Anna L. Hudson

Spatial distribution of inspiratory drive to the parasternal intercostal muscles in humans

The human parasternal intercostal muscles are obligatory inspiratory muscles with a diminishing mechanical advantage from cranial to caudal interspaces. This study determined whether inspiratory neural drive to these muscles is graded, and whether this distribution matches regional differences in inspiratory mechanical advantage. To determine the neural drive, intramuscular EMG was recorded from the first to the fifth parasternal intercostals during resting breathing in six subjects. All interspaces showed phasic inspiratory activity but the onset of activity relative to inspiratory flow in the fourth and fifth spaces was delayed compared with that in cranial interspaces. Activity in the first, second and third interspaces commenced, on average, within the first 10% of inspiratory time, and sometimes preceded inspiratory airflow. In contrast, activity in the fourth and fifth interspaces began after an average 33% of inspiratory time. The peak inspiratory discharge frequency of motor units in the first interspace averaged 13.4 ± 1.0 Hz (mean ±s.e.m.) and was significantly greater than in all other interspaces, in particular in the fifth space (8.0 ± 1.0 Hz). Phasic inspiratory activity was sometimes superimposed on tonic activity. In the first interspace, only 3% of units had tonic firing, but this proportion increased to 34% in the fifth space. In five subjects, recordings were also made from the medial and lateral extent of the second parasternal intercostal. Both portions showed phasic inspiratory activity which began within the first 6% of inspiratory time. Motor units from the lateral and medial portions fired at the same peak discharge rate (10.4 ± 0.7 versus 10.7 ± 0.6 Hz). These observations indicate that the distribution of neural drive to the parasternal intercostals in humans has a rostrocaudal gradient, but that the drive is uniform along the mediolateral extent of the second interspace. The distribution of inspiratory neural drive to the parasternal intercostals parallels the spatial distribution of inspiratory mechanical advantage, while tonic activity was higher where mechanical advantage was lower.

The effect of lung volume on the co‐ordinated recruitment of scalene and sternomastoid muscles in humans

The human scalenes are obligatory inspiratory muscles that have a greater mechanical advantage than sternomastoid, an accessory muscle. This study determined scalene and sternomastoid recruitment during voluntary inspiratory tasks, and whether this activity varied with lung volume, when feedback from the lungs and inspiratory muscles would differ. If afferent feedback has a major role in determining the recruitment of the scalenes and sternomastoid, then at each lung volume, activity would be altered. Intramuscular EMG from scalene and sternomastoid muscles, and oesophageal pressure were recorded while subjects (n= 7) performed inspiratory isovolumetric ramps to maximal inspiratory pressure (MIP) and dynamic inspirations from functional residual capacity (FRC) to total lung capacity (TLC). The static inspiratory ramps were repeated at three lung volumes: FRC, FRC + tidal volume, and TLC. To determine the profile of inspiratory activation, i.e. the initial and ongoing recruitment of the muscles, the root mean square of the EMG was measured throughout the tasks. Scalene was recruited early, and EMG increased with pressure, reaching a plateau at 80% MIP. In contrast, sternomastoid activity began later, but then increased with pressure from 20 to 100% MIP. Similar profiles of activation occurred at all three lung volumes (n.s.). The ratio of sternomastoid to scalene EMG was also the same irrespective of the initial lung volume (n.s.). In dynamic inspirations, scalene and sternomastoid activation had similar stereotypical profiles to the static tasks, but scalene EMG was 15–40% greater (P < 0.05). Sternomastoid activation was the same in both tasks (n.s.). These results suggest that in voluntary tasks, scalene and sternomastoid are recruited in the order of their mechanical advantages, and that alterations in feedback related to changes in lung volume failed to alter their activation. Thus, in humans, the mechanism responsible for the differential activation of these two inspiratory muscles has an element that is preset.

Reproducible measurement of human motoneuron excitability with magnetic stimulation of the corticospinal tract.

