Monitoring respiratory function and sleep in the obese Vietnamese pot-bellied pig

Miniature Companion Pig Sedation and Anesthesia

With their increase in popularity in North America as pets, miniature companion pigs are in need of veterinary professionals familiar with sedation and anesthesia for the species. This article provides a review of the agents used for sedation, premedication, induction, and maintenance of anesthesia for miniature companion pigs. This review also covers species-specific anatomic and physiologic factors of miniature companion pigs with respect to administration of anesthetics, endotracheal intubation, anesthetic maintenance, and common complications so that the reader can make an informed anesthetic plan for the species.

Obstructive sleep apnea in obese minipigs

Objective:

Obesity has reached epidemic proportions and is a strong risk factor for obstructive sleep apnea (OSA). However, the underlying mechanisms are poorly understood and current treatment strategies for OSA and obesity have critical limitations. Thus, establishment of an obesity-related large animal model with spontaneous OSA is imperative.

Materials and methods:

Natural and sedated sleep were monitored and characterized in 4 obese (body mass index - BMI>48) and 3 non-obese (BMI<40) minipigs. These minipigs were instrumented with the BioRadio system under sedation for the wireless recording of respiratory airflow, snoring, abdominal and chest respiratory movements, electroencephalogram, electrooclulogram, electromyogram, and oxygen saturation. After instrumentation, the minipigs were placed in a dark room with a remote night-vision camera for monitoring all behaviors. Wakefulness and different sleep stages were classified, and episodes of apneas and/or hypopneas were identified during natural and/or sedated sleep.

Results:

No hypopnea episodes were observed in two of the non-obese minipigs, but one non-obese minipig had 5 hypopnea events. Heavy snoring and 27-58 apnea and/or hypopnea episodes were identified in all 4 obese minipigs. Most of these episodes occurred in the rapid eye movement stage during natural sleep and/or sedated sleep in Yucatan minipigs.

Conclusions:

Obese minipigs can experience naturally occurring OSA, thus are an ideal large animal model for obese-related OSA studies.

Translational approaches to understanding metabolic dysfunction and cardiovascular consequences of obstructive sleep apnea

Obstructive sleep apnea (OSA) is known to be independently associated with several cardiovascular diseases including hypertension, myocardial infarction, and stroke. To determine how OSA can increase cardiovascular risk, animal models have been developed to explore the underlying mechanisms and the cellular and end-organ targets of the predominant pathophysiological disturbance in OSA-intermittent hypoxia. Despite several limitations in translating data from animal models to the clinical arena, significant progress has been made in our understanding of how OSA confers increased cardiovascular risk. It is clear now that the hypoxic stress associated with OSA can elicit a broad spectrum of pathological systemic events including sympathetic activation, systemic inflammation, impaired glucose and lipid metabolism, and endothelial dysfunction, among others. This review provides an update of the basic, clinical, and translational advances in our understanding of the metabolic dysfunction and cardiovascular consequences of OSA and highlights the most recent findings and perspectives in the field.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

An animal model of obstructive sleep apnea in rabbit

OBJECTIVES/HYPOTHESIS

An animal model of obstructive sleep apnea (OSA) may help to investigate the pathophysiology of this disorder and develop appropriate treatments. We investigated the feasibility of a rabbit model of OSA.

STUDY DESIGN

Animal study.

METHODS

Twelve New Zealand white rabbits were injected at the base of their tongues under endoscopic guidance with liquid silicone (experimental group, n = 6) or normal saline (control group, n = 6). Polysomnography was performed before and after injection. The development of OSA and changes in sleep parameters were compared between the two groups.

RESULTS

Before injection, all rabbits showed normal breathing during sleep without hypopnea. In the silicone group, the rabbits had a mean of 29.9 ± 6.9 hypopneas/hour and a mean of 10.4 ± 3.1 apneas/hour 1 month after silicone injection and 28.4 ± 6.9 hypopneas/hour and 10.0 ± 3.3 apneas/hour 3 months after silicone injection (P < 0.05). Mean total sleep time decreased from 260.3 ± 70.2 minutes at baseline to 152.5 ± 38.8 minutes 1 month and 206.8 ± 60.3 minutes 3 months after injection, with a decrease in stage II sleep. In the saline group, however, there were no breathing events during sleep.

CONCLUSIONS

These results show that silicone injections into the tongue base of rabbits can result in OSA.

