Role of costal and crural diaphragm and parasternal intercostals during coughing in cats

Divergent expiratory braking activity of costal and crural diaphragm

BACKGROUND

There is increasing clinical interest in understanding the contribution of the diaphragm in early expiration, especially during mechanical ventilation. However, current experimental evidence is limited, so essential activity of the diaphragm during expiration and diaphragm segmental differences in expiratory activity, are unknown.

OBJECTIVES

To determine if: 1) the diaphragm is normally active into expiration during spontaneous breathing and hypercapnic ventilation, 2) expiratory diaphragmatic activity is distributed equally among the segments of the diaphragm, costal and crural.

METHODS

In 30 spontaneously breathing male and female canines, awake without confounding anesthetic, we measured directly both inspiratory and expiratory electrical activity (EMG), and corresponding mechanical shortening, of costal and crural diaphragm, during room air and hypercapnia.

RESULTS

During eupnea, costal and crural diaphragm are active into expiration, showing significant and distinct expiratory activity, with crural expiratory activity greater than costal, for both magnitude and duration. This diaphragm segmental difference diverged further during progressive hypercapnic ventilation: crural expiratory activity progressively increased, while costal expiratory activity disappeared.

CONCLUSION

The diaphragm is not passive during expiration. During spontaneous breathing, expiratory activity -"braking"- of the diaphragm is expressed routinely, but is not equally distributed. Crural muscle "braking" is greater than costal muscle in magnitude and duration. With increasing ventilation during hypercapnia, expiratory activity -"braking"- diverges notably. Crural expiratory activity greatly increases, while costal expiratory "braking" decreases in magnitude and duration, and disappears. Thus, diaphragm expiratory "braking" action represents an inherent, physiological function of the diaphragm, distinct for each segment, expressing differing neural activation.

THE INFLUENCE OF CO2 ON SPATIOTEMPORAL FEATURES OF MECHANICALLY INDUCED COUGH IN ANESTHETIZED CATS

Effective cough requires a significant increase in lung volume used to produce the shear forces on the airway to clear aspirated material. This increase in tidal volume during cough, along with an increase in tidal frequency during bouts of paroxysmal cough produces profound hyperventilation and thus reduces arterial CO₂. While there are several reports in the literature regarding the effects of hypercapnia, hyperoxia, and hypoxia on cough, there is little research quantifying the effects of hypocapnia on the cough reflex. We hypothesized that decreased CO₂ would enhance coughing. In 12 spontaneously breathing adult male cats, we compared bouts of prolonged mechanically stimulated cough, in which cough induced hyperventilation (CHV) was allowed to occur, with isocapnic cough trials where we maintained eupneic end-tidal CO₂ by adding CO₂ to the inspired gas. Isocapnia slightly increased cough number and decreased esophageal pressures with no change in EMG magnitudes or phase durations. The cough-to-eupnea transition was also analyzed between CHV, isocapnia, and a third group of animals that were mechanically hyperventilated to apnea. The transition to eupnea was highly sensitive to added CO₂, and CHV apneas were much shorter than those produced by mechanical hyperventilation. We suggest that the cough pattern generator is relatively insensitive to CHV. In the immediate post-cough period, the appearance of breathing while CO₂ is very low suggests a transient reduction in apneic threshold following a paroxysmal cough bout.

Parasternal intercostal, costal, and crural diaphragm neural activation during hypercapnia

The parasternal intercostal is an obligatory inspiratory muscle working in coordination with the diaphragm, apparently sharing a common pathway of neural response. This similarity has attracted clinical interest, promoting the parasternal as a noninvasive alternative to the diaphragm, to monitor central neural respiratory output. However, this role may be confounded by the distinct and different functions of the costal and crural diaphragm. Given the anatomic location, parasternal activation may significantly impact the chest wall via both mechanical shortening or as a "fixator" for the chest wall. Either mechanical function of the parasternal may also impact differential function of the costal and crural. The objectives of the present study were, during eupnea and hypercapnia, 1) to compare the intensity of neural activation of the parasternal with the costal and crural diaphragm and 2) to examine parasternal recruitment and changes in mechanical action during progressive hypercapnia, including muscle baseline length and shortening. In 30 spontaneously breathing canines, awake without confounding anesthetic, we directly measured the electrical activity of the parasternal, costal, and crural diaphragm, and the corresponding mechanical shortening of the parasternal, during eupnea and hypercapnia. During eupnea and hypercapnia, the parasternal and costal diaphragm share a similar intensity of neural activation, whereas both differ significantly from crural diaphragm activity. The shortening of the parasternal increases significantly with hypercapnia, without a change in baseline end-expiratory length. In conclusion, the parasternal shares an equivalent intensity of neural activation with the costal, but not crural, diaphragm. The parasternal maintains and increases its active inspiratory shortening during augmented ventilation, despite high levels of diaphragm recruitment. Throughout hypercapnic ventilation, the parasternal contributes mechanically; it is not relegated to chest wall fixation.NEW & NOTEWORTHY This investigation directly compares neural activation of the parasternal intercostal muscle with the two distinct segments of the diaphragm, costal and crural, during room air and hypercapnic ventilation. During eupnea and hypercapnia, the parasternal intercostal muscle and costal diaphragm share a similar neural activation, whereas they both differ significantly from the crural diaphragm. The parasternal intercostal muscle maintains and increases active inspiratory mechanical action with shortening during ventilation, even with high levels of diaphragm recruitment.

