Dyspnea as a side effect of subthalamic nucleus deep brain stimulation for Parkinson's disease

Bilateral subthalamic nucleus deep brain stimulation for Parkinson's disease improves limb function. Unpublished observations from our clinic noted that some subthalamic nucleus deep brain stimulation patients complain of post-operative dyspnea. Therefore, we designed a prospective, longitudinal study to characterize this in greater depth. We used specific questionnaires to assess dyspnea in patients with electrodes in the subthalamic nucleus (n=13) or ventral intermediate thalamus (n=7). St. George's Hospital Respiratory Questionnaire symptom subscale scores were greater in subthalamic nucleus patients (median=18.60, interquartile range=40.80) than ventral intermediate thalamus patients (median = 0.00, interquartile range=15.38) at greater than 6 months post-operatively (p<0.05). Several of the subthalamic nucleus patients exhibited functional impairments as judged by the St. George's Hospital Respiratory Questionnaire impact subscale, the Medical Research Council Dyspnoea Scale, and the Dyspnoea-12 Questionnaire. There was no correlation between limb function ratings, stimulation parameters, or precise electrode position and dyspnea severity. We have shown, for the first time, that dyspnea can be a side effect of subthalamic nucleus deep brain stimulation, and that this dyspnea may be highly disabling.

Physiological noise in brainstem FMRI

The brainstem is directly involved in controlling blood pressure, respiration, sleep/wake cycles, pain modulation, motor, and cardiac output. As such it is of significant basic science and clinical interest. However, the brainstem's location close to major arteries and adjacent pulsatile cerebrospinal fluid filled spaces, means that it is difficult to reliably record functional magnetic resonance imaging (fMRI) data from. These physiological sources of noise generate time varying signals in fMRI data, which if left uncorrected can obscure signals of interest. In this Methods Article we will provide a practical introduction to the techniques used to correct for the presence of physiological noise in time series fMRI data. Techniques based on independent measurement of the cardiac and respiratory cycles, such as retrospective image correction (RETROICOR, Glover et al., 2000), will be described and their application and limitations discussed. The impact of a physiological noise model, implemented in the framework of the general linear model, on resting fMRI data acquired at 3 and 7 T is presented. Data driven approaches based such as independent component analysis (ICA) are described. MR acquisition strategies that attempt to either minimize the influence of physiological fluctuations on recorded fMRI data, or provide additional information to correct for their presence, will be mentioned. General advice on modeling noise sources, and its effect on statistical inference via loss of degrees of freedom, and non-orthogonality of regressors, is given. Lastly, different strategies for assessing the benefit of different approaches to physiological noise modeling are presented.

Pseudo-continuous arterial spin labelling MRI for non-invasive, whole-brain, serial quantification of cerebral blood flow following aneurysmal subarachnoid haemorrhage

Delayed cerebral ischaemia (DCI) is the major cause of mortality and morbidity following aneurysmal subarachnoid haemorrhage (SAH). Recent experimental evidence from animal models has highlighted the need for non-invasive and robust measurements of brain tissue perfusion in patients in order to help understand the pathophysiology underlying DCI. Quantitative, serial, whole-brain cerebral perfusion measurements were obtained with pseudo-continuous arterial spin labelling (PCASL) magnetic resonance imaging (MRI) in six SAH patients acutely following endovascular coiling. This technique requires no injected contrast or radioactive isotopes. MRI scanning was well tolerated. Artefact from endovascular coils was minimal. PCASL MRI was able to detect time-dependent and patient-specific changes in voxel-wise and regional cerebral blood flow. These changes reflected changes in clinical condition. Data obtained in healthy controls using the same experimental protocol confirm the reliability and reproducibility of these results. This is the first study to use whole-brain, quantitative PCASL to identify time-dependent changes in cerebral blood flow at the tissue level in the acute period following SAH. This technique has the potential to better understand changes in cerebral pathophysiology as a consequence of aneurysm rupture.

