Keywords
intraoperative intracranial pressure monitoring - intraventricular catheter - subdural
catheter - perioperative management
Introduction
In 1783, Alexander Monro proposed that the skull is a rigid box with a fixed internal
volume of 1,400 to 1,700 mL.[1] The contents include 80% brain parenchyma, 10% cerebrospinal fluid (CSF), and 10%
blood.[2] As per the Monro–Kellie doctrine, an increase in one component will cause a decrease
in one or both of the other components, mostly CSF and blood.[2] Such a compensatory reserve is called spatial compensation, and the failure of compensatory
reserve results in severe brain damage. Normal intracranial pressure (ICP) values
change with age and posture. It is 5 to 15 mm Hg in healthy supine adults, 3 to 7 mm
Hg in children, and 1.5 to 6 mm Hg in infants.[3] Several monitors and transducers are available for monitoring ICP.[4]
Intraventricular ICP monitoring is the gold standard and comprises a ventricular catheter
attached to an external fluid-filled pressure transducer.[4] Other advantages of the intraventricular ICP monitoring system are that it allows
the drainage of CSF, and ICP waveforms are distinct and readily analyzed.[4] Monitors which measure ICP from extraventricular sites display the absolute ICP
values and neither allow CSF drainage nor permit ICP waveform analysis.[5] These monitors have transducers either at the tip of the catheter or externally,
and they can be placed in the brain parenchyma, subdural/subarachnoid, or extradural
space. The main advantages of extraventricular ICP monitors are less invasiveness,
less infection rate, and easy maintenance.[1] However, these monitors are primarily used in the intensive care unit, and their
utility during the intraoperative period is not frequently explored. Knowledge of
preoperative and intraoperative ICP will enable the neuroanesthesiologist to target
optimal cerebral perfusion pressure (CPP). Intraventricular ICP monitoring may not
be feasible in cases where cerebral ventricles are compressed. Extraventricular ICP
monitoring is an alternative in such cases. Presently validated extraventricular ICP
monitors are expensive, and the sensors are not reusable.
We hypothesize in this pilot study that the subdural ICP measurement using an intravenous
cannula is a reliable surrogate for the gold standard technique of intraventricular
ICP measurement. The primary objective of this study was to correlate the subdural
ICP values with the gold standard intraventricular ICP values.
Materials and Methods
This is a prospective validation and feasibility study of neurosurgical patients whose
ICP was monitored intraoperatively. Patients were studied for 4 months, from July
2021 to October 2021. Institutional ethics committee approval was not obtained for
this study as ICP monitoring is standard of care at our institution. Informed consent
from patients was obtained for scientific publication of the data with patient anonymity.
Intraventricular ICP and/or subdural ICP were monitored in patients recruited into
the study.
For the intraventricular monitoring, the lateral ventricle was hit using a ventricular
cannula through the Kocher's point, 3 cm lateral to the midline, and 1 cm in front
of the coronal suture.[6] On confirming the CSF flow, the cannula was immediately connected to a de-aired
system of an external fluid-filled pressure transducer (IpeX pressure monitoring kit,
BL Lifesciences, Uttar Pradesh, India) to record the opening pressure ([Fig. 1A] and [B]). Both values and waveform get displayed on the monitor (Intellivue MX850, Philips,
GmbH, Germany and Carescape B850, Wipro GE Healthcare, Karnataka, India). After noting
the opening pressure, the ventricular cannula was replaced with a ventricular catheter,
connected to the same transducer, and retained for further intraoperative monitoring.
If the patient was scheduled for ventriculoperitoneal shunt procedure, the intraventricular
cannula was internalized into a shunt after recording the opening CSF pressure.
Fig. 1 (A and B) Intraventricular cannula and pressure transducer system. (C and D) Subdural cannula and pressure transducer system.
For subdural ICP monitoring, an intravenous cannula connected to a fluid-filled pressure
transducer was used. The de-aired system was placed subdurally once the first burr
hole was made by the neurosurgeon ([Fig. 1C] and [D]), and the value gets displayed on the monitor.
The transducer was zeroed at the level of the tragus of the patient on all occasions.
The system was not pressurized. When deemed necessary, the CSF was drained from the
proximal three-way stopcock of the transducer in the intraventricular ICP monitoring
system. ICP waveforms were distinct with intraventricular monitoring and were not
appreciable during subdural ICP monitoring ([Fig. 2A] and [B]).
Fig. 2 (A) Appreciable intraventricular pressure (ICP No) waveform and value (white arrow).
(B) Absolute ICP value without appreciable ICP waveform (white arrow).
