Cough

CHAPTER 57 Cough



Virginia Buechner-Maxwell


Cough is a reflex that protects the lungs from damage resulting from aspiration or inhalation of excessive noxious debris, and it aids in removal of excessive airway secretions and foreign bodies. Persistent cough is a nonspecific clinical sign associated with many infectious and noninfectious diseases of the lower and upper portions of the airway of horses. In most instances, cough is only one manifestation of a disease complex. Therefore, resolution of cough is dependent on identification and treatment of the primary disease process. Understanding the cough reflex and recognizing the stimuli that provoke it help provide direction to the practitioner evaluating the coughing horse.



MECHANISM OF COUGH


In humans and small animal species, the cough reflex has been well studied but less is known about horses. The density of cough receptors is highest in the larynx, trachea, and bronchi. Cough results from chemical, mechanical, or pharmacologic stimulation. Cough originating rostral to the thoracic inlet (extrathoracic cough) is initiated by stimulation of receptors in the nose, oropharynx, and upper trachea. In humans, receptors have also been identified in the tympanic membrane of the ear and in the diaphragm and stomach. Intrathoracic receptors are located in the epithelium of the lower trachea and bronchi.


Depending on which receptors are activated, the afferent message travels to the brain by several routes. Stimulation of receptors in the larynx, trachea, and bronchi results in transmission of impulses via the afferent fibers of the vagus nerve. The trigeminal nerve sends messages from the nose and paranasal sinus area; the glossopharyngeal nerve sends messages from the pharyngeal area; the vagus nerve sends messages from the ear, stomach, and pleura; and the phrenic nerve sends messages from the heart and pericardium. These impulses arrive at the cough center in the medulla. Efferent pathways originating from this center are carried by the vagus nerve, which coordinates the function of the larynx, trachea, and bronchi; and the phrenic and spinal nerves, which transmit impulses to the diaphragm and other respiratory muscles. Induction of voluntary cough, which is unlikely in horses, also requires input from the cortex.


Cough begins with deep, rapid inspiration of a volume of air that exceeds resting tidal volume. The glottis closes momentarily, and pleural, abdominal, and alveolar pressure builds as expiratory muscles contract and compress the air against the closed glottis. This pressure reaches 50 to 100 mm Hg in humans and may reach 500 mm Hg in horses. As pressure builds, the glottis opens and air is rapidly expelled through the mouth. In humans this event takes approximately 30 to 50 ms. Peak expiratory airflow reaches 12 L/s in humans and 65 to 90 L/s in horses. Oscillations of tissue and gas in the upper airway and mouth account for the explosive noise associated with initiation of a cough. As air is rapidly expelled, the intrathoracic portion of the trachea and bronchi are compressed, leading to a transient velocity increase and more sustained expiratory airflow. This occurs because the positive pressure in the pleural space exceeds that within the trachea and central bronchi, causing the membranous portion of the airways to fold inward and nearly obliterate the lumen. The linear velocity of air flowing through these narrowed channels is markedly increased, much like the increase in water flow that occurs when the diameter of a hose is reduced by pinching the end so that the opening is partially closed. This yields a shearing force that serves to dislodge secretions and particles from the airway surface.


Studies in humans indicate that cough sensitivity is decreased by administration of topical anesthetics, by chronic inflammation, and by chronic irritation. Cough receptors also adapt rapidly to stimuli so that cough frequency may diminish over time when animals are continuously exposed to the same noxious stimuli. Central nervous depressants such as narcotics, sedatives, and alcohol suppress the neurologic drive to cough. Injury to any of the neuronal pathways that control cough can also impair this reflex. In horses, cough provoked by bronchoalveolar lavage can be attenuated by topical application of 60 mL of a 0.66% lidocaine solution onto the carina and large bronchi at the time of sample collection or by intravenous administration of butorphanol tartrate (0.02 mg/kg) 20 minutes before the procedure. However, other medications, such as topical application of 60 mL of 0.33% lidocaine or parenterally administered detomidine, codeine, and glycopyrrolate, are not effective.


In healthy humans, mucociliary clearance and alveolar macrophage function effectively control minor insults to the airway that develop with inhalation of environmental debris and microbes. However, when these mechanisms are overwhelmed or damaged, cough plays a primary role in dislodging inhaled debris and accumulated secretions. Coughing that does not result in significant removal of secretions is considered dry or nonproductive and is more likely to arise from mechanical irritation of airway cough receptors or airway hyperreactivity. When mucus accumulates in the airway secondary to inflammation or dysfunction of the mucociliary clearance mechanism, coughing contributes to removal of debris and is considered wet or productive. Wet coughs are often associated with inflammation of the lower or upper airways (including paranasal sinuses) and may also occur in animals with masses or abscesses. Coughing eliminates accumulated secretions.


In disease states, excessive mucus accumulates along the airway, and as air (gas) flows across this layer, it imparts motion to the mucus (liquid) layer. Air velocity determines the pattern of mucus flow. For example, in humans, when air velocity is between 0 and 60 cm/s, the interaction between gas and liquid causes only the formation of small bubbles, resulting in minimal mucus elimination. If airflow exceeds 2500 cm/s, the mucus layer is dislodged from the surface in the direction of flow. This flow rate is achieved in large airways when humans cough. These measurements have not been made in horses, but it is presumed that a greater air velocity is required to move mucus along the length of a horse’s trachea.


Besides surface velocity, thickness of the mucus layer, rheologic properties of the secretions, and airflow pattern affect the success of two-phase gas-liquid transport in predictable ways. For example, increased viscosity decreases the ability of cough to clear these secretions. Animals that are weak or that have had thoracic trauma may be unable to fill and expel the lungs in such a way as to generate effective surface velocities. Body position may also be important. Recumbent humans can generate less than half the airflow velocity as standing humans, suggesting that animals that are lying down (such as sick foals) might not be able to generate adequate airflow velocities to cough excessive secretions from the lungs. In these instances, cough is of little benefit to the patient. In smaller airways, airflow does not achieve adequate velocity to clear secretions effectively, yet radioactive particles are cleared from the peripheral airways in voluntarily coughing humans. The effect of cough on lower airways in ill animals is dependent on the disease characteristics. For example, in diseases that impair surfactant production, excessive cough may contribute to collapse of small airways.


In some cases, cough occurs simultaneously with bronchospasm, also known as bronchoconstriction. In horses with recurrent airway obstruction (RAO), increased airway resistance and cough frequency are correlated. Mechanical deformation of larger airways secondary to bronchospasm is the likely cause of the associated cough. However, bronchospasm of the lower airways is not always associated with cough; this finding may explain why some horses with airway disease do not cough.



EXAMINING THE COUGHING HORSE


Many diseases elicit cough as part of the clinical complex (Table 57-1). Careful examination of affected horses typically reveals the underlying cause.


Table 57-1 Causes of Cough, Sorted by Anatomic Site of the Lesion









Site Causes of Cough
Nasal passages and paranasal sinuses
Septic rhinitis/sinusitis

Ethmoid hematoma

Granulomatous mass or lesion: cryptococcosis, nocardiosis, coccidioidomycosis

Foreign body: grass awn, thistle

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Cough

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