Abstract
Monitoring the cerebrum is a complex phenomenon mainly because a small part of the brain will be controlling a major physiological function and the damage can result in a devastating complication. To ensure that there is no major injury to the brain, all parts of the brain need to be monitored continuously, both anatomically and physiologically. This is an onerous task. Thus, various modalities of cerebral function monitoring have come into vogue. This chapter mainly deals with the physiological monitoring of the brain. The cerebral blood flow monitoring is done either as a global aspect or a regional aspect. The various intracranial pressure monitoring techniques and devices are discussed. Electrophysiological monitoring of the brain and its role in assessing the functioning of the neuronal cells and the tracts are discussed. Cerebral oxygenation is assessed by jugular venous oximetry, regional cerebral oximetry, direct brain tissue oxygen monitoring. Cerebral microdialysis in assessing cerebral oxygenation is also discussed.
Keywords
EEG, Evoked potentials, Monitoring, Neuromonitoring, Transcranial Doppler
Outline
Introduction 134
Cerebral Blood Flow 134
Transcranial Dopplerf0010 134
Assumptions 135
Technique 135
Pulsatility Index 135
Uses of Transcranial Doppler 135
Limitations of Transcranial Doppler 138
Transcranial Sonography 139
Thermal Diffusion Flowmetry 139
Laser Doppler Flowmetry 139
Intra-Arterial 133 Xenon 139
CT Perfusion 139
Xenon Enhanced CT 139
Positron Emission Tomography 140
Single Photon Emission Computed Tomography 140
Magnetic Resonance Imaging 140
Intracranial Pressure 140
Technology 140
Values 141
Pathophysiology 141
Waveform Analysis 141
Pressure–Volume Relationship 142
Pressure Reactivity Index 142
Indications for Intracranial Pressure Monitoring 142
Electroencephalogram 143
Evoked Potential Monitoring 145
Somatosensory Evoked Potentials 145
Brain Stem Auditory Evoked Potential 147
Stimulus Characteristics 147
Normal Waveforms 148
Factors Affecting Brain Stem Auditory Evoked Potentials 148
Uses 148
Visual Evoked Potential 149
Motor Evoked Potentials 149
Depth of Anesthesia 150
Cerebral Oxygenation Monitoring 152
Jugular Venous Oximetry 152
Technology 153
Normal Values 153
Indications 153
Limitations 154
Complications 154
Contraindications 154
Regional Cerebral Oximetry 154
Equipment 155
Normative Values (Based on INVOS Device) 156
Factors Influencing rSO 2 Values 156
Indications 156
Limitations 156
Brain Tissue Oxygen Monitoring 156
Technology 156
Probe Placement 157
PbtO 2 Values 157
O 2 Reactivity Index 157
Uses 157
Limitations 157
Complications 158
Cerebral Microdialysis 158
Conclusion 159
References 159
Introduction
It is important to monitor continuously the organ that we want to see perform correctly at all the times. Brain is a very complex organ to monitor, and in fact we can call the brain as an organ of organs. There are many structures that have to be monitored separately and continuously, if we have to ensure the correct functioning of all those structures. However, with so many independent structures, it is impossible to monitor all of them continuously. Thus, the neuromonitoring is always a challenging task and the information gained may not always transform into clinical utility. However, with so many modalities available, and in combination, we are able to gain some insight into the pathophysiology of the brain. We are now able to use the neuromonitoring in clinically managing the patients.
Cerebral Blood Flow
Transcranial Dopplerf0010
TCD is the most commonly used technique to measure the cerebral blood flow (CBF).
The TCD uses a principle of Doppler sound waves where the waves reflected from a moving object are at a higher frequency than the origin frequency ( Fig. 8.1 ). Similarly, reflected waves from the object moving away will be at a lower frequency than the origin frequency. The ultrasound waves were used to measure the CBF velocity at basal arteries. The RBCs in the vessels act as a moving object toward the probe or away from the probe.
Assumptions
- 1.
TCD is measuring the blood flow velocity (FV) in the vessel and not the actual flow. However, we presume the FV is equivalent to flow.
- 2.
The diameter of the vessel remains constant. This is debatable but few studies have shown that vessel diameter indeed remains constant in many conditions.
- 3.
The angle of insonation has to remain constant for comparing the measurements.
