Philip Nolan

Automated processing of the single-lead electrocardiogram for the detection of obstructive sleep apnoea

A method for the automatic processing of the electrocardiogram (ECG) for the detection of obstructive apnoea is presented. The method screens nighttime single-lead ECG recordings for the presence of major sleep apnoea and provides a minute-by-minute analysis of disordered breathing. A large independently validated database of 70 ECG recordings acquired from normal subjects and subjects with obstructive and mixed sleep apnoea, each of approximately eight hours in duration, was used throughout the study. Thirty-five of these recordings were used for training and 35 retained for independent testing. A wide variety of features based on heartbeat intervals and an ECG-derived respiratory signal were considered. Classifiers based on linear and quadratic discriminants were compared. Feature selection and regularization of classifier parameters were used to optimize classifier performance. Results show that the normal recordings could be separated from the apnoea recordings with a 100% success rate and a minute-by-minute classification accuracy of over 90% is achievable.

Microdialysis perfusion of 5-HT into hypoglossal motor nucleus differentially modulates genioglossus activity across natural sleep-wake states in rats

1 Serotonin (5‐hydroxytryptamine, 5‐HT) excites hypoglossal (XII) motoneurons in reduced preparations, and it has been suggested that withdrawal of 5‐HT may underlie reduced genioglossus (GG) muscle activity in sleep. However, systemic administration of 5‐HT agents in humans has limited effects on GG activity. Whether 5‐HT applied directly to the XII motor nucleus increases GG activity in an intact preparation either awake or asleep has not been tested. 2 The aim of this study was to develop a novel freely behaving animal model for in vivo microdialysis of the XII motor nucleus across sleep‐wake states, and test the hypothesis that 5‐HT application will increase GG activity. 3 Eighteen rats were implanted with electroencephalogram and neck muscle electrodes to record sleep‐wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the XII motor nucleus and perfused with artificial cerebrospinal fluid (ACSF) or 10 mm 5‐HT. 4 Normal decreases in GG activity occurred from wakefulness to non‐rapid eye movement (non‐REM) and REM sleep with ACSF (P < 0.01). Compared to ACSF, 5‐HT caused marked GG activation across all sleep‐wake states (increases of 91‐251 %, P < 0.015). Importantly, 5‐HT increased sleeping GG activity to normal waking levels for as long as 5‐HT was applied (3‐5 h). Despite tonic stimulation by 5‐HT, periods of phasic GG suppression and excitation occurred in REM sleep compared with non‐REM. 5 The results show that sleep‐wake states differentially modulate GG responses to 5‐HT at the XII motor nucleus. This animal model using in vivo microdialysis of the caudal medulla will enable the determination of neural mechanisms underlying pharyngeal motor control in natural sleep.

Role of inhibitory amino acids in control of hypoglossal motor outflow to genioglossus muscle in naturally sleeping rats

The hypoglossal motor nucleus innervates the genioglossus (GG) muscle of the tongue, a muscle that helps maintain an open airway for effective breathing. Rapid‐eye‐movement (REM) sleep, however, recruits powerful neural mechanisms that can abolish GG activity even during strong reflex stimulation such as by hypercapnia, effects that can predispose to sleep‐related breathing problems in humans. We have developed an animal model to chronically manipulate neurotransmission at the hypoglossal motor nucleus using in vivo microdialysis in freely behaving rats. This study tests the hypothesis that glycine receptor antagonism at the hypoglossal motor nucleus, either alone or in combination with GABAA receptor antagonism, will prevent suppression of GG activity in natural REM sleep during room air and CO2‐stimulated breathing. Rats were implanted with electroencephalogram and neck muscle electrodes to record sleep–wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the hypoglossal motor nucleus for perfusion of artificial cerebrospinal fluid (ACSF) and strychnine (glycine receptor antagonist, 0.1 mm) either alone or combined with bicuculline (GABAA antagonist, 0.1 mm) during room air and CO2‐stimulated breathing. Compared to ACSF controls, glycine receptor antagonism at the hypoglossal motor nucleus increased respiratory‐related GG activity in room air (P= 0.010) but not hypercapnia (P= 0.221). This stimulating effect of strychnine in room air did not depend on the prevailing sleep–wake state (P= 0.625) indicating removal of a non‐specific background inhibitory glycinergic tone. Nevertheless, GG activity remained minimal in those REM sleep periods without phasic twitches in GG muscle, with GG suppression from non‐REM (NREM) sleep being > 85% whether ACSF or strychnine was at the hypoglossal motor nucleus or the inspired gas was room air or 7% CO2. While GG activity was minimal in these REM sleep periods, there was a small but measurable increase in GG activity after strychnine (P < 0.05). GG activity was also minimal, and effectively abolished, in the REM sleep periods without GG twitches with combined glycine and GABAA receptor antagonism at the hypoglossal motor nucleus. We conclude that these data in freely behaving rats confirm that inhibitory glycine and GABAA receptor mechanisms are present at the hypoglossal motor nucleus and are tonically active, but that such inhibitory mechanisms make only a small contribution to the marked suppression of GG activity and reflex responses observed in periods of natural REM sleep.

