Clinical, clinicopathologic, and radiographic findings in dogs with aspiration pneumonia_88 cases

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Clinical, clinicopathologic, and radiographic findings in dogs with aspiration pneumonia: 88 cases (2004–2006) David A. Kogan, bs; Lynelle R. Johnson, dvm, phd, dacvim; Karl E. Jandrey, dvm, dacvecc; Rachel E. Pollard, dvm, phd, dacvr Objective—To evaluate clinical, clinicopathologic, and radiographic findings in dogs with aspiration pneumonia.

Design—Retrospective case series.

Animals—88 dogs with aspiration pneumonia.

Procedures—History, physical examination findings, and clinicopathologic data were obtained from medical records and analyzed for all 88 dogs. Thoracic radiographic findings for all dogs were reviewed to determine the type and location of pulmonary infiltrates.

Results—Aspiration pneumonia was evident at admission to the hospital in 65 (74%) dogs and developed during hospitalization in 23 (26%) dogs. Less than half of these affected dogs had high values for rectal temperature, heart rate, or respiratory rate; however, most (68%) affected dogs had increased, decreased, or adventitious lung sounds. Neutrophilia with a left shift was a common finding. Hypoalbuminemia was detected in 31 of 58 (53%) dogs. Hypoxemia and a high alveolar-arterial gradient in partial pressure of oxygen were detected in 22 of 28 (79%) dogs and 27 of 28 (96%) dogs, respectively. Among the 88 dogs, thoracic radiography revealed a predominantly alveolar infiltrate in 65 (74%) dogs and an interstitial pattern in 23 (26%) dogs; a single lung lobe was affected in 46 (52%) dogs, most commonly the right middle lung lobe (21/46 [46%] dogs).

Conclusions and Clinical Relevance—In dogs, aspiration pneumonia was often associated with abnormalities in pulmonary auscultation in the absence of objective changes in physical examination findings. However, neutrophilia, hypoalbuminemia, and hypoxemia were frequently detected, and radiographic evidence of infiltrates in the right middle lung lobe was common. (J Am Vet Med Assoc 2008;233:1742–1747)

A

spiration pneumonia is a serious and potentially life-threatening inflammatory lung process. Pathologic damage to the lungs results from insults to the alveolar capillary membranes, and loss of surface area for gas exchange leads to ventilation-perfusion mismatch and hypoxemia. Gastric acid aspiration directly causes alterations in surfactant function that result in loss of surface tension and atelectasis.1 Subsequent bronchoconstriction increases airway resistance and the effort required for breathing. Acid injury adversely impacts pulmonary resistance to infection,2 and the presence of oral or gastrointestinal bacteria in aspirated fluid can result in development of severe bacterial pneumonia.3 Although aspiration pneumonia is encountered commonly in humans and other animals, definitive diagnosis of the disorder is challenging. In people, aspiration events are categorized as aspiration pneumonitis or aspiration pneumonia depending on whether injury From the Veterinary Medical Teaching Hospital (Kogan), and the Departments of Veterinary Medicine and Epidemiology (Johnson) and Surgical and Radiological Sciences (Jandrey, Pollard), School of Veterinary Medicine, University of California, Davis, CA 95616. Dr. Kogan’s present address is Veterinary Medical and Surgical Group, 2199 Sperry Ave, Ventura, CA 93003.

Supported in part by the S.T.A.R. (Students Training in Advanced Research) program, School of Veterinary Medicine, University of California, Davis, and the Bailey Wrigley Fund.

Address correspondence to Dr. Johnson.

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Abbreviations Fio2 Fraction of inspired oxygen Pao2 - Pao2 Alveolar-to-arterial gradient in partial pressure of oxygen

is related to aspiration of sterile gastric contents that results in acid-induced damage or related to pulmonary inflammation that develops in response to infectious organisms contained within the aspirant.4,5 Definitive diagnosis of aspiration pneumonia is difficult to obtain because of the lack of specific identifying features.5–9 In humans, aspiration pneumonia is a clinical diagnosis based on the presence of respiratory tract signs and radiographic evidence of a newly developed pulmonary infiltrate in a patient at risk. Similar criteria appear to be applied in veterinary medicine, although identification of respiratory tract signs, risk factors, and newly developed lung infiltrates in a patient is often problematic. Various underlying disorders associated with aspiration have been identified in humans and other animals, including dysphagia, laryngeal disease, decreased consciousness, and neurologic disease.5,7,10–13 Although aspiration pneumonia appears to develop quite commonly in dogs, little information is available on specific features of the disease, hematologic changes, or the specific radiographic changes seen in dogs with naturally occurring disease. The purpose of JAVMA, Vol 233, No. 11, December 1, 2008