Central neural circuits for coordination of swallowing, breathing, and coughing: predictions from computational modeling and simulation

The purpose of this article is to update the otolaryngologic community on recent developments in the basic understanding of how cough, swallow, and breathing are controlled. These behaviors are coordinated to occur at specific times relative to one another to minimize the risk of aspiration. The control system that generates and coordinates these behaviors is complex, and advanced computational modeling methods are useful tools to elucidate its function.

Recovery of airway protective behaviors after spinal cord injury

Pulmonary morbidity is high following spinal cord injury and is due, in part, to impairment of airway protective behaviors. These airway protective behaviors include augmented breaths, the cough reflex, and expiration reflexes. Functional recovery of these behaviors has been reported after spinal cord injury. In humans, evidence for functional recovery is restricted to alterations in motor strategy and changes in the frequency of occurrence of these behaviors. In animal models, compensatory alterations in motor strategy have been identified. Crossed descending respiratory motor pathways at the thoracic spinal cord levels exist that are composed of crossed premotor axons, local circuit interneurons, and propriospinal neurons. These pathways can collectively form a substrate that supports maintenance and/or recovery of function, especially after asymmetric spinal cord injury. Local sprouting of premotor axons in the thoracic spinal cord also can occur following chronic spinal cord injury. These mechanisms may contribute to functional resiliency of the cough reflex that has been observed following chronic spinal cord injury in the cat.

Cough motor mechanisms

Cough is a defensive airway mechanisms which involves the sequential activation of several laryngeal and respiratory muscles in the generation of the typical four-phase motor pattern. Activation of such muscles can be considered to represent the "primary" cough motor mechanism, and its functional significance, although complex, appears to be fairly well established. Nonetheless, the outflows of cough are numerous, and may additionally involve the reflex or mechanical activation of other respiratory and non-respiratory motor systems. These additional, or "secondary", outflows of cough can mainly be regarded as being involved in either enhancing the defensive function of cough, or opposing the possible noxious effects exerted by the mechanical stresses of coughing. In addition, both the primary and secondary cough motor mechanisms are known to play multiple functional roles, thus considerably complicating the cough panorama. Finally, some of the secondary cough motor responses, such as the changes in the pattern of breathing, seem to be devoid of any favourable action and their functional meaning, if any, is not fully understood. Although it is well known that all patterns of cough can be produced voluntarily, the extent to which also volitional cough is accompanied by an array of motor responses similar to that of reflex cough remains to be elucidated.

Phrenic and iliohypogastric nerve discharges during tussigenic stimulation in paralyzed and decerebrate guinea pigs and rats

Although effects of antitussive drugs have been examined in inbred small animals using a whole body plethysmography, neuronal mechanisms underlying the cough reflex are not fully understood. The present study analyzed the reflex discharge patterns of the phrenic (PN) and iliohypogastric nerves (IHN) evoked in decerebrate and paralyzed guinea pigs and rats. In guinea pigs, electrical stimulation of the superior laryngeal nerve, chemical stimulation with capsaicin and mechanical stimulation to the intratracheal mucosa equally produced a serial PN-IHN response. This response was characterized by an increased PN discharge and following spindle-shaped burst of the IHN. The evoked discharges overlapped for 20 ms. In rats, by contrast, mechanical stimulation was without effect while capsaicin and electrical stimulation produced two types of responses, both of which differed from that observed in guinea pigs. The first type consisted of an augmented burst of the IHN that was immediately followed by an increased PN discharge. The second type was a large spindle-shaped burst of the IHN that occurred 80 ms after the end of the preceding PN discharge. Codeine (3 mg/kg i.v.) depressed all types of responses evoked in guinea pigs and rats. The present study demonstrated that the fictive cough comparable with those induced in other experimental animals was produced consistently in guinea pigs, but not in rats. Therefore, guinea pigs are suitable for investigation of the neuronal mechanisms underlying the cough reflex and assessment of antitussive drugs.

