Transcranial Doppler Ultrasonography in Intensive Care Unit. Report of a Case with Subarachnoid Hemorrhage and Brain Death and Review of the Literature

Transcranial Doppler (TCD) is increasingly utilized in patients with lifethreatening neurologic injury and has several practical applications in neurocritical care. It holds promise for the diagnosis and monitoring of vasospasm (VSP) in patients with subarachnoid hemorrhage (SAH) and the detection of increased intracranial hypertension. In addition, it has the ability to estimate flow velocity alterations, associated with critical decrease in cerebral perfusion pressure, in patients with clinical diagnosis of brain death (BD). It is easily performed, it is relatively inexpensive and non invasive and can aid intensivists in optimizing neurovascular dynamics for the individual patient. In general, TCD findings in different clinical scenarios can be useful tools for screening neurocritical patients during clinical trials for the development of new therapeutic treatments, leading to improvement in final outcome. In this article we will try to describe basic technical issues regarding TCD instrument and examination and we will also try to provide a description of indications and findings in patients with subarachnoid hemorrhage and brain death clinical diagnosis, through the presentation of a case report with SAH and subsequent vasospasm leading to BD, treated in the Intensive Care Unit of a University Hospital. Monitoring of physiologic functions is an important element of patient care in the Intensive Care Unit (ICU). Cardiovascular and pulmonary functions can be continuously monitored in ICU; however, the development of continuous monitoring of cerebrovascular function has lagged behind. Bedside measurement of cerebral blood flow (CBF) is a cumbersome task and requires expensive techniques that are not available in the ICU setting. Transcranial Doppler ultrasonography (TCD) was used in Neurology for the first time by Aaslid in 1982 [1] and is a non-invasive monitoring method that can be useful for bedside measurement of cerebral blood flow velocity (FV) and treatment response. It is portable, inexpensive, easily repeatable and non-invasive. Analysis of the FV waveform can indirectly provide information about CBF, cerebrovascular resistance and intracranial pressure (ICP) in neurocritically ill patients[2]. When monitoring cerebral heamodynamics, TCD can assess vessel patency, alterations in diameter of basal arteries of the Willis circle through the changes in FV, autoregulation (stable FV within a mean arterial pressure range of approximately 50-150 mmHg) and vascular reactivity to CO2[2-5]. TCD can also monitor vasospasm after SAH and estimate blood flow patterns in patients with stroke, head trauma, hydrocephalus and intracranial masses [6-10]. Furthermore, TCD can be of significant value for guiding therapeutic decisions and prediction of outcome. In this study we discuss methodology and basic guidelines for using TCD ultrasonography in Intensive Care Unit, Department of Neurosurgery Democritus University of Thrace, Alexandroupolis University Hospital The Greek E-Journal of Perioperative Medicine 2008; 6:95-104 (ISSN 1109-6888) www.anesthesia.gr/ejournal Ελληνικό Περιοδικό Περιεγχειρητικής Ιατρικής 2008; 6:95-104 (ISSN 1109-6888) www.anesthesia.gr/ejournal ©2008 Society of Anesthesiology and Intensive Medicine of Northern Greece ©2008 Εταιρεία Αναισθησιολογίας και Εντατικής Ιατρικής Βορείου Ελλάδος 96 the ICU, along with clinical applications in the diagnosis and monitoring of SAH and brain death. Furthermore, we present major TCD findings of a patient with SAH, complicated by cerebral vasospasm with severe intracranial hypertension that led to cerebral circulatory arrest. Examination and Doppler instrument Ultrasound examination of a vessel by means of TCD is referred to as insonation. The TCD probe is placed over different ‘acoustic windows’ that are specific areas of skull where the cranial bone is thin (Figure 1). The transtemporal window is used to insonate the middle cerebral artery (MCA), the anterior cerebral artery (ACA), the posterior cerebral artery (PCA) and the terminal portion of the internal carotid artery (TICA) before its bifurcation. The transorbital window is used for insonating the ophthalmic artery (OA) and the internal carotid artery at the siphon level, whereas the transforaminal (occipital) window allows insonation of the distal vertebral arteries (VA) and the basilar artery (BA)[11,12]. Although the temporal bone allows the best insonation of the brain and its vessels, there are still important limitations. An inadequate temporal acoustic window is the major cause of inaccurate TCD measurements and has a prevalence of approximately 14.5%. In addition, bone has the highest acoustic impedance, resulting in great attenuation of the ultrasonic signal before it reaches the brain[13]. All TCD devices use a pulsed Doppler system with a low-frequency, 1-2 MHz ultrasonic signal that provides adequate penetration through thin areas of the cranium and exploits the Doppler effect, in order to determine the speed and direction (velocity) of flow in blood vessels. According to Christian Andreas Doppler, the Doppler effect is the change in the frequency or wavelength of a wave due to relative movement between the sound source and the receiver. The change in frequency is called Doppler frequency shift and is the difference between the transmitted and reflected