Understanding dyspnea as a complex individual experience

Dyspnea is the highly threatening experience of breathlessness experienced by patients with diverse pathologies, including respiratory, cardiovascular, and neuromuscular diseases, cancer and panic disorder. This debilitating symptom is especially prominent in the elderly and the obese, two growing populations in the Western world. It has further been found that women suffer more strongly from dyspnea than men. Despite optimization of disease-specific treatments, dyspnea is often inadequately treated. The immense burden faced by patients, families and the healthcare system makes improving management of chronic dyspnea a priority. Dyspnea is a multidimensional sensation that encompasses an array of unpleasant respiratory sensations that vary according to underlying cause and patient characteristics. Biopsychological factors beyond disease pathology exacerbate the perception of dyspnea, increase symptom severity and reduce quality of life. Psychological state (especially comorbid anxiety and depression), hormone status, gender, body weight (obesity) and general fitness level are particularly important. Neuroimaging has started to uncover the neural mechanisms involved in the processing of sensory and affective components of dyspnea. Awareness of biopsychological factors beyond pathology is essential for diagnosis and treatment of dyspnea. Increasing understanding the interactions between biopsychological factors and dyspnea perception will enhance the development of symptomatic treatments that specifically address each patient's most pressing needs at a specific stage in life. Future neuroimaging research can provide objective markers to fully understand the role of biopsychological factors in the perception of dyspnea in the hope of uncovering target areas for pharmacologic and non-pharmacologic therapy.

The effects of altered intrathoracic pressure on resting cerebral blood flow and its response to visual stimulation

Investigating how intrathoracic pressure changes affect cerebral blood flow (CBF) is important for a clear interpretation of neuroimaging data in patients with abnormal respiratory physiology, intensive care patients receiving mechanical ventilation and in research paradigms that manipulate intrathoracic pressure. Here, we investigated the effect of experimentally increased and decreased intrathoracic pressures upon CBF and the stimulus-evoked CBF response to visual stimulation. Twenty healthy volunteers received intermittent inspiratory and expiratory loads (plus or minus 9cmH2O for 270s) and viewed an intermittent 2Hz flashing checkerboard, while maintaining stable end-tidal CO2. CBF was recorded with transcranial Doppler sonography (TCD) and whole-brain pseudo-continuous arterial spin labeling magnetic resonance imaging (PCASL MRI). Application of inspiratory loading (negative intrathoracic pressure) showed an increase in TCD-measured CBF of 4% and a PCASL-measured increase in grey matter CBF of 5%, but did not alter mean arterial pressure (MAP). Expiratory loading (positive intrathoracic pressure) did not alter CBF, while MAP increased by 3%. Neither loading condition altered the perfusion response to visual stimulation in the primary visual cortex. In both loading conditions localized CBF increases were observed in the somatosensory and motor cortices, and in the cerebellum. Altered intrathoracic pressures, whether induced experimentally, therapeutically or through a disease process, have possible significant effects on CBF and should be considered as a potential systematic confound in the interpretation of perfusion-based neuroimaging data.

Delayed cerebral ischaemia after subarachnoid haemorrhage: looking beyond vasospasm

Despite improvements in the clinical management of aneurysmal subarachnoid haemorrhage over the last decade, delayed cerebral ischaemia (DCI) remains the single most important cause of morbidity and mortality in those patients who survive the initial bleed. The pathological mechanisms underlying DCI are still unclear and the calcium channel blocker nimodipine remains the only therapeutic intervention proven to improve functional outcomes after SAH. The recent failure of the drug clazosentan to improve functional outcomes despite reducing vasoconstriction has moved the focus of research into DCI away from cerebral artery constriction towards a more multifactorial aetiology. Novel pathological mechanisms have been suggested, including damage to cerebral tissue in the first 72 h after aneurysm rupture ('early brain injury'), cortical spreading depression, and microthrombosis. A greater understanding of the significance of these pathophysiological mechanisms and potential genetic risk factors is required, if new approaches to the prophylaxis, diagnosis, and treatment of DCI are to be developed. Furthermore, objective and reliable biomarkers are needed for the diagnosis of DCI in poor grade SAH patients requiring sedation and to assess the efficacy of new therapeutic interventions. The purpose of this article is to appraise these recent advances in research into DCI, relate them to current clinical practice, and suggest potential novel avenues for future research.