Rehabilitation of pelvic floor muscles utilizing trunk stabilization

The pelvic floor muscles (PFM) are part of the trunk stability mechanism. Their function is interdependent with other muscles of this system. They also contribute to continence, elimination, sexual arousal and intra-abdominal pressure. This paper outlines some aspects of function and dysfunction of the PFM complex and describes the contribution of other trunk muscles to these processes. Muscle pathophysiology of stress urinary incontinence (SUI) is described in detail. The innovative rehabilitation programme for SUI presented here utilizes abdominal muscle action to initiate tonic PFM activity. Abdominal muscle activity is then used in PFM strengthening, motor relearning for functional expiratory actions and finally impact training.

A simple non-invasive method to measure the cough reflex in dogs

INTRODUCTION

This study describes a method to measure the cough reflex in dogs that is simple to perform, requires no surgical intervention and can be used to profile efficacy and side-effect liabilities of antitussive drugs.

METHODS

Experiments were performed in propofol-anesthetized dogs in which cardiopulmonary functions were non-invasively monitored before and after the induction of cough produced by spraying 0.75 ml of distilled water into the trachea.

RESULTS

The magnitude of the cough response, measured by the frequency and amplitude was not different for individual dogs performed with repeated trials on different days. Treatment with the opioid antitussive drug, torbutrol (0.055-0.0055 mg/kg, s.c.) inhibited the cough frequency but not the amplitude induced by the water challenge. Furthermore, side effects of torbutrol were identified as mild respiratory depression and an anesthetic-sparing effect with propofol.

DISCUSSION

This method offers many distinct advantages to evaluate efficacy of antitussive drugs including the fact that no surgery is required, it takes only 15-20 min to complete an experiment, and it can be used to simultaneously profile antitussive and side effect liabilities of drugs developed for the treatment of cough.

Volume-timing relationships during cough and resistive loading in the cat

The relationship between pulmonary volume-related feedback and inspiratory (CTI) and expiratory (CTE) phase durations during cough was determined. Cough was produced in anesthetized cats by mechanical stimulation of the intrathoracic tracheal lumen. During eupnea, the animals were exposed to single-breath inspiratory and expiratory resistive loads. Cough was associated with large increases in inspiratory volume (VI) and expiratory volume (VE) but no change in phase durations compared with eupnea. There was no relationship between VI and CTI during coughing. A linear relationship with a negative slope existed between VI and eupneic inspiratory time during control and inspiratory resistive loading trials. There was no relationship between VE and CTE during all coughs. However, when the first cough in a series or a single cough was analyzed, the VE/CTE relationship had a positive slope. A linear relationship with a negative slope existed between VE and eupneic expiratory time during control and expiratory resistive loading trials. These results support separate ventilatory pattern regulation during cough that does not include modulation of phase durations by pulmonary volume-related feedback.

Responses of the anterolateral abdominal muscles during cough and expiratory threshold loading in the cat

The present study was conducted to determine the pattern of activation of the anterolateral abdominal muscles during the cough reflex. Electromyograms (EMGs) of the rectus abdominis, external oblique, internal oblique, transversus abdominis, and parasternal muscles were recorded along with gastric pressure in anesthetized cats. Cough was produced by mechanical stimulation of the lumen of the intrathoracic trachea or larynx. The pattern of EMG activation of these muscles during cough was compared with that during graded expiratory threshold loading (ETL; 1-30 cmH(2)O). ETL elicited differential recruitment of abdominal muscle EMG activity (transversus abdominis > internal oblique > rectus abdominis congruent with external oblique). In contrast, both laryngeal and tracheobronchial cough resulted in simultaneous activation of all four anterolateral abdominal muscles with peak EMG amplitudes 3- to 10-fold greater than those observed during the largest ETL. Gastric pressures during laryngeal and tracheobronchial cough were at least eightfold greater than those produced by the largest ETL. These results suggest that, unlike their behavior during expiratory loading, the anterolateral abdominal muscles act as a unit during cough.

