Aspiration pneumonia (AP) is a secondary event that occurs in emergency and critically ill patients. The precise incidence in veterinary medicine is not known, but it is most likely underdiagnosed and undertreated as is the case in human medicine.
Aspiration pneumonitis is caused by the patient's regurgitation of stomach acid which then migrates from the oropharynx down into the bronchial tree. Strictly speaking, aspiration pneumonitis is a 'sterile' inflammatory process, but it is inflammatory nevertheless and can have significant impact upon pulmonary function. Aspiration pneumonitis often precedes aspiration pneumonia. The term aspiration pneumonia is used when the patient's lungs are colonised by enteric bacteria and/or stomach contents, which then create a bacterial infection of the pulmonary system. For the purposes of this lecture, we will be treating the two diseases as one entity.
Many different kinds of disease and patients are susceptible to AP. Fundamentally, AP most readily occurs in patients who do not have normal control of their gastrointestinal function and swallowing or gag reflexes. Any patient that is in prolonged recumbency, has neuromuscular weakness, has impaired consciousness or has impaired oesophageal or laryngeal function is at real risk of AP. Examples of these conditions are:
Oesophageal disorders - megaoesophagus, oesophageal motility disorders, hiatal hernia
Vomiting - primary gastrointestinal disease, liver disease, toxins, foreign bodies, pancreatic disease
Neuromuscular disease - myasthenia gravis, spinal cord disease
Altered level of consciousness - seizures, stroke, brainstem injury, toxicities, head trauma, anaesthesia
Laryngeal disease - laryngeal paralysis, previous arytenoid lateralisation
Postanaesthetic aspiration
Associated risk factors in people include pregnancy, gastric distension, gastric outflow obstruction, tube feeding and bronchoscopy, as well as increased age and poor oral hygiene.
Basic precautions should be taken to prevent AP in at-risk patients. Anaesthesia-related strategies are well known, such as ensuring the patient has an empty stomach, accomplishing rapid-sequence induction and ensuring that the endotracheal tube cuff is well seated.
The unanaesthetised patient can also be at risk, however, as mentioned above. For those patients, the simplest strategy is to keep them in an inclined position, at about a 30-degree angle with their head up. Given that most of our patients are on a cage floor rather than a reclining hospital bed, some creativity is needed to organise an inclined bed. Bags of dog food can be used to prop up a piece of plywood or a cage divider panel. A rigid board of some sort is necessary, otherwise the patient will just sag into the blankets.
Organising and maintaining an inclined bed may seem like extraordinary hassle, but by the time you know the patient has aspiration pneumonia it's too late to do anything about it. A study has shown that human patients who were being tube-fed or mechanically ventilated in an inclined position had an 8% incidence of nosocomial pneumonia. Those patients who were left flat in bed had a 34% incidence of pneumonia.
Another simple nursing strategy that is used routinely in human intensive care is to swab the patient's mouth with a dilute chlorhexidine rinse every 4–6 hours. Additionally, if there is any green or yellow material on the swab, that is an early warning sign that regurgitation has occurred so aspiration may have occurred.
The clinical signs of AP are not particularly obvious. In a recent study of 88 dogs with aspiration pneumonia, only 42% of dogs had an increased respiratory rate greater than 30/min. Only 31% had a temperature greater than 39.2°C and only 12% of dogs had a tachycardia greater than 160/min. A cough was present in about half of the dogs. Abnormal lung sounds were described in 69% of dogs. From these results it can be seen that careful auscultation is the most sensitive part of the physical examination. Careful auscultation of lung fields means ausculting each side in at least four places - cranial, mid-dorsal, dorsocaudal, mid-ventral. Auscultation findings should be recorded on the patient's chart. A simple diagram and 'plus or minus' notation can be used to communicate findings effectively.
Radiography is a key part of confirming the diagnosis of AP. In human medicine, the development of 'new' respiratory signs in an at-risk patient, along with radiographic signs, is sufficient for a diagnosis of AP. When taking radiographs on AP suspects, it is important to take both right and left lateral views as well as a dorsoventral view. The left lateral view will give the best view of the right lung fields. In dogs, the right middle lung is most commonly affected but not always, and other lung lobes can be affected as well. In the study of 88 dogs by Kogan and Johnson, about 75% of the dogs had an alveolar pattern and 25% had an interstitial pattern.
