Neuromonitoring

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 ).

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.

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.

  

• 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.

• 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 ).

  

• 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.

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.

      

  • 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.

    $\text{Static}\text{rate}\text{of}\text{regulation}\left(\text{sRoR}\right)=100\times \%\frac{\text{CVRe}}{\text{MAP}}$

    where CVRe = MAP/FV; sRoR of “1” indicates perfect autoregulation and “0” indicates an absence of autoregulation.

• 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

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

(A1—amplitude of fundamental frequency component of arterial pressure, F1—amplitude of fundamental frequency component of FV.) Fundamental frequency is calculated by fast Fourier transformation of the waveform.

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.

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.

• 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.

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.

Values

Normal: 5–15 mm Hg (healthy adults, supine); 3–7 mm Hg (children); 1–5 mm Hg (infants). Cutoff values for treating ICP depends on the intracranial pathology. For head injury, treatment is initiated when the ICP exceeds 20–25 mm Hg.

Pathophysiology

High ICP is an indicator of brain injury and also results in secondary brain injury by reducing CBF ( Fig. 8.6 ).

Waveform Analysis

ICP is made up of three components: arterial vascular component, cerebrospinal fluid (CSF) circulatory component, and cerebral venous outflow component. Normal waves have three peaks: percussion (P1), tidal (P2), and dicrotic (P3) ( Fig. 8.7 ).

During reduced compliance, the P2 merges with P1 or exceeds P1.

Three types of waveforms (Lundberg) has been reported:

• A waves (plateau waves) : In patients with reduced intracranial compliance, systemic hypotension results in cerebral vasodilation leading to increase in CBV and hence ICP. The ICP values reach up to 40 mm Hg and stays there for 5–15 min. When the duration of plateau waves exceeds >30 min, there are high chances for cerebral ischemia. Due to intact autoregulation, blood pressure rises that reverses the phenomenon. This condition should always be treated to prevent cerebral ischemia and herniation syndromes.

• B waves : The frequency is around 0.5–2/min with amplitudes going up to 20–30 mm Hg. These waves indicate vasomotor center instability due to low CPP or at the lower end of cerebral autoregulation.

• C waves : Frequency is around 4–8/min with amplitudes around 20 mm Hg. This wave has been documented in healthy individuals.

Pressure–Volume Relationship

Pressure and volume relationship within the intracranial compartment is nonlinear. Intracranial compliance is defined as the changes in volume for a given change in pressure. The inverse of compliance is called elastance and the relationship is nonlinear. The slope of this relationship in the logarithmic scale is linear and is described in terms of pressure–volume index (PVI). PVI is the volume required to change the ICP by 10-fold (PVI = ΔV/log 10 Po/pm). Normal value is around 20–25 mL. This is calculated by withdrawal of around 2 mL of CSF and noting the pressure change. This procedure is repeated multiple times with aspirating and injecting saline into the catheter, and the pressure changes noted. Reduced compliance occurs even before the ICP values go high. On the other hand, high PVI values can be seen in patients with normal CPP but defective cerebral autoregulation. Caution should be exercised when interpreting PVI inpatients whose CPP is below the autoregulatory threshold.

Pressure Reactivity Index

Pressure reactivity index (PRx) describes the changes in smooth muscle tone of the arterial walls in relation to the changes in transmural pressure. It is calculated as a linear correlation coefficient between averaged ICP and arterial blood pressure over a 3–5 min period. Negative PRx index indicates good pressure reactivity index. The index helps in identifying patients who might benefit from targeted CPP therapy as increasing CPP will reduce ICP only in patients with intact vasomotor reactivity. CPP at which PRx is lowest is the optimal CPP.

Indications for Intracranial Pressure Monitoring

• Severe head injury with abnormal CT head scan

• Severe head injury with normal head CT with any of the two—age >40 years, systolic blood pressure <90 mm Hg, and abnormal motor posturing

• Systemic diseases causing raised ICP, e.g., Rye’s syndrome, hepatic failure

• In patients with head injury where clinical neurological examination is not possible for prolonged periods of time (e.g., patients undergoing general surgery)

• In intracerebral and SAH

• Malignant cerebral infarction

• Hydrocephalus, meningitis

Recording

EEG is recorded from the cup electrodes or needle electrodes from the scalp. Meticulous cleaning of the area is important to remove the oil and dead cells from the surface. Thus the electrical resistance (impedance) is decreased and signal conduction is increased. A pair of electrodes is called as a montage. The montage can be bipolar or referential. In the bipolar montage both the electrodes are active (i.e., electrodes lie on the underlying cerebral cortex and are capable of recording the electrical activity of the cortex), and the voltage difference between the two electrodes is recorded. In referential montage only one electrode is active and the other one is common referential electrode which is commonly on the mastoid, ear lobe, or the shoulder. The focal lesions/changes are better picked up by the bipolar and diffuse changes are better picked up by referential (unipolar) montages. Intraoperative monitoring is more commonly done with bipolar montage.

The 10–20 system of electrode placement is commonly employed. It is 10% or 20% of the nasion–inion line or preauricular line or hemi-circumference line. The midline electrodes are designated as “z” and the right-sided electrodes as even numbers and left-sided electrodes as odd numbers. There are frontal prominence, frontal, central, parietal, and occipital electrodes. Artifacts are common during EEG recording. Many filters are used to decrease these artifacts. However, most of the filters are frequency filters which will filter low-frequency and high-frequency waveforms. Artifacts within clinically useful range of frequency are difficult to eliminate [electromyogram (EMG), blinking, motion, cautery, etc.] ( Fig. 8.8 ).

Normal EEG

It is a plot of voltage versus time of 1–35 Hz frequency. Based on the frequency, the waveforms are classified into different bands. Normally higher is the frequency, lower is the amplitude ( Table 8.4 ).