Cardiac measurements on thoracic radiographs of cats

Measurements of the heart and thorax were made on lateral and ventrodorsal thoracic radiographs of normal cats. The degree of correlation between selected cardiac and thoracic measurements was determined. Absolute measurements and ratios of measurements in normal cats and in cats with certain cardiac diseases were compared and their diagnostic value was evaluated. It was concluded that on the lateral thoracic radiograph the ratio of the cranio-caudal width of the heart to the distance between the cranial border of the fifth rib and the caudal border of the seventh rib was a highly reliable indicator of cardiac enlargement. On the ventro-dorsal radiograph the ratio of the maxium width of heart to the width of the thorax at the same level was the best indicator of increases in cardiac size.     

THE EFFECT OF DEXMEDETOMIDINE ON RADIOGRAPHIC CARDIAC SILHOUETTE SIZE IN HEALTHY CATS

Dexmedetomidine, an alpha₂‐adrenergic agonist, may be used in companion animals for chemical restraint, including cardiac evaluation. Echocardiographic changes associated with alpha₂‐adrenergic agonists have been described; however reports of radiographic changes in cats were not found at the time of this study. Aims of this observational, prospective, experimental study were to describe the effects of dexmedetomidine on the radiographic appearance of the cardiac silhouette in healthy, adult cats. Fourteen healthy adult cats received dexmedetomidine 40 mcg/kg IM. Right lateral, left lateral, ventrodorsal, and dorsoventral thoracic radiographs were obtained for each cat at three time points: presedation, intrasedation, and postsedation (≥ two hours after reversal with atipamezole). Radiographs were evaluated in a blinded, randomized fashion by two independent observers using the vertebral heart score on all four views, the number of intercostal spaces on lateral projections, and the percent width of thorax on ventrodorsal and dorsoventral projections. Median vertebral heart score on right lateral view was significantly increased intrasedation (median = 7.8; range = 7.25–8.25) compared to presedation (median = 7.5; range = 7–8 [P = 0.001]). Median percentage width was significantly higher intrasedation (70% on VD; range 65–80 [P = 0.001], and 75% on DV; range 65–80 [P = 0.006]) compared to presedation (65%; range 65–75 on both projections). Dexmedetomidine was associated with a small but significant increase in cardiac silhouette size on right lateral (vertebral heart score), ventrodorsal (percentage width), and dorsoventral (percentage width) radiographs in healthy adult cats. This effect should be taken into consideration for future interpretation of thoracic radiographs in dexmedetomidine‐sedated cats.     

Radiographic and electrocardiographic evaluation of cardiac morphology and function in captive cheetahs (Acinonyx jubatus).

In a prospective study, eight (four males and four females) healthy, adult captive cheetahs (Acinonyx jubatus) were immobilized with a combination of tiletamine-zolazepam (4 mg/kg, i.m.), administered with a remote drug delivery system, to define normal cardiac morphology and function. Standard lateral and ventrodorsal (VD) radiographs were then taken to measure heart and thorax using a metric and vertebral scale system. Standard six-lead electrocardiograms were obtained with the animals in right lateral recumbency under isoflurane anesthesia. Mean chest depth and width was 18.7 +/- 1.3 cm and 13.0 +/- 0.6 cm, respectively. The mean lateral cardiac short axis (X) was 9.1 +/- 0.6 cm. the mean cardiac long axis (Y) was 13.6 +/- 0.7 cm, and the mean lateral heart sum (X + Y) was 22.6 +/- 1.2 cm. In the VD projection, mean cardiac short axis (V) was 10.1 +/- 0.7 cm, mean cardiac long axis (W) was 14.9 +/- 1.2 cm, and the heart sum (V + W) was 24.9 +/- 1.8 cm. The vertebral heart size was 8.2 +/- 0.9. All cheetahs had sinus rhythm, and no arrhythmias were noted. Mean heart rate was 126 +/- 15 beats/min, and the mean electrical axis was 82 + 5 degrees. P waves were always positive on lead II and had a width of 0.04 +/- 0.01 sec and a height between 0.1 and 0.3 mV. PR intervals were 0.11 +/- 0.01 sec. The height of the QRS complex was 1.25 +/- 0.24 mV and the width 0.06 +/- 0.01 sec. The ST segment was 0.04 sec, and the T wave (height: 0.25 +/- 0.05 mV) was positive in all cheetahs examined. Although these cardiac and thoracic measurements were larger than those of domestic cats (Felis catus), ratios of cardiac parameters were similar in both species. Electrocardiographic findings were similar to those reported from domestic cats.     