Anesthetic agents and complications in Vietnamese potbellied pigs: 27 cases (1999-2006)

OBJECTIVE

To document complications associated with preanesthetic and anesthetic agents used in Vietnamese potbellied pigs and identify predictors of complications.

DESIGN

Retrospective case series.

ANIMALS

27 potbellied pigs (14 female and 13 male) ranging in age from 0.25 to 15 years old and ranging in body weight from 5.9 to 169 kg (13.0 to 371.8 lb) that were anesthetized on 32 occasions between 1999 and 2006.

PROCEDURES

Data, including perianesthetic management, anesthetic agents and dosages, complications, and outcome, were retrieved from medical records. Patient information, anesthetic agents, and duration of anesthesia were evaluated as predictors for development of complications.

RESULTS

Anesthesia was maintained with isoflurane or sevoflurane during 30 anesthetic episodes. Commonly used premedicants were butorphanol, atropine, and midazolam administered in combination with xylazine or medetomidine and a combination of tiletamine-zolazepam and butorphanol. Anesthesia was induced with an inhalation agent on 15 occasions, via injection of ketamine on 10 occasions, and via injection of propofol on 3 occasions. Complications included hypoventilation (16/24 [67%]), hypotension (16/25 [64%]), hypothermia (15/31 [48%]), bradycardia (9/32 [28%]), and prolonged recovery time (7/32 [22%]). None of the factors evaluated were associated with development of these complications. All pigs survived anesthesia.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that a variety of anesthetic agent combinations can be used to provide anesthesia in potbellied pigs with satisfactory outcomes. Although there were high incidences of hypoventilation, hypotension, and hypothermia, no specific anesthetic agent was associated with development of these complications.

Cardiovascular consequences of sleep apnea

Sleep apnea is a common health concern that is characterized by repetitive episodes of asphyxia. This condition has been linked to serious long-term adverse effects such as hypertension, metabolic dysregulation, and cardiovascular disease. Although the mechanism for the initiation and aggravation of cardiovascular disease has not been fully elucidated, oxidative stress and subsequent endothelial dysfunction play major roles. Animal models, which have the advantage of being free of comorbidities and/or behavioral variables (that commonly occur in humans), allow invasive measurements under well-controlled experimental conditions, and as such are useful tools in the study of the pathophysiological mechanisms of sleep apnea. This review summarizes currently available information on the cardiovascular consequences of sleep apnea and briefly describes common experimental approaches useful to sleep apnea in different animal models.

A new animal model of obstructive sleep apnea responding to continuous positive airway pressure

STUDY OBJECTIVES

An improved animal model of obstructive sleep apnea (OSA) is needed for the development of effective pharmacotherapies. In humans, flexion of the neck and a supine position, two main pathogenic factors during human sleep, are associated with substantially greater OSA severity. We postulated that these two factors might generate OSA in animals.

DESIGN

We developed a restraining device for conditioning to investigate the effect of the combination of 2 body positions-prone (P) or supine (S)-and 2 head positions-with the neck flexed at right angles to the body (90°) or in extension in line with the body (180°)-during sleep in 6 cats. Polysomnography was performed twice on each cat in each of the 4 sleeping positions-P180, S180, P90, or S90. The effect of continuous positive airway pressure (CPAP) treatment was then investigated in 2 cats under the most pathogenic condition.

SETTING

NA.

PATIENTS OR PARTICIPANTS

NA.

INTERVENTIONS

NA.

MEASUREMENTS AND RESULTS

Positions P180 and, S90 resulted, respectively, in the lowest and highest apnea-hypopnea index (AHI) (3 ± 1 vs 25 ± 2, P < 0.001), while P90 (18 ± 3, P<0.001) and S180 (13 ± 5, P<0.01) gave intermediate values. In position S90, an increase in slow wave sleep stage 1 (28% ± 3% vs 22% ± 3%, P<0.05) and a decrease in REM sleep (10% ± 2% vs 18% ± 2%, P<0.001) were also observed. CPAP resulted in a reduction in the AHI (8 ± 1 vs 27 ± 3, P<0.01), with the added benefit of sleep consolidation.

CONCLUSION

By mimicking human pathogenic sleep conditions, we have developed a new reversible animal model of OSA.

Biology of obesity: lessons from animal models of obesity

Obesity is an epidemic problem in the world and is associated with several health problems, including diabetes, cardiovascular disease, respiratory failure, muscle weakness, and cancer. The precise molecular mechanisms by which obesity induces these health problems are not yet clear. To better understand the pathomechanisms of human disease, good animal models are essential. In this paper, we will analyze animal models of obesity and their use in the research of obesity-associated human health conditions and diseases such as diabetes, cancer, and obstructive sleep apnea syndrome.

