• View in gallery

    Dorsal (A), right lateral (B), and left lateral (C) reconstructed 3-D CT views of the lungs of a healthy adult alpaca in a study to characterize and quantitatively assess the typical pulmonary anatomy in this species. The right lung extends farther in the cranial and caudal directions, compared with the left lung. The caudal aspects of the lungs extended farther caudally at the medial aspect than at the lateral aspect. No lobation of the lungs is evident in these views. L = Left. R = Right.

  • View in gallery

    Computed tomographic images of the thorax in a healthy adult alpaca. A—Transverse image obtained at the level of T7 prior to administration of contrast medium, depicting regions of interest where right and left ventral (RV and LV) and dorsal (RD and LD) lung parenchyma density measurements were obtained. B—Transverse image obtained at the same location as in panel A 60 seconds after IV administration of iodinated contrast medium (iopamidol [1 mL/kg, IV]). C—Transverse image obtained at the level of the thoracic inlet. Lines represent the measurements for minimum (Min) and maximum (Max) diameters and height (H) of the tracheal lumen. The half moon–shaped structure to the right of the trachea is the air-filled esophagus. D—Midsagittal image illustrating the measurement of the angle between the thoracic vertebral column and trachea at the base of the heart (white lines). E—Transverse image obtained at the level of T6 depicting luminal diameter measurements of the right and left first caudal bronchi (RCd1 and LCd1, respectively). F—Transverse image obtained at the level of T7 showing luminal diameter measurements of a bronchus (B) and its associated artery (A) and vein (V). The artery is in closer proximity to the bronchus than is the vein. See Figure 1 for remainder of key.

  • View in gallery

    Typical branching patterns of the trachea and bronchi in alpacas. A—Schematic illustration of the most common branching pattern detected in alpacas of the present study. The cranial aspect of the right lung is supplied by a bronchus originating from the trachea cranial to the bifurcation (R cranial). The trachea divides into a right and left mainstem bronchus; the left mainstem bronchus gives off a cranial bronchus just caudal to the bifurcation (L cranial), and both mainstem bronchi divide further into right or left first, second, and third caudal bronchi (R caudal 1 to 3 and L caudal 1 to 3, respectively). The accessory bronchus originates from the right mainstem bronchus caudal to the bifurcation of the trachea and supplies the accessory lung lobe. Both mainstem bronchi taper and end in a terminal bronchus. B—Reconstructed sagittal plane CT image of bronchial branching in the left lung. The left cranial bronchus supplies the cranioventral aspect of the lung. The first, second, and third left caudal bronchi supply the ventral portion, midportion, and dorsal portion of the caudal aspect of the lung, respectively. The left mainstem bronchus tapers into the left terminal bronchus after giving off the caudal bronchi.

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Computed tomographic characterization of the pulmonary system in clinically normal alpacas

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  • 1 Department of Clinical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97333.
  • | 2 Department of Clinical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97333.
  • | 3 Department of Clinical Sciences, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97333.

Abstract

Objective—To characterize and quantitatively assess the typical pulmonary anatomy of healthy adult alpacas with multidetector row CT.

Animals—10 clinically normal adult female alpacas.

Procedures—CT examination of the thorax was performed before and after IV administration of iodinated contrast medium in sedated alpacas in sternal recumbency. Measurements of the trachea, bronchi and related blood vessels, and selected vertebrae as well as the extent and density of lung parenchyma were performed with a Digital Imaging and Communications in Medicine (DICOM) viewer. Morphometric and quantitative data were summarized.

Results—Separation of individual lung lobes could not be identified, except for the accessory lung lobe. In all alpacas, both lungs extended farther caudally at the medial aspect than at the lateral aspect. The right lung extended farther in both cranial and caudal directions than did the left lung. The branching pattern of the bronchial tree varied only slightly among alpacas and consisted of 1 cranial bronchus and 3 caudal bronchi bilaterally, with a right accessory bronchus. Luminal diameters of first-generation bronchi ranged from 3 to 9 mm. Mean ± SD parenchymal lung density was −869 ± 40 Hounsfield units (HU) before contrast injection and −825 ± 51 HU after contrast injection. Mean difference in diameter between bronchi and associated arteries or veins was 0.8 ± 0.9 mm.

