Experimental studyTranscranial Doppler-derived indices of cerebrovascular haemodynamics are independent of depth and angle of insonation
Introduction
Transcranial Doppler ultrasonography (TCD) has developed into a widespread and diversely used tool in neurological diagnostics and brain-neurocritical care [1], [2], [3], [4], [5], [6] since its first description in 1982 by Aaslid et al. [7]. Especially TCD of the middle cerebral artery (MCA), monitoring cerebral blood flow velocity (CBFV), has established as a non-invasive instrument to assess multiple properties and aspects of cerebrovascular haemodynamics [8], [9] in the setting of neuromonitoring. The TCD-derived mean index of cerebral autoregulation (Mxa) has prognostic significance in a number of acute conditions relevant to neurocritical care, such as traumatic brain injury, aneurysmal subarachnoid haemorrhage and stroke [10], [11], [12]. Other relevant parameters of cerebrovascular function obtained from TCD are the critical closing pressure (CrCP) of the cerebral circulation [13], [14], [15], [16], the time constant of the cerebral arterial bed (τ or tau) [17], [18], [19] and the non-invasive estimator of ICP (nICP) [20]. These indices were selected for this study as they are based on continuous TCD readings and do not require external stimulation or manipulation of the patient’s cardiovascular system to test or monitor cerebrovascular function or autoregulation. In this respect they are suitable to investigate the influence of insonation of TCD reading on their values.
One of the main advantages of TCD for monitoring these various indices of cerebrovascular function is its non-invasive technique, as compared to invasive methods using surgically implanted probes to measure intracranial pressure or brain tissue oxygen. However, TCD is highly operator-dependent and the angle and depth of blood vessel insonation can significantly change the measured CBFV [21], [22], [23]. For example, slight variation in the position of a standard 2 MHz TCD probe only for a couple of millimetres on the patient’s head, thus changing the angle of MCA insonation, can significantly change the CBFV readings. Whilst investigators would typically aim to obtain the best TCD signal with a handheld probe, this can be more difficult when the TCD probe is mounted to a headframe, as commonly done for continuous TCD measurements over prolonged periods of time. It is unknown if this variation in CBFV readings due to probe position also affects the calculation of the TCD-derived parameters Mxa, CrCP, tau or nICP.
The aim of this study was to evaluate the effect of different angles and depths of MCA insonation on multiple TCD-derived parameters describing the function of the cerebral circulation. We investigated this by performing continuous synchronous ipsilateral CBFV recordings of the MCA using two adjacent TCD probes at two different depths of insonation each in patients with acute cerebral insults.
Section snippets
Methods
Eight patients were included in this study. Table 1 presents demographic information and clinical characteristics of these patients.
The ethics committee of South Western Sydney health district approved this study (HREC/12/LPOOL/110; SSA/12/LPOOL/205; project number 12/069). Written informed consent was obtained from the next of kin. All patients required sedation and artificial ventilation due to injury severity and were treated according to accepted disease specific clinical standards.
Primary acquired data and resulting secondary indices
An overview of the primarily recorded measurements, averaged over the whole period of the recording for each patient are provided in table 3. These demonstrate that the synchronously acquired intraindividual CBFV measurements 1–4 diverge markedly. Table 4 shows the values of Mxa, CrCP, tau and nICP for each patient.
Statistical comparison of the synchronous measurements and derived indices demonstrates that the secondary indices Mxa, tau, CrCP are not influenced by the modality of insonation
As required, the longitudinal clustering algorithm identified two cluster curves (pairings) within each individual and index. The average (across patients) ratio BSS/TSS was
Discussion
This study analysed the effects of different angles and depths of TCD insonation of the MCA on the readings of CBFV and derived indices of cerebrovascular function Mxa, tau, CrCP and nICP in a small cohort of patients with acute cerebral insults.
While there are several other indices established to test and monitor cerebrovascular function and autoregulation, these four indices were selected because they allow to investigate the influence of TCD insonation characteristics. Other indices, as the
Conclusion
Our pilot study showed that the mean index of cerebral autoregulation, Mxa, is likely not significantly affected by the depth and angle of TCD insonation. This also accounts for the cerebrovascular indices CrCP and tau. Hence, despite inter-operator variability in measuring CBFV by TCD, this non-invasive method can be used to calculate and monitor these indices of cerebrovascular function. Robustness of nICP to the TCD insonation conditions was inconclusive and requires further investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
RG was partially supported by CMUP (UID/MAT/00144/2013), which is funded by FCT (Portugal) with national (MEC) and European structural funds (ERDF), under the partnership agreement PT2020.
MC was supported by NIHR Medical Informatics Cooperative. PS and MC have interest in a part of ICM+ software licensing fee (Cambridge Enterprise Ltd, UK).
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