Statistical Analysis
Data were analyzed using R software, version 3.5.2. A correlation was conducted using
Spearman's correlation coefficient and data graphed as a scatter plot. Agreement analysis
was conducted using the Bland–Altman plot, and a single-sample t-test was performed on the mean difference of ICP between the modalities. Null hypothesis
(H0) was that the difference between the techniques was considered as 0 mm Hg.
Results
Twelve patients were recruited into this study. Both subdural and intraventricular
ICP were monitored in nine patients and were included for analysis of the agreement.
The remaining three patients were included for assessing the feasibility of intraoperative
ICP monitoring using the subdural technique. Seven patients included in the agreement
analysis underwent insertion of a ventriculoperitoneal shunt, one underwent intracranial
aneurysm clipping, and one patient underwent strip craniectomy for craniosynostosis.
The subdural ICP and intraventricular ICP measured in each of the nine study subjects
are given in [Table 1]. There was a strong positive correlation between the two ICP monitoring techniques
with r
s = 0.93 (p = 0.01) ([Fig. 3A]). The mean difference of ICP between modalities was found to be 1.44 mm Hg (95%
confidence interval, −0.6 to 3.49, p = 0.122) ([Fig. 3B]).
Table 1
Subdural ICP and Intraventricular ICP values in study subjects
|
Sl. no.
|
Diagnosis
|
Subdural ICP (mm Hg)
|
Opening pressure of CSF (mm Hg)
|
GCS (immediate postop)
|
|
1
|
Left CP angle lesion
|
28
|
25
|
E4V5M6
|
|
2
|
GH secreting pituitary macroadenoma
|
27
|
23
|
E4V5M6
|
|
3
|
Thalamic lesion
|
33
|
35
|
E4V5M6
|
|
4
|
Sellar lesion
|
21
|
24
|
E4V5M6
|
|
5
|
Cerebellar hematoma
|
37
|
34
|
E3M4VT
|
|
6
|
Right CP angle lesion
|
31
|
31
|
E4V5M6
|
|
7
|
Pineal gland tumor
|
29
|
26
|
E3V3M6
|
|
8
|
Subarachnoid hemorrhage
|
24
|
21
|
E3VTM5
|
|
9
|
Craniosynostosis
|
16
|
14
|
E4V5M6
|
Abbreviations: CP, cerebellopontine; CSF, cerebrospinal fluid; E, eye opening; GCS,
Glasgow Coma Scale; GH, growth hormone; ICP, intraventricular pressure; M, motor response;
V, verbal response; VT, patient on tracheal tube.
Fig. 3 (A) Spearman's correlation coefficient scatter plot. (B) Bland–Altman agreement plot (mentioned as cm of H2O in the x-axis. It should be mm Hg. To be verified).
From the ICP values, it is apparent that the subdural ICP had consistently higher
values than intraventricular ICP. However, the mean difference was found to be 1.44 mm
Hg, and the difference from zero is not statistically significant (p = 0.12). The 95% confidence levels were also found to be large, which may be interpreted
as imprecise estimates. Both the points are probably related to the small sample size
of the study.
To explore the feasibility of continuous intraoperative ICP monitoring, subdural ICP
was measured in three patients using an intravenous cannula connected to the pressure
transducer. Subdural ICP monitoring in these patients helped to make critical intraoperative
clinical decisions based on the ICP values. Details of cases and the intraoperative
course are shown in [Table 2].
Table 2
Intraoperative course of patients in whom intraoperative subdural ICP was measured
|
Sl. no
|
Case details
|
ICP monitoring
|
ICP mm Hg
(baseline)
|
Interventions
|
Final ICP (mm Hg)
|
Target CPP in mm Hg
|
Patient outcome at discharge
|
|
1
|
20 year/male/right temporal contusion/admission GCS: E4V5M6/
8 hours after admission, GCS deteriorated to E3V2M4 with midline shift of 7 mm on
CT brain
Initial surgical plan: Right frontotemporal craniotomy and evacuation of contusion
and replacement of bone flap
|
Subdural
|
49
|
Head end elevation from 30 to 60 degrees
Brief hyperventilation (ETCO2 28 mm Hg)
|
39
25
|
60–70
|
Decompressive craniectomy performed based on baseline ICP
GCS E4V5M6, moving all four limbs
|
|
2
|
38 y/F/headache for 8 months, vomiting for 4 months. Headache is relieved by vomiting
suggesting poor cerebral compliance/admission GCS E4V5M6
Diagnosis: Supratentorial space-occupying lesion
|
Subdural
|
25
(with head end elevation,
Mannitol)
|
Change over from inhalational anesthesia to total intravenous anesthesia
|
8
|
60–70
|
GCS E4V5M6, moving all four limbs
|
|
3
|
6 months/M/craniosynostosis
|
Subdural
|
22
|
Strip craniectomy
|
9
|
Maintain ward MAP
|
Conscious, alert, playful and moving all four limbs
|
Abbreviations: CPP, cerebral perfusion pressure; CT, computed tomography; GCS, Glasgow
Coma Scale; ICP, intraventricular pressure; MAP, mean arterial pressure.
