Continuing Education Activity

According to the Monro-Kellie theory, the cranial compartment is incompressible, and its contents (blood, CSF, and brain tissue) are in an internal milieu of volume balance; thus, an increase in one must be offset by a reduction in the other two components. Intracranial pressure (ICP) guided therapy has been the cornerstone in managing severe traumatic brain injury. ICP monitoring allows for judicious use of interventions with a defined target point, thereby avoiding potentially harmful aggressive treatment. This activity outlines the current recommendations for intracranial pressure (ICP) monitoring and reviews the role of current invasive and noninvasive monitoring methods. The activity also addresses the need for the collaborative efforts of an interprofessional team to minimize complications regarding the process and thereby safeguard patient safety.

Objectives:

• Identify the indications for ICP monitoring.
• Describe the equipment, personnel, preparation, and technique regarding ICP monitoring.
• Review the evaluation of the potential complications of ICP monitoring.
• Outline interprofessional team strategies for improving care coordination and communication to advance care bundle approaches in ICP monitoring and improve clinical outcomes.

Introduction

The normal intracranial pressure (ICP) ranges from 7 to 15 mm Hg, while in the vertical position, it does not exceed -15 mm Hg. Overnight sleep monitoring is considered the “gold standard” in conscious patients.[1]

Typically, ICP lowering therapy initiates when ICP is greater than 20 to 25 mm Hg.[2] Refractory elevated ICP reduces cerebral perfusion pressure (CPP), thereby accounting for cerebral ischemia and initiating herniation syndromes that eventually lead to death.[3][4]

The application of multimodal monitoring (MMM) in conjunction with adherence to ICP-guided therapy has been the cornerstone in managing severe traumatic brain injury. Thus, ICP monitoring allows for judicious use of interventions with a defined target point, thereby avoiding potentially harmful aggressive treatment. Brain Trauma Foundation (BTF) guidelines during patient care bundle approaches have shown positive outcomes as well as the minimized cost of acute care.[5][6]

Anatomy and Physiology

Monro-Kellie Doctrine

According to the Monro-Kellie theory, the cranial compartment is incompressible and its contents (blood, CSF, and brain tissue) are in an internal milieu of volume balance; thus, an increase in one must be offset by a reduction in the other two components. The Monro-Kellie hypothesis is based on a pressure-volume relationship that tries to keep the non-compressible aspect of the skull in a steady state.[7][8] CSF and cerebral blood volume (CBV) are the primary buffers for any extra volume increment.

Historical Perspective

Alexander Monro Secundus mentioned in his 1783 monograph that the brain tissue volume and the blood volume remained constant and emphasized the necessity to egress blood equal to any foreign liquid injected inside the skull. George Kellie reinforced the assumption that the total amount of blood flowing within the skull remains constant. John Abercrombie was primarily responsible for propagating this doctrine across the medical world. However, they were unable to finish the hypothesis owing to the lack of information about the anatomy and physiology of the nervous system during that period. They disregarded the CSF's involvement, if not its mere existence, in ICP control. In his book 'On the disorders of the cerebral circulation,' Burrows disputed the conception of the skull as a perfect sphere and the concept of invariability of blood volume and claimed a significant function of the CSF in the control of ICP. He mentioned that cerebral blood volume could change, but only to the benefit or harm of the brain and CSF volumes.[9]

Harvey Cushing established the clinical and physiological significance of this doctrine in the twentieth century by confirming Burrows' findings of the CSF's role in intracranial dynamics and the existence of three volumes that operate as compensators for volume depletion or insertion inside the skull.

Role of the Doctrine in Intracranial Hypertension

According to Monro and Kellie's 1783 pressure-volume equations, when intracranial pressure (ICP) rises, vascular blood and CSF are moved as part of a dynamic counterbalance to maintain normal pressure inside the inelastic skull, while brain tissue stays unaffected. Under pathological circumstances, if one of these compartments expands or a fourth one forms (as a result of a mass-effect lesion such as a tumor or hematoma), one or more of the other components must contract to prevent any subsequent rise in ICP. Since the parenchymal compartment is incapable of compensating for a rapid increase in ICP, CBV and CSF are responsible for this process. CSF is the primary buffer mechanism; it migrates into the perimedullary subarachnoid area until displaced brain structures obstruct its flow. The vascular compartment's capacity to correct for ICP is activated later and consists of lowering the CBV through jugular drainage.