RADIOGRAPHIC MEASUREMENT OF CARDIAC SIZE IN NORMAL FERRETS

Right lateral, left lateral and ventrodorsal radiographs were obtained in 20 normal ferrets (11 male). Three independent observers recorded measurements of the cardiac silhouette and results for each parameter were averaged. Long axis (length), short axis (width) and total of length plus width (L+W) were recorded in each view. Comparative measurements were calculated, including the ratio of L+W/length of thoracic vertebrae 5-8, and a modified vertebral heart score (VHS) method, measuring the heart in vertebral units. Measurements made in ventrodorsal views were usually larger than corresponding measurements in lateral views. Weight and most absolute measurements differed significantly between male and female ferret (P<0.05), but the differences in absolute measurements were not apparent when related to measures of body size (Ratio and modified VHS methods). Given the variability in body weight and size in ferrets, measurements of the cardiac silhouette normalized for body size may be more universally applicable than absolute measurements.     

VERTEBRAL HEART SIZE RANGES SPECIFIC FOR WHIPPETS

To assess the influence of breed, breeding lines, and training on heart size, the vertebral heart size (VHS) was evaluated on left-to-right lateral, right-to-left lateral, dorsoventral, and ventrodorsal thoracic radiographs from 44 whippets free from cardiac and pulmonary disease. In lateral views, the VHS was 11.0 +/- 0.5 vertebrae (mean +/- SD) on right-to-left lateral and 11.3 +/- 0.5 vertebrae on left-to-right lateral radiographs, being larger than the 9.7 +/- 0.5 vertebrae proposed by Buchanan (P<0.0001). The VHS on left-to-right lateral views was larger than on right-to-left lateral views (P<0.0001). The VHS was 10.5 +/- 0.6 vertebrae on dorsoventral radiographs and 11.1 +/- 0.6 vertebrae on ventrodorsal radiographs. Both values were larger than the 10.2 +/- 1.5 vertebrae (dorsoventral) (P<0.0082) or 10.2 +/- 0.8 vertebrae (ventrodorsal) (P<0.0001) proposed by Buchanan. In addition, the VHS on ventrodorsal views was larger than on dorsoventral views (P<0.0001). Dogs out of racing pedigree lines had a significantly larger VHS than those out of show pedigree lines, and trained dogs had a significantly larger VHS than nontrained dogs. Because most trained dogs came out of racing pedigree lines, and all nontrained dogs came out of show pedigree lines, however, it is difficult to determine whether the higher VHS for trained dogs is due to genetic influences or training, or both. In conclusion, it is important to take into account the breed and the radiographic view when evaluating heart size in thoracic radiographs of whippets to avoid overinterpretation of cardiac enlargement in these dogs.     