Altered upper airway and soft tissue structures in the New Zealand Obese mouse

RATIONALE

The effect of obesity on upper airway soft tissue structure and size was examined in the New Zealand Obese (NZO) mouse and in a control lean mouse, the New Zealand White (NZW).

OBJECTIVES

We hypothesized that the NZO mouse has increased volume of neck fat and upper airway soft tissues and decreased pharyngeal airway caliber.

METHODS

Pharyngeal airway size, volume of the upper airway soft tissue structures, and distribution of fat in the neck and body were measured using magnetic resonance imaging (MRI). Dynamic MRI was used to examine the differences in upper airway caliber between inspiration and expiration in NZO versus NZW mice.

MEASUREMENTS AND MAIN RESULTS

The data support the hypothesis that, in obese NZO versus lean NZW mice, airway caliber was significantly smaller (P < 0.03), with greater parapharyngeal fat pad volumes (P < 0.0001) and a greater volume of other upper airway soft tissue structures (tongue, P = 0.003; lateral pharyngeal walls, P = 0.01; soft palate, P = 0.02). Dynamic MRI showed that the airway of the obese NZO mouse dilated during inspiration, whereas in the lean NZW mouse, the upper airway was reduced in size during inspiration.

CONCLUSIONS

In addition to the increased volume of pharyngeal soft tissue structures, direct fat deposits within the tongue may contribute to airway compromise in obesity. Pharyngeal airway dilation during inspiration in NZO mice compared with narrowing in NZW mice suggests that airway compromise in obese mice may lead to muscle activation to defend upper airway patency during inspiration.

A neurotoxinological approach to the treatment of obstructive sleep apnoea

Current treatment approaches to the problem of obstructive sleep apnoea (OSA) have limitations. Specifically, invasive anatomical-based surgery and dental appliances typically do not alleviate obstruction at an acceptable rate, and compliance to continuous positive airway pressure (CPAP) devices is frequently suboptimal. Neurotoxinological treatment approaches are widespread in the field of medicine, but as yet have not been evaluated as a treatment for sleep-disordered breathing. In this review, it is argued that despite widespread recognition of the loss of upper airway (UA) muscular tone and/or reflexes in the expression of OSA, most treatment interventions to date have focused on anatomical principles alone. Several hypothesised neurotoxinological interventions aimed at either enhancing UA neuromuscular tone and/or reflexes are proposed, and some preliminary data is presented. Although in its early infancy, with considerable toxicity studies in animals yet to be done, a neurotoxinological approach to the problem of OSA holds promise as a future treatment, with the potential for both high effectiveness and patient compliance.

An animal model of a spontaneously reversible obstructive sleep apnea syndrome in the monkey

The anatomies of the tongue and uvula in monkeys share many similarities with humans, such that this species has the closest approximation to the human upper airway than any other species. In this study, we investigated the feasibility of using small monkeys as experimental animals for an obstructive sleep apnea model. Monkeys received intradermal injections of liquid collagen in the uvula, tongue, and lateral pharyngeal walls every 2 weeks. Polysomnography was performed bi-monthly in order to control the impact of injections on breathing events, respiratory effort (as measured by esophageal pressure), and sleep. Before injections, the three animals showed normal breathing during sleep with a mean of 4.8 +/- 2.0 events/h. After injections, a mean of 27.9 +/- 19.7 hypopneas/h was recorded (P = 0.023). Total sleep time was significantly reduced, with a decrease of REM sleep and stage II sleep; however, stage I sleep increased. Collagen injections in monkey's upper airways can create sleep-disordered breathing and abnormal sleep, as seen in sleep apneic patients.

Mechanical properties of the passive pharynx in Vietnamese pot-bellied pigs. II. Dynamics

We described the dynamic mechanical properties of the passive pharynx in Vietnamese pot-bellied pigs and the effects of caudal tracheal displacement. During general anesthesia and neuromuscular blockade, airflow through the upper airway (V) and pharyngeal cross-sectional area were measured during ramp decreases in pressure downstream from the pharynx (Pdown). Measurements were made with 0, 1, and 2 cm of caudal tracheal displacement. Airflow limitation and/or negative pressure dependence (NPD) were observed in all animals. Tracheal displacement (2 cm) increased maximal V (V(max)) by 205.1 +/- 105.1% (P < 0.05) relative to the value with no displacement and increased the magnitude of NPD, expressed as percent decrease in V from V(max), from 22.9 +/- 27.4 to 56.6 +/- 37.5% (P < 0.05). Initial decreases in Pdown narrowed all levels of the pharynx, but, once V(max) was reached, further decreases in Pdown narrowed the hypopharynx but not the nasopharynx and oropharynx. We conclude that the hypopharynx is the flow-limiting site in the pig pharynx. Tracheal displacement not only improved airflow dynamics as V(max) increased but also resulted in pronounced NPD.