Response of respiratory muscles to intravenous nicotine challenge in anaesthetized cats

The effects of an intravenous nicotine challenge on the ventilation and activity of rib cage muscles were studied in 33 pentobarbitone-chloralose anaesthetized cats. Bolus injection of nicotine (200 microg) into the right femoral vein evoked in 19 of the intact animals prompt, short-lived apnoea, or prolongation of the first expiration after the drug, the occurrence of which was significantly reduced by midcervical vagotomy (P < 0.001). In breaths that followed the apnoea, peak activity of the parasternal intercostal muscles increased from a baseline of 3.1 +/- 0.8 to 9.2 +/- 1.8 arbitrary units (P < 0.001). Nicotine produced a similar increase in peak phrenic ENG (7.0 +/- 0.5 to 14.5 +/- 1.2 arbitrary units; P < 0.001). Peak triangularis sterni muscle EMG was reduced from 8.9 +/- 1.2 to 6 +/- 1.7 arbitrary units (P < 0.05) and the onset of response was delayed to 30 s after the challenge. The changes of respiratory effectors induced by nicotine were independent of vagal integrity. The results show that post-nicotine apnoea is to large extent vagally dependent though the response of the respiratory muscles is mediated by non-vagal influences.

Influence of central antitussive drugs on the cough motor pattern

The present study was conducted to determine the effects of administration of centrally active antitussive drugs on the cough motor pattern. Electromyograms of diaphragm and rectus abdominis muscles were recorded in anesthetized, spontaneously breathing cats. Cough was produced by mechanical stimulation of the intrathoracic trachea. Centrally acting drugs administered included codeine, morphine, dextromethorphan, baclofen, CP-99,994, and SR-48,968. Intravertebral artery administration of all drugs reduced cough number (number of coughs per stimulus trial) and rectus abdominis burst amplitude in a dose-dependent manner. Codeine, dextromethorphan, CP-99,994, SR-48,968, and baclofen had no effect on cough cycle timing (CTtot) or diaphragm amplitude during cough, even at doses that inhibited cough number by 80-90%. Morphine lengthened CTtot and inhibited diaphragm amplitude during cough, but these effects were not dose dependent. Only CP-99,994 altered the eupneic respiratory pattern. Central antitussive drugs primarily suppress cough by inhibition of expiratory motor drive and cough number. CTtot and inspiratory motor drive are relatively insensitive to the effects of these drugs. CTtot can be controlled independently from cough number.

Inspiratory and expiratory patterns of the pectoralis major muscle during pulmonary defensive reflexes

Experiments were conducted to determine the discharge pattern of the pectoralis major muscle during pulmonary defensive reflexes in anesthetized cats (n = 15). Coughs and expiration reflexes were elicited by mechanical stimulation of the intrathoracic trachea or larynx. Augmented breaths occurred spontaneously or were evoked by the same mechanical stimuli. Electromyograms (EMGs) were recorded from the diaphragm, rectus abdominis, and pectoralis major muscles. During augmented breaths, the pectoralis major had inspiratory EMG activity similar to that of the diaphragm, but during expiration reflexes the pectoralis major also had purely expiratory EMG activity similar to the rectus abdominis. During tracheobronchial cough, the pectoralis major had an inspiratory pattern similar to that of the diaphragm in 10 animals, an expiratory pattern similar to that of the rectus abdominis in 3 animals, and a biphasic pattern in 2 animals. The pectoralis major was active during both the inspiratory and expiratory phases during laryngeal cough. We conclude that, in contrast to the diaphragm or rectus abdominis muscles, the pectoralis major is active during both inspiratory and expiratory pulmonary defensive reflexes.

Effect of codeine on the inspiratory and expiratory burst pattern during fictive cough in cats

Experiments were conducted to study the effect of the opioid, codeine, on different components of the cough motor pattern. Midcollicular decerebrate cats were paralyzed and artificially ventilated by a pump triggered by the phrenic neurogram. Inspiratory (phrenic) and expiratory (cranial iliohypogastric) neurograms were recorded. Fictive cough was produced by mechanical stimuli applied to the intrathoracic trachea. Codeine (0.03-1.0 mg.kg-1, i.v.) decreased cough frequency (average number of coughs per stimulus trial), expiratory burst amplitude, and inspiratory burst amplitude in a dose-dependent manner. The maximum reduction in cough frequency and expiratory amplitude produced by codeine was 80-90% for both parameters. However, codeine was more potent in reducing cough frequency (ED50 = 0.1 mg.kg-1) than expiratory burst amplitude (ED50 = 0.35 mg.kg-1). The maximum observed reduction of inspiratory burst amplitude elicited by codeine was approximately 40%. There was a positive linear relationship between phrenic and cranial iliohypogastric burst amplitudes during fictive cough (r = 0.82, P < 0.001). Codeine destabilized the motor pattern during fictive cough by disrupting this relationship between inspiratory and expiratory burst amplitudes. We conclude: (a) the central pattern generator for cough is functionally organized into a cough frequency generator, an expiratory burst amplitude generator and an inspiratory burst amplitude generator, each of which have different sensitivities to codeine (b) there exists a specific codeine-sensitive neural mechanism matching the relative magnitude of central drive to inspiratory and expiratory motoneurons during cough.

