Treating canine bacterial pneumonia: Beyond antibiotics (Proceedings)

In most patients with pneumonia, antibiotic therapy should be considered part of an overall management scheme, rather than the only treatment. Since resolution of pneumonia largely depends on clearance of secretions from the airway via the cough reflex and the mucociliary escalator, measures must be taken to ensure that the secretions are liquid and easily expectorated. Additionally, chest physiotherapy can play a vital role in promoting the cough reflex. Supportive care should include oxygen therapy if the patient is hypoxic, and other nursing care including nutritional support and routine monitoring and care of the recumbent patient if appropriate. In some situations, all of this may be virtually ineffective if unaccompanied by other therapy. For example, since many antibiotics cannot penetrate abscesses effectively, and are inactivated by the presence of pus, therapy in patients with lung abscesses or pyothorax must be accompanied by other aggressive forms of treatment, for example lung lobectomy or pleural drainage. Additionally, attempts should be made to resolve the cause of pneumonia, if possible, in order to effect complete resolution of the bacterial infection.

Airway hygiene and clearance of secretions

Clearance of secretions from the airways occurs via the mucociliary escalator and cough reflex, and is delayed if the secretions are extremely viscous and tenacious. In dogs and cats with pneumonia, large amounts of viscous secretions are produced, and attempts to resolve the infection must include attention to the character of the respiratory secretions. Productive coughing must be actively encouraged, and the secretions must be maintained as liquid as possible. More than 90% of the mucus in the respiratory tract is water, so even a mild degree of dehydration leads to drying of the secretions. The most important means by which this is achieved is by parenteral fluid therapy. Unless extreme respiratory distress is present, these patients should not be allowed to become dehydrated, and diuretic use should be avoided.

The tenacity of mucus also depends on the structure of the mucopolysaccharides that it contains. N-acetylcysteine can be administered orally, and acts as a mucolytic by opening disulfide bonds, thereby decreasing the viscosity of the mucus. It can also be administered by nebulization, but it can cause bronchospasm by this route, which is usually manifested by coughing. If coughing or dyspnea occurs, the patient may be pre-treated with bronchodilators prior to nebulization. Orally administered expectorants such as ammonium bicarbonate and potassium iodide act by irritating the mucosa of the gastrointestinal tract, thereby stimulating a vagal gastropulmonary reflex that results in increased secretion by the bronchial glands. Phenolic compounds such as guaiacol, and inhaled volatile oils such as Eucalyptus oil, may directly stimulate production of increased amounts of watery mucus.

Nebulization is a technique in which tiny spherical droplets of water are generated and inhaled by the patient. The droplets then "shower out" at various levels of the respiratory tract, depending on their size, due to changes in direction of air flow, brownian motion, and gravity. Droplets greater than 10 microns reach only the upper airway and trachea. In the range of 1-10 microns, the smaller the droplet, the deeper it is able to penetrate into the respiratory tract. Droplets less than 0.5 microns reach the alveoli and are exhaled. Most ultrasonic nebulizers create droplets in the 2-5 micron range.

Once the respiratory tract secretions have been moistened and increased in volume, clearance of the material depends on normal function of the other respiratory defense mechanisms. Atelectasis predisposes to pneumonia because bacteria can be trapped and proliferate in collapsed airspaces and cannot effectively be cleared by the mucociliary escalator. In addition, animals with prolonged or recurrent atelectasis are often recumbent, and because they are weak and sometimes painful they may also have a depressed cough reflex, further impairing their ability to clear organisms and material from their airways. In particular, the cough reflex is a vital part of recovery from serious pneumonia. The simplest method of stimulating coughing is simply to stimulate an increased tidal volume during respiration, usually by mild exercise. Dogs with pneumonia should not be allowed to lie in one place for long periods of time. The amount of exercise needed to increase the tidal volume and respiration rate is variable depending on the severity of disease. In some, simply turning the animal from one side to the other in lateral recumbency is enough. The next step may be to stand the patient for brief periods of time, then to take a few steps, gradually building strength and mobility. Mild to moderate exercise often stimulates productive coughing which should be encouraged by coupage.

Coupage is the action of firmly striking the chest wall of the patient with a cupped hand, which helps to stimulate the cough reflex and to "break up" secretions in the airways. Coupage should be performed several times daily, especially in patients that are unable to stand and move around. It is usually well tolerated, except in patients that have experienced thoracic trauma or thoracic surgery.

Monitoring the pneumonia patient during therapy

Pneumonia patients must be monitored carefully to ensure that they are continuing to respond appropriately to therapy. Radiographs of the chest should be obtained periodically during hospitalization (about every 3-4 days) to confirm that the alveolar disease is resolving. Failure to achieve clinical or radiographic improvement should prompt reconsideration of antibiotic therapy, repeat tracheal wash culture, or repeated attempts to resolve the underlying cause of the pneumonia.

