Intracranial Pressure Monitoring in the Critically Ill

Intracranial Pressure Monitoring in the Critically Ill: A Practical Review for the Modern Intensivist

Author: Dr Neeraj Manikath 
 

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Abstract

Intracranial pressure (ICP) monitoring is a cornerstone of neurocritical care, providing crucial data to guide therapies aimed at preventing secondary brain injury. While its roots are in traumatic brain injury (TBI), its application has expanded to a variety of medical and surgical conditions encountered in the general intensive care unit (ICU), including subarachnoid hemorrhage, large ischemic strokes, and fulminant hepatic failure. For the critical care postgraduate, mastering the principles, practical application, and interpretation of ICP data is essential. This review provides a comprehensive overview of ICP monitoring, moving beyond the basics to explore the nuances of waveform analysis, troubleshooting common bedside problems, and integrating ICP data into a multimodal patient management strategy. We present evidence-based indications, compare monitoring modalities, and offer practical "pearls and oysters" to enhance clinical practice. The goal is to equip the modern intensivist with the knowledge to use ICP monitoring not just as a number generator, but as a sophisticated diagnostic tool to optimize cerebral perfusion and patient outcomes.

Keywords: Intracranial Pressure, Neurocritical Care, Cerebral Perfusion Pressure, External Ventricular Drain, Waveform Analysis, Traumatic Brain Injury

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1. Introduction: Beyond the Monroe-Kellie Doctrine

The cranial vault is a rigid, fixed-volume container housing three components: brain parenchyma (~80%), blood (~10%), and cerebrospinal fluid (CSF) (~10%). The Monroe-Kellie doctrine posits that an increase in the volume of one component must be compensated by a decrease in another to maintain normal intracranial pressure (ICP) [1]. When these compensatory mechanisms are exhausted, ICP rises exponentially, leading to reduced cerebral perfusion pressure (CPP), ischemia, and potentially, cerebral herniation.

The primary goal of ICP monitoring is to serve as an early warning system and to guide therapies that maintain adequate CPP, defined as CPP = Mean Arterial Pressure (MAP) – ICP. While historically associated with severe traumatic brain injury (TBI), the utility of ICP monitoring is now recognized in a broader spectrum of critical illnesses that threaten intracranial homeostasis. However, the landmark BEST-TRIP trial, which showed no outcome benefit of ICP monitoring over a strategy of imaging and clinical examination in TBI, has forced a re-evaluation [2]. The contemporary view is that ICP monitoring is not a therapeutic intervention in itself; its value lies entirely in its ability to guide timely and effective treatments. This review aims to provide a practical, evidence-based guide for the critical care trainee on the indications, techniques, interpretation, and troubleshooting of ICP monitoring in the modern ICU.

2. Indications for ICP Monitoring: Who Needs a Monitor?

Deciding which patient will benefit from invasive monitoring requires careful clinical judgment, balancing the potential benefits against the risks of the procedure.

Established Indications:

• Severe TBI: This remains the most common indication. The Brain Trauma Foundation (BTF) guidelines recommend ICP monitoring in all salvageable patients with severe TBI (Glasgow Coma Scale [GCS] 3-8) and an abnormal CT scan. It should also be considered in patients with a normal CT scan if they have two or more of the following: age >40 years, unilateral or bilateral motor posturing, or systolic blood pressure <90 mmHg [3].

Expanding and Relative Indications:

• Subarachnoid Hemorrhage (SAH): In poor-grade (Hunt-Hess IV-V) SAH patients, ICP monitoring is valuable for detecting hydrocephalus and managing CPP, especially in sedated and ventilated patients where the neurological exam is limited.

• Fulminant Hepatic Failure (FHF): Cerebral edema and intracranial hypertension are major causes of death in patients with FHF awaiting transplant. ICP monitoring can guide osmotherapy and help determine the safety of liver transplantation [4].

• Large Territorial Ischemic or Hemorrhagic Stroke: Malignant cerebral edema following a large middle cerebral artery (MCA) stroke or hematoma expansion in intracerebral hemorrhage (ICH) can lead to fatal ICP elevation. Monitoring can guide medical management and the timing of surgical interventions like decompressive craniectomy.

• Meningitis/Encephalitis: Severe bacterial or viral CNS infections can cause communicating hydrocephalus and diffuse cerebral edema, making ICP monitoring a useful adjunct in comatose patients.