Routine biochemistry and haematology are not helpful in establishing a diagnosis of AP. Neutrophilia with a left shift was seen in only 55% of dogs.
Pulse oximetry is not an early indicator of pulmonary dysfunction. By the time that pulse oximeter readings are abnormal, the patient is already seriously hypoxaemic.
More sensitive diagnostic tests are measurements that are performed with the information obtained from an arterial blood gas sample. There are two of these measurements. The first is called the oxygenation ratio, which is defined as the partial pressure of oxygen in arterial blood divided by the percentage of inspired oxygen:
Oxygenation ratio = PaO2 / FiO2
The PaO2 is measured from an arterial blood gas. It should be around 100 mmHg if the patient is breathing room air.
An example of a normal oxygenation ratio is:
PaO2 = 105 mmHg
FiO2 = 0.21
105/0.21 = 500
The oxygenation ratio in this example is 500. A normal oxygen ratio should be between 300 and 500. Oxygenation ratios less than 200 are indicative of compromised pulmonary function. The oxygenation ratio can be approximated for animals that are on supplemental oxygen by measuring the PaO2 and using an estimated figure for the FiO2. Patients on mask oxygen are generally agreed to have an an FiO2 of 0.3–0.6, depending on flow rates and the fit of the mask. Patients on nasal prongs or nasal oxygen catheters have an estimated FiO2 of 0.4. Patients who are intubated and on 100% oxygen have an FiO2 of 1.0.
The second diagnostic test that can be performed with blood gas analysis is calculation of the difference between alveolar and arterial oxygen concentrations, known as the A-a gradient. This is the most sensitive test of pulmonary function available to most practitioners. To make the best interpretation of this test, the patient needs to be on room air for 5 minutes before the sample is drawn.
The Alveolar oxygen, PAO2 is calculated by using the PCO2 from blood gas results in the following equation
PAO2 = FiO2 (PB-PH2O) - (PCO2/ R)
Where, at sea level, PB (barometric pressure) = 720 mmHg, PH2O = 47 mmHg and R = 0.9
These numbers when substituted into the equation simplify as follows:
PAO2 = 0.21 (720-47) - PCO2/ 0.9
PAO2 = 141-PCO2/ 0.9
The arterial oxygen, PaO2 is obtained directly from the blood gas analysis results.
Simply subtract the PaO2 from the answer of the alveolar oxygen equation to get the A-a gradient. In normal patients the difference should be less than 15 mmHg. In a recent study 27 out of 28 dogs with AP had an abnormal A-a gradient.
Treatment of AP is accomplished with supplemental oxygen, good nursing care and antimicrobial therapy. Supplemental oxygen can most easily be provided with nasal oxygen prongs or nasal oxygen catheter(s).
Coupage, postural management and patient mobilisation are the key elements of nursing care.
Antimicrobial therapy should be intravenous and broad spectrum, covering for gram-positive and Gram-negative aerobes and anaerobes. The antibiotic choice in humans did not affect the survival rate. However, methicillin-resistant Staphylococcus aureus developed in about 30% of various antibiotic groups except for the group that received clindamycin.
Patients with AP may be in the hospital for some period of time. In a recent study the average stay in hospital was 5.4 ± 4.5 days, ranging from 1–23 days. The survival rate was 77% in this study of 88 dogs. Survival was not associated with radiographic degree of severity, length of time in hospital or the underlying cause.
References
1. Kogan DA, Johnson LR, et al. Clinical, clinicopathologic and radiologic findings in dogs with aspiration pneumonia: 88 cases (2004–2006). Journal of the American Veterinary Medical Association 2008;233(11):1742–1747.
2. Kogan DA, Johnson LR, et al. Etiology and clinical outcome in dogs with aspiration pneumonia: 88 cases (2004–2006). Journal of the American Veterinary Medical Association 2008;233(11):1748–1755.
3. Shigemitsu H, Afshar K. Aspiration pneumonias - under-diagnosed and under-treated. Current Opinion in Pulmonary Medicine 2007;13(3):192–198.
4. Drakulovic MB, Torres A, et al. Supine body position as a risk factor for nosocomial pneumonia in mechanically ventilated patients: a randomised trial. The Lancet 1999;354(9193):1851.