Effect of various thoracic radiographic projections on the appearance of selected thoracic viscera

OBJECTIVES: To assess the effect of different radiographic projections on thoracic width, as well as position and visibility of the trachea, principal bronchi, cardiac silhouette, aorta, caudal vena cava (CVC) and oesophagus. METHODS: Right lateral recumbency (RLR), left lateral recumbency (LLR), dorsoventral (DV) and ventrodorsal (VD) thoracic radiographs of 42 dogs were reviewed retrospectively. RESULTS: In 78 per cent of cases the thoracic width was significantly larger on the VD projection than on the DV projection. The angle of divergence formed by the principal bronchi was significantly larger on the VD projection than on the DV projection in 80 per cent of dogs. A cardiac silhouette bulge at 1 to 2 o'clock was apparent on the VD projection in 22 per cent of dogs but was never seen on DV projections. The descending aorta was more visible at the 4 to 5 o'clock cardiac silhouette level on the DV projection and laterally at the T8 level on LLR projections. The CVC was better seen on VD and LLR projections. The oesophagus was visible as a soft tissue opacity in LLR in large dogs with normal thoracic conformation in 35 per cent of cases. CLINICAL SIGNIFICANCE: The DV projection appears to be more reliable for assessing the cardiac silhouette, the descending aorta and the angle of divergence of the principal bronchi. The VD projection should be considered for evaluating the CVC. LLR should be used for assessing the descending aorta and CVC.     

CARDIAC AND PULMONARY ARTERY MENSURATION IN FELINE HEARTWORM DISEASE

A retrospective study was undertaken to quantify thoracic radiographic changes in cats with heartworm diseases, ( Dirofilaria immitis ). Using a blinded study format, the cardiac silhouette, thoracic cavity and pulmonary arteries were measured from thoracic radiographs of 21 cats with feline heartworm disease and 30 cats without known cardiac or pulmonary vessel pathology. Measured data were normalized to the thoracic cavity or bony structures within the radiographic field of view. The measurements were compared between the two groups of cats using an unpaired, two-tailed Student's t -test, with a p value of < 0.05 being considered significant. Cats with feline heartworm disease had enlargement of the craniocaudal aspect of the cardiac silhouette and normalized cardiac:thoracic ratio (p < 0.05) on the lateral view. Also, there was significant enlargement of the central and peripheral caudal lobar pulmonary arteries and their normalized ratios (p < 0.05) in the heartworm infected cats as visualized on the ventrodorsal projection. Tortuosity of the pulmonary arteries was seen in three of the 21 infected cats. Eleven of the 21 cats with feline heartworm disease had pulmonary parenchymal changes. Based on the present study, central and peripheral pulmonary artery enlargement as viewed on the ventrodorsal radiograph was the single best radiographic indicator of feline heartworm disease.     

THORACIC RADIOGRAPHIC APPEARANCE IN THE NORMAL LLAMA

The goal of this study was to collect quantitative and qualitative radiographic information of the normal adult llama thorax. Standing right-left lateral radiographs of the thorax of 16 normal llamas were made. Normal ratios of cardiac height, width, and height plus width to thoracic vertebrae 3-5 and thoracic height were calculated. Normal values determined for tracheal angle of divergence from the thoracic spine, cardiophrenic and cardiosternal contact are additional potential indicators of cardiac enlargement. Ratios of normal pulmonary artery and vein, caudal vena cava and trachea to the height of the fourth thoracic vertebra should allow identification of pathology of these structures. Observations regarding pulmonary vessels and airways, thoracic spine, sternebrae and portions of the gastrointestinal tract observed on thoracic radiographs are also included. It is proposed that these normal values and observations can be used to better evaluate diseases of the cardiovascular and respiratory systems of adult llamas.     

Assessment of the value of the vertebral heart scale in the radiographic diagnosis of cardiac disease in dogs

In order to assess the influence of the vertebral heart scale (VHS) on the accuracy of the radiographic diagnosis of cardiac disease, thoracic radiographs of 50 dogs with proven cardiac disease, 26 with other thoracic diseases, and 50 with no clinical signs of cardiovascular or respiratory disease were mixed and examined by three independent, blinded observers chosen to represent a range of radiographic abilities. They first examined all the radiographs without making measurements of VHS and made a diagnosis. They then re-examined the radiographs, and measured VHS on both lateral and dorsoventral or ventrodorsal radiographs before again recording a diagnosis without reference to their original diagnoses. For all the observers, the dogs with cardiac disease had a higher mean VHS than the normal dogs. A VHS over 10.7 on the lateral radiograph was a moderately accurate sign of cardiac disease. The observers' accuracy of diagnosis did not change significantly as a result of using VHS as an adjunct to a subjective assessment of the radiographs.     