Lung function should be repeatedly evaluated by monitoring arterial blood gases or pulse oximetry. The pulse oximeter can be used for intermittent monitoring of oxygen saturation, or alternatively it can be used to provide a continuous real-time read-out, which is particularly useful for monitoring general anesthesia or sedation. The pulse oximeter can also be used to monitor changes in saturation when stressful procedures are being performed, for example transtracheal washes or radiographs. This technique allows the clinician to determine whether a need exists for oxygen supplementation, and also to objectively assess the response in terms of an increase in oxygen saturation. Pulse oximetry readings of < 90% are clinically significant, and should be addressed immediately with oxygen supplementation. Desaturation in dogs that are already on oxygen supplementation is a serious situation. Arterial blood gas analysis is the gold standard for direct assessment of pulmonary function, and it also provides information about the metabolic acid-base status of the body. Normal partial pressure of oxygen is expected to be 90-100 mmHg when the animal is breathing room air, and results < 80 mmHg are clinically significant. Normal partial pressure of carbon dioxide is 35-45 mmHg, and clinically significant hypoventilation occurs when carbon dioxide is > 50 mmHg. Most dogs with bacterial pneumonia experience hyperventilation and slightly low carbon dioxide concentrations. Sequential analysis of arterial blood gas results is the most accurate tool for objectively assessing trends in response to treatment.

Once the lung function has returned to normal, and the patient is feeling better, eating well, and is active and alert, oral antibiotic therapy can be instituted and discharge from the hospital can be considered. In most patients, this occurs 3-14 days from hospital admission. The patient should be re-examined in about one week for chest radiographs to confirm that the pneumonia is continuing to resolve. In severe cases, a number of weeks of therapy are required for complete resolution of radiographic signs of pneumonia. As long as the animal is doing well clinically, it should be radiographed approximately every 2 weeks until the radiographs are normal. Oral antibiotic therapy should be continued for a further 2 weeks after radiographic resolution of the disease, in order to assure that the bacterial infection has been completely eliminated. The total duration of antibiotic therapy may be as long as 2-3 months in severely affected patients.

Persistent lung lobe abnormalities despite treatment

In animals with pneumonia, review of a series of sequential radiographs should reveal a gradual progression of improvement and eventually resolution if all is going well. Occasionally, a patient may have persistent alveolar disease that eventually plateaus and fails to improve further despite ongoing appropriate therapy. If the alveolar disease persists, then several possibilities should be considered

1. Incorrect antibiotic choice: If a culture is not obtained, empiric antibiotic therapy may prove to be incorrect. For example, if a dog has pneumonia caused by E. coli, then use of a first generation cephalosporin such as cephalexin (primarily effective against gram positive organisms) is likely to be ineffective.

2. Failure to optimize airway defenses during treatment: If the animal is dehydrated, weak or recumbent, then it will be unable to remove secretions from its airways and pneumonia can persist in spite of appropriate antibiotic therapy.

3. Insufficient duration of therapy/follow-up: If antibiotic therapy is not continued for a long enough duration of time, then recurrence of disease is possible. This is an easy trap to fall into, as many of these animals can feel very well and show minimal signs of disease, despite still having abnormal radiographs.

If radiographic evidence of disease persists in one lung lobe, and each of these three possibilities has been considered and ruled out, other options must be investigated. It is possible that severe inflammation in one lung lobe has caused some degree of fibrosis or chronic changes that will never completely resolve and do not represent an active pneumonia. If this is the case the radiographic changes in that lung lobe, while appearing as alveolar disease, are usually wispy and minor. Alternatively, one lung lobe may appear completely atelectatic, with movement of the mediastinum towards that side of the thorax because the lung lobe is smaller than normal. If treatment has appeared to be clinically effective and the animal is doing well, then a decision must eventually be made about whether to discontinue antimicrobial therapy even though the radiographs have not completely returned to normal. If antibiotic treatment is terminated prematurely, there is a risk of recurrence of pneumonia and the animal should be monitored carefully. In this situation, a bronchoalveolar lavage or repeat tracheal wash after antibiotics have been discontinued can provide useful information.

Alternatively, if a significant amount of alveolar disease persists but is consistently localized to one lung lobe, especially if the animal continues to have clinical signs of respiratory disease, then further investigation should be considered. An area of lung necrosis or abscessation may be present, which can occasionally be associated with aspiration of a foreign body. It is also possible that the diagnosis of bacterial pneumonia was incorrect, and other diagnoses such as neoplasia or lung lobe torsion should be considered. Bronchoscopy and bronchoalveolar lavage can provide useful information that may help direct therapy. Ultimately, if the disease is localized, is severe, and is failing to resolve with appropriate therapy, then surgical exploration and resection of the offending lung lobe may be required in order to completely resolve the problem.

References available from the author on request