Electrical modulation of the sympathetic nervous system in order to augment cerebral blood flow: a protocol for an experimental study.

Introduction Cerebral blood flow (CBF) is regulated by several mechanisms. Neurogenic control has been a matter of debate, even though several publications reported the effects of changes in sympathetic tone on CBF. Transcutaneous electrical nerve stimulation and spinal-cord stimulation have been shown to influence peripheral and cerebral blood flow through a sympathetic pathway. The authors hypothesise that certain pathological conditions result in a relative increase in the neurogenic regulation of CBF and that this regulation can be modulated electrically. Methods and analysis Patients with cerebral vasospasm after subarachnoid haemorrhage will be included. The experimental set-up measures several parameters that are involved in cerebral blood flow regulation in patients with cerebral vasospasm after subarachnoid haemorrhage. Measurements are taken at baseline and with stimulation in several frequencies. An ad hoc statistical analysis is used to evaluate different settings of the electrical stimulation. Autoregulation is evaluated with transfer function analysis and autoregulatory index calculations. Ethics and dissemination Ethical registration was granted by Medical Review Ethics Committee Groningen (ID METc 2010.123). All participants provide written informed consent on participation. Upon finishing a pilot study to investigate feasibility and effect, either future prospective (randomised) studies will be designed, or other modalities of electrical stimulation will be explored using the same set-up. Trial Registration Dutch Trial Registry: NTR2358.

Introduction

Cerebral blood flow (CBF) is determined by cerebral perfusion pressure and cerebral vascular resistance (CVR) (figure 1). Both are regulated by complex mechanisms. Cerebral perfusion pressure depends on intracranial pressure (ICP) and mean arterial blood pressure (MAP), which in turn is a resultant of cardiac output and systemic vascular resistance. CVR is mainly regulated by cerebral vasomotor autoregulation, chemoreflex control (based on arterial carbon dioxide and oxygen concentrations (Paco2 and Pao2)), local metabolic processes and nervous activity.1–4 The anatomical and physiological bases of neurogenic control of CBF have been extensively studied. The cerebral vessels are thought to be directly sympathetically innervated in two ways: (1) extrinsic by the cervical sympathetic nervous system (analogous to other parenchymal vascular territories) and (2) intrinsic by central pathways of the locus coeruleus and other brainstem vasomotor centre origin.2 5–7

Figure 1

Regulatory mechanisms of cerebral blood flow. CBF, cerebral blood flow; CO, cardiac output; CPP, cerebral perfusion pressure; CVR, cerebral vascular resistance; ICP, intracranial pressure; MAP, mean arterial blood pressure; P, pulse; PaCO2, arterial carbon dioxide pressure; PaO2, arterial oxygen pressure; SV, stroke volume; SVR, systemic vascular resistance; 8/π×η×l/r4, Poiseuille's law; where η=blood viscosity, l=vessel length and r=vessel radius. Overlap and interactions between metabolic, chemical, neurogenic and vasomotor autoregulation are not shown.

Even though several studies have shown changes in CBF as a result of sympathetic blockage or modulation in humans,1 8–12 the influence of the latter remains debatable.13–16 In physiological resting conditions, the sympathetic effects on CVR seem to be minor, but in non-resting and particularly pathological conditions, the effects of sympathetic tone on CVR and consequently CBF become more apparent.12 17 18 Indirect effects of the autonomic nervous system influence CBF by altering MAP, pulse and cardiac output—for example, through the arterial and cardiopulmonary baroreflexes.11 12

All pathways can be affected by pathological conditions. Known examples are arterial hypertension, carotid stenosis, ischaemic or haemorrhagic cerebral vascular accidents, traumatic brain injury and cerebral vasospasm.18–22 It is postulated that in pathological conditions, such as subarachnoid haemorrhage or ischaemic stroke, cerebral autoregulation is (focally) decreased, resulting in a relative increase in sympathetic regulation. Therefore, in these conditions, sympathetic pathways can be relevant, while in a normal resting state, they are over-ruled by stronger mechanisms. In these conditions, modulation of the sympathetic nerve activity on cerebral vessels could be of therapeutic importance.