Vertebral scale system to measure heart size in radiographs of cats

OBJECTIVE: To determine absolute and relative heart size in clinically normal cats by correlating heart size and selected skeletal structures. DESIGN: Prospective radiographic study. ANIMALS: 100 cats that did not have thoracic radiographic abnormalities. PROCEDURE: Standardized measurements of the long and short axes of the heart, midthoracic vertebrae, and other structures were made. Measurements were recorded in millimeters and number of thoracic vertebral lengths spanned by each dimension, measured caudally from T4 in a lateral radiograph. The long- and short-axis measurements of the heart, expressed in vertebral lengths, were added to yield vertebral heart size. RESULTS: Mean +/- SD vertebral heart size in lateral radiographs was 7.5 +/- 0.3 vertebrae. The long-axis dimension correlated with the length of 3 sternebrae, measured from S2 to S4. The cardiac short-axis dimension correlated moderately with the length of 3.2 vertebrae, measured from T4 to T6. The cardiac short-axis dimension in ventrodorsal radiographs was 3.4 +/- 0.25 vertebrae. CONCLUSIONS AND CLINICAL RELEVANCE: The vertebral heart-size method is easy to use, allows objective assessment of heart size, and may be helpful in determining cardiomegaly and comparing heart size in sequential radiographs.     

INITIAL INFLUENCE OF RIGHT VERSUS LEFT LATERAL RECUMBENCY ON THE RADIOGRAPHIC FINDING OF DUODENAL GAS ON SUBSEQUENT SURVEY VENTRODORSAL PROJECTIONS OF THE CANINE ABDOMEN

Identification of the duodenum and potential abnormalities on survey abdominal radiographs is often difficult unless it contains gas. This study investigated the effect of patient positioning on the presence of duodenal gas in survey abdominal radiographs. One hundred dogs receiving a three‐view survey abdominal radiographic study were enrolled in a prospective, randomized study where all dogs were divided into two groups. Group A (n = 51) dogs had a left lateral projection first, followed by a ventrodorsal projection, ending with a right lateral projection. Group B (n = 49) dogs had a right lateral projection first, followed by a ventrodorsal projection, ending with a left lateral projection. The presence of gas within the duodenum and level of distribution of gas throughout the duodenum were recorded for all three projections. In addition, the presence or absence of duodenal pseudoulcers was evaluated on all three projections for each dog. The results for the two groups were compared using Chi‐square analysis with a P‐value of less than 0.05 being considered significant. Results showed that dogs first placed in left lateral recumbency were significantly more likely to have duodenal gas on the subsequent ventrodorsal and right lateral radiographic projections compared to dogs first placed in right lateral recumbency (P‐value < 0.0001). Pseudoulcers were seen in 11 dogs that had duodenal gas making the visualization of pseudoulcers on survey abdominal radiographs somewhat commonplace. This study emphasizes the benefit of using initial left lateral abdominal projections prior to other views for subsequent evaluation of the duodenum.     

