Respiratory disease and emergencies (Proceedings)

The pulmonary system is complex with various anatomical structures performing highly specialized functions. When evaluating the system it is useful to examine each structure for its unique function and associated potential complications. Physical assessment and monitoring tools such as pulse oximetry and arterial blood gas analysis are used to localize respiratory problems and guide treatment which may include supplemental oxygen therapy, appropriate drugs or pulmonary physiotherapy. Generally, problems are divided into two categories; ventilatory, that is the ability to move air in and out of the pulmonary system and respiratory, that is the ability to effectively exchange oxygen and carbon dioxide. Regardless of the specific problem, animals in respiratory distress are always regarded as medical emergencies due to the high potential for brain damage, respiratory failure and cardiac arrest. Rapid localization of the problem is the key to successful outcome

The upper airway

Upper airway anatomy includes the nose, mouth, pharynx, larynx and trachea. The primary purpose of these structures is the act as a conduit for the passage of air into the lower airway (ventilation). Although considered anatomical "dead space" because gas exchange (respiration) does not occur, several vital functions are provided by the upper airway. Humidification and protection from infection are provided by mucous production. Cilia that line the respiratory tract move mucous and bacteria out of the airway in a sweeping motion.

Problems with the upper airway are related to complete or partial obstruction of one or more of its structures. Obstruction may be caused by a number of factors including:

     • foreign body

     • laryngeal spasm/paralysis

     • brachycephalic disease

     • edema

     • neoplasia

Specific breathing patterns and presenting signs are unique to patients with upper airway obstruction. Most notably these patients have audible "noisy" inspiratory sounds (stridor) and present in a state of hysteria or panic due to the acute onset of disease. In animals with partial obstruction, respiration will most likely be prolonged and forceful on inspiration in an effort to move air through a narrowed airway. Tachypnea, cyanosis and foaming are likely to be present in patients with more complete obstruction.

Diagnosis of upper airway disease is made chiefly by observing breathing pattern and visualization of the airway itself. Radiographs may be helpful as well but often patients are not stable enough to undergo radiography, initially. In fact, many patients will require sedation immediately to eliminate stress, relax the airway and allow removal of the obstruction if possible. Patients require oxygen supplementation until airway patency can be established. In some cases this may necessitate endotracheal intubation or tracheostomy.

The lower airway

Lower airway anatomy includes the bronchi, bronchioles, alveoli, and lung tissue. Gas exchange (respiration) is the primary function of the lower airway.

Problems of the lower airway are related to interference, by a number of processes, with the ability to take in oxygen and expel carbon dioxide. Common lower airway diseases/disorders are:

     • asthma(bronchial irritation/inflammation/constriction)

     • pulmonary edema

     • pneumonia

     • pulmonary contusions

     • smoke inhalation/near drowning

     • pulmonary thromboembolism

Many of the breathing patterns associated with lower airway disease are related to damaged or fluid filled alveoli. Patients are often tachypnea with shallow respiration. Moist sounds, harshness and wheezing are often heard on auscultation. Because patients with lower airway problems are often hypoxic, they frequently present with pale or cyanotic mucous membranes.

Diagnosis of lower airway disorders can be confirmed with radiographs. Arterial blood gas analysis and pulse oximetry are useful tools in assessing the degree of respiratory compromise as well as in guiding therapy. Treatment for lower airway disorders may include bronchodilators, steroids, and/or antibiotics. In most cases, supplemental oxygen is beneficial. In extreme cases, mechanical ventilation may be required.

The thorax

Thoracic anatomy includes the pleural cavity, chest wall, ribs and diaphragm. Thoracic disorders are separated into two broad categories. Restrictive disorders; those that inhibit the ability of the lungs to expand adequately into the pleural space. Examples of restrictive disorders are various pleural effusions and pneumothorax (see upcoming article on pleural effusion and pneumothorax), and those that involve trauma to the chest wall, rib cage or diaphragm. The second group which includes, rib fractures, penetrating chest wounds and flail chest inhibit effective breathing by interfering with the mechanics of negative pressure ventilation. Diaphragmatic hernia is a disorder with both components. The mechanics of ventilation may be inhibited by the tear in the diaphragm while lung expansion may be restricted by abdominal organs that have herniated into the pleural space through the tear.

The typical breathing pattern is rapid and shallow. Some patients may show dramatic abdominal effort when adequate chest excursions are not possible. Mucous membranes are often pale or cyanotic particularly if the thoracic condition is acute. In chronic cases, the patient may be habituated to the reduced respiration and may show little or no sign of acute respiratory distress. When there is fluid or air in the pleural space, heart and lung sounds will be dulled on auscultation, "Rubbing" sounds can also be auscultated as the lung tissue expands against effused fluids.