GABAA receptor antagonism at the hypoglossal motor nucleus increases genioglossus muscle activity in NREM but not REM sleep

The pharyngeal muscles, such as the genioglossus (GG) muscle of the tongue, are important for effective lung ventilation since they maintain an open airspace. Rapid‐eye‐movement (REM) sleep, however, recruits powerful neural mechanisms that can abolish GG activity, even during strong reflex respiratory stimulation by elevated CO2. In vitro studies have demonstrated the presence of GABAA receptors on hypoglossal motoneurons, and these and other data have led to the speculation that GABAA mechanisms may contribute to the suppression of hypoglossal motor outflow to the GG muscle in REM sleep. We have developed an animal model that allows us to chronically manipulate neurotransmission at the hypoglossal motor nucleus using microdialysis across natural sleep‐wake states in rats. The present study tests the hypothesis that microdialysis perfusion of the GABAA receptor antagonist bicuculline into the hypoglossal motor nucleus will prevent the suppression of GG muscle activity in REM sleep during both room‐air and CO2‐stimulated breathing. Ten rats were implanted with electroencephalogram and neck muscle electrodes to record sleep‐wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the hypoglossal motor nucleus for perfusion of artificial cerebrospinal fluid (ACSF) or 100 μm bicuculline during room‐air and CO2‐stimulated breathing (7 % inspired CO2). GABAA receptor antagonism at the hypoglossal motor nucleus increased respiratory‐related GG activity during both room‐air (P= 0.01) and CO2‐stimulated breathing (P= 0.007), indicating a background inhibitory GABA tone. However, the effects of bicuculline on GG activity depended on the prevailing sleep‐wake state (P < 0.005), with bicuculline increasing GG activity in non‐REM (NREM) sleep and wakefulness both in room air and hypercapnia (P < 0.01), but GG activity was effectively abolished in those REM periods without phasic twitches in the GG muscle. This abolition of GG activity in REM sleep occurred regardless of ACSF or bicuculline at the hypoglossal motor nucleus, or room‐air or CO2‐stimulated breathing (P > 0.63). We conclude that these data in freely behaving rats confirm previous in vitro studies that GABAA receptor mechanisms are present at the hypoglossal motor nucleus and are tonically active, but the data also show that GABAA receptor antagonism at the hypoglossal motor nucleus does not increase GG muscle activity in natural REM sleep.

5-HT at hypoglossal motor nucleus and respiratory control of genioglossus muscle in anesthetized rats

Serotonin (5-HT) from medullary raphe neurons excites hypoglossal motoneurons innervating genioglossus (GG) muscle. Since some raphe neurons also show increased activity in hypercapnia, we tested the hypothesis that serotonergic mechanisms at the hypoglossal motor nucleus (HMN) modulate GG activity and responses to CO2. Seventeen urethane-anesthetized, tracheotomized and vagotomized rats were studied. Microdialysis probes were used to deliver mianserin (5-HT receptor antagonist, 0 and 0.1 mM) or 5-HT (eight doses, 0-50 mM) to the HMN during room air or CO2-stimulated breathing. Mianserin decreased respiratory-related GG activity during room air and CO2-stimulated breathing (P<0.001), and also suppressed GG responses to CO2 (P=0.05). In contrast, GG activity was increased by 5-HT at the HMN, and was further increased in hypercapnia (P<0.02). However, 5-HT increased respiratory-related GG activity at levels lower (1 mM) than those eliciting tonic GG activity (10-30 mM 5-HT). The results show that 5-HT at the HMN contributes to the respiratory control of GG muscle.

Electromyogram median frequency, spectral compression and muscle fibre conduction velocity during sustained sub-maximal contraction of the brachioradialis muscle

Changes in the median frequency of the power spectrum of the surface electromyogram (EMG) are commonly used to detect muscle fatigue. Previous research has indicated that changes in the median frequency are related to decreases in muscle fibre conduction velocity (MFCV) during sustained fatiguing contractions. However, in experimental studies the median frequency has been consistently observed to decrease by a relatively greater amount than MFCV. In this paper, a new estimate of EMG frequency compression, the Spectral Compression Estimate (SCE), is compared with the median frequency of the EMG power spectrum, the median frequency of the EMG amplitude spectrum and MFCV measured during sustained, isometric, fatiguing contractions of the brachioradialis muscle at 30, 50 and 80% maximum voluntary contraction (MVC). The SCE is found to provide a better estimate of the observed changes in MFCV than the median frequency of either the EMG power spectrum or EMG amplitude spectrum.