• Post-Cardiac Arrest: Following resuscitation from cardiac arrest, anoxic brain injury can lead to cytotoxic edema and elevated ICP. While not routine, it may be considered in select patients to optimize neuroprotection protocols [5].

Pearl: The decision to monitor ICP should be based on the patient's underlying pathology and the ability of the clinical team to act upon the data provided. If you are not prepared to escalate therapy based on a high ICP value, the risks of monitoring may outweigh the benefits.

3. Modalities of ICP Monitoring: Choosing the Right Tool

A. The Gold Standard: External Ventricular Drain (EVD)

The EVD, or ventriculostomy, is a fluid-filled catheter placed into the lateral ventricle. It remains the gold standard because it is highly accurate, can be recalibrated at any time, and uniquely allows for therapeutic drainage of CSF.

• Setup: The catheter is connected to a closed system with a transducer and a drainage burette. The transducer must be zeroed at the level of the Foramen of Monro, which is anatomically estimated at the tragus of the ear or the outer canthus of the eye in the supine patient.

• Advantages: High accuracy, therapeutic capability, allows for CSF sampling.

• Disadvantages: Most invasive, highest risk of infection (ventriculitis rates ~5-10%) and hemorrhage, potential for obstruction or misplacement.

B. Intraparenchymal Monitors

These devices are placed directly into the brain parenchyma. Common types include fiber-optic (e.g., Camino) or strain-gauge (e.g., Codman) systems.

• Advantages: Lower risk of infection and hemorrhage compared to EVDs, easier insertion (can be done at the bedside without precise ventricular cannulation).

• Disadvantages: Cannot drain CSF. Subject to "transducer drift" over time and cannot be re-zeroed in-situ. This means accuracy can degrade over several days.

C. Non-Invasive ICP Monitoring: The Future is Here (Almost)

Non-invasive methods are a rapidly evolving area, primarily used for screening or as an adjunct when invasive monitoring is contraindicated or unavailable. They are not yet accurate enough to replace the gold standard for guiding therapy.

• Optic Nerve Sheath Diameter (ONSD): The optic nerve sheath is contiguous with the dura mater. As ICP rises, CSF is forced into this space, causing the sheath to distend. Measured with bedside ultrasound, an ONSD > 5.0-5.8 mm is suggestive of raised ICP (>20 mmHg) [6].

• Transcranial Doppler (TCD): TCD measures blood flow velocities in the basal cerebral arteries. By calculating the Pulsatility Index (PI = [systolic velocity - diastolic velocity] / mean velocity), one can estimate downstream resistance. A high PI (>1.2) suggests high distal resistance, often due to elevated ICP.

• Pupillometry: Automated pupillometers provide an objective measure of pupil size and reactivity (Neurological Pupil Index, NPi). A declining NPi can be an early sign of third nerve compression from rising ICP.

Oyster: Non-invasive methods are best used for trend analysis. A single ONSD measurement has limited value, but a documented increase from 4.5 mm to 5.5 mm over 6 hours in a patient with worsening mentation is a powerful signal to obtain definitive imaging or invasive monitoring.

4. Practical Hacks and Troubleshooting at the Bedside

An ICP monitor is only as good as the clinician interpreting it. Here are common problems and solutions.

Hack #1: Zeroing is Everything

• Problem: Inaccurate ICP readings.

• Solution: Always re-zero the transducer after any significant patient repositioning (e.g., turning, transfer to CT scanner). Ensure the transducer is precisely at the level of the tragus. An incorrectly placed transducer is the most common cause of erroneous readings. A transducer placed too low will falsely elevate the ICP, and one placed too high will falsely lower it.

Hack #2: Taming the Damped Waveform

• Problem: The ICP waveform appears flattened or "damped," with loss of distinct P1, P2, and P3 peaks. The numerical value may be unreliable.

• Causes & Solutions:

  1. Check the System First: Start at the monitor and work your way to the patient. Are there any loose connections, three-way taps turned the wrong way, or air bubbles in the line? Air is a common culprit; carefully flush it away from the patient.

  2. Kinks or Clots: Is the EVD tubing kinked? Is there a small clot at the catheter tip? A gentle, aseptic aspiration/flush by a trained provider (e.g., neurosurgeon) may be required. Never flush aggressively towards the patient.