Materials and Methods Case selection—The electronic medical database at the Veterinary Medical Teaching Hospital of the University of California, Davis, was searched for the terms aspiration pneumonia and canine in records dated January 1, 2004, to January 1, 2006, to identify all dogs with this clinical diagnosis. This time frame was chosen to generate a sufficient number of cases for evaluation from the period prior to transition from conventional film screen radiography to digital radiography. This was specifically done to eliminate the confounding variability caused by differing imaging techniques. Dogs were included in the study if aspiration pneumonia was listed in the medical record under the record field for the final clinical diagnoses. Dogs were included whether the diagnosis was made at the time of admission to the hospital or during hospitalization. Dogs were excluded from the study if radiographic images illustrating pulmonary infiltrates were not available for review. Records of dogs with forms of pneumonia other than aspiration pneumonia were not evaluated.

Medical records review—Medical records were examined by one of the authors (LRJ) to verify the clinical diagnosis of aspiration pneumonia by use of criteria (variably documented in individual records) as follows: witnessed or suspected regurgitation or vomiting episodes followed by acute onset of respiratory difficulty, cough, or tachypnea; physical examination findings consistent with lower respiratory tract disease or radiographic detection of pulmonary infiltrates in a dog at risk for aspiration; and identification of systemic disorders or interventions potentially associated with aspiration, including anesthesia, esophageal dysfunction, laryngeal disease, gastrointestinal tract disease, and decreased consciousness in dogs with clinical or radiographic evidence of pneumonia. Medical records and radiographic reports were also scrutinized for comments that reflected clinical suspicion of an aspiration event and clinical or clinicopathologic evidence of hypoxemia. Cases were excluded if supportive information for an aspiration event was not found or if an alternate cause for lower respiratory tract signs or radiographic changes supported diagnoses such as primary infectious pneumonia, pulmonary embolization, or neoplasia.

Information on age, breed, weight, and reproductive status was abstracted from the records. These data were compared with data from the total hospital population of dogs during the study period. Reasons for initial evaluation at the hospital, physical examination findings (including rectal temperature, pulse rate, respiratory rate and effort, presence of cough, and detection of adventitious lung sounds), results of clinicopathologic testing JAVMA, Vol 233, No. 11, December 1, 2008

(ie, results of a CBC, serum biochemical and arterial blood gas analyses, and urinalysis), and radiographic findings were also obtained. Physical examination findings and clinical data obtained soon after the aspiration event were collated, although it was not possible to determine specifically the number of hours between the aspiration event and diagnostic testing. Complete blood cell count results were evaluated for responses compatible with pneumonia, including abnormally high or low Hct values or WBC counts, detection of a neutrophil band response, and presence of nucleated RBCs. Arterial blood gas data (room air conditions) were evaluated for evidence of hypoxemia via assessment of Pao2 and calculation of the Pao2 - Pao2 by use of a formula as follows: Pao2 - Pao2 = Fio2 (PB – PH2O) – (Paco2•R−1) – Pao2

where Pao2 is the partial pressure of oxygen, alveolar, Fio2 is 0.21 in room air, PB is the barometric pressure (in mm Hg), PH2O is the water vapor pressure (47 mm Hg at 37oC), and R is the respiratory quotient (0.9).

A value of Pao2 - Pao2 > 15 mm Hg was considered indicative of lung dysfunction.14 In dogs receiving supplemental oxygen, the oxygenation ratio (fraction of Pao2 to Fio2) was estimated as the measured Pao2 (mm Hg) divided by Fio2.

The value used for Fio2 (range, 0.3 to 0.6) was estimated from the oxygen flow rate used when administering mask oxygen or from the value displayed on the oxygen cage in which 4 of the dogs were placed; Fio2 was considered 100% in 5 dogs that were intubated and ventilated. Direct measurement of tracheal oxygen concentration was not performed in any dog. An oxygenation ratio > 400 was considered normal.15 Thoracic radiographs (2-, 3-, and 4-view sets) for each dog were reviewed by a board-certified radiologist (REP), who was unaware of the clinical course of disease.