Suppression of genioglossus muscle tone and activity during reflex hypercapnic stimulation by GABAa mechanisms at the hypoglossal motor nucleus in vivo

The genioglossus muscle is involved in the maintenance of an open airway for effective breathing. Inhibitory neurotransmitters may be responsible for the major suppression of hypoglossal motor output to genioglossus muscle that occurs in certain behaviours such as rapid-eye-movement sleep. There is evidence for GABA(A) receptor-mediated inhibition of hypoglossal motoneurons in vitro. However, comparable studies have not been performed in vivo and the interactions of such mechanisms with integrative reflex respiratory control have also not been determined. Urethane-anaesthetised, tracheotomized and vagotomized rats were studied whilst diaphragm and genioglossus muscle activities, blood pressure and the electroencephalogram were recorded. Microdialysis probes were implanted into the hypoglossal motor nucleus, with sites verified by histology. Genioglossus responses to microdialysis perfusion of muscimol (GABA(A) agonist: 0, 0.1, 1 and 10 microM in artificial cerebrospinal fluid) were recorded at inspired CO(2)s of 0, 5 and 7.5% in six rats. Responses to bicuculline (GABA(A) antagonist, 0, 1, 10, 100 and 1000 microM) were also studied in six rats with and without CO(2) stimulation. Genioglossus activity decreased with muscimol (P<0.0001), with major suppression at 1 and 10 microM during air breathing (decreases=70.2% and 92.8%, P<0.005). Genioglossus activity increased with CO(2) (P=0.003), but genioglossus activation with 5 and 7.5% CO(2) were almost abolished with 10-microM muscimol. Responses were specific to genioglossus muscle as there were no changes in diaphragm, respiratory rate or blood pressure with muscimol (P>0.144). Antagonism of GABA(A) receptors increased genioglossus activity (P<0.001). These results show that GABA(A) receptor stimulation at the hypoglossal motor nucleus suppresses both genioglossus muscle tone and activity in the presence of reflex stimulation produced by hypercapnia. Recruitment of such mechanisms may contribute to the major suppression of genioglossus activity observed with and without CO(2) stimulation in behaviours such as rapid-eye-movement sleep.

Reflex respiratory response to changes in upper airway pressure in the anaesthetized rat

1 We examined the upper airway (UA) motor response to upper airway negative pressure (UANP) in the rat. We hypothesized that this response is mediated by superior laryngeal nerve (SLN) afferents and is not confined to airway dilator muscles but also involves an increase in motor drive to tongue retractor and pharyngeal constrictor muscles, reflecting a role for these muscles in stabilizing the UA. 2 Experiments were performed in 49 chloralose‐anaesthetized, tracheostomized rats. Subatmospheric pressure in the range 0 to −30 cmH2O was applied to the isolated UA. Motor activity was recorded in separate experiments from the main trunk of the hypoglossal nerve (XII, n= 8), the pharyngeal branch of the glossopharyngeal nerve (n= 8), the medial and lateral branches of the XII (n= 8) and the pharyngeal branch of the vagus (n= 8). Afferent activity was recorded from the whole SLN in six experiments. 3 All UA motor outflows exhibited phasic inspiratory activity and this was significantly (P < 0.05) increased by UANP. Tonic end‐expiratory activity increased significantly in response to pressures more negative than −20 cmH2O. Bilateral section of the SLN also increased (P < 0.05) motor activity and abolished the responses to UANP. Electrical stimulation of the SLN inhibited inspiratory XII activity. SLN afferents were tonically active and inhibited by UANP. 4 We conclude that UANP causes a reflex increase in motor drive to pharyngeal dilator, tongue retractor and pharyngeal constrictor muscles via afferent fibres in the SLN. Tonic activity in SLN afferent fibres at zero transmural pressure exerts a marked inhibitory effect on UA motor outflow.

Automatic classification of sleep apnea epochs using the electrocardiogram

This study investigated the automatic prediction of epochs of sleep apnea from the electrocardiogram. A large independently validated database of 70 single lead ECGs, each of approximately 8 hours in duration, was used throughout the study. Thirty five of these records were used for training and 35 retained for independent testing. After considering a wide variety of features the authors found that features based on the power spectral density estimates of the R-wave maxima and R-R intervals to be the most discriminating. Results show that a classification rate of approximately 89% is achievable.

The effects of changes in laryngeal airway CO2 concentration on genioglossus muscle activity in the anaesthetized cat

In the anaesthetized cat the larynx was isolated in situ, artificially ventilated and used to assess reflex effects exerted by respiration‐related laryngeal stimuli on genioglossus electromyographic activity (Gg EMG) and respiratory frequency (RF). Phasic Gg EMG was not observed when the larynx was unventilated but was evoked, with a concurrent decrease in RF, when negative pressures or oscillatory pressures similar to those of normal ventilation were applied to the larynx. Increases in laryngeal airway CO2 concentration also enhanced Gg EMG and reduced RF. All reflex effects were abolished by bilateral section of the superior laryngeal nerves. We propose that negative intralaryngeal pressure and CO2 may act together to restore pharyngeal patency during obstructive apnoea.