Discussion
Preoperative and postoperative ICP monitoring has been utilized for patient management,
particularly in patients with traumatic brain injury.[6] However, intraoperative ICP monitoring is seldom practiced. In neurosurgical patients
with other intracranial pathologies, perioperative ICP monitoring is limited to research
and not a routine practice. Through this feasibility and pilot validation study, the
utility of intraoperative ICP monitoring for clinical decision-making has been highlighted.
This study also proposes that subdural ICP is a satisfactory alternative to intraventricular
ICP monitoring in the intraoperative period. There are limited studies in the literature
comparing intraventricular ICP with subdural ICP. Olson et al, in their meta-analysis,
argued that intraventricular and intraparenchymal ICP values are not interchangeable
and there exists significant differences.[7] No such comparative studies exist for subdural ICP monitoring.
Use of intravenous cannula for subdural ICP monitoring is safe considering the short
length and narrow lumen of the catheter. In this study, subdural ICP was monitored
intermittently at various surgical points by inserting intravenous cannula in the
subdural location. Hence, chance of infection is rare. Hemorrhage is a possibility
as the insertion of cannula is a blind procedure. However, craniotomy would be relatively
large in patients with traumatic brain injury and bleeding could be well controlled.
Seizures are rare again as patients are operated for traumatic brain injury under
cover of antiseizure medication. While use of intraoperative subdural ICP monitoring
is an invasive technique, intraoperative ultrasonography can be a noninvasive technique
to detect mass effects and midline shift.
Monitoring subdural ICP in three patients demonstrated the feasibility of intraoperative
ICP monitoring and helped clinicians for the goal-directed management of patients
([Table 2]).
Case 1: Indications for ICP monitoring as per the Brain Trauma Foundation guidelines include
severe head injury with abnormal computed tomography (CT) scan. This patient presented
with Glasgow Coma Scale of 15 and normal CT; hence, ICP monitoring was not initiated.
However, he deteriorated during observation and was taken up for surgery. The decision
for decompressive craniectomy (DC) was based on the baseline intraoperative subdural
ICP (= 49 mm Hg). Demetriades studied ICP in patients after DC to understand the effectiveness
of DC for ICP reduction.[8] Nevertheless, literature is lacking for decisions on DC based on intraoperative
ICP values.
Case 2: In this case, the choice of total intravenous anesthesia (TIVA) was made based on
the subdural ICP values before the craniotomy was completed. A subdural ICP of 25 mm
Hg following the first burr hole with controlled intraoperative hemodynamics and ventilation
is deemed high. Thus, an objective criterion (ICP) was used to decide on the choice
of anesthetics. The use of TIVA versus inhalational agents for intraoperative management
of neurosurgical patients with raised ICP is a dilemma for most neuroanesthesiologists.
This case illustrates that decisions can be made based on intraoperative ICP values.
However, Petersen et al observed in their study that subdural ICP was comparable in
patients of supratentorial tumors with either inhalational agents or TIVA.[9]
Case 3: In this case, ICP was monitored before and after strip craniectomy. Incidence of
hydrocephalus in patients with craniosynostosis and syndromic association is anywhere
between 30 and 100%.[10]
[11] Whenever the ICP is higher after strip craniectomy, close patient observation in
the postoperative period becomes crucial. One should look for either symptoms suggestive
of raised ICP or objective ICP value to decide the need for CSF diversion.
Limitations
This study had several limitations. First, small sample size of the study is a major
limitation. More studies on the clinical outcome of patients with a larger sample
size are needed to stress the importance of intraoperative ICP monitoring. Second,
ICP waveform was not elicitable on the monitor during subdural monitoring. This could
be due to the rigid nature of the intravenous catheter. However, kinking and blockage
was ruled out at each instance of ICP monitoring by flushing the cannula. The ICP
values were reliable as the ICP changed with clinical scenarios in each patient.
Conclusion
Intraoperative subdural ICP monitoring is feasible using an inexpensive intravenous
cannula. Simple external pressure transducers may be used to monitor ICP values. Intraoperative
ICP can guide the neuroanesthesiologist and the neurosurgeon regarding optimal CPP,
the need for DC, and insertion of external ventricular drain as appropriate for efficient
clinical management. More studies on the clinical outcome of patients with a larger
sample size are needed to stress the importance of intraoperative ICP monitoring.