Conclusions and Clinical Relevance—Knowledge of the typical anatomy of the lungs and bronchial tree in healthy alpacas as determined via CT will aid veterinarians in clinical assessment and bronchoscopic evaluation of alpacas.

Abstract

Objective—To characterize and quantitatively assess the typical pulmonary anatomy of healthy adult alpacas with multidetector row CT.

Animals—10 clinically normal adult female alpacas.

Procedures—CT examination of the thorax was performed before and after IV administration of iodinated contrast medium in sedated alpacas in sternal recumbency. Measurements of the trachea, bronchi and related blood vessels, and selected vertebrae as well as the extent and density of lung parenchyma were performed with a Digital Imaging and Communications in Medicine (DICOM) viewer. Morphometric and quantitative data were summarized.

Results—Separation of individual lung lobes could not be identified, except for the accessory lung lobe. In all alpacas, both lungs extended farther caudally at the medial aspect than at the lateral aspect. The right lung extended farther in both cranial and caudal directions than did the left lung. The branching pattern of the bronchial tree varied only slightly among alpacas and consisted of 1 cranial bronchus and 3 caudal bronchi bilaterally, with a right accessory bronchus. Luminal diameters of first-generation bronchi ranged from 3 to 9 mm. Mean ± SD parenchymal lung density was −869 ± 40 Hounsfield units (HU) before contrast injection and −825 ± 51 HU after contrast injection. Mean difference in diameter between bronchi and associated arteries or veins was 0.8 ± 0.9 mm.

Conclusions and Clinical Relevance—Knowledge of the typical anatomy of the lungs and bronchial tree in healthy alpacas as determined via CT will aid veterinarians in clinical assessment and bronchoscopic evaluation of alpacas.

Camelids develop many of the same respiratory diseases as domestic livestock, including tuberculosis,1 fungal disease,2 pneumonia,3 neoplasia,4 parasite migration,3 and various viral diseases, including bluetongue,5 bovine viral diarrhea virus,6 and alpaca coronavirus.7,8 However, differentiation among various forms of respiratory disease is difficult because of a variety of complicating factors. First, camelids generally have very few, if any, clinical signs associated with respiratory dysfunction until they are severely compromised. Additionally, auscultation of the camelid pulmonary system can be difficult in heavily fleeced animals and is particularly restricted in the caudal lung fields because of interference from gastrointestinal sounds.3 Conventional radiography is generally performed in standing position in adult animals and is limited to lateral views because of their large body mass.9 Therefore, interpretation can be difficult because of overlapping lung lobes, summation with extremities, and the lack of orthogonal views. Finally, disease-compromised camelids tend to rapidly decompensate with even minimal handling and, therefore, more definitive and invasive diagnostic methods such as transtracheal washes are often avoided. Because of these factors, diseases such as pneumonia are diagnosed presumptively on the basis of clinical signs rather than physical examination or imaging findings.