Systemic blood pressure, arterial stiffness and pulse waveform analysis at altitude

OBJECTIVES

Systemic arterial pressure rises on acute exposure to high altitude and changes in blood pressure (BP) and endothelial function may be important in the pathogenesis of clinical syndromes occurring at high altitude.

METHODS

Arterial BP, stiffness (SI) and tone (RI) were studied over 11 days in 17 subjects (three having mild hypertension) ascending to 3,450m and 4,770m using a non-invasive, finger photoplethysmography technique.

RESULTS

At 3,450m BP rose from mean 131/75 mmHg (SD 23/12) to 145/86 (23/12) and was maintained at this level (p < 0.001). SI did not change significantly from 8.5 m/sec (2.5) to 9.7 (3.2). RI fell during the first day at 3,450m from 74.4% (7.9) to 70.5% (13.8) (NS p > 0.05) and to 69.9% (12.0) (p < 0.02) at 4,770m but then reverted to baseline. Changes in SI and RI did not relate to changes in blood pressure. Changes in both arterial stiffness and tone were similar in those who developed AMS compared with those who did not. Baseline SI tended to be higher in the three subjects with hypertension 11.1m/sec (SD 2.7)) compared with the normotensives 8.3 m/sec (SD 2.7) (NS) and baseline RI lower 74.7% (7.0) compared with the normotensives 76.5% (8.5) (NS). Changes in SI and RI at altitude in the hypertensive subjects were similar to the non-hypertensive subjects.

CONCLUSIONS

We conclude that acute exposure temporarily affected endothelial function as measured by a change in vascular tone but this did not predict the development of AMS. The rise in arterial BP was not related to changes in arterial stiffness or tone.

Measurement of relative cerebral blood volume using BOLD contrast and mild hypoxic hypoxia

Relative cerebral blood volume (CBV) was estimated using a mild hypoxic challenge in humans, combined with BOLD contrast gradient-echo imaging at 3 T. Subjects breathed 16% inspired oxygen, eliciting mild arterial desaturation. The fractional BOLD signal change induced by mild hypoxia is expected to be proportional to CBV under conditions in which there are negligible changes in cerebral perfusion. By comparing the regional BOLD signal changes arising with the transition between normoxia and mild hypoxia, we calculated CBV ratios of 1.5 ± 0.2 (mean ± S.D.) for cortical gray matter to white matter and 1.0 ± 0.3 for cortical gray matter to deep gray matter.

Brainstem functional magnetic resonance imaging: disentangling signal from physiological noise

PURPOSE

To estimate the importance of respiratory and cardiac effects on signal variability found in functional magnetic resonance imaging data recorded from the brainstem.

MATERIALS AND METHODS

A modified version of the retrospective image correction (RETROICOR) method (Glover et al, [2000] Magn Reson Med 44:162-167) was implemented on resting brainstem echo-planar imaging (EPI) data in 12 subjects. Fourier series were fitted to image data based on cardiac and respiratory recordings (pulseoximetry and respiratory turbine), including multiplicative terms that accounted for interactions between cardiac and respiratory signals. F-tests were performed on residuals produced by regression analysis. Additionally, we evaluated whether modified RETROICOR improved detection of brainstem activation (in 11 subjects) during a finger opposition task.

RESULTS

The optimal model, containing three cardiac (C) and four respiratory (R) harmonics, and one multiplicative (X) term, "3C4R1X," significantly reduced signal variability without overfitting to noise. The application of modified RETROICOR to activation data increased group Z-statistics and reduced putative false-positive activation.