Radiographic views were assessed for the primary type of infiltrate (designated as interstitial, alveolar [including lobar consolidation], or bronchial); a mixed pattern was categorized on the basis of the predominant type of infiltrate detected. Location of the primary infiltrate was defined by the lobar branching pattern of the airways in dogs as described by Amis and McKiernan.16 The left cranial lung lobe in dogs is comprised of cranial and caudal segments; for the purposes of this study, each segment was considered independently. Therefore, the location of infiltrates was described as left cranial-cranial segment, left cranial-caudal segment, or left caudal, right cranial, right middle, accessory, or right caudal lobe. The number of affected lobar segments was identified for each dog.

For comparison of selected clinical variables, dogs were allocated to 1 of 2 groups. Group 1 dogs were those that had aspiration pneumonia at the time of admission to the hospital. Group 2 dogs were those that developed aspiration pneumonia during hospitalization. Causes and clinical outcome among these dogs are reported elsewhere.17 Statistical analysis—Data are presented as mean ± SD, and ranges are provided where appropriate; characteristics are reported for all dogs. Hematologic and serum biochemical values in groups 1 and 2 were compared by use of an unpaired t test. A Fisher exact test was used to detect difScientific Reports

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the study reported here was to evaluate clinical, clinicopathologic, and radiographic findings in dogs with aspiration pneumonia. In addition, specific features in dogs that had aspiration pneumonia at the time of admission to the hospital (ie, unknown duration) and in dogs that developed aspiration pneumonia during hospitalization were compared to elucidate discriminatory findings in dogs during the acute stages of the disorder (ie, signs within 12 hours of aspiration).

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ferences between groups on the basis of the frequency of abnormal findings. Signalment characteristics for dogs with aspiration pneumonia were compared with those of dogs in the general hospital population by use of χ2 analysis.

Statistical analyses were performed by use of commercial software,a and significance was set at a value of P < 0.05.

Results Search of the electronic medical database yielded medical records of 105 dogs for which aspiration pneumonia was listed as a final clinical diagnosis. Two cases were excluded because review of the medical records failed to identify supportive clinical evidence for aspiration pneumonia, and 15 cases were excluded because of the lack of radiographs for review. Medical records for the remaining 88 dogs were comprehensively reviewed.

Among the 88 dogs, there were 20 mixed-breed dogs and 68 purebred dogs. Sixteen (18%) dogs were considered small-breed dogs (< 10 kg [22 lb]), 18 (21%) dogs were medium-sized breeds (10 to 20 kg [22 to 44 lb]), and 54 (61%) dogs were large-breed dogs (> 20 kg [44 lb]). This breed distribution contrasted somewhat with that of the general hospital population of 8,053 dogs, of which 27% were small-breed dogs, 17% were mediumsized breeds, and 56% were large-breed dogs. However, the sizes of affected study dogs did not differ significantly from sizes of dogs in the hospital population. Breeds that were represented more than twice among the study dogs included Golden Retriever (n = 10), Labrador Retriever (9), and English Springer Spaniel, German Shepherd Dog, Rottweiler, Pug, and Cocker Spaniel (3 of each breed).

The percentages of Labrador Retrievers, German Shepherd Dogs, and Rottweilers with aspiration pneumonia (13.2%, 4.4%, and 4.4%, respectively) were similar to the percentages of those breeds in the hospital population (14.2%, 4.5%, and 3.5%, respectively). However, compared with the breed percentages in the hospital population, there were more Golden Retrievers (15% vs 6%), Cocker Spaniels (4.4% vs 2.5%), English Springer Spaniels (4.4% vs 0.6%), and Pugs (4.4% vs 1.6%) with aspiration pneumonia. Mean ± SD weight of dogs with aspiration pneumonia was 26.7 ± 17.2 kg (58.7 ± 37.8 lb; range, 2.6 to 91 kg [5.72 to 200.2 lb]). There were slightly more male (50/88 [57%])

than female (38/88 [43%]) dogs. Among the male dogs, 6 were sexually intact and 44 were castrated; among the female dogs, 5 were sexually intact and 33 were spayed. The age of the dogs ranged from 4 months to 16 years (mean age, 8.4 ± 4.3 years). Eight (9%) dogs were < 1 year old, and 45 (51%) were > 8 years old. In comparison, 11% of dogs were < 1 year old and 41% were > 8 years old in the general hospital population of dogs.