Role of Doctrine in Intracranial Hypotension

In individuals with intracranial hypotension, a decrease in CSF volume may result in a compensatory increase in brain and/or intracranial blood volume. Since the amount of brain tissue is typically regarded constant, compensation would occur through an increase in blood volume, notably venous blood, because veins are more adaptable than arteries.[10] The Monro-Kellie theory may account for many MRI anomalies seen in intracranial hypotension or CSF volume depletion, such as meningeal augmentation, subdural fluid collections, engorgement of cerebral venous sinuses, prominence of the spinal epidural venous plexus, and pituitary gland enlargement.[8] As a second consequence of the lack of the buoyancy forces given by CSF, brain tissue is forced downward, resulting in herniation. These effects are amplified by gravity while the patient is standing up, and they serve as a foundation for understanding both the clinical presentations of the condition as well as the imaging results.[11]

Interaction Between Cerebral Volume and ICP

Cerebral compliance is defined as the volume required to produce a specific change in pressure. Thus, cerebral compliance may be defined as the cranial vault's adaptive ability to increase volume.[12] The cerebral elastance (pressure resulting from a known change in volume) is interpreted as the opposition to intracranial volume growth. The pressure-volume curve is divided into three phases.[12]

• Initial stage (phase 1): This stage is characterized by a high level of brain compliance and a low ICP. Despite the volume rise, there is no evidence of a subsequent increase in ICP (CSF and CBV nullify the increase in volume).
• Transition stage (phase 2): This stage is characterized by poor brain compliance and a low ICP, although the latter gradually increases.
• Ascending stage (phase 3): This stage exhibits poor or non-existent compliance with elevated ICP (beginning decompensation). Compensatory processes become inactive, and tiny volume changes result in significant pressure increments.

Limitations of the Doctrine:

• No mention of the role of CSF in the brain's adaptive physiology
• No mention of the effect of gravitation on the volume and pressure inside the skull
• It does not hold up in children with open fontanels.

Indications

The guidelines (Level II recommendations) implemented by the BTF for managing severe Traumatic Brain Injury (TBI) recommend an ICP monitor in the situations listed below:

1. Patients with Glasgow Coma Scale (GCS) less than 8
2. An abnormal computed tomogram (CT) scan of the head
3. Two or more of the following are present:

• Age greater than 40 years
• Unilateral or bilateral motor posturing
• Systolic blood pressure less than 90 mm Hg.[13]

Other ICP Monitoring Recommendations (Level III Recommendations)

• Patients with an initial normal CT scan or with minor changes in CT images but later show features of neurologic worsening or progression on the repeat scan
• Evidence of brain swelling, e.g., compressed or absent basal cisterns
• Patients with large bifrontal contusions independent of the neurological condition
• When sedation interruption to check neurological function is not justified, e.g., respiratory failure from lung contusions and flail chest
• When the neurological examination is not reliable, e.g., maxillofacial trauma or spinal cord injury
• A decompressive craniectomy is performed as a last resort for intracranial hypertension refractory to medical management
• Following craniotomy wherein there are relevant risk factors for the propagation of brain edema, e.g., confounding hypoxia, hypotension, pupil abnormalities, midline shift greater than 5 mm.[14]

Contraindications

Contraindications for placement of an invasive mode of ICP monitoring include cases of:  

1. Concurrent use of anticoagulant drugs
2. Bleeding disorders
3. Scalp infection
4. Brain abscess.[15]

Equipment

Intraventricular monitoring with the aid of ventriculostomy or the use of intraparenchymal strain gauge or fiber-optic monitors is the recommendation for ICP monitoring. So appropriate monitoring devices should be available. There must be utmost care for strict adherence to aseptic conditions during these procedures. There also is the paramount importance of implementing algorithmic management guidelines in all patients with invasive ICP monitors for safeguarding all monitor sets.

Personnel

Necessary personnel includes a composite healthcare team comprising of:

• A neurosurgeon
• A qualified assistant
• An attending nurse
• An anesthetist
• A general duty assistant (GDA).

Preparation

The patient and next of kin/relatives should receive a thorough explanation regarding the indication for the procedure and the risks involved before the procedure, and written consent should be obtained.

Strict adherence to aseptic guidelines is a cornerstone in preventing the risk of infection, and prophylactic antibiotics must be administered just at the beginning of the procedure. A meticulous technique is vital in minimizing procedure-related complications.[15]

The patient should be well sedated, assuring patency of his airway, and a local anesthetic should be administered at the allocated point of ventriculostomy or insertion of ICP monitoring devices.

Technique or Treatment

The insertion of the device is aided by the placement of either a burr hole or a twist drill technique. Kocher's point is the choice for the ventriculostomy, which is 3 cm lateral of the midline and 1 cm anterior to the coronal suture. Other points of ventricular puncture include Keen's point, Dandy's point, and Frazier's point.