EVALUATION OF TRADITIONAL AND NOVEL RADIOGRAPHIC VERTEBRAL RATIOS IN GREAT DANES WITH VERSUS WITHOUT CERVICAL SPONDYLOMYELOPATHY

Great Danes are predisposed to osseous‐associated cervical spondylomyelopathy (Wobbler syndrome). The first aim of this prospective study was to compare values measured using previously published intravertebral and intervertebral ratio methods and a novel ventrodorsal ratio method in radiographs of clinically normal and affected Great Danes. The second aim was to determine whether these ratios could be used as predictors of sites of spinal cord compression based on magnetic resonance imaging (MRI). Thirty dogs (15 normal, 15 affected) were prospectively enrolled. Lateral and ventrodorsal radiographs were obtained and six measurements were recorded from C3‐T1. For each vertebral location, intravertebral ratios and intervertebral ratios were calculated from lateral views, and the ratio of the distance between the articular process joints vs. vertebral body width (novel ventrodorsal ratio) was calculated from ventrodorsal views. Values for these three ratios were compared, by vertebral location and dog group. Intravertebral and intervertebral ratios did not differ between dog groups. The ventrodorsal ratio was significantly smaller in affected Great Danes at C5–6 (P = 0.005) and C6–7 (P < 0.001). The ventrodorsal ratio was significantly associated with MRI presence of spinal cord compression. For each 0.1 unit increase in this ratio value, there was a 65% decrease in the odds of spinal cord compression being present at that site, independent of vertebral location (P = 0.002). Findings from this study supported use of the novel ventrodorsal ratio as an initial radiographic screening method for Great Danes with suspected cervical spondylomyelopathy.     

Radiographic measurement of vertebral heart size in healthy stray cats

The aims of this study were to determine vertebral heart size (VHS) in stray cats and to compare different radiographic views. This study was performed on 50 adult stray cats. All cats were short-haired and non-obese and were considered to be healthy based on physical examination and electrocardiography. Left and right lateral, dorsoventral and ventrodorsal radiographs were taken. The long and short axes of the heart were measured in millimetres. The thoracic vertebral length spanned by each dimension was measured caudally from the fourth thoracic vertebra. Mean+/-SD and the correlation coefficient between the measurements were calculated with standard statistical software. The sum of the long and short axes of the heart expressed as VHS was 7.3+/-0.49 vertebrae in right lateral, 7.3+/-0.55 vertebrae in left lateral, 7.5+/-0.68 vertebrae in dorsoventral and 7.5+/-0.53 vertebrae in ventrodorsal. The differences between right and left lateral as well as dorsoventral and ventrodorsal views were not significant (P>0.05). Absolute measurements and vertebral heart scale values were slightly smaller than those reported in the literature for mixed population of cats. It is, therefore, important to take the breed in to account.     

A Pasteurella-like bacterium associated with pneumonia in captive megachiropterans.

A novel Pasteurella-like organism was recovered postmortem from lung tissue of two captive Wahlberg's epauleted fruit bats (Epomophorus wahlbergi), with severe, unilateral pneumonia. The bats had been recently shipped and died shortly after release from a 30-day quarantine. One presented with clinical signs of anorexia and lethargy before death; the other died without prior clinical symptoms. The same Pasteurella-like organism was recovered antemortem from subcutaneous abscesses in two captive little golden mantled flying foxes (Pteropus pumilus) housed with additional E. wahlbergi. The organism was also cultured on tracheal wash from one Malaysian flying fox (Pteropus vampyrus) and another E. wahlbergi, both demonstrating clinical signs of pneumonia. All recovered isolates appeared morphologically and biochemically similar to the initial isolates and were further characterized as either a Pasteurella or Actinobacillus organism on the basis of biochemical and cellular fatty acid profiles. Screening of the current collection using pharyngeal swabs isolated this organism from 12 of 15 E. wahlbergi, two of three P. vampyrus, one of 26 island flying foxes (Pteropus hypomelanus), and one of nine Rodrigues fruit bats (Pteropus rodricensis). The organism was not identified in pharyngeal culture from eight Indian flying foxes (Pteropus giganteus), nine Egyptian fruit bats (Rousettus aegypticus), or an additional 16 P. pumilus.     