Computed tomography, especially high-resolution CT, has proven to be a valuable technique for diagnosis and management of respiratory disease in human patients, especially when results of conventional thoracic radiography are normal.10,11 In humans, CT has been shown to be more sensitive than radiography for diagnosis of various parenchymal lung diseases, such as chronic diffuse infiltrative disease,12 emphysema,13 fibrosing alveolitis,14 pneumonia,15 and metastatic pulmonary disease.16 Similarly, in dogs and cats, thoracic CT has provided additional information, especially regarding the location and extent of pathological changes of the lungs, and its use has altered diagnoses in some cases.17 Computed tomography offers numerous advantages over conventional thoracic radiography with cross-sectional imaging of very thin (0.5- to 2.0-mm) slices, superior soft tissue contrast resolution, and the capability for multiplanar reconstruction.18 Additionally, the availability of newer multidetector row CT scanners provides rapid scan times that enable an entire thoracic scan to be performed with sedation only. However, currently, there is extremely limited information available regarding normal anatomy of the respiratory system of camelids.3 For CT to be a valuable diagnostic tool in camelids, better knowledge of their bronchial and lung anatomy is necessary. The objective of the study reported here was to characterize and quantitatively assess the typical pulmonary anatomy of healthy adult alpacas by use of multidetector row CT and to provide baseline values for use in evaluation of thoracic CT studies and bronchoscopy in this species.

Materials and Methods

Animals—Ten female alpacas (mean ± SD age, 6.5 ± 4.5 years; range, 2 to 17 years) without any clinical signs of thoracic disease were included in the study. These alpacas were referred to the Oregon State University Veterinary Teaching Hospital to be evaluated for disease unrelated to the respiratory system. All evaluations were performed with informed owner consent as part of each patient's clinical assessment, and institutional animal care and use committee approval was not required for the study. Alpacas were considered healthy on the basis of history and physical examination, which included cardiac and pulmonary auscultation. The alpacas weighed 44.5 to 80.9 kg (mean, 60.5 ± 12.6 kg). Food, but not water, was withheld from all animals for 12 hours before the CT study was performed.

CT imaging of the thorax—All alpacas received butorphanola (0.1 mg/kg, IM), and a jugular catheter was placed via aseptic technique. Alpacas were then sedated with diazepama (0.25 mg/kg, IV) and ketamineb (0.3 mg/kg, IV), and scans were completed with the animals in sternal recumbency. All CT examinations were performed with a 64-slice CT scanner.c First, a nonenhanced thoracic CT scan was performed from approximately 10 cm cranial to the thoracic inlet to midlevel of the left kidney with the following scan parameters: 120 to 135 kV, 152 to 400 mA, a helical pitch of 53, a pitch factor of 0.828, and 0 tilt. Subsequently, a contrast-enhanced CT was performed after administration of iopamidold (1 mL/kg, IV) with a power injector at a flow rate of 3 mL/s; images were obtained following a 60-second delay from the start of injection. The thin collimated CT volume data were used to create transverse, dorsal, and sagittal reconstructed images of the thorax with 3 mm slice thickness. A bone, soft tissue, and lung algorithm was used to create bone, soft tissue, and lung window images. One alpaca did not receive contrast medium, although all other variables were the same. Images were sent to a server for offline analysis.

Evaluation of CT images—A DICOM viewerd was used by 2 investigators (SDC and SMSV) to perform all measurements. At least 3 measurements for each location were obtained, and the mean of these values was used in subsequent analysis. In each alpaca, various characteristics of the thorax and lungs were described and the most consistent bronchial mapping pattern was identified. Additionally, the following measurements were obtained: cranial and caudal extent of lungs, height and width of the thorax at 3 sites, diameter of the trachea, location of the tracheal bifurcation relative to the thoracic vertebrae, diameters of the bronchi and related blood vessels, and density and contrast enhancement of the lung parenchyma.

The 3 sites where the height and width of the thorax were measured were at the levels of the thoracic inlet, T4, and T8. As an internal control for patient size, the minimum and maximum height and length of the body of T4 were measured on midline sagittal images via a bone window (window width, 2,700 HU; window level, 350 HU). The maximum height of T4 was determined to be the largest measurement made at the cranial or caudal endplates of T4. The minimum height of T4 was measured at the midpoint of the central concavity in the vertebral body. The maximum vertebral length of T4 was determined halfway between the dorsal and ventral borders of the vertebral body. The vertebral bodies were rectangular, and thus, no minimum vertebral length was identified. The lower limit for all measurements was 1 mm. The maximum height and width of the thorax were measured at the 3 described sites by drawing a line from the ventral aspect of the vertebral body to the dorsal aspect of the sternum in a bone window.