In this context, electrical nervous stimulation is attractive, since several studies have shown the effects of electrical nervous stimulation on peripheral and cerebral blood flow. For example, it has been demonstrated that cervical spinal cord stimulation (cervical SCS) increases CBF,23–25 which is thought to be caused by inhibition of the sympathetic nervous system26 27 and the release of vasoactive substances.28 Both intrinsic and extrinsic systems have been postulated to be the path of action for cervical spinal cord stimulation induced increase in CBF.27 29 30 Vascular calibre can directly reflect adrenergic tone and sympathetic receptor sensitivity, as demonstrated in experimental and animal models of vasospasm in SAH.31 32 Studies with spinal cord stimulation as a treatment of patients with coronary vasospasm showed ameliorated coronary perfusion independent of MAP, as did studies using transcutaneous electrical nerve stimulation (TENS).33–35 Also, studies showed an inhibitory effect of TENS on sympathetically mediated reflexes.36–38

The decision to study the effects of TENS on CBF is empowered by the following motivations:

Cervical SCS has the ability to increase CBF.23–25 This effect must be indirect because the electrical field does not surpass the sympathetic pathways.39

If the effect of SCS is indeed indirect, and (antidromic or orthodromic) neuronal pathways lead to modulation of sympathetic pathways in the spinal cord, then TENS may have the same effect.

TENS may have a mild effect on reflexes involving the autonomic nervous system.37

TENS and SCS are interchangeable in pain management, suggesting they affect similar pathways.

TENS is non-invasive and safe.40

The aim of the study set-up is to investigate the following hypotheses:

CBF can be influenced by electrical stimulation (either TENS or SCS) through the sympathetic nervous system.

Neurogenic regulation of CBF is relatively increased when cerebral autoregulation is diminished in certain cerebrovascular diseases.

As a model, patients with cerebral vasospasm were selected, since, in those patients, the strong autoregulatory mechanisms are diminished by the disease. This makes them better candidates than healthy subjects with intact cerebral autoregulation in order to show proof of concept. Since cerebral vasospasm after SAH is often asymmetrical, these patients are ideal candidates to test both hypotheses at the same time. If we can show that autoregulation is asymmetrically impaired, and CBF can be electrically augmented on one side, a neurogenic pathway is likely to play a different role on each side.

Methods and analysis

Sample selection

The patient-selection criteria are listed in box 1. In order to study the feasibility of application of TENS in subjects after SAH and in order to collect data for a future power analysis, a pilot study will be performed in 10 subjects.

Confirmed aneurysmatic subarachnoid haemorrhage

Cerebral vasospasm demonstrated by transcranial Doppler, defined as a middle cerebral artery/internal carotid artery ratio >3

Aneurysm is treated with a surgical or endovascular procedure

Age >18 years

Treatment can be started promptly

Informed consent signed by patient or family

History of cervical spine or skull-base surgery

Known adverse reaction to trancutaneous electrical neurostimulation pads

Presence of any implanted electronic device (including pacemakers)

Pre-existing disease that can obscure follow-up

Unacceptable interference with ECG registration (in case intensive care is necessary)

Insufficient temporal bony windows

Use of sympathicomimetic or sympathicolytic agents

Intervention

The subjects are treated with conventional TENS, providing a continuous flow of symmetrical rectangular biphasic pulses (Schwa Medico, Pierenkemper GMBH, Ehringhausen, Germany). Stimulation will be performed at 90% of the highest tolerated current, with a pulse-width of 200 μs. Stimulation frequencies are 20, 50, 100 and 120 Hz. In this way, the entire range of possible TENS frequencies and the most used frequencies in current TENS treatments are captured. Subjects are supine during the entire experiment. The TENS electrodes are applied cervical at the height of the mandible on both sides of the nuchal ligament (dermatome C2–C3).

Data collection and integration

An experimental set-up was designed to measure and influence the sympathetic regulatory mechanisms of CBF (by TENS). The set-up is planned to determine CBF (represented by cerebral blood-flow velocity), as well as to continuously monitor as many factors that influence CBF as possible (figure 1).