Technique
The technique commonly used is 2 MHz probe as the waves have to penetrate through the bone. Higher the frequency, lower is the capability to penetrate the bone. Most commonly used site is transpterional, where the bone thickness is the least. It is just above the zygus, about 1–2 cm anterior to tragus. Keeping the probe perpendicular to the skin and directed slightly anterior will frequently encounter the middle cerebral artery (MCA). The MCA is identified with flow toward the probe at a depth of 50–65 mm and with characteristic flow sound ( Fig. 8.2 ). From that point, with slight manipulation, it is possible to trace back to internal carotid artery (ICA) bifurcation: anterior cerebral artery (ACA) and posterior cerebral artery. Transorbital (carotid artery) and suboccipital (basilar and vertebral arteries) approaches are also used in some situations ( Table 8.1 ).
| Artery | FV (cm/s) | Depth of Insonation (mm) | Direction of Flow | Effect of Ipsilateral Carotid Compression |
|---|---|---|---|---|
| Middle cerebral artery | 40–60 | 45–60 | Toward | Decreases |
| Anterior cerebral artery | 40–50 | 60–75 | Away | Decreases |
| Posterior cerebral artery | 30–45 | 60–75 | Toward | No change |
| Carotid art (orbital) | 35–50 | 60–80 | Toward | Decreases |
| Basilar | 35–50 | 60–80 | Away | No change |
| Vertebral | 30–45 | 80–110 | Away | No change |
Pulsatility Index
Pulsatility refers to peak systolic to lowest diastolic FV. With constant cerebral perfusion pressure (CPP), any change in pulsatility reflects the change in the cerebrovascular resistance (CVR), i.e., higher the CVR, higher is the pulsatility index (PI). The normal PI is 0.5–1.0, which is a dimensionless number.
PI = FVsys − FVdia FVmean
Uses of Transcranial Doppler
- 1.
Subarachnoid hemorrhage (SAH): Commonly used to diagnose the vasospasm to quantify the degree of vasospasm and its response to treatment. Any FV more than 120 cm/s with PI of more than 1.5 is considered indicative of vasospasm ( Table 8.2 & Fig. 8.3 ). Many centers use TCD regularly to monitor SAH patients .
Table 8.2
Transcranial Doppler Classification of the Severity of Vasospasm
From Kassab MY, Majid A, Farooq MU, Azhary H, Hershey LA, Bednarczyk EM, Graybeal DF, Johnson MD. Transcranial Doppler: an introduction for primary care physicians. J Am Board Fam Med 2007; 20 :65–71.
Severity of Vasospasm
MFV Value (cm/s)
MCA/ICA Ratio
Normal
<85
<3
Mild
<120
<3
Moderate
120–150
3–5.9
Severe
151–200
>6
Critical
>200
>6
ICA , anterior cerebral artery; MCA , middle cerebral artery.
Figure 8.3
Middle cerebral artery in cerebral vasospasm with increased pulsatility index of 1.83 and peak FV of 92 cm/s.
- 2.
Carotid endarterectomy (CEA): TCD is used to identify many things:
- a.
Ischemia: During cross clamping, FV decrease <40% of baseline is considered as ischemia. Simultaneous monitoring of electroencephalogram (EEG) will help in better delineation of ischemia. Generally, surgeon considers placement of shunt if the FV < 40%. There is a study further classified the decrease as mild (16–40% of baseline value) and ≤15% as severe.
- b.
Emboli: Detection of emboli is easy with TCD. Surgeon can modify their technique to decrease emboli. Detection of emboli will also help in the prediction of postoperative cognitive deficits.
- c.
Identification of the shunt malfunction.
- d.
Hyperemia: Postoperatively patient may develop sudden hyperemia due to vasoparalysis in the ischemic area. This can lead to cerebral edema and hemorrhage. TCD can identify the patients early and thus, preventive measures can be instituted.
- e.
Postoperative ischemia: Carotid occlusion at operative site is a lethal complication. TCD can help in identifying these patients before total occlusion by identifying decrease in the FV.
- a.
- 3.
Head Injury: It is useful in identifying cerebral vasospasm. Cerebral vasospasm develops in 20–30% of patients with head injury. TCD is used to measure intracranial pressure (ICP) and CPP noninvasively. It is also used to assess the presence or absence of autoregulation and carbon dioxide reactivity. It is also used to diagnose the brain death where typical oscillatory flow is seen with intact skull ( Fig. 8.4 ).
Figure 8.4
Transcranial Doppler in brain death. Only little flow is seen entering the middle and anterior cerebral arteries (near carotid bifurcation) with each beat during systole.
- 4.
Other uses:
- a.
Cardiac surgery: TCD is used to detect emboli during cardiac surgery and also to measure the CBF during cardiopulmonary bypass.
- b.
Hepatic encephalopathy: TCD is used to assess the ICP and CPP noninvasively because of bleeding risks with invasive monitoring.
- c.
Eclampsia: TCD is used to assess the ICP and CPP noninvasively.
- d.
TCD is used in noninvasive assessment of CBF in diverse conditions.
- a.
TCD is also used in:
- 1.
Testing of pressure autoregulation : With intact autoregulation, any changes to the arterial pressure does not affect any change in CBF (measured by FV with TCD). Both the static and dynamic autoregulation can be tested using TCD.