  3. Catheter Position: The catheter tip may be abutting the ventricular wall or choroid plexus. A slight change in head position can sometimes free it.

Hack #3: The Square Wave Test (Fast-Flush Test)

• Just like with an arterial line, activating the fast-flush device on the transducer system should produce a square wave on the monitor, followed by a few oscillations.

• Over-damped system: A slurred upstroke and only one or no oscillations indicates a problem with the system (air, clot, kink) and will lead to falsely low systolic and falsely high diastolic pressures (affecting waveform morphology).

• Under-damped system: Excessive "ringing" or oscillations can be caused by long, compliant tubing and can overestimate the pulse pressure component of the ICP.

5. Interpreting the Data: More Than Just a Number

A single ICP value of "25" is concerning, but the trend and waveform morphology provide far richer diagnostic information. A normal ICP is <15 mmHg; sustained ICP >20-22 mmHg warrants intervention.

A. The ICP Waveform

The arterial pulsation transmitted through the CSF creates a characteristic waveform with three peaks:

• P1 (Percussion Wave): The initial sharp peak. Represents the arterial pulsation transmitted from the choroid plexus. It should be the tallest peak.

• P2 (Tidal Wave): A more rounded, delayed peak. This is the compliance wave. It reflects the brain's ability to accommodate changes in volume.

• P3 (Dicrotic Wave): A smaller peak following P2, related to the dicrotic notch of the arterial pressure wave.

Pearl: The most important relationship is between P1 and P2. When P2 rises to become taller than P1 (P2 > P1), it is a sign of decreasing intracranial compliance. The brain is becoming "tight" and losing its ability to buffer additional volume. This can be an early warning of impending dangerous ICP elevation, even if the absolute ICP number is not yet critical.

B. Lundberg Waves

These are spontaneous, cyclical changes in ICP described by Niels Lundberg [7].

• A-Waves (Plateau Waves): These are pathological. They represent sudden, sharp elevations of ICP to 50 mmHg or higher, lasting for 5-20 minutes. They are associated with critically low cerebral compliance and often precede clinical herniation. Seeing A-waves is a neurological emergency.

• B-Waves: Rhythmic oscillations occurring 1-2 times per minute. They often reflect fluctuations in respiratory or vasomotor tone. While not as ominous as A-waves, a predominance of B-waves can indicate instability and failing compensatory mechanisms.

C. Derived Indices: The Pressure Reactivity Index (PRx)

PRx is an advanced index available on some bedside software that measures the state of cerebral autoregulation. It calculates the moving correlation coefficient between slow waves of ABP and ICP.

• PRx near -1: Intact autoregulation. When blood pressure rises, cerebral vessels constrict to keep blood flow constant, causing a slight drop in ICP (negative correlation).

• PRx near +1: Impaired autoregulation. When blood pressure rises, the vessels are passive and distend, causing ICP to rise as well (positive correlation).

• Clinical Utility: A high PRx (>0.2) suggests that augmenting MAP may be harmful, as it will directly increase ICP. This allows for an individualized CPP target, aiming for the MAP at which PRx is lowest [8].

Oyster: Instead of a universal CPP target of 60-70 mmHg, using PRx to find a patient's optimal CPP may be a more physiological approach. If PRx is high, raising MAP to reach a target CPP may paradoxically worsen secondary injury by promoting vasogenic edema.

6. Integrating ICP Data into a Tiered Management Protocol

ICP data should guide a stepwise approach to management.

• Tier 0 (Foundation): Head of bed elevation (30°), maintain neck in neutral position, control fever and pain, adequate sedation.

• Tier 1 (First-line): CSF drainage via EVD (intermittent or continuous), osmotherapy (Mannitol vs. Hypertonic Saline [9]).

• Tier 2 (Refractory ICP): Brief, controlled hyperventilation (Target PaCO2 30-35 mmHg) as a bridge to other therapies, barbiturate coma, or optimization of CPP using PRx.

• Tier 3 (Rescue): Decompressive craniectomy, profound hypothermia (controversial).

Hack: When using an EVD for continuous drainage, setting the height of the burette relative to the patient’s tragus acts as a "pop-off" valve. For example, setting the drain at 15 cmH₂O above the tragus means CSF will only drain when ICP exceeds that level, preventing over-drainage while automatically treating ICP spikes.