Basic equipment sets should include the following:

1. Non-sterile gloves, soap, brush, hand towel, razor, and a marker pen for parts preparation and marking of the site of placement of monitor devices.[15]
2. For the procedure itself, face mask, sterile gown and gloves, an antiseptic solution, a drape, a local anesthetic agent, a 5-ml syringe, a 15 or 11-number surgical blade, and an ICP-monitoring kit. A drill with a drill bit, a bolt, an ICP sensor, and a transducer per the methods utilized. A suture material and a sterile dressing.[15]

Stringent analysis of the pressure and pulse amplitude of ICP waveforms is favored contrary to its assessment from the height of the CSF column.[1]

The pulse component of the waveform shows three peaks:

1. P1 (percussion wave) due to arterial pulsation
2. P2 (tidal wave) represents brain compliance
3. P3 (dicrotic wave) owing to aortic valve closure.

With decreasing brain compliance, waveform amplitude will increase, rising P2 above P1 and P3, rounding of waveform and appearance of plateau waves, and Lundberg B and A waves.

Ventricular catheters represent a "global" ICP with minimal chances of drift and influence from pressure gradients between the parenchyma and ventricular system.

It is the most reliable method of achieving maximum accuracy with minimal expense. There are added therapeutic benefits of cerebrospinal fluid (CSF) drainage, instilling medications like antibiotics and thrombolytic agents.

Strain gauge or fiber optic-based systems inserted into the ventricles or brain parenchymas are more accurate than fluid-coupled or pneumatic devices.

Parenchymal monitors are easier to insert in cases of midline shift or malignant brain swelling, with no risks of blockage by hemorrhage or debris.

However, they cannot be re-calibrated in vivo, only measure a localized pressure and have drift issues in long-term usage.

The current non-invasive techniques are simply not accurate enough to replace traditional invasive techniques.

CT and MRI images provide only provide a single-point snapshot picture. Pulsatility index from transcranial doppler (TCD), tympanic membrane displacement (otoacoustic emissions), Near-infrared spectroscopy, as well as optic nerve sheath diameter (ONSD) assessments have a study error margin of +/- 10 to 15 mm Hg with high inter-rater variability.

Complications

An underlying assumption is that an ICP reading at one point is equivocal, and the mirror reflects the global pressure throughout the brain. However, it is confounded by the pressure gradient within the ventricular system as well as the parenchyma brain interface. The accuracy and precision over time (drift) and in vivo calibration of different ICP measurement systems are also a concern.[2][16]

Difficulties in catheter placement in severe brain edema with slit ventricles can complicate intraventricular monitoring of ICP.

Complications of Ventricular Catheter-based ICP Monitoring

1. Intracranial and tract hemorrhage - 10%
2. Infection (ventriculitis) - 20%
3. Technical failure (failure to tap ventricle or misplacement) - 5%
4. The cost for placing an external ventricular drain (EVD) amounts to around $200, with transducers costing an extra $400 to 600
5. Over drainage can lead to aneurysmal rebleed and complicate the upward transtentorial herniation in cases of hydrocephalus
6. Kinks and blockage by air, blood, and debris are also frequent, leading to the poor and false recording of the ICP
7. There can be localized elevations of ICP due to compartmentalization from mass lesions.[14]

Clinical Significance

ICP, rather than a simple numeric value or a mere threshold, is an epiphenomenon of multispectral interlinking brain processes such as compliance, hemodynamic strain, metabolic dysfunction, etc.[14]

ICP management based on the stair step-type linear algorithmic protocol has significant flaws. Assumptions that patterns and sequelae of ICP increment are the same; the only pivotal difference being their pattern of response or their resistance to them are rather empirical.

Therefore a tiered system approach has been formulated for managing refractory ICP through the implementation of an “individualized ICP threshold” aided by waveform analysis through correlation coefficient (R) between the pulse pulsation amplitude and mean ICP (RAP) or pressure-volume index (PVI).[1]

Higher RAP (correlation coefficient (R) between the pulse pulsation amplitude and mean ICP) values indicate less compliance.

As the brain ICP increases, RAP gradually falls below zero signifying exhaustion of autoregulatory capacity. There will be a right shift of the pressure-volume curve, and thereby further increase in cerebral perfusion pressure leads to the paradoxical passive collapse of the arterioles.[1]

Pressure-volume index (PVI) or the apparent volume implementing a ten-fold ICP increase is 25 to 30 ml. ICP waveform analysis also reveals a gradual increase in the amplitude of P2 becoming greater than P1.[14]

Similarly, pathological Lundberg waves also appear. 