Among the dogs with aspiration pneumonia, 34 of 80 (42%) had a high respiratory rate (ie, > 30 breaths/min), 26 of 83 (31%) had high rectal temperature (ie, > 39.2°C [102.5°F]), and 10 of 82 (12%) had a high heart rate (ie, > 160 beats/min). Most dogs (65/88 [74%]) had evidence of aspiration pneumonia at the time of admission to the hospital (group 1); the others (23/88 [26%]) developed aspiration pneumonia during hospitalization (group 2). General physical examination characteristics (including rectal temperature, pulse rate, and respiration rate) did not differ (P > 0.05) between dogs in groups 1 and 2. Compared with dogs in group 1, proportionally more dogs in group 2 had a high rectal temperature (10/20 [50%] dogs vs 16/63 [25%] dogs), although this difference was not significant (P = 0.053).

In the records of group 1, cough was reported for 37 of 65 (57%) dogs and increased respiratory effort was reported for 36 of 65 (55%) dogs. Lung sounds were considered normal or clear in 21 (32%) dogs and harsh or loud in 20 (31%) dogs. In the 65 dogs, adventitious lung sounds (crackles or wheezes) were detected in 16 (25%), whereas lung sounds were quieter than expected in 8 (12%). Fifteen of 65 (23%) dogs had a combination of cough, increased respiratory effort, and abnormal lung sounds. Eight (12%) dogs had no clinical evidence of respiratory tract disease. Lung sounds were described for 15 of the 23 dogs in group 2 and were considered normal or clear in 5 of those dogs. For the remaining 10 of the 15 dogs, louder than expected or adventitious lung sounds were reported. Among 80 dogs with aspiration pneumonia for which lung sounds were described, abnormal lung sounds were reported for 55 (69%).

Hematologic values were available for 77 of 88 dogs; data from 1 dog with immune-mediated hemolytic anemia were excluded from analysis. Dogs in group 2 had significantly (P = 0.033) lower Hct than dogs in group 1 (mean values, 36.5 ± 2.0% vs 40.8 ± 7%; Table 1).

Table 1—Values of hematologic variables in 76 of 88 dogs with aspiration pneumonia. Dogs were allocated to 1 of 2 groups on the basis of whether they had aspiration pneumonia at the time of admission to the hospital (group 1; n = 56) or developed aspiration pneumonia during hospitalization (group 2; 20). Mean values of each variable did not differ (P  0.05) between groups.

Reference Variable range Mean  SD

Group 1 No. of dogs with a high value

No. of dogs with a low value Mean  SD

Group 2 No. of dogs with a high value

Hct (%) 40–55 40.8  7 2 21 35.7  9.5 0 Reticulocytes† (cells/µL) 7–65,000 39,781  55,361 1 NA 43,650  41,798 3 Nucleated RBCs (cells/µL) 0 1  1 6 NA 7  6 3 WBCs (cells/µL) 6–13,000 14,960  6,300 35 4 19,460  12,809 14‡

Neutrophils (cells/µL) 3–10,500 11,910  5,724 30 0 15,390  10,437 13 Band neutrophils (bands/µL)  100 2,050  1,768 30 NA 1,757  1,546 14 Lymphocytes (cells/µL) 1–4,000 1,385  1,001 2 25 1,316  981 1 Monocytes (cells/µL) 0.15–1,200 991  627 16 0 1,402  1,902 9 Eosinophils (cells/µL) 0–1,500 348  273 0 NA 228  174 0 Basophils (cells/µL) 0–50 39  25 6 NA 28  18 0

No. of dogs with a low value 10* NA NA 3 2 NA 12 2 NA NA

*Number of dogs in this category was significantly (P = 0.033) different from that of group 1. †For this variable, data were available from 11 dogs in group 1 and 10 dogs in group 2. ‡Number of dogs in this category was significantly (P = 0.047) different from that of group 1.

NA = Not applicable.

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with involvement of 2 lung regions, the left cranial-caudal segment was most commonly affected (n = 16), followed by the right cranial lobe (14), left cranial-cranial segment (13), and right middle lobe (11) in decreasing order of frequency.