Diagnosis of a thoracic disorder is easily made with radiographs. Patients with severe effusions or pneumothorax should have their chests evacuated using thoracocentesis prior to radiography. Fluid obtained by thoracocentesis is evaluated for definitive diagnosis. Treatment for thoracic disorders involves continual and effective pleural space evacuation to allow lung re-expansion. Indwelling chest tube(s) may be indicated in cases of chronic, recurrent effusion. Thoracic trauma, such as diaphragmatic hernia and fail chest, requires surgical intervention. Chest tubes are also placed at surgery, in all thoracotomy patients, for post-operative chest evacuation and pain management. Supplemental oxygen may be helpful early in the treatment of thoracic problems.

Other causes of respiratory distress

There are a surprising number of diseases and disorders that cause respiratory distress that are not directly related to the respiratory system. While these problems may or may not be life threatening, is essential to consider them when trying to identify the cause of respiratory distress.

      Anemia results decreased oxygen carrying capability due to decreased circulating red blood cells. The breathing pattern is usually panting/tachypnea. Patients may have pale or cyanotic mucous membranes. These patients require red blood cell transfusion rather than oxygen to improve respiration.

      Metabolic acidosis (decreased bicarbonate) causes a compensatory hyperventilation. The body tries to decrease carbon dioxide by increasing respiratory rate in an effort to restore normal blood pH. These patients may require supplementation with sodium bicarbonate to correct acidosis. Once pH is normalized, respiration slows down responsively.

      Increased abdominal pressure can be due to ascites (fluid accumulation), hemoabdomen (blood accumulation) or organomegaly (enlarged abdominal organ(s) or tumor. Dramatic increases in intra-abdominal pressure interfere with diaphragmatic function and disturb the mechanics of ventilation. Patients appear to have exaggerated chest excursions to compensate for diaphragmatic restriction. Reducing intra-abdominal pressure will correct respiratory signs

      Neurologic disease can affect the brain's respiratory center and alter respiratory drive. Patients with central nervous system impairment may breathe quite rapidly or barely breathe and often require mechanical ventilation to regulate rate and maintain normal carbon dioxide levels. Animals in status epilepticus may also pant from hyperthermia due to continual seizing.

      Tachypnea (rapid respiration) is also associated with fever, pain, anxiety and the need to urinate or defecate in a caged housebroken animal.

Oxygen therapy

Normal cellular function is dependent on a constant supply of oxygen sufficient to meet energy needs. Hypoxia (lack of sufficient oxygen available to tissues) results in cellular dysfunction. Diseases of the pulmonary system such as pneumonia, v/q mismatch and shunting may result in hypoxia. Hypoxemia results from disorders affecting oxygen transport such as hemoglobin dysfunction, anemia and diminished cardiac output.

Patients with hypoxemia (PaO2 < 80% or SpO2 < 85%) may benefit from the addition of supplemental oxygen alone. Severe cases that are nonresponsive to oxygen supplementation may require mechanical interventions such as positive end expiratory pressure (PEEP) to maximize gas exchange at the alveolar level. Also, patients with hypoxia secondary to severe anemia may not benefit from O2 therapy alone and may require transfusion of red blood cells to increase hemoglobin carrying capacity. In any case the benefit of oxygen therapy must be weighed against the potential complications associated with prolonged exposure to excessive amounts of oxygen (>60% for >12 hrs), i.e. oxygen toxicity. Oxygen toxicity is a serious complication of prolonged O2 supplementation because excess oxygen free radicals can cause severe cellular damage particularly in the lungs. For this reason it is generally recommended that O2 supplementation not exceed 50% whenever possible and that O2 therapy is decreased/discontinued promptly.

Supplementation

      Oxygen mask

High concentration oxygen supplementation can be provided, on a short-term basis, via a close fitting oxygen mask. This is only useful in recumbent patients with depressed mentation. Too tight fitting a mask results in rebreathing CO2 and consequent hypercapnea

      Oxygen bag (hood)

A clear plastic bag can be placed over the patient's head or over an Elizabethan collar placed around the patient's neck. Oxygen is provided via an oxygen delivery tube or catheter placed inside the bag near the patient's nose. The recommended flow rate is 5 to 8 liters per minute and the bag is inflated to a volume that equals several breath exchanges. The bag remains open along the patient's neck to allow expired carbon dioxide rich air to escape. This method can provide at least a 75% oxygen concentration and is better tolerated than a facemask.

      Oxygen cage

The disadvantages of an oxygen cage include the fact that close observation of the patient is difficult and progressive respiratory failure may go undetected. Dramatic decrease in the oxygen concentration whenever the cage door is opened may also significantly affect the patient. Diagnostic tests such as auscultation and radiography are impossible to perform. However, oxygen cages do provide a means of administering variable known inspired concentrations of oxygen and often allow control of ambient temperature and humidity.

      Nasal oxygen

Nasal cannulization with a feeding tube is tolerated well by most animals following local installation of a few drops of Proparacaine or Lidocaine. Long-term tolerance is good. A humidified oxygen flow rate approximately 100ml/kg per minute will result in a 40% oxygen concentration in the trachea. Higher flow rates will result in increased oxygen concentrations but may be less well tolerated.