index.

Figure 5

Increase in ICP above 20 and then above 80 mmHg. PRx in this case deteriorates almost half of a day before. ICP: intracranial pressure. ABP: arterial blood pressure. CPP: cerebral perfusion pressure. PRx: pressure reactivity index.

Figure 6

Mortality rate, expressed as a function of CPP, shows distinctive “U shape” curve. CPP: cerebral perfusion pressure.

Figure 7

An algorithm to trace the PRx/CPP curve. CPP: cerebral perfusion pressure. PRx: pressure reactivity index.

Figure 8

Multimodal monitoring during a plateau wave of ICP, demonstrating a decrease in CPP, PbtO₂, blood flow velocity (which was assessed with TCD), and HbO₂ during the curve, resulting in an ischaemic insult. ICP: intracranial pressure. NIRS THI: near infrared spectroscopy, tissue haemoglobin index. CPP: cerebral perfusion pressure. TCD FV: Trans Cranial Doppler Flow Velocity. PtiO2: brain tissue oxygenation. NIRS TOI: near infrared spectroscopy, tissue oxygenation index.