Among the 88 dogs, aspiration into the right lung occurred in 47 (53%) dogs, the left lung in 29 (33%) dogs, and both lungs in 12 (14%) dogs. The most commonly affected lung region in all dogs was the right middle lobe (42/88 [48%] dogs), and the least commonly affected was the accessory lung lobe (3/88 [3%] dogs).

The right cranial lung lobe and the left cranial-caudal segment were infiltrated in 33 dogs each; the left cranial-cranial segment, left caudal lobe, and right caudal lobe were infiltrated in 27, 9, and 6 dogs, respectively.

Discussion In the present study, clinical, clinicopathologic, and radiographic findings in a large population of dogs with aspiration pneumonia were evaluated. The majority of dogs (61%) with aspiration pneumonia were large breeds, and more than half (51%) of the dogs were > 8 years old; however, the breed and age distributions mirrored those of the general hospital population of dogs.

In humans, a predisposition for aspiration pneumonia in aged patients who are hospitalized for treatment of various diseases has been reported.18,19 In people, defective or weak swallowing reflexes17,18 or reduced respiratory tract defense mechanisms3,4 may play a role in development of aspiration pneumonia.

For purposes of the study, dogs were allocated into 1 of 2 groups: those that developed aspiration pneumonia prior to admission to the hospital and those that aspirated and developed pneumonia during hospitalization. Interestingly, few clinical characteristics that were considered specifically associated with pneumonia were noted in the records of either group of dogs. Despite the fact that aspiration is associated with multiple stimuli for inflammation or infection,4,20 only approximately a third of all dogs had high rectal temperature or high respiratory rate at the time of diagnosis. Similarly, it has been reported21,22 that fever develops in < 50% of adult dogs with bacterial bronchopneumonia and in < 50% of puppies with community-acquired pneumonia. Even in dogs that developed aspiration pneumonia during hospitalization in the present study, systemic signs of disease were commonly absent, suggesting that altera-

Table 2—Location of pulmonary infiltrates determined via thoracic radiography and number of lung regions* affected in 88 dogs with aspiration pneumonia.

Lung region* No. of lung regions No. of dogs Left cranial- Left cranial- Left caudal Right cranial Right middle Accessory Right affected† affected cranial segment caudal segment lobe lobe lobe lobe caudal lobe 1 2 3 4 7 Total (%)

46 28 8 5 1 88

3 13 6 4 1 27 (31)

6 16 6 4 1 33 (38)

3 1 4 0 1 9 (10)

11 14 3 4 1 33 (38)

21 11 4 5 1 42 (48)

0 0 0 2 1 3 (3)

2 1 1 1 1 6 (7)

*For the purposes of the study, the cranial and caudal segments of the left cranial lung lobe were considered independently. Therefore, the location of infiltrates was described as the left cranial-cranial segment, left cranial-caudal segment, or left caudal, right cranial, right middle, accessory, or right caudal lobe. None of the dogs had 5 or 6 affected lung regions.

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The mean WBC count in group 2 dogs was significantly (P = 0.047) higher than that in group 1 dogs (19,763 ± 12,809 cells/µL vs 14,962 ± 6,300 cells/µL).

Serum biochemical analyses were performed in 58 of 77 dogs. High liver enzyme activity was detected in 35 of those 58 dogs. Serum alkaline phosphatase activity ranged from 16 to 6,182 U/L (mean, 590 ± 1,242 U/ L) and was high (reference range, 15 to 127 U/L) in 35 of 58 (60%) dogs. Serum alanine transaminase activity ranged from 9 to 531 U/L (mean, 132 ± 345 U/L) and was high (reference range, 19 to 67 U/L) in 21 (36%) dogs. Serum albumin concentration ranged from 1.1 to 3.6 g/dL (mean, 2.8 ± 0.6 g/dL) and was low (reference range, 2.9 to 4.2 g/dL) in 31 (53%) dogs. There were no significant differences in serum liver enzyme activities or albumin concentration between groups 1 and 2.