Data are collected using a continuous transcranial Doppler (TCD) monitor (DIGI-LITEtm, Raanana, Israel) to measure cerebral blood flow velocities (CBFV) in the middle cerebral artery (MCA) on both sides, a plethysmograph for assessing blood pressure and pulse (Finometer-Pro, Finapress Medical Systems, Amsterdam, The Netherlands), a capnograph (Capnomac Ultima, GE Healtcare, Chalfont St Giles, UK) to measure respiration rate and end-tidal carbon dioxide concentration (ETCO2), and a near-infrared spectroscope (Invos 5100C, Somanetics, Troy, Michigan) to measure cerebral oxygenation. All analogue output is routed to a computer via a digitiser. The data are continuously registered using Labview 9.0 software. Raw data are sampled at 250 Hz. Beat-to-beat averages are calculated using the arterial blood pressure curve for triggering. To verify data quality and any relevant changes in all variables, both raw data and the calculated averages are plotted in waveform charts that are continuously updated. Both the raw data and the calculated averages will be stored in digital format on a PC for further off-line analysis.

Ad hoc statistical analysis

In order to determine the optimal frequency of stimulation, an ad hoc analysis is performed. Each time, a data stream acquired with a specific TENS frequency is compared with baseline. Stable sections of data of the same length with least artefacts will be selected for analysis after visual inspection. In order to clean the data from artefacts, the top and bottom 5% of data will be deleted, replacing those by linear interpolation using in-house written routines in the Matlab (6.5) environment. A visual inspection of the plotted filtered data will take place, and when necessary the same filter can be run a second time, or a data section with less noise will be selected.

Using the Matlab environment, matrices of data are compared with baseline using the Student t test, since a sufficient number of observations will be available. An effect size is calculated for the significant differences over time (with baseline as anchorpoint) in order to determine the frequency with the greatest amount of change since baseline. If none of the frequencies shows an effect of more than 20% of the pooled SD,41 no superiority can be shown.

Analysis of autoregulation

Most patients will have more severe vasospasm on one side. This allows for a comparison of dynamic autoregulation.

Since the absolute values for MCA velocity can be influenced by many factors, including slight alteration of probe position and change in spontaneous autonomic activity, a frequency domain analysis will be performed. This is largely independent of minor fluctuations in the absolute values, since normalisation is used, and phase differences are less dependent on absolute values. This facilitates comparison of serial measurements.

The transfer function analysis42 and the Autoregulatory Index (ARI) are evaluated,43 estimated from spontaneous oscillations in blood pressure and TCD parameters, which results in a phase-difference parameter for the transfer function analysis, and an ARI index. Previously, values for the phase difference between the mean ABP and TCD values in the low-frequency range of >50° were suggested to represent intact cerebral autoregulation.44 45 The values for the ARI index range from 0 to 9, with 9 indicating perfect autoregulation, and 0 meaning complete absence of autoregulation.

The beat-to-beat data are resampled at 10 Hz to create a uniform timebase. Also, the data are detrended, normalised and subtracted by 1, creating zero-mean signals. A Hanning window is applied to the data. Segments of data with 512 samples each are used to estimate the cross-spectra and transfer function between the mean ABP and TCD signals. Spectral averaging was employed using the Welch method, using segments with 50% overlap, which results in a spectral average that is calculated over at least nine segments of data (ie, five epochs of 51.2 s). The phase difference is determined in the 0.06–0.12 frequency range according to preset rules, provided coherence is >0.5.42 Data on other frequency ranges will also be provided. For the ARI index, the inverse fast Fourier transform is calculated, using a cut-off frequency of 1.0 Hz. The first 10 s of the impulse response function is integrated to yield an estimate of the step response. The resulting curve is compared with the original Tiecks curves46 by using a least-squares fitting procedure.

Interpretation

The described set-up is based on a few assumptions regarding several factors that play a role in CBF regulation. ICP probably remains constant during the experiments, even though fluctuations in CBF and venous pressure can affect ICP. We have no reason to suspect blood-viscosity changes during a measurement. We assume metabolic autoregulatory mechanisms to remain constant during the experiments; especially in our patients with cerebral vasospasm, we think they are absent. Oxygenation can be safely assumed to be constant when respiratory rate and room oxygen concentration remain constant. In order not to disturb these factors, all experiments take place in a quiet room and in a supine position.

