Infectious Bacterial Pneumonia in Puppies
Puppies are predisposed to acute infectious tracheobronchitis, which can progress to pneumonia, because they are often physiologically stressed by changes in ownership and new environments. In addition, poor nutrition, overcrowding, poor hygiene, and concurrent diseases such as parasitism all predispose them to development of contagious respiratory tract infections. A variety of organisms are implicated, including viruses such as parainfluenza, adenovirus and canine distemper virus; and bacteria such as Bordetella, Streptococci, and Mycoplasma sp. Puppies that have infections confined to the upper respiratory tract are usually clinically healthy, eating and afebrile. Most of these puppies will respond favorably to time, good husbandry, and antibiotic therapy. The youngest, most immunosuppressed puppies, or those of breeds such as English bulldogs with congenital abnormalities including brachycephalic airway syndrome or hypoplastic trachea, have a decreased ability to resolve respiratory tract infections. In these patients, pneumonia is a real and life-threatening risk. Puppies with infectious bronchopneumonia can be recognized because they are usually systemically sick, often febrile, and they may have significant respiratory distress.
Representative cultures should be obtained from the respiratory tract prior to initiation of antibiotic therapy. In most puppies, cultures are best obtained by endotracheal lavage. Once samples have been obtained for culture, antibiotic therapy should be instituted immediately. The initial antibiotic should provide broad-spectrum coverage for the most likely organisms, bearing in mind the possibility of polymicrobial infection. As a general rule, oral antibiotics can be used if the pneumonia puppy is systemically healthy and is not dyspneic. Antibiotics should be administered by parenteral routes (ideally intravenously) in puppies that are dyspneic, febrile, debilitated, or depressed. Intravenous antibiotics are the best way of ensuring that adequate plasma concentrations are achieved, because there is no guarantee of adequate absorption of drugs from the gut in such sick animals. Our experience suggests that the best initial antibiotic choice in puppies with severe pneumonia is a combination of ampicillin and an aminoglycoside (once dehydration has been corrected). When ampicillin is combined with an aminoglycoside, a synergistic effect provides excellent broad spectrum coverage in serious respiratory infections. If Bordetella pneumonia is suspected, intravenous azithromycin can be administered. Other options such as enrofloxacin or tetracyclines must be avoided because of their respective adverse effects on joints and teeth. Interestingly, the beta lactams such as amoxicillin, ampicillin and ticarcillin do not penetrate well into the mucus lining the bronchi, and therefore are often ineffective in puppies with Bordetella pneumonia. Once culture and sensitivity results are available, a specific and narrow spectrum antibiotic can then be chosen for ongoing care. Once the crisis has resolved, azithromycin has proven to be a useful oral antibiotic for puppies with bordetellosis.
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 puppies with pneumonia, large amounts of viscous secretions are produced, and must be moved up through a very narrow airway. 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. 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.
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. N-acetylcysteine can also be given intravenously. Drug therapy can also include a bronchodilator such as aminophylline or terbutaline, ideally administered parenterally.
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. 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. Puppies 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. Mild to moderate exercise often stimulates productive coughing, which should be encouraged by coupage. Coupage is the action of gently tapping the chest wall of the puppy 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.
Oxygen supplementation should be delivered as required to keep the puppy comfortable. Many of the sickest puppies require oxygen supplementation for prolonged periods of time, sometimes as long as 2-3 weeks.
Puppies with pneumonia 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 4-7 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 determination of 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. Many puppies with bacterial pneumonia experience hypoventilation and slightly high 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, the radiographs are improving, and the puppy 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 adult dogs, this occurs 3-14 days from hospital admission, but in puppies it can be a very protracted process sometimes requiring as long as 3-4 weeks of hospitalization and oxygen supplementation. The puppy should be re-examined about one week after discharge with chest radiographs to confirm that the pneumonia is continuing to resolve. In severe cases, several weeks or even months 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 3-6 months in severely affected puppies.
Recognizing and Managing Hypoventilation
Ventilation is the ability of the chest wall and diaphragm to move an adequate volume of air into the chest. The term 'minute ventilation' refers to the volume of air moved into and out of the lung during a 60 second period. The volume of air is a product of the tidal volume (volume of a single breath), and the respiratory rate. For normal ventilation to occur, the animal must have a normal brain-stem respiratory control center, normal spinal cord function to the level of C3/C4, normal spinal and phrenic nerve function and neuromuscular transmission, normal muscle and chest wall integrity, absence of pleural disease, and a patent airway. If there is an abnormality of any of these functions, inadequate volumes of air may enter the body, resulting in hypoventilation. Thus, hypoventilation occurs if there is: neuromuscular disease affecting the chest wall and/or diaphragm, respiratory depressant drugs (e.g., anesthetics) which can suppress the central nervous system, or lesions such as flail chest or pleural effusion which affect the ability of the animal to move adequate quantities of air. Sometimes, severe pulmonary disease such as pneumonia can also lead to hypoventilation, but this is unusual.
Examples of the most common causes of hypercarbia include:
Neurologic disease or anesthesia affecting central medullary respiratory drive
Spinal cord dysfunction cranial to C4-C5
Phrenic nerve dysfunction
Chest wall injury
Respiratory muscle dysfunction
Pleural diseases: effusion, severe pneumothorax, diaphragmatic hernia
Because CO2 diffuses through tissues very easily (about 20x more soluble than O2), we use the amount of CO2 in arterial blood (PaCO2) as a measure of the extent of ventilation. Normally, if ventilation is adequate, then CO2 is 'blown off' easily; normal dogs have a PaCO2 of 35-45 mm Hg. If hypoventilation occurs, CO2 increases, leading to respiratory acidosis by formation of carbonic acid:
CO2 + H2O = H2CO3 = H+ + HCO3-
As well as causing an increase in CO2, hypoventilation also leads to hypoxia, since there is less oxygen in the alveoli for gas exchange. Increased PaCO2 in hypoventilation is therefore accompanied by decreased PaO2. In a hypoventilating patient, oxygen supplementation will increase the PaO2, but will result in no change in PaCO2 values because it does not change the total volume of air moved into and out of the lungs per minute.
If PaCO2 is high, the animal experiences dyspnea. Profound respiratory acidosis may result from the hypercarbia, which can be life-threatening by causing decreased cardiac output, hypotension and neurologic depression due to carbon dioxide narcosis. Thus, if hypoventilation is found in clinical patients, the clinician must consider measures to improve ventilatory status by addressing the cause of hypoventilation. Hypoventilation can often be managed by treatment of the underlying problem e.g., thoracocentesis for a pleural effusion, reversing anesthetic drugs, or surgery for an airway obstruction. If conservative methods are not adequate for the management of the hypoventilating patient, then positive pressure ventilation is the only effective option.
References are available upon request.