It is difficult to test responses of human motoneurons in a controlled way or to make longitudinal assessments of adaptive changes at the motoneuron level. These studies assessed the reliability of responses produced by magnetic stimulation of the corticospinal tract. Cervicomedullary motor evoked potentials (CMEPs) were recorded in the first dorsal interosseus (FDI) on 2 separate days. On each day, four sets of stimuli were delivered at the maximal output of the stimulator, with the final two sets>or=10 min after the initial sets. Sets of stimuli were also delivered at different stimulus intensities to obtain stimulus-response curves. In addition, on the second day, responses at different stimulus intensities were evoked during weak voluntary contractions. Responses were normalized to the maximal muscle compound action potential (Mmax). CMEPs evoked in the relaxed FDI were small, even when stimulus intensity was maximal (3.6+/-2.5% Mmax) but much larger during a weak contraction (e.g., 26.2+/-10.2% Mmax). CMEPs evoked in the relaxed muscle at the maximal output of the stimulator were highly reproducible both within (ICC=0.83, session 1; ICC=0.87, session 2) and between sessions (ICC=0.87). ICCs for parameters of the input-output curves, which included measures of motor threshold, slope, and maximal response size, ranged between 0.87 and 0.62. These results suggest that responses to magnetic stimulation of the corticospinal tract can be assessed in relaxation and contraction and can be reliably obtained for longitudinal studies of motoneuronal excitability.

An algorithm for the safety of costal diaphragm electromyography derived from ultrasound.

Introduction: Costal diaphragm electromyography (EMG) remains unpopular due to the risk of pneumothorax. In this study we assessed the safety of the “trans‐intercostal” method of diaphragm EMG using B‐mode ultrasound. Methods: Twenty healthy subjects participated in this investigation. The diaphragm and the lung were visualized in the most distal intercostal space (dICS) with ultrasound. The risk of pneumothorax was assessed at the mid‐clavicular, anterior, and mid‐axillary levels, during normal and deep breathing, in supine and upright postures. Results: The dICS at the anterior axillary level was the safest landmark for diaphragm EMG during normal breathing, with the subject supine. The mid‐clavicular level is the least optimal location for EMG. The upright position and deep breaths increase the risk of pneumothorax. Conclusions: The safety of the trans‐intercostal method of diaphragm EMG depends on the anatomic level chosen to insert the needle, patient position, and breathing pattern. Hence, we have developed a safety algorithm for electromyographers. Muscle Nerve, 2012

Coupling between mechanical and neural behaviour in the human first dorsal interosseous muscle

The neural drive to a muscle and its biomechanical properties determine the force at a joint. These factors may be centrally linked. We studied the relationship between the ability of first dorsal interosseous muscle (FDI) to generate index flexion force around the metacarpophalangeal joint and the neural drive it receives in a voluntary contraction. The role of FDI was assessed in two thumb postures, thumb ‘down’ (thumb abducted) and thumb ‘up’ (thumb extended), and at different thumb carpometacarpal angles. These postures were designed to change acutely the flexion moment arm for FDI. The flexion twitch force evoked by supramaximal stimulation of the ulnar nerve was measured in the two postures and the change in moment arm was assessed by ultrasonography. Subjects also made voluntary flexion contractions of the index finger of ∼5 N in both postures during which neural drive to FDI and the long finger flexor muscles was measured using surface EMG. Recordings of FDI EMG were normalized to the maximal M wave. Five of the 15 subjects also had a radial nerve block to eliminate any co‐contraction of the extensor muscles, and extensor muscle EMG was monitored in subjects without radial nerve block. Compared to thumb up, flexion twitch force was ∼60% greater, and the flexion moment arm was ∼50% greater with the thumb down. There was minimal effect of altered carpometacarpal angle on flexion twitch force for either thumb posture. During voluntary flexion contractions, normalized FDI EMG was ∼28% greater with thumb down, compared to thumb up, with no consistent change in neural drive to the long flexors. Hence, the contribution of FDI to index finger flexion can be altered by changes in thumb position. This is linked to changes in neural drive to FDI such that neural drive increases when the mechanical contribution increases, and provides a central mechanism to produce efficient voluntary movements.

Does the supplementary motor area keep patients with Ondine's curse syndrome breathing while awake?

Background Congenital central hypoventilation syndrome (CCHS) is a rare neuro-respiratory disorder associated with mutations of the PHOX2B gene. Patients with this disease experience severe hypoventilation during sleep and are consequently ventilator-dependent. However, they breathe almost normally while awake, indicating the existence of cortical mechanisms compensating for the deficient brainstem generation of automatic breathing. Current evidence indicates that the supplementary motor area plays an important role in modulating ventilation in awake normal humans. We hypothesized that the wake-related maintenance of spontaneous breathing in patients with CCHS could involve supplementary motor area. Methods We studied 7 CCHS patients (5 women; age: 20–30; BMI: 22.1±4 kg.m−2) during resting breathing and during exposure to carbon dioxide and inspiratory mechanical constraints. They were compared with 8 healthy individuals. Segments of electroencephalographic tracings were selected according to ventilatory flow signal, from 2.5 seconds to 1.5 seconds after the onset of inspiration. After artefact rejection, 80 or more such segments were ensemble averaged. A slow upward shift of the EEG signal starting between 2 and 0.5 s before inspiration (pre-inspiratory potential) was considered suggestive of supplementary motor area activation. Results In the control group, pre-inspiratory potentials were generally absent during resting breathing and carbon dioxide stimulation, and consistently identified in the presence of inspiratory constraints (expected). In CCHS patients, pre-inspiratory potentials were systematically identified in all study conditions, including resting breathing. They were therefore significantly more frequent than in controls. Conclusions This study provides a neurophysiological substrate to the wakefulness drive to breathe that is characteristic of CCHS and suggests that the supplementary motor area contributes to this phenomenon. Whether or not this “cortical breathing” can be taken advantage of therapeutically, or has clinical consequences (like competition with attentional resources) remains to be determined.