frequencies. A positive shift indicates flow towards the transducer and a negative shift implies the opposite. Accurate estimation of the Doppler frequency shift requires knowledge of the angle between the sound beam and the flow direction (angle of insonation θ) and is inversely related to the cosine of this angle (cos θ)[11]. The ultrasonic beam is produced by piezoelectric crystals that have been stimulated electrically. This beam bounces off the erythrocytes within the insonated artery. The reflected signal is received by the transducer and converted to an electrical signal. This information is subtracted from the transmitting signal and then processed to obtain a waveform that allows accurate determination of blood flow velocities and direction of flow. Because the calculation of blood flow velocity depends on the cosine of the angle of insonation, the highest velocities are found with a 0-degree angle (cos θ=1), whereas it becomes impossible to accurately determine velocity at insonation angles approaching 90 degrees (cos 90=0). There are two categories of Doppler transducers: those that transmit and receive frequencies continuously (continuous wave-CW Doppler) and those that transmit or receive intermittently (pulse wave-PW Doppler). During TCD ultrasonography, CW Doppler transducers cannot localize the depth of reflecting signal, Figure 1: TCD probe position over different acoustic windows of the skull. The Greek E-Journal of Perioperative Medicine 2008; 6:95-104 (ISSN 1109-6888) www.anesthesia.gr/ejournal Ελληνικό Περιοδικό Περιεγχειρητικής Ιατρικής 2008; 6:95-104 (ISSN 1109-6888) www.anesthesia.gr/ejournal ©2008 Society of Anesthesiology and Intensive Medicine of Northern Greece ©2008 Εταιρεία Αναισθησιολογίας και Εντατικής Ιατρικής Βορείου Ελλάδος 97 whereas PW Doppler transducers permit knowledge of depth of insonated vessels, and are exclusively used for blood flow velocity estimation. A hand-held transducer, which operates as both transmitter and receiver, is used by the technician who takes measurements at selected sites, 25-100 mm from the transducer[11,14]. In addition, transcranial ultrasonography may allow the examination of the parenchyma of the central nervous system (CNS) through the intact skull by means of B-mode ultrasonographic imaging techniques, with the aid of many different transducers of relative low frequency (2-3.5 MHz), allowing at the same time direct visualization of insonated vessels. Ultrasonographic B-mode imaging yields twodimensional image plan segments that are liberally angled and have relatively free axial or coronal orientations. This method is the standard procedure for static imaging and allows vessel visualization through transcranial colorcoded ultrasonography techniques (TCCS), (Figure 2)[15]. Except for flow velocities [peak systolic (PS), end-diastolic (ED), mean] estimation, TCD permits calculation of pulsatility index (PI) that is considered a reliable marker of resistance distal to the insonated site. It is easily calculated by the Gosling equation: PI= (Peak systolic-end diastolic FV)/Mean FV. Another useful measure is Pourcelot’s resistance index (RI) that is a measure of peripheral flow resistance. R I= (Peak systolic-end diastolic FV)/Peak systolic FV. High vascular resistance is characterized by low diastolic flow velocities and a high RI (>0.8), whereas reduced RI (<0.8) is associated with high diastolic flow and low vascular resistance[16-18]. Different vascular scenarios are related to different patterns of blood flow velocities and PI[19]. 1. Pure focal narrowing at the site of insonation will cause an increase in FV. 2. Narrowing or obstructing lesions proximal to the insonation site will cause a decrease in FV at the insonation site. 3. Distal (downstream) increased vascular resistance will decrease FV and increase PI proximal to lesion (PI>1.2). Different depth range, flow direction and normal age-related flow velocities have been established for each vessel (Table 1)[12]. Figure 2: Transcranial color-coded B-Mode ultrasonography (TCCS) of the left anterior cerebral circulation. The upper red coded segment is the middle cerebral artery (MCA), followed by the blue coded A1 segment of ipsilateral anterior cerebral artery (ACA). The two spots at the bottom of the image that are placed next to each other (red and blue coded) are the controlateral right A1 segment of ACA and the right P1 segment of the posterior cerebral artery (PCA) respectively. Table 1: Accepted Guidelines for a normal TCD examination Artery Window Depth (mm) Direction Mean flow velocity MCA ACA PCA TICA ICA (siphon) OA VA BA Temopral Temporal Temporal Temporal Orbital Orbital Occipital Occipital 30 to 60 60 to 85 60 to70 55 to 65 60 to 80 40 to 60 60 to 80 80 to 110 Toward probe Away Bidirectional Toward Bidirectional Toward Away Away 55 ±12 cm/sec 50 ±11 cm/sec 40 ±10 cm/sec 39 ±10 cm/sec 45 ±15 cm/sec 20 ±10 cm/sec 38 ±10 cm/sec 41 ±10 cm/sec MCA: mean cerebral artery, ACA: anterior cerebral artery, PCA: posterior cerebral artery, TICA: terminal internal carotid artery, ICA: internal carotid artery, VA: vertebral artery, BA: basilar artery. The Greek E-Journal of Perioperative Medicine 2008; 6:95-104 (ISSN 1109-6888) www.anesthesia.gr/ejournal Ελληνικό Περιοδικό Περιεγχειρητικής Ιατρικής 2008; 6:95-104 (ISSN 1109-6888) www.anesthesia.gr/ejournal ©2008 Society of Anesthesiology and Intensive Medicine of Northern Greece ©2008 Εταιρεία Αναισθησιολογίας και Εντατικής Ιατρικής Βορείου Ελλάδος 98