CONCLUSION

In addition to cardiac and respiratory effects, their interaction was also a significant source of physiological noise. The modified RETROICOR model improved detection of brainstem activation and would be usefully applied to any study examining this brain region.

The effect of remifentanil on respiratory variability, evaluated with dynamic modeling

Opioid drugs disrupt signaling in the brain stem respiratory network affecting respiratory rhythm. We evaluated the influence of a steady-state infusion of a model opioid, remifentanil, on respiratory variability during spontaneous respiration in a group of 11 healthy human volunteers. We used dynamic linear and nonlinear models to examine the effects of remifentanil on both directions of the ventilatory loop, i.e., on the influence of natural variations in end-tidal carbon dioxide (Pet(CO(2))) on ventilatory variability, which was assessed by tidal volume (Vt) and breath-to-breath ventilation (i.e., the ratio of tidal volume over total breath time Vt/Ttot), and vice versa. Breath-by-breath recordings of expired CO(2) and respiration were made during a target-controlled infusion of remifentanil for 15 min at estimated effect site (i.e., brain tissue) concentrations of 0, 0.7, 1.1, and 1.5 ng/ml, respectively. Remifentanil caused a profound increase in the duration of expiration. The obtained models revealed a decrease in the strength of the dynamic effect of Pet(CO(2)) variability on Vt (the "controller" part of the ventilatory loop) and a more pronounced increase in the effect of Vt variability on Pet(CO(2)) (the "plant" part of the loop). Nonlinear models explained these dynamic interrelationships better than linear models. Our approach allows detailed investigation of drug effects in the resting state at the systems level using noninvasive and minimally perturbing experimental protocols, which can closely represent real-life clinical situations.

Determination of the human brainstem respiratory control network and its cortical connections in vivo using functional and structural imaging

This study combined functional and structural magnetic resonance imaging techniques, optimized for the human brainstem, to investigate activity in brainstem respiratory control centres in a group of 12 healthy human volunteers. We stimulated respiration with carbon dioxide (CO(2)), and utilized novel methodology to separate its vascular from its neuronal effects upon the blood oxygen level dependent (BOLD) signal. In the brainstem we observed activity in the dorsal rostral pons (representing the Kölliker-Fuse/parabrachial (KF/PB) nuclei and locus coeruleus), the inferior ventral pons and the dorsal and lateral medulla. These areas of activation correspond to respiratory nuclei identified in recent rodent studies. Our results also reveal functional participation of the anteroventral (AV), ventral posterolateral (VPL) ventrolateral thalamic nuclei, and the posterior putamen in the response to CO(2) stimulation, suggesting that these centres may play a role in gating respiratory information to the cortex. As the functional imaging plane was limited to the brainstem and adjacent subcortical areas, we employed diffusion tractography to further investigate cortical connectivity of the thalamic activations. This revealed distinct connectivity profiles of these thalamic activations suggesting subdivision of the thalamus with regards to respiratory control. From these results we speculate that the thalamus plays an important role in integrating respiratory signals to and from the brainstem respiratory centres.

Measuring the effects of remifentanil on cerebral blood flow and arterial arrival time using 3D GRASE MRI with pulsed arterial spin labelling

Arterial spin labelling (ASL) has proved to be a promising magnetic resonance imaging (MRI) technique to measure brain perfusion. In this study, volumetric three-dimensional (3D) gradient and spin echo (GRASE) ASL was used to produce cerebral blood flow (CBF) and arterial arrival time (AAT) maps during rest and during an infusion of remifentanil. Gradient and spin echo ASL perfusion-weighted images were collected at multiple inflow times (500 to 2,500 ms in increments of 250 ms) to accurately fit an ASL perfusion model. Fit estimates were assessed using z-statistics, allowing voxels with a poor fit to be excluded from subsequent analyses. Nonparametric permutation testing showed voxels with a significant difference in CBF and AAT between conditions across a group of healthy participants (N=10). Administration of remifentanil produced an increase in end-tidal CO(2), an increase in CBF from 57+/-12.0 to 77+/-18.4 mL/100 g tissue per min and a reduction in AAT from 0.73+/-0.073 to 0.64+/-0.076 sec. Within grey matter, remifentanil produced a cerebrovascular response of 5.7+/-1.60 %CBF per mm Hg. Significant differences between physiologic conditions were observed in both CBF and AAT maps, indicating that 3D GRASE-ASL has the sensitivity to study changes in physiology at a voxel level.