Mechanical properties of the passive pharynx in Vietnamese pot-bellied pigs. I. Statics

The static mechanical properties of the passive pharynx were investigated in Vietnamese pot-bellied pigs by using an isolated upper airway preparation. During general anesthesia and neuromuscular blockade, cross-sectional area (A) of the pharynx was measured while airway pressure (Paw) was held at various pressures in the absence of airflow. The static A-Paw relationship was measured during application of 0, 1, and 2 cm of caudal tracheal displacement. Relative to humans, closing pressures (Pclose) of the pig pharynx were very low (-15 to -35 cmH(2)O). Tracheal displacement significantly decreased compliance of the hypopharynx (from 0.074 +/- 0.02 cm(2)/cmH(2)O with no displacement to 0.052 +/- 0.01 cm(2)/cmH(2)O with 2 cm of displacement) and decreased Pclose of the oropharynx (from -18.2 +/- 9.9 cmH(2)O to -24.1 +/- 10.5 and -28.7 +/- 12.3 cmH(2)O with 1 and 2 cm of displacement, respectively). Tracheal displacement did not affect A of the pharyngeal segments. In conclusion, tracheal displacement decreased collapsibility of the passive pharynx. The pharynx of the pot-bellied pig is structurally more resistant to collapse than the human pharynx.

A model of sleep-disordered breathing in the C57BL/6J mouse

To investigate the pathophysiological sequelae of sleep-disordered breathing (SDB), we have developed a mouse model in which hypoxia was induced during periods of sleep and was removed in response to arousal or wakefulness. An on-line sleep-wake detection system, based on the frequency and amplitude of electroencephalograph and electromyograph recordings, served to trigger intermittent hypoxia during periods of sleep. In adult male C57BL/6J mice (n = 5), the sleep-wake detection system accurately assessed wakefulness (97.2 +/- 1.1%), non-rapid eye movement (NREM) sleep (96.0 +/- 0.9%) and rapid eye movement (REM) sleep (85.6 +/- 5.0%). After 5 consecutive days of SDB, 554 +/- 29 (SE) hypoxic events were recorded over a 24-h period at a rate of 63.6 +/- 2.6 events/h of sleep and with a duration of 28.2 +/- 0.7 s. The mean nadir of fraction of inspired O(2) (FI(O(2))) on day 5 was 13.2 +/- 0.1%, and 137.1 +/- 13.2 of the events had a nadir FI(O(2)) <10% O(2). Arterial blood gases confirmed that hypoxia of this magnitude lead to a significant degree of hypoxemia. Furthermore, 5 days of SDB were associated with decreases in both NREM and REM sleep during the light phase compared with the 24-h postintervention period. We conclude that our murine model of SDB mimics the rate and magnitude of sleep-induced hypoxia, sleep fragmentation, and reduction in total sleep time found in patients with moderate to severe SDB in the clinical setting.

Braking of expiratory airflow in obese pigs during wakefulness and sleep

Braking of expiratory airflow is a phenomenon prominently seen in neonates where it is thought to defend end-expiratory lung volume. This paper describes pronounced expiratory braking in an adult animal, the obese Vietnamese pot-bellied pig. Three obese pigs were chronically instrumented for recording of intrapleural pressure and bioelectric signals related to sleep. Airflow was measured by a pneumotachograph attached to a facemask. Expiratory airflow resistance was calculated for 10 consecutive expirations during wakefulness, NREM, and REM sleep. All animals demonstrated a biphasic expiratory flow pattern characterized by an initial plateau in flow at a low value followed by a rapid increase later in expiration. Airflow resistance during early expiration was on average four-fold higher than during late expiration. A striking observation was the maintenance of pronounced expiratory braking during NREM and REM sleep. Expiratory braking in these animals is likely due to laryngeal mechanisms and may serve to preserve end-expiratory lung volume or improve hemodynamics.

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.