Among various armamentarium to calculate autoregulation (AR) such as brain tissue oxygen (PbtO), near-infrared spectroscopy (NIRS), thermal dilution regional CBF (td-rCBF), microdialysis, PbtO2 has the greatest evidence base for signifying the same.[17]

Benchmark Evidence from South American Trials: Treatment of Intracranial Pressure (BEST TRIP) trial concluded that ICP-based management showed no significant favorable outcome compared with patient management under the guidance of serial CTs and clinical examination.[18] There were various limitations of the study, foremost being a lack of specific recommendations regarding diagnosis, inclusion criteria, and governing of patterns of interventions. There was also methodological heterogeneity as well as the bias relating to missing data.[4]There were also concerns regarding the generalized validity of the results, which also provoked concerns regarding the ethical standards of the study.[19] There are deleterious effects of the ICP-reducing therapy, such as prolonged hyperventilation, reducing cerebral blood flow (CBF), and possibly propagating cerebral ischemia. Similarly, fluids and vasopressors for maintaining CPP greater than or equal to 70 mm Hg carry the risk of acute lung injury (ALI).

Enhancing Healthcare Team Outcomes

To ensure better clinical outcomes and to prioritize patient safety by minimizing complications, there need to be mandatory patient safety checklists to be implemented by the health care team involved in the process. The following guidelines have to be adhered to:

• Valid treatment order sheet
• All reportable limits clearly specified
• EVD drainage point at a prescribed level with the transducer leveled to the tragus of the ear
• The EVD column is oscillating
• Monitoring for normal ICP waveform
• ICP waveform is pulsatile on the monitor
• No soakage in the wound site or any junctions within the monitor set
• Judicious assessment of drained CSF volume
• Stringent evaluation of the neurological status of the patient
• Coordination between monitoring specialty trained nurses and clinicians
• Family education by nursing staff and clinicians; providing strong family support.

Intracranial pressure monitoring requires an interprofessional team approach, including clinicians (MDs, DOs, NPs, and PAs), specialists, and specialty-trained nurses, collaborating across disciplines to achieve optimal patient results. The critical care nurse is essential for close monitoring of intracranial pressure and communicating any change to the medical team. The neurology and critical care nurse assists the medical team with hourly neurological and hemodynamic evaluations to ensure prompt intervention when needed. An interprofessional team working in unison with collaboration can significantly enhance patient outcomes in patients undergoing intracranial pressure monitoring.

Nursing, Allied Health, and Interprofessional Team Interventions

The advantage of the ventricular monitoring device is the facility for egress of CSF in cases of a sustained rise in ICP (greater than or equal to 20 mm Hg for 5 minutes or longer), but the disadvantage is that simultaneous monitoring, as well as CSF drainage, is not possible. The amount of CSF to be drained can be guided as per the recommended target ICP (commonly set as 10 mm Hg) or can be aided with the visual guidance in improving the ICP waveform analysis obtained from the concurrent application of intraparenchymal monitors or through clinical neurological examination.[20] Care always needs to be taken in preventing paradoxical upward transtentorial herniation due to over jealous drainage of CSF.

Surgical decompression is the usual recommendation; there is a refractory rise in ICP and clinical deterioration despite the stepwise escalation in the management tiers aimed to counteract the same such as sedation, neuromuscular blockade, mild hyperventilation, hyperosmolar therapies, and barbiturate coma.[20]

The ICP monitoring devices get removed once the ICP is normalized with sustained or improved clinical neurology (motor score at least 5) for at least 48  to 72 hours without any interventions. In cases of ventricular devices, the EVD can undergo clamping, or more ideally gradual increment in its height (training of the EVD) is attained to watch for any clinical deterioration in the patient for at least 48 hours. 

Strict aseptic precautions and care also need to be implemented during its removal. The head end should be lowered down to prevent the risk of pneumocephalus and pneumoventricle. The catheter tip can be sent for bacteriological analysis in cases of persisting fever with features of meningitis. The wound is closed in layers to minimize the risk of CSF leak and infection. The patient should be strictly monitored for any signs of clinical deterioration for at least 24 hours, with all preparations made for emergency placement of a new EVD or ICP monitor device.

Nursing, Allied Health, and Interprofessional Team Monitoring

Monitor Should Include

• Hourly CSF drainage
• Ensure CSF oscillation inside the tubes
• Confer no soakage of the wound
• Ensure the correct height of EVD
• Zeroing of the EVD height at the level of the foramen of Monro or tragus of the ear
• Stringent neurological monitoring of the patient
• Monitor hourly ICP.