Anaesthesia and high altitude: a history

During an expedition to climb Everest in 1933, expedition doctor Raymond Greene administered an open-drop chloroform anaesthetic to a Tibetan patient at an altitude of more than 14,000 feet. The patient's subsequent apparent cardiopulmonary arrest has long been attributed to the effects of altitude on anaesthetic delivery. However, anaesthetics can be safely administered at a wide variety of altitudes by adequately trained and experienced anaesthetists. The problems may have arisen from an inadequate depth of anaesthesia consequent to decreased chloroform vaporisation in a cold environment, Greene's concern about potential depression of ventilation and the contemporary lack of a precise approach to assessing depth of anaesthesia.

Delayed acclimatization of the ventilatory threshold in healthy trekkers

OBJECTIVE

To test the hypothesis that acclimatization to high altitude results in an improvement of the ventilatory threshold (VT).

METHODS

Eight lowlanders underwent cardiopulmonary exercise testing with a cycle ergometer to determine VT and peak oxygen uptake (Vo2peak) in Coventry, United Kingdom (altitude: 80 m), on arrival in leh, india (altitude: 3500 m), and after 12 days of acclimatization that included a 5-day high altitude trek up to 4770 m.

RESULTS

Vo2peak fell on arrival at 3500 m and remained depressed at 12 days. VT was depressed on arrival at high altitude and was further depressed at 12 days. VT as a proportion of the Vo2peak was decreased on arrival at high altitude, and after acclimatization, this relationship was further decreased.

CONCLUSIONS

Individuals who are sedentary or not participating in regular physical training appear to require a longer period of acclimatization than trained athletes. With the increasing numbers participating in high-altitude trekking and charity climbs of peaks, such as Mt. Kilimanjaro, this information has clinically significant practical implications for those leading or acting as medical advisors.

Dynamic forcing of end-tidal carbon dioxide and oxygen applied to functional magnetic resonance imaging

Investigations into the blood oxygenation level-dependent (BOLD) functional MRI signal have used respiratory challenges with the aim of probing cerebrovascular physiology. Such challenges have altered the inspired partial pressures of either carbon dioxide or oxygen, typically to a fixed and constant level (fixed inspired challenge (FIC)). The resulting end-tidal gas partial pressures then depend on the subject's metabolism and ventilatory responses. In contrast, dynamic end-tidal forcing (DEF) rapidly and independently sets end-tidal oxygen and carbon dioxide to desired levels by altering the inspired gas partial pressures on a breath-by-breath basis using computer-controlled feedback. This study implements DEF in the MRI environment to map BOLD signal reactivity to CO(2). We performed BOLD (T2(*)) contrast FMRI in four healthy male volunteers, while using DEF to provide a cyclic normocapnic-hypercapnic challenge, with each cycle lasting 4 mins (PET(CO(2)) mean+/-s.d., from 40.9+/-1.8 to 46.4+/-1.6 mm Hg). This was compared with a traditional fixed-inspired (FI(CO(2))=5%) hypercapnic challenge (PET(CO(2)) mean+/-s.d., from 38.2+/-2.1 to 45.6+/-1.4 mm Hg). Dynamic end-tidal forcing achieved the desired target PET(CO(2)) for each subject while maintaining PET(O(2)) constant. As a result of CO(2)-induced increases in ventilation, the FIC showed a greater cyclic fluctuation in PET(O(2)). These were associated with spatially widespread fluctuations in BOLD signal that were eliminated largely by the control of PET(O(2)) during DEF. The DEF system can provide flexible, convenient, and physiologically well-controlled respiratory challenges in the MRI environment for mapping dynamic responses of the cerebrovasculature.