Results of arterial blood gas analyses obtained in room air (Fio2, 0.21) within 48 hours of the aspiration event were available for 28 dogs. Among these dogs, mean Pao2 was 68.5 ± 14.6 mm Hg (range, 39.2 to 93.7 mm Hg). Hypoxemia (Pao2 < 80 mm Hg) was detected in 22 of 28 (79%) dogs (range, 39.2 to 77.7 mm Hg). Partial pressure of carbon dioxide ranged from 20.8 to 52.5 mm Hg (reference range, 35 to 45 mm Hg); hypercapnia was detected in 2 (7%) dogs, and hypocapnia was detected in 14 (50%) dogs. Mean Pao2 - Pao2 was 43.5 ± 17.0 mm Hg (range, 12.8 to 82.8 mm Hg), and the Pao2 - Pao2 value was within the reference range (< 15 mm Hg) in only 1 dog. In 9 dogs, arterial blood gas analysis was repeated during administration of supplemental oxygen (ranging from 30% to 100%); the oxygenation ratio ranged from 93 to 532 mm Hg (mean, 277 mm Hg; median, 225 mm Hg), and values in 6 of 9 dogs were < 200 mm Hg. All 88 dogs underwent thoracic radiography (2 views were available for 31 dogs, 3 views were available for 51 dogs, and 4 views were available for 6 dogs).

Review of the radiographic views revealed a predominantly interstitial pattern in 23 (26%) dogs and alveolar infiltrates in 65 (74%) dogs. Of the 7 lung regions, the number affected ranged from 1 to 7 (mean, 1.7; median, 1). One region was affected in 46 dogs, 2 regions were affected in 28 dogs, 3 regions were affected in 8 dogs, 4 regions were affected in 5 dogs, and all 7 regions were affected in 1 dog. In the 46 dogs with involvement of 1 lung region, the right middle lobe was most commonly affected (n = 21), followed by the right cranial lobe (11), and the left cranial-caudal segment (6) in decreasing order of frequency (Table 2). In the 28 dogs

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tions in rectal temperature, pulse rate, and respiratory rate are not always reliable indicators of respiratory tract dysfunction.

Importantly, cough and respiratory difficulty were noted historically or detected during physical examination in over half of the affected dogs in the present study. Abnormal lung sounds were reported in most dogs (68%), indicating the importance of a thorough physical examination (particularly thoracic auscultation) in localization of diseases of the lower respiratory tract. Crackles and louder than expected breath sounds were the most consistent finding in dogs with bacterial bronchopneumonia also.21 Proficiency in thoracic auscultation is one of the more difficult skills to learn; however, it was clearly of importance in leading to the clinical diagnosis of pneumonia in dogs of the present study. However, specific adventitious lung sounds were reported less often than harsh or loud lung sounds, which suggests physical signs of pathologic changes in the lungs of dogs with aspiration pneumonia may be subtle.

In the group of dogs included in the present study, neutrophilia with a left shift was common as one would expect in animals with inflammation or infection associated with pneumonia. Dogs that developed aspiration pneumonia during hospitalization had lower Hct values, higher WBC counts, and higher (albeit not significantly) neutrophil counts than dogs that had aspiration pneumonia at the time of admission. The lower Hct in the former group may have resulted from IV administration of fluids to hospitalized patients. The significantly higher WBC count in dogs that developed aspiration pneumonia during hospitalization could be evidence of a more acute inflammatory response. Among all of the affected dogs, hypoalbuminemia was detected in slightly more than half of the dogs for which data were available, although serum globulin concentration was not high. This could be suggestive of lung inflammation and vasculitis that resulted in leakage of albumin into the alveolar space. Conversely, it may have been associated with concurrent systemic disease. High liver enzyme activities were also common, and a limitation of the study is that it did not evaluate the effect of comorbid conditions on clinicopathologic variables.

Hypoxemia was detected in 79% of 28 dogs for which arterial blood gas data were available. Such decrements in Pao2 values are indicative of variable dysfunction in pulmonary gas exchange. It is possible that hypoxemia was common among the dogs in the present study because this test is perhaps more typically performed in dogs that appear to be more severely affected.

Interestingly, 50% of the 28 dogs were hypocapnic. Low Paco2 values in dogs with pulmonary thromboembolism have also been reported23; in that study, hypocapnia was thought to represent altered control of ventilation. In dogs with aspiration injury, low values of Paco2 could be attributable to a similar mechanism or could result from compensatory hyperventilation induced by hypoxemia associated with ventilation-perfusion mismatch. In the present study, only 2 of the evaluated dogs were hypercapnic, indicating that alveolar hypoventilation was not a common problem. In studies23â&#x20AC;&#x201C;25 of pulmonary thromboembolism in dogs and humans, high Pao2 - Pao2 was more common than was 1746

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hypoxemia, suggesting that calculation of this variable results in improved detection of abnormalities in gas exchange. Calculation of the Pao2 - Pao2 is also useful because it can be used to follow response to treatment, although this was not performed in the study of this report. Arterial blood gas analyses in dogs treated with supplemental oxygen revealed poor oxygen responsiveness, thereby confirming that aspiration pneumonia can cause severe lung damage.