Vertebral heart size in healthy Australian cattle dog

The vertebral heart score or size (VHS) measurement is routinely used to provide a more objective measurement of cardiomegaly in dogs. However, breed or body conformation can influence the VHS. To assess the specific VHS for the Australian Cattle Dog, left‐to‐right lateral, right‐to‐left lateral, dorsoventral and ventrodorsal thoracic radiographs from 20 individuals free from cardiac and pulmonary disease were obtained. The mean VHS was significantly higher in Australian Cattle Dog (10.5 ± 0.4 vertebrae), when compared with the average VHS for 100 normal dogs of different breeds that had been initially published (9.7 ± 0.5 vertebrae). This emphasizes the importance of breed‐specific VHS ranges. In our study group of normal Australian Cattle Dogs, the mean VHS was 10.5 ± 0.5 vertebrae (mean ±SD) on right lateral and 10.3 ± 0.5 vertebrae on left lateral radiographs. The VHS on right lateral views was significantly larger than on left lateral views. The VHS was 10.5 ± 0.6 vertebrae on dorsoventral and 11.1 ± 0.6 vertebrae on ventrodorsal radiographs. The VHS on ventrodorsal views was significantly larger than on dorsoventral views.     

Newly discovered viruses of flying foxes.

Flying foxes have been the focus of research into three newly described viruses from the order Mononegavirales, namely Hendra virus (HeV), Menangle virus and Australian Bat Lyssavirus (ABL). Early investigations indicate that flying foxes are the reservoir host for these viruses. In 1994, two outbreaks of a new zoonotic disease affecting horses and humans occurred in Queensland. The virus which was found to be responsible was called equine morbillivirus (EMV) and has since been renamed HeV. Investigation into the reservoir of HeV has produced evidence that antibodies capable of neutralising HeV have only been detected in flying foxes. Over 20% of flying foxes in eastern Australia have been identified as being seropositive. Additionally six species of flying foxes in Papua New Guinea have tested positive for antibodies to HeV. In 1996 a virus from the family Paramyxoviridae was isolated from the uterine fluid of a female flying fox. Sequencing of 10000 of the 18000 base pairs (bp) has shown that the sequence is identical to the HeV sequence. As part of investigations into HeV, a virus was isolated from a juvenile flying fox which presented with neurological signs in 1996. This virus was characterised as belonging to the family Rhabdoviridae, and was named ABL. Since then four flying fox species and one insectivorous species have tested positive for ABL. The third virus to be detected in flying foxes is Menangle virus, belonging to the family Paramyxoviridae. This virus was responsible for a zoonotic disease affecting pigs and humans in New South Wales in 1997. Antibodies capable of neutralising Menangle virus, were detected in flying foxes.     

THE EFFECT OF POSITIONING ON THE APPEARANCE OF SELECTED CRANIAL THORACIC STRUCTURES IN THE DOG

In this retrospective study the effect of thoracic positioning on the visibility and size of selected cranial thoracic structures in dogs was investigated. Dorsoventral (DV), ventrodorsal (VD) as well as left lateral recumbent (LLR) and right lateral recumbent (RLR) thoracic radiographs of 17 large, 15 medium, and 10 small skeletally mature dogs were evaluated. The craniodorsal and cranioventral mediastinum, the cupula pleura and sternal lymph nodes were examined. The effect of obesity was also evaluated. The craniodorsal mediastinum was better delineated on DV radiographs and was wider on VD radiographs. The craniodorsal mediastinal width: width of T2 cranial end-plate on VD radiographs was 2.41 for all groups combined and obesity significantly influenced this value. The cranioventral mediastinum was more visible in RLR and VD radiographs. A normal sternal lymph node soft tissue opacity was seen most commonly in RLR in large breed dogs and had a mean length of 30 mm. The pulmonary cupula extended beyond the first rib on all views and extended more cranially and was better visualised on VD than DV views.     