Pharmacological FMRI: measuring opioid effects on the BOLD response to hypercapnia

Opioid binding to the cerebral blood vessels may affect vascular responsiveness and hence confound interpretation of blood oxygen level-dependent (BOLD) responses, which are usually interpreted as neuronal in origin. Opioid binding varies in different brain regions. It is unclear whether opioids alter neurovascular coupling, or whether their effects are purely neuronal. This study used BOLD functional magnetic resonance imaging (FMRI) to investigate the effect of a mu-opioid agonist remifentanil, on cerebrovascular CO(2) reactivity (being one component of neurovascular coupling). Hypercapnic challenges were delivered to human volunteers, while controlling potential opioid-induced respiratory depression. The BOLD signal increase to hypercapnia was compared before and during remifentanil administration. Remifentanil was shown not to have a generalised effect on CO(2) responsiveness in the cerebral vasculature. However, it caused a significant reduction in the positive BOLD response to hypercapnia in the bilateral primary sensorimotor cortices, bilateral extrastriate visual areas, left insula, left caudate nucleus, and left inferior temporal gyrus. We conclude that remifentanil does not modulate cerebrovascular CO(2) reactivity, as we saw no difference in BOLD response to hypercapnia in areas with high opioid receptor densities. We did however see a focal reduction in areas related to motor control and putative task activation, which we conclude to be related to changes in neuronal activity related to the sedative effects of remifentanil. Our method of controlling CO(2) levels effectively mitigated the potential confound of respiratory depression and allowed comparison over a similar range of CO(2) levels. We suggest that similar methodology should be used when investigating other potentially vasoactive compounds with FMRI.

Efficient breathing circuit for use at altitude

We describe a case report of a subject suffering high-altitude cerebral and pulmonary edema successfully treated with low flow rates of supplemental oxygen administered with a breathing system designed to conserve oxygen supplies at high altitude.

Effect of exercise on cerebral perfusion in humans at high altitude

The effects of submaximal and maximal exercise on cerebral perfusion were assessed using a portable, recumbent cycle ergometer in nine unacclimatized subjects ascending to 5,260 m. At 150 m, mean (SD) cerebral oxygenation (rSo2%) increased during submaximal exercise from 68.4 (SD 2.1) to 70.9 (SD 3.8) ( P < 0.0001) and at maximal oxygen uptake (V̇o2 max) to 69.8 (SD 3.1) ( P < 0.02). In contrast, at each of the high altitudes studied, rSo2 was reduced during submaximal exercise from 66.2 (SD 2.5) to 62.6 (SD 2.1) at 3,610 m ( P < 0.0001), 63.0 (SD 2.1) to 58.9 (SD 2.1) at 4,750 m ( P < 0.0001), and 62.4 (SD 3.6) to 61.2 (SD 3.9) at 5,260 m ( P < 0.01), and at V̇o2 max to 61.2 (SD 3.3) at 3,610 m ( P < 0.0001), to 59.4 (SD 2.6) at 4,750 m ( P < 0.0001), and to 58.0 (SD 3.0) at 5,260 m ( P < 0.0001). Cerebrovascular resistance tended to fall during submaximal exercise ( P = not significant) and rise at V̇o2 max, following the changes in arterial oxygen saturation and end-tidal CO2. Cerebral oxygen delivery was maintained during submaximal exercise at 150 m with a nonsignificant fall at V̇o2 max, but at high altitude peaked at 30% of V̇o2 max and then fell progressively at higher levels of exercise. The fall in rSo2 and oxygen delivery during exercise may limit exercise at altitude and is likely to contribute to the problems of acute mountain sickness and high-altitude cerebral edema.