Among the dogs of the present study, the most common thoracic radiographic pattern was an alveolar infiltrate (detected in 65/88 [74%] dogs). Slightly more than 50% of dogs had involvement of a single lung region, most commonly the right middle lobe. Involvement of this lung lobe in an aspiration event is most likely a consequence of the ventral orientation of the bronchus to this lobe from the right mainstem bronchus.16 Mucus pooling from ventral dependence might also increase the likelihood that this lung lobe will be affected. Overall, aspiration into the right lung occurred more commonly than aspiration into the left lung in dogs of the present study. In an investigation26 of dogs with experimentally induced lung aspiration, the right lung was involved more commonly than the left lung when dogs were maintained in dorsoventral or ventrodorsal positioning. The straighter angle of the right principal bronchus at the carina (compared with the angle of the left principal bronchus) was considered likely to contribute to this phenomenon. This is particularly important to consider when performing thoracic radiography on a dog that is suspected of having aspiration pneumonia. The left lateral radiographic view is not routinely acquired but yields the best depiction of the right lung. Therefore, 3 radiographic views of the thorax should be obtained in dogs suspected of having aspiration pneumonia, particularly because if the infiltrates are distributed ventrally, the left lateral view may be the only one in which right cranial or middle lung lobe infiltrates are seen. Of equal importance is the fact that 26% of dogs in the present study had a predominantly interstitial pattern of infiltrates identified radiographically. Aspiration pneumonia traditionally has been described as resulting in ventrally distributed alveolar infiltrates; however, that radiographic appearance was not evident in a subset of dogs in our study.

Given the retrospective nature of the study, it was not possible to determine whether this lack of ventrally distributed alveolar infiltrates in some dogs was related to the timing of thoracic radiography in relationship to the aspiration event; however, it is of clinical importance that identification of an interstitial pattern in thoracic radiographic views could be consistent with aspiration pneumonia.

In dogs that had involvement of 2 lung regions, the left and right cranial lung lobes and right middle lung lobes were most commonly affected. Radiographic pattern recognition is considered highly important in the diagnosis and characterization of pneumonia in humans; aspiration pneumonia is characterized by multicentric opacities in the lower rather than upper lobes.8 In dogs, a similar distribution of lesions should raise the suspicion for pneumonia. Nevertheless, the presence of infiltrates in atypical lung regions should not rule out JAVMA, Vol 233, No. 11, December 1, 2008

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consideration of aspiration pneumonia because 20% of dogs in the present study had involvement of the caudal and accessory lung lobes. Positioning during an aspiration event plays a role in the location of radiographically detectable infiltrates5; in a study26 in which contrast medium was infused into the trachea of anesthetized dogs, dogs placed in ventrodorsal recumbency were more likely to aspirate into the right caudal lung lobe.

Some limitations of the present study should be mentioned. Diagnosis of aspiration pneumonia was based on clinical information obtained through retrospective analysis of medical record data. Various clinicians were involved in planning the diagnostic assessments, and various tests were performed for individual cases. Definitive diagnosis of aspiration pneumonia is challenging. In people with aspiration pneumonia, a high proportion of lipid-laden macrophages is found in bronchoalveolar lavage fluid,27 although this finding has relatively low specificity for discrimination of aspiration pneumonia from other chronic pulmonary conditions.6 Also, dogs have a high proportion of lipid-laden macrophages, compared with other species, and experimental induction of gastroesophageal reflux did not increase this index value.28,29 Detection of pepsin in bronchoalveolar lavage fluid can be used to diagnose aspiration injury in children with gastroesophageal reflux.30 Unfortunately, collection of airway samples was rarely performed in the dogs of the present study, thereby precluding assessment of lipid-laden macrophages or pepsin concentration. Thus, it was not possible to differentiate aspiration pneumonia from pneumonitis or to describe either the pulmonary inflammatory responses or the bacteria involved in the aspiration injury. Prospective studies would be useful in providing additional information on the relative roles of acid injury and bacteria-induced damage in dogs with aspiration pneumonia.