A.J. Venker-van Haagen, DVM, PhD, DECVS
Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University
Utrecht, The Netherlands
What is dyspnea?
Dyspnea is difficult or labored breathing. It is caused by hypoxemia and hypercapnia, which are detected by the peripheral and central chemoreceptors, respectively. Dyspnea may be caused by insufficient ventilation or insufficient oxygen in the inspired air, or by insufficient circulation, or anemia, or abnormal hemoglobin. Respiration and circulation regulate the oxygen, carbon dioxide, and hydrogen ion environment of the cells. Respiration is controlled by central respiratory centers, central and peripheral chemoreceptors, pulmonary reflexes, and nonrespiratory neural input.
What are the clinical signs of dyspnea?
In the dog and the cat, labored breathing is recognized by increased respiratory frequency and strong outward movement of the thoracic wall. It is accompanied by labial respiration, the mouth held slightly open and the corners of the lips retracted. Distress and fear of being handled are also common in dyspneic patients. When an obstruction in the upper airways causes the dyspnea, there may be a stridor or wheeze. A stridor is the sound produced by the passage of the respiratory airflow through a narrowed pathway. When the narrowing is in the nose, breathing through the nose produces a rustling sound or wheezing. Obstruction of the larynx is recognized as the sound of a handsaw sawing wood, and it occurs during both inspiration and expiration. In the dog these sounds are produced through the open mouth and are loud. When dyspnea is caused by lung dysfunction or severe anemia, respiration is labored, there is distress, but there is no stridor.
Risk assessment in dyspnea
Even when the history indicates stationary or slowly progressive signs of dyspnea, there could be a rapid increase of the signs of dyspnea due to the stress of the visit to the veterinarian. During the general physical examination of the dyspneic patient, physical and emotional stress should therefore be avoided. While handling the patient gently, continuously observe the character of the respiration, the sound of the stridor, and the color of the tongue. The changing of a laryngeal stridor into a high-pitched inspiratory sound indicates the need for immediate intravenous sedation (medetomidine and propofol) and endotracheal intubation. The color of the mucosa changes rapidly to a dull grayish-blue color and without immediate sedation and intubation, lung edema and death may follow. Oxygen and intravenous infusion of fluid may help to overcome the critical situation. The examination can then continue.
When there is no stridor and the mucosa of the dyspneic patient is not anemic but dark red or cyanotic, oxygen may be administered via the nose during the physical examination and during further examination such as radiography. Remember to avoid stress as much as possible and never attempt to press an oxygen mask on a conscious dyspneic dog or cat. The ensuing struggle may cause death in these patients.
Especially in dyspneic dogs with laryngeal obstruction, hypoplasia of the larynx, or laryngeal paralysis, the forced, panting respiration may cause hyperthermia. Body temperature may rise to over 40 °C, even within a matter of minutes, and at that point, while the mucosa is still red, cooling is more important than oxygen. Spraying or sponging cool water over the entire surface of the dog will lower the body temperature to normal in about 20 minutes. If the panting does not stop, sedation and endotracheal intubation may then be required, but oxygen is not always necessary. In this situation, intravenous administration of fluid is a useful addition, for the dog has probably lost a large amount of fluid by prolonged panting. When the dog is stable, further diagnostic examination can be carried out.
Indications for tracheotomy
Tracheotomy is a surgical intervention to provide a direct connection between the trachea and the outside. It is useful when the airway through the larynx, the mouth, or the nose is obstructed, that is, when the airway obstruction is located cranial to the opening in the trachea. It is most often used as a temporary bypass for the time that it takes to remove the obstruction or for obstructive edema to disappear.
Laryngeal surgery is one of the indications for tracheotomy. During and after laryngeal surgery, edema of the laryngeal mucosa may cause life-threatening dyspnea. Another indication is severe trauma to the nose. Obstruction of the nose obviously does not preclude respiration when the dog or cat breathes through its mouth, but neither cats nor dogs appear to be able to do this continuously when at rest or sleeping. Hence they usually become hypoxic and hypercapnic during sleep if the nose is obstructed. Tracheotomy can prevent this distress and even death due to insufficient oxygenation of the heart and brain and respiratory acidosis.
Surgical technique of tracheotomy
A tracheotomy is always performed in a previously intubated patient. Remember that in the living dyspneic animal there is always an opening to the airway, however small and difficult to find, for without it the animal would not be alive. Introducing an endotracheal tube always requires anesthesia. In the dyspneic patient the brain may be more sensitive to anesthetic drugs, and the safest approach is to use an intravenously administered anesthetic in low doses, while observing the effect during injection. Medetomidine and propofol is an appropriate combination. Once the endotracheal tube is in place and the patient's condition is stable, the patient is placed in dorsal recumbency and the ventral side of the neck is clipped and otherwise prepared for sterile surgery. Sterile tracheal cannulas in several sizes are made ready. The diameter should be just large enough to permit insertion of the cannula, as a rule it is a round hole with a diameter of the distance across two tracheal rings. The trachea cannula should hang loosely in the tracheal lumen, with the inner opening pointing in the direction of the lungs. A small transverse skin incision is made over the cervical trachea and the subcutis is removed over the trachea until the tracheal rings are visible. A pointed scalpel is used to make a round opening in the trachea exactly fitting the tracheal cannula. Before inserting the cannula the previously placed endotracheal tube should be slowly removed until it is no longer visible through the opening. The cannula is then inserted, sutured to the skin, and also anchored with a soft cord around the neck. In cats it is important to also tape the cannula in place, because cats often try to remove the cannula, as dogs seldom do. The string and tape should be loose enough to avoid jugular vein obstruction.
Risks of dyspnea after tracheotomy
A tracheal cannula introduces dry air into the trachea and bronchi. Any mucus or pus tends to congeal in the lumen of the trachea cannula, causing obstruction. The best way to handle this problem is to use cannulas consisting of an outer and an inner cannula. The inner cannula can be removed and cleaned without disturbing the fixation of the outer cannula. The cleaning of the cannula should be performed every two hours, 24 hours a day.
A tracheal cannula should be made of rigid material and not plastic that softens at body temperature. The latter often flatten in the tracheal wound so that their lumen becomes dangerously narrow. The cannula should be extra long for dogs, so that it does not slip out of the tracheal opening and become lodged in the subcutis, which is very dangerous and may easily escape the caretaker's attention.
Bronchopneumonia is another risk for an unprotected airway. The surroundings should be kept clean, the hands should be washed and gloves should be worn to handle the inner cannula, and a broad-spectrum antibiotic should be administered for the duration of the use of the tracheal cannula.
Removal of the trachea cannula
The cannula can be safely removed when inspection under anesthesia reveals satisfactory resolution of the initial obstructive lesion. After the cannula is removed, the patient is kept under close observation for 20 minutes or longer. To avoid the development of subcutaneous emphysema, the wound in the trachea and skin is never sutured. Its spontaneous closure is effective in 20 minutes and complete in three days. The transverse incision of the skin over the trachea aids rapid skin healing.
A tracheotomy is a wonderful tool in experienced hands and every careful veterinarian should be able to place it correctly. The care of the patient with a tracheal cannula should be planned, for the cleaning procedure is a burden and should be shared among those available.