Nasal air puff stimulations and laryngeal, thoracic and abdominal muscle activities

In cats, we studied the activity of laryngeal, thoracic and abdominal muscles and the variations in oesophageal pressure in response to air puff stimulations of the nasal mucosa. Following single stimulations, inspiratory and laryngeal dilator muscles were transiently inhibited. During inspiratory inhibition, expiratory muscles and laryngeal constrictor were transiently activated. Repetitive air puff stimulations, which induced sneeze, evoked a similar pattern of transient activities during the inspiratory preparation of sneeze. This resulted in transient fluctuations of the oesophageal pressure, whose mean value became more negative as the preparatory inspiration enhanced. Our results suggest that the entire pool of respiratory neurons (bulbospinal, vagal and facial) works together in the sneeze reflex. Study of transient inspiratory inhibition demonstrates two periods during the preparatory inspiration phase of sneeze. In the first period transient effects are related to each shock of the stimulation. The second period is characterized by a diminution or a lack of transient effects associated with the stimulation. At the end of the expulsive phase, the diaphragm and the glottal dilator were further activated.

Postnatal development of the anterior ethmoidal nerve in cats: unmyelinated and myelinated nerve fiber analysis

This is the first quantitative electron microscopic study of anterior ethmoidal nerve in adult and newborn cats. The adult nerve comprises about 1,000 myelinated fibers including A delta (65%) and A beta (35%) fibers and 6,000 unmyelinated fibers. At birth, only 27% of the adult myelinated fibers complement is already present. The immaturity of the nerve is discussed in relation to that of the sneeze reflex.

Antitussive effects of GABAB agonists in the cat and guinea-pig

1. GABAB agonists inhibit neuronal processes which are important in the pathogenesis of airway disease, such as bronchospasm. Cough is a prominent symptom of pulmonary disease, but the effects of GABAB agonists on this airway reflex are unknown. Experiments were conducted to determine the antitussive effect of GABAB receptor agonists in comparison to the known antitussive agents, codeine and dextromethorphan. 2. Unanaesthetized guinea-pigs were exposed to aerosols of 0.3 mM capsaicin to elicit coughing, which was detected with a microphone and counted. Cough also was produced in anaesthetized cats by mechanical stimulation of the intrathoracic trachea and was recorded from electromyograms of respiratory muscle activity. 3. In guinea-pigs, the GABAB agonists baclofen and 3-aminopropyl-phosphinic acid (3-APPi) produced dose-dependent inhibition of capsaicin-induced cough when administered by subcutaneous or inhaled routes. The potencies of baclofen and 3-APPi compared favourably with codeine and dextromethorphan. 4. The GABAB antagonist, CGP 35348 (0.3- 30 mg kg-1, s.c.) inhibited the antitussive effect of baclofen (3.0 mg kg-1, s.c.). However, CGP 35348 (10 mg kg-1, s.c.) had no effect on the antitussive activity of codeine (30 mg kg-1, s.c.). The antitussive effect of baclofen was not influenced by the GABAA antagonist, bicuculline (3 mg kg-1, s.c.) or naloxone (0.3 mg kg-1, s.c.). 5. In the cat, baclofen (0.3-3.0 mg kg-1, i.v.) decreased mechanically-induced cough in a dose-dependent manner. In this model, baclofen (ED50 = 0.63 mg kg-1) was less potent than either codeine or dextromethorphan. The antitussive effect of baclofen in the cat was antagonized by the GABAB antagonists, CGP 35348 (10 mg kg-1, i.v.) and 3-aminopropylphosphonic acid (3 mg kg-1, i.v.).6. We show that baclofen and 3-APPi have antitussive effects in the guinea-pig and cat and these effects are mediated by GABAB receptors.