7. Complications of ICP Monitoring

• Infection: Ventriculitis is the most feared complication of EVDs. Strict aseptic technique during insertion and handling is paramount. The utility of antibiotic-impregnated catheters is debated but may reduce infection rates [10].

• Hemorrhage: Can occur along the catheter track upon insertion.

• Malfunction: Obstruction, drift (for intraparenchymal monitors), or accidental dislodgement.

• Over-drainage of CSF: Can lead to "slit ventricle syndrome" and subdural hematoma formation if the brain pulls away from the dura.

8. Conclusion

Intracranial pressure monitoring is a powerful but invasive tool. For the postgraduate intensivist, mastery extends beyond simply reading a number from the monitor. It requires an understanding of the underlying physiology, the technical nuances of the equipment, and the sophisticated interpretation of waveforms and trends. By integrating ICP data with the clinical examination, imaging, and other multimodal monitoring tools, we can move from generic protocols to individualized, physiology-based care. In the post-BEST-TRIP era, the challenge is not simply to monitor ICP, but to use the information wisely to guide therapies that definitively improve outcomes for our most critically ill neurological patients.

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Summary of Pearls and Oysters

Pearls (Established Wisdom)	Oysters (Nuanced Insights)
1. The ICP monitor is a diagnostic, not a therapeutic, tool. Its value is in guiding effective treatment.	1. Non-invasive methods (ONSD, TCD) are best for screening and trend-monitoring, not for replacing the gold standard when definitive treatment decisions are needed.
2. Zero the transducer at the Foramen of Monro (tragus of the ear). Re-zero with every position change.	2. The trend of ICP and the shape of the waveform are often more informative than a single static number. A rising P2 wave can predict trouble before the absolute ICP value becomes critical.
3. A P2 wave taller than the P1 wave is a clear sign of poor intracranial complianceand impending danger.	3. The BEST-TRIP trial challenges us to prove that monitoring leads to better outcomes. This emphasizes the need to link data to effective action.
4. Lundberg A (Plateau) waves are a neurological emergency signaling critically low compliance and impending herniation.	4. Using PRx to define an "optimal CPP" for each patient may be superior to a one-size-fits-all target, preventing harm from iatrogenic hypertension in patients with impaired autoregulation.

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References

[1] Mokri B. The Monro-Kellie hypothesis: applications in CSF volume depletion. Neurology. 2001;56(12):1746-1748.
[2] Chesnut RM, Temkin N, Carney N, et al. A trial of intracranial-pressure monitoring in traumatic brain injury. N Engl J Med. 2012;367(26):2471-2481.
[3] Carney N, Totten AM, O'Reilly C, et al. Guidelines for the Management of Severe Traumatic Brain Injury, Fourth Edition. Neurosurgery. 2017;80(1):6-15.
[4] Wijdicks EFM, Plevak DJ, Rakela J, Wiesner RH. Clinical and radiologic features of cerebral edema in fulminant hepatic failure. Mayo Clin Proc. 1995;70(2):119-124.
[5] Sekhon MS, Griesdale DE, Ainslie PN, et al. Intracranial pressure and cerebral autoregulation in post-cardiac arrest patients. Neurocrit Care. 2014;20(2):256-262.
[6] Dubourg J, Javouhey E, Geeraerts T, Messerer M, Kassai B. Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: a systematic review and meta-analysis. Intensive Care Med. 2011;37(7):1059-1068.
[7] Lundberg N. Continuous recording and control of ventricular fluid pressure in neurosurgical practice. Acta Psychiatr Scand Suppl. 1960;36(149):1-193.
[8] Aries MJH, Czosnyka M, Budohoski KP, et al. Continuous determination of optimal cerebral perfusion pressure in traumatic brain injury. Crit Care Med. 2012;40(8):2456-2463.
[9] Rickard S, Sawyer S, Blanco J, et al. Mannitol versus hypertonic saline for the treatment of elevated intracranial pressure: a meta-analysis of randomized controlled trials. Neurocrit Care. 2014;20(2):292-300.
[10] Tamber MS, Klimo P Jr, Nicol K, et al. The role of antibiotic-impregnated shunts in the treatment of pediatric hydrocephalus: a systematic review and meta-analysis. J Neurosurg Pediatr. 2014;13(4):369-376.