- a.
Dynamic tests:
- i.
Dynamic autoregulation : Sudden hypotension is induced by deflation of a large thigh cuff [at least 20 mm Hg drop in mean arterial pressure (MAP)]. The FV also drops immediately, but recovers within few seconds. With continuous TCD monitoring this drop and recovery are mapped. This map will be compared with standardized graphs and the degree of autoregulation would be said [autoregulatory index (ARI)]. Normally the return of autoregulation is complete within 5 s, i.e., dynamic rate of autoregulation (dRoR) is 20%/s. This measurement has to be done within 10 s of deflation of thigh cuff to avoid confusion due to carbon dioxide (CO 2 ) changes.
- ii.
Transient hyperemic response test : This is the most commonly used test due to ease of testing. The baseline FV is recorded and the carotid artery is compressed for 5–8 s and released. With the compression the FV would decrease and after the release, FV would increase due to cerebral vascular dilation in response to ischemia. This increase would return back to baseline within 5 s with intact autoregulation. If the autoregulation is impaired, there would be no hyperemia as there would be no cerebral vasodilation in response to ischemia ( Fig. 8.5 ). The success of the test depends on the adequate compression of the carotid artery.
Figure 8.5
Transient hyperemic response test in a patient with normal autoregulation.
- i.
- b.
Static tests : The MAP is raised by 20 mm Hg by vasopressor (phenylephrine) infusion. The change in the FV would be minimal if the autoregulation is intact. The FV would increase if the autoregulation is impaired.
<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='Staticrateofregulation(sRoR)=100×%CVReMAP’>Staticrateofregulation(sRoR)=100×%CVReMAPStaticrateofregulation(sRoR)=100×%CVReMAPwhere CVRe = MAP/FV; sRoR of “1” indicates perfect autoregulation and “0” indicates an absence of autoregulation.
Static rate of regulation ( sRoR ) = 100 × % CVRe MAP
- a.
- 2.
Cerebrovascular CO 2 reactivity : With every mm Hg change in CO 2 , there will be 3–4% change in the CBF in the same direction until limitation/saturation develops, i.e., within a range of 20–80 mm Hg CO 2 levels. TCD FV can be used instead of CBF measurements in assessing the change in CBF to change on CO 2 levels. It is considered that the diameter of basal arteries remains constant with the changes in CO 2 levels. It is also assumed that only the distal vessel diameter is altered with CO 2 . Therefore TCD is used to assess the CBF changes to CO 2 levels, i.e., percentage of change in the FV to percentage of change in CO 2 levels. T is commonly used in head-injured patients to assess the CO 2 reactivity and to prognosticate these patients.
- 3.
Noninvasive assessment of ICP : One of the important uses of TCD is noninvasive assessment of ICP and CPP. Many techniques of assessment have been described but still they have not achieved the perfection to be used in clinical management of patients.
The estimated CPP by TCD is calculated using the formula
CPPe = MAP × FVd / FVm + 14 mmHg
It is calculated both the sides and averaged. Authors were able to achieve <15 mm Hg error in 92% (<10 mm Hg error in 89%) of measurements.
Initially, Aaslid et al. described
CPPe = FVm × A 1 / F 1
Other techniques are based on the following:
- 1.
Increased pulsatility
- 2.
Decreased diastolic FV
- 3.
Decreased ratio of diastolic to mean FV
Limitations of Transcranial Doppler
- 1.
It is a blind procedure. The accuracy depends on the individual doing it.
- 2.
In 5–10% of patients, insonation is not possible because of thick bone.
- 3.
Difficult to detect the distal branches.
Transcranial Sonography
Recently transcranial B-mode ultrasound is used to monitor brain parenchyma. Repeated measurements of ventricular size, midline shift, intracerebral hemorrhage size, optical nerve sheath diameter to monitor raised ICP is possible. The technology is used in the early diagnosis of Parkinson disease, other movement disorders, sleep disorders, treatment of stroke, etc. Even though at present the utility is very limited, the technology holds great promise to future.
Thermal Diffusion Flowmetry
The principle is that two sensors are placed nearby and one sensor is heated and the temperature is measured by the other sensor. The temperature difference between the sensors is inversely proportional to the thermal conductivity of the brain tissue between the sensors. There are many types of sensors and many techniques of heating and measuring the temperature difference. The assumption is thermal conductivity is constant in all the individuals. The probe is about 1 mm thickness, which is placed deep into the brain tissue in the arterial territory of interest. It gives the absolute values of CBF and almost instantaneous (1–2 s) and continuous measurement. Operative lights, irrigation of surgical fields, febrile patients can cause problems in measuring. However, thermal diffusion flowmetry is slowly getting popular and many feel that there is a greater role for this modality in monitoring the CBF in neurological patients.