Thoractic radiographic appearance during lung inflation in neonatal calves

The effect of lung inflation on thoracic radiographic anatomy was determined in seven anesthetized calves less than one week of age. Radiographs were taken with the calves in dorsoventral (DV), ventrodorsal (VD), left and right lateral recumbency at functional residual capacity (FRC), at half inflation of the lung and at total lung capacity (TLC). Lung volumes were maintained by positive pressure ventilation. Increases in areas and linear dimensions during lung inflation were not uniform, being greatest in the caudal and especially the caudal ventral regions of the thorax. With inflation, heart size decreased and the heart shadow moved cauded as the diaphragm flattened. When the calf was shifted from the DV to the VD position, the heart displaced toward the left thoracic wall. Lung areas were larger on VD than on DV projections with the right lung being larger than the left. On lateral radiographs the most reliable, easily observed indication of the stage of lung inflation was the area bounded by the vena cava, the caudal heart border and the diaphragm. On the DV or VD radiographs the transthoracic width at the diaphragm was the best indicator of the degree of lung inflation.     

Comparative immunogenicity and efficacy studies with oral rabies virus vaccine SAD P5/88 in raccoon dogs and red foxes.

A comparative study of immunogenicity and efficacy of the oral rabies virus vaccine SAD P5/88 in raccoon dogs and foxes was conducted. The raccoon dogs received 10(6.9) (n = 6), 10(6.3) (n = 6) or 10(5.7) FFU SAD P5/88 (n = 5) by direct oral application, and subsequently all animals seroconverted. The foxes received 10(7.2) (n = 4), 10(6.2) (n = 4), 10(5.2) (n = 4) and 10(4.2) FFU SAD P5/88 (n = 5) by the same route. On days 106 and 196 post vaccination 10 raccoon dogs and 16 foxes were challenged with a relevant street virus, respectively. All 10 raccoon dogs vaccinated with 10(6.3) (n = 5) or 10(5.7) FFU SAD P5/88 (n = 5) survived the challenge, whereas all control animals (n = 5) died of rabies. Two foxes vaccinated with 10(4.2) FFU and one fox vaccinated with 10(5.2) FFU died of rabies on day 7, 17 and 12 post infection, respectively. Also all control foxes succumbed to rabies. Our findings demonstrate that SAD P5/88 is not only an effective vaccine for oral vaccination of foxes but also for that of raccoon dogs.     

RADIOGRAPHIC AND ECHOCARDIOGRAPHIC ASSESSMENT OF LEFT ATRIAL SIZE IN 100 CATS WITH ACUTE LEFT‐SIDED CONGESTIVE HEART FAILURE

The aims of this study were to evaluate left atrial size in cats with acute left‐sided congestive heart failure. We hypothesized that left atrial size as determined by thoracic radiography can be normal in cats with acute left‐sided congestive heart failure. One hundred cats with acute left‐sided congestive heart failure in which thoracic radiography and echocardiography were performed within 12 h were identified. Left atrial size was evaluated using right lateral and ventrodorsal radiographs. Measurements were compared to two‐dimensional echocardiographic variables of left atrial size and left ventricular size. On echocardiography, left atrial enlargement was observed in 96% cats (subjective assessment) whereas maximum left atrial dimension was increased (>15.7 mm) in 93% cats. On radiographs left atrial enlargement (subjective assessment) was found in 48% (lateral view), 53% (ventrodorsal view), and 64% (any view) of cats whereas left atrial enlargement was absent in 36% of cats in both views. Agreement between both methods of left atrial size estimation was poor (Cohen's kappa 0.17). Receiver operating characteristic curve analysis identified a maximum echocardiographic left atrial dimension of approximately 20 mm as the best compromise (Youden index) between sensitivity and specificity in the prediction of radiographic left atrial enlargement. Left atrial enlargement as assessed by thoracic radiography may be absent in a clinically relevant number of cats with congestive heart failure. Therefore, normal left atrial size on thoracic radiographs does not rule out presence of left‐sided congestive heart failure in cats with clinical signs of respiratory distress.