Several factors (figure 1) have to be taken into account before the conclusion can be drawn that a change in CBFV measured in the MCA is most probably caused by a change in sympathetic activity. First of all, there should be no major changes in blood pressure and Paco2 (represented by ETco2 in our set-up), since changes in these variables can have a profound effect on CBFV. Furthermore, any large changes in pulse are also undesirable, since this might indicate a generalised sympathetic activation, and not a more focal sympathetic activation by TENS. Since we cannot be certain if the effects of TENS, if any, are mediated by the intrinsic or extrinsic sympathetic pathways, we can expect changes in vessel diameter of the MCA or the cerebral arterioles, or possibly even both.

CBF is determined indirectly by measuring CBFV in the MCA with TCD. In normal subjects, the MCA diameter is fairly constant, so flow velocity is proportional to cerebral flow, and arteriolar diameter is the most important regulator. If arteriolar diameter increases, resistance to flow will decrease, and CBFV (in the MCA) will increase, and vice versa. By contrast, in cerebral vasospasm, the opposite situation exists. In vasospasm, TCD measures intrastenotic blood-flow velocity, and blood-flow velocity is inversely related to CBF. In this situation, the relation CBFV=CBF/cross-sectional area can be applied, that is, if the MCA diameter increases, CBFV decreases.

Therefore, in this experiment, the finding of a decrease or increase in CBFV cannot be interpreted unambiguously. Based on TCD alone, even if we include indices of vascular resistance (pulsatility), we cannot determine with sufficient certainty if proximal or distal diameter changes may have occurred. We use the measurement of cerebral oxygenation (by NIRS) of the frontal lobe (downstream of the CBFV measurement in the MCA) as another modality to indirectly estimate changes in CBF. Provided brain metabolism, oxygen extraction, blood pressure and Paco2 remain unaltered, an increase in oxygenation indicates vasodilatation and an increase in CBF, while a decrease indicates vasoconstriction and a decrease in CBF. Using both TCD and NIRS facilitates interpretation of data, provided the other factors remain stable (table 1). Once again, any substantial alterations in MAP, ETco2 and pulse would make interpretation more difficult. Still, even though a raised ETco2 or MAP can explain an increase in CBFV, a decrease in CBFV under these circumstances cannot be disregarded and must be explained otherwise.

Table 1

Theoretically possible measurements and the most probable consequences for vessel diameter, provided the mean arterial blood pressure and end tidal carbon dioxide concentration remain unchanged

Measurement		Vessel diameter		Interpretation
Cerebral blood-flow velocity	Cerebral oxygen saturation	Middle cerebral artery	Arteriolar
↑	↓	↓	↔	Vasospasm increase
↑	↑	↔	↑	Arteriolar dilatation
↓	↑	↑	↔	Vasospasm decrease
↓	↓	↔	↓	Arteriolar constriction

Ethics and dissemination

Ethics and safety considerations

All potential participants will be informed fully about the study procedures and known risks. All subjects or their relatives will provide written informed consent. They will have the opportunity to withdraw from the study at any time.

Studies on SCS and CBF have produced no stimulation-related complications, only surgery-related complications such as infection, electrode displacement, etc. The TENS electrodes will be attached to the skin, which may cause skin irritation or rash comparable with sunburn. No serious adverse events are known from TENS in general. A previous study has reported that TENS can be safely applied in the cervical region.40

Our study protocol has been approved by the Medical Review Ethics Committee Groningen (ID METc 2010.123).

Dissemination

We hope to disseminate our study results through conferences and journal publications. If effects of TENS can be found in patients with cerebral vasospasm after SAH and TENS is shown to be a feasible application of electrical stimulation in this patient population, this set-up will be used in future prospective (randomised) controlled trials. If TENS is shown not to be feasible, other methods of neurostimulation such as subcutaneous electrical stimulation or spinal cord stimulation will be explored using the same set-up.

In conclusion, this set-up can be used to investigate regulation of CBF in several cerebrovascular diseases. Besides this, the application of ad hoc statistical analysis allows for the optimisation of several settings (frequency, current, etc) of electrical stimulation in one session, which facilitates research on electrical modulation of CBF.