The Neural Control of Human Inspiratory Muscles

The neural control of inspiratory muscles can be assessed in human subjects by measurement of the behavior of populations of single motor unit from the various inspiratory muscles. The discharge frequencies and patterns of firing of the motor units directly reflect the output of the motoneurons that innervate them. With the use of these methods, our work has revealed several features of the way the output of different inspiratory motoneuron pools are controlled. The output of inspiratory motoneurons is nonuniform across pools during quiet breathing and this coordinates the contraction of all the different muscles. This output is geared to the mechanical advantage of the muscles that they innervate. For the intercostal muscles, there is recruitment of the motor units by a principle of neuromechanical matching in which neural drive is higher in the muscles with the greatest mechanical advantage for inspiration, presumably to minimize the metabolic cost of ventilation. We summarize some evidence that this principle is likely to be organized at the spinal cord, although the exact underlying mechanisms are not known. The specific differences in the output from motoneurones innervating parasternal intercostal and diaphragm muscles during trunk rotation suggest that the output of inspiratory motoneurones engaged in a nonrespiratory voluntary task involve integration of corticospinal and bulbospinal drives at the spinal cord. An evolutionary argument is presented to support the importance of a role for spinal integration in ventilatory control.

Do we “drive” dyspnoea?

Electromyography (EMG) measures neural drive, and is routinely used to investigate movement control and pathophysiology in human subjects. The electrical signals recorded from a muscle indicate the recruitment and discharge of spinal motor neurones by voluntary and reflex activation. EMG recordings are typically made with surface electrodes placed on the skin over the muscle of interest, or intramuscular needle or wire electrodes inserted into the muscle of interest. There are advantages and disadvantages of both techniques [1] but the disadvantages are somewhat amplified for EMG recordings from the respiratory muscles. As many respiratory muscles are small and located close to one other, often in layers of muscle with different functions (e.g. the external and internal intercostal muscles), surface recordings are easily contaminated by activity from neighbouring muscles. Intramuscular recordings are more selective but the risks associated with needle use are more serious for the respiratory muscles because of the underlying lung. In saying that, inspiratory muscle surface EMG recordings are possible [2, 3] and electrode position can be optimised for the diaphragm [4] for some protocols. With intramuscular electrodes, the risk of pneumothorax can be minimised by using ultrasound to visualise the muscle and lung and estimate maximal insertion depth prior to recordings [5], and on-line audio and visual feedback of EMG activity during recordings [6]. Changes between neural drive and dyspnoea were determined during exercise in severe COPD patients by measuring EMGdi http://ow.ly/FCAs3

Riemannian Geometry Applied to Detection of Respiratory States From EEG Signals: The Basis for a Brain–Ventilator Interface

Goal: During mechanical ventilation, patient-ventilator disharmony is frequently observed and may result in increased breathing effort, compromising the patients comfort and recovery. This circumstance requires clinical intervention and becomes challenging when verbal communication is difficult. In this study, we propose a brain–computer interface (BCI) to automatically and noninvasively detect patient-ventilator disharmony from electroencephalographic (EEG) signals: a brain–ventilator interface (BVI). Methods: Our framework exploits the cortical activation provoked by the inspiratory compensation when the subject and the ventilator are desynchronized. Use of a one-class approach and Riemannian geometry of EEG covariance matrices allows effective classification of respiratory states. The BVI is validated on nine healthy subjects that performed different respiratory tasks that mimic a patient-ventilator disharmony. Results: Classification performances, in terms of areas under receiver operating characteristic curves, are significantly improved using EEG signals compared to detection based on air flow. Reduction in the number of electrodes that can achieve discrimination can be often desirable (e.g., for portable BCI systems). By using an iterative channel selection technique, the common highest order ranking, we find that a reduced set of electrodes (

Activation of human inspiratory muscles in an upside-down posture.

n=6