Laser Doppler Flowmetry
Laser Doppler flowmetry (LDF) measures the CBF noninvasively (with open skull), semi quantitatively, but continuously over the cortical surface. It detects the Doppler shift of laser light reflected from the RBCs in a small volume of cortical tissue. It can be used to scan a large surface of cortical tissue. It has been used intraoperatively to detect both ischemia and hyperemia.
Intra-Arterial 133 Xenon
Kety–Schmidt technique using the Fick principle perfected the measurement of CBF. The method quantifies the difference between cerebral washin and washout of freely diffusible inert gas (N 2 O) by serial measurements of arterial and jugular bulb blood concentrations of the tracer. Now it is well known that the N 2 O is not an inert gas, thus 133 Xe is used. The tracer is injected into carotid artery and washout is recorded by multiple external scintillation counters. The rate at which the tracer is washed out is proportional to CBF. With appropriate mathematical equations, gray and white matter blood flow calculation is possible.
Carotid puncture is not possible every time and thus, noninvasive techniques have been developed, i.e., inhalational 133 Xe and intravenous 133 Xe. These noninvasive techniques also provide reproducible results with a reasonable spatial resolution.
CT Perfusion
The iodinated contrast is injected and simultaneously images are acquired using a helical CT multislice scanner in a cine mode which allows for the measurement of CBF and cerebral blood volume (CBV). This technique is relatively fast and can be done in most of the CT scanners. Clinically this method can be used to measure the perfusion of the brain in many clinical scenarios, e.g., perfusion of the brain in severe traumatic brain injury (TBI) and hypoperfused areas in the SAH patients.
Positron Emission Tomography
Both CBF and metabolism can be measured with positron emission tomography (PET) scan. Regional CBF, regional CBV, regional oxygen extraction fraction (rOEF), and regional cerebral metabolic rate of oxygen (rCMRO 2 ) from the whole brain can be obtained. Kety–Schmidt technique is used to measure CBF. Resolution is about 4–6 mm. However, it is not useful in emergency settings and used mainly in the research settings because of high cost.
Single Photon Emission Computed Tomography
Single photon emission computed tomography is similar to 133 Xe technique described earlier but is reconstructed in three dimensions with a rotating camera. Whole brain is covered and takes about 10–15 min for the study. Absolute values are difficult to obtain. It has slightly less resolution but much cheaper than the PET scan.
Magnetic Resonance Imaging
Many magnetic resonance imaging (MRI) techniques measuring CBF have been developed. The most successful approaches are dynamic tracking of a bolus of a paramagnetic contrast agent (dynamic susceptibility contrast) or on arterial spin labeling. Whole brain is covered and anatomical localization is possible. Good resolution is possible but absolute values are difficult to obtain.
Intracranial Pressure
It is defined as the pressure within cranial cavity relative to the atmospheric pressure.
Technology
ICP can be measured by either invasive or noninvasive techniques.
- 1.
Invasive
- a.
Fluid-filled external pressure transducer : This is similar to arterial pressure and central venous pressure monitoring. A catheter is inserted in to the lateral ventricle and is connected to the transducer (Wheatstone bridge) through fluid-filled tubing.
- b.
Miniature strain gauge transducer (Codman) : In this, the sensor is placed in the tip of the catheter. Changes in pressures cause change in resistance of the circuit within the sensor and is interpreted as a waveform. Zeroing has to be done preinsertion and once placed cannot be rezeroed in vivo. Hence, zero drifting can occur over a period of days and results in false ICP values.
- c.
Fiberoptic (Camino) : The sensor at the catheter tip uses a light source. Pressure changes cause change in the light reflection and this is quantified as pressure change. Zeroing has to be done preinsertion and once placed cannot be rezeroed in vivo.
- d.
Spielberg ICP system : In this system, a fluid-filled catheter has an air balloon pouch at the tip of the catheter. A fluctuation in balloon pressure is interpreted as ICP change.
- a.
- 2.
Noninvasive:
- a.
Tympanic membrane displacement.
- b.
TCD: Various formulae have been described to estimate CPP noninvasively (see the detailed discussion earlier).
- c.
Optic nerve sheath diameter: Measurement is taken 3 mm behind the globe. It is qualitative. Diameter >6 mm is highly indicative of raised ICP.
- a.
As ICP measures the pressure inside the cranial cavity, the catheter can be placed in the epidural/subdural/intraparenchymal or ventricular space ( Table 8.3 ). Ventricular measurement is considered as gold standard. Epidural (Richmond, Gaeltec) and subdural sites are less commonly used. For intraparenchymal monitoring (Codman, Camino), a burr hole is placed separately or as a part of triple bolt system into the nondominant frontal region. The tip is usually placed in to the white matter.
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