Original Contribution
Improved Parameterization of the Transcranial Doppler Signal

https://doi.org/10.1016/j.ultrasmedbio.2012.03.016Get rights and content

Abstract

The great potential of transcranial Doppler (TCD) as a tool for neuromonitoring is limited by the current parameterization of the signal. This article proposes a set of new parameters that more accurately represents the shape of the waveform and eliminates a number of confounding factors. This set of parameters was tested in 227 patients with ipsilateral carotid artery stenosis and compared with 31 normal subjects recruited at our laboratory. From the TCD waveform, we calculated on a beat-to-beat basis the maximal change in flow velocity at stroke onset (acceleration or acc), the maximal flow velocity during the first 100 ms of systole (sys1) and the maximal flow velocity in the remaining part of systole (sys2). All data were normalized relative to the mean diastolic flow velocity over an interval ranging from 520 till 600 ms after stroke onset (dias@560). For the group with carotid stenosis compared with the normal controls the average ± SD for acc (20.2 ± 9.5 vs. 20.2 ± 6.7; p = 0.98) and sys1 (1.82 ± 0.38 vs. 1.77 ± 0.56; p = 0.35) did not differ significantly. The average ±SD for sys2 (1.94 ± 0.33 vs. 1.50 ± 0.12; p < 0.001), however, was significantly higher in the group with carotid stenosis than in the group of normal subjects. The difference between sys1 and sys2 (“sys1–sys2”) was lower in the patient group than in controls (−0.12 ± 0.16 vs. 0.27 ± 0.22; p < 0.001). For the acc, there was a significantly higher variance in the group with stenosis than without (p < 0.001). Of the old parameters, the beat-to-beat mean (37.0 ± 13.1 vs. 41.3 ± 15.9; p = 0.17) and the pulsatility index (PI; 1.00 ± 0.26 vs. 0.91 ± 0.23; p = 0.06) were not significantly different between groups. Graphed together the acc and “sys1–sys2” parameters allowed a clear demarcation of both groups whereas in a graph of the old parameters mean and PI both groups overlapped considerably. In conclusion, the proposed set of new parameters not only has theoretical and practical benefits but also has excellent discriminative power in a group of carotid patients compared with normal controls.

Introduction

Transcranial Doppler (TCD) can theoretically contribute to the monitoring of critically ill patients in whom brain circulation is at stake (Aaslid et al. 1982; Bhatia and Gupta 2007). However, despite, the well defined relation between pulsatility index (PI) and intracranial pressure (ICP) (e.g., Bellner et al. 2004), TCD on its own has not achieved a prominent position on (neuro-) intensive care (ICU). TCD in combination with a continuous registration of arterial pressure (ABP) and intracranial pressure (ICP) is still under investigation (e.g., Czosnyka et al. 2003). In most ICUs, therapy remains restricted to optimization of systemic ABP and breathing parameters without detailed information about its effects on brain circulation.

To improve the capability of TCD for neurovascular monitoring, two important problems need to be addressed. First, performing a TCD investigation is operator dependent. This problem is addressed by self-searching TCD probes currently being introduced to the market. Second, the interpretation of the TCD signal depends on a great number of technical and physiologic factors contributing to the signal in various ways.

Regarding the technical factors, it should be emphasized that TCD measures flow velocity (FV) and not flow. Therefore, the measurements are expressed in meters per second instead of volume per second. The flow velocity may vary with the vessel's diameter, which is usually unknown (e.g., Lunt et al. 2004). Furthermore, the angle of the insonating beam with the flow direction of the artery may differ from one investigation to another. This, too, causes variation in FV measurements.

Regarding the physiologic factors, the middle cerebral artery flow velocity (MCAFV) depends on proximal influences such as heart rate, stroke volume, arterial blood pressure, possible obstruction in carotid arteries, on distal factors such as peripheral resistance, intracranial pressure etc., as well as on rheologic factors such as hematocrit and blood viscosity. All these factors, of which many are not readily known, should ideally be taken into account when interpreting the MCAFV signal (McCartney et al. 1997).

Many clinical TCD studies are conveniently based upon the parameters provided by commercial TCD apparatus (e.g., Hanlo et al. 1995; Rainov et al. 2000). We should, however, realize that this set of parameters has major theoretical drawbacks. These are summarized in Table 1. First, peak systolic flow velocity is the maximal flow velocity encountered anywhere during systole. TCD-manufacturers have made no distinction between early and late systole. This article will demonstrate that it is important to distinguish two phases in systole: stroke onset (or sys1) and the remaining part of systole (sys2). Second, the variation in insonation angle and vessel diameter result in large standard deviations in measurements of mean, diastolic and systolic flow velocities, which complicate interindividual comparison. This effect can be removed when all FV measurements are taken relative to a reference measurement of diastolic FV. Finally, for the calculation of a pulsatility index (PI), most commercial apparatus rely on a mean flow velocity and an end-diastolic flow velocity. By their definition, both measurements largely depend on heart rate. To avoid heart rate dependency, we propose that the reference diastolic flow velocity mentioned above should be taken at a fixed time interval with respect to stroke onset.

These theoretical considerations have led to the formulation of a new set of parameters. We tested this new set of parameters in a group of patients with significant internal carotid artery stenosis in comparison with a group of normal subjects without stenosis.

Section snippets

Materials and Methods

At our hospital, all patients selected for carotid surgery routinely undergo a preoperative assessment of intracranial hemodynamics by means of TCD. The rationale for this assessment is formed by the belief that patients with more severe hemodynamic disturbance from carotid stenosis have a greater risk upon postoperative hyperperfusion (Sbarigia et al. 1993; Reigel et al. 1987; Jorgensen and Schroeder 1993; Schaafsma et al. 2002). Hemodynamic status can be determined by measuring CO2-reactivity

Results

To better grasp the differences in FV wave morphology we determined values for acceleration, meaning the maximal steepness of the FV increase at stroke onset and for the first and second systolic peak normalized with respect to the dias@560 (see Methods section). These new parameters were compared with the parameters used traditionally: the mean and PI.

Table 2 contains data on the statistical analysis of all parameters when comparing the group with significant carotid stenosis to a group of

Discussion

As outlined in the Introduction section and summarized in Table 1, the current parameterization of the TCD signal suffers from important theoretical drawbacks. This is illustrated by Figure 3. The TCD parameters most often used, namely the mean and PI, are unable to distinguish a patient group with significant ipsilateral stenosis from a normal group, even though, in addition, there was a significant age difference between both groups. This article introduces a new set of parameters aimed to

Conclusion

The present data shows that the proposed set of TCD parameters allows for a good discrimination of carotid artery patients from normal controls. In the future, we aim to further test these parameters in health and disease and thereby increase our understanding of the interpretation of the TCD signal. Ultimately, we hope that TCD will evolve into the neurovascular monitoring tool it was anticipated, given its bedside availability and high temporal resolution.

Acknowledgments

The author gratefully acknowledges the support of Anuschka Niemeijer, Research Institute Martini Ziekenhuis Groningen, for statistical analyses and textual remarks and of Stefan van Duijvenboden and Bernard J. Geurts for help with the automatic analysis of sys1 and sys2 components.

The author is owner of MEAR Holding BV, an institution aiming to improve neuromonitoring on the ICU. MEAR Holding BV. holds an international patent on ‘PaR Technology’, based on a simultaneous analysis of arterial

References (18)

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