Cranial and caudal extents of the right and left lungs were determined with transverse and sagittal images viewed side-by-side in a bone window (window width, 2,700 HU; window level, 350 HU). By use of the linear localization tool of the DICOM viewer, the cranial and caudal extent of both lungs was identified in the transverse image and the vertebral body was identified in the corresponding sagittal image. The caudomedial and caudolateral extents of both lungs were determined in the dorsal plane. Parallel lines were drawn (with DICOM viewer tools) at the level of the caudomedial and caudolateral extents of the lungs perpendicular to the vertebral column. The distance between these lines was measured in centimeters and recorded as the difference between the caudomedial and caudolateral lung margins.

Mean lung density was measured in a lung window (window width, 1,500 HU; window level, −600 HU), which provided the best visualization of small pulmonary vessels. All visible vessels and bronchi were avoided in obtaining lung parenchyma density measurements. For the 9 alpacas administered contrast medium, pre- and postcontrast transverse images of the thorax were viewed side-by-side. One representative image was chosen from the cranial aspect, and 2 each were selected from the midregion and the caudal aspect of the thorax for further evaluation. In each selected image, a 1.4-cm2 region of interest was drawn in the right dorsal, right ventral, left dorsal, and left ventral aspects of the lungs in the same anatomic area simultaneously in the pre- and postcontrast images. Both pre- and postcontrast values were recorded, and the mean value for all animals was determined for right dorsal, right ventral, left dorsal, and left ventral lung fields. Because these values were obtained in the same anatomic area in pre- and postcontrast images, they were considered paired samples. The differences between pre- and postcontrast values were calculated, and the mean of this value was determined for the described lung regions.

Measurements of the trachea, bronchi, and intrathoracic vascular structures—Measurements were performed in a bone window (window width, 2,700 HU; window level, 350 HU). The minimum and maximum luminal diameter (measured as a line drawn across the minimum and maximum diameter of the trachea independent of its position relative to the spine) and luminal height (measured as a vertical line drawn through the middle of the trachea on a transverse image at the level of the thoracic inlet) of the trachea were measured and recorded in a transverse CT image at the level of the thoracic inlet. The ratio of the luminal tracheal height to that of the thoracic inlet was calculated and recorded as a percentage. The angle of the trachea was determined in the sagittal plane as the angle between a line drawn through the middle of the vertebral bodies of T1 through T4 and a line drawn along the ventral aspect of the trachea.

The lumens of bronchi and the outer edges of each corresponding artery and vein were measured and recorded as the minimum cross-sectional diameter at the level of the bronchial division from the trachea or mainstem bronchus in a transverse CT image. The smallest diameter was recorded to eliminate the effect of obliquity. Measurements of the mainstem bronchi were performed just caudal to the bifurcation of the trachea. Additionally, measurements of the bronchi and parallel vessels were obtained in the cranial aspect of the thorax at the level of the body of T3 or T4 and in the caudal aspect of the thorax at the levels of T7, T8, and T9 when inclusion criteria were met. For the values to be included, the bronchus, artery, and vein all had to be visible in the same image and all structures were required to be > 3.0 mm away from a division to avoid recording falsely large measurements at branching sites. If a trio of measurements could not be obtained within the limits of a single vertebral body measurement, the bronchus and a single vessel (either the artery or vein) were measured.

Statistical analysis—Data were tested for normality via D'Agostino and Pearson omnibus normality tests. Paired t tests were performed for selected measurements, including the pre- and postcontrast administration lung parenchymal density measurements and measurements of right and left cranial and right and left first, second, and third caudal bronchi. When data were not normally distributed, measurements were compared between right and left lungs via a Mann-Whitney test. The mean, SD, and range were calculated for each measurement of interest. All analyses were done with a commercial software package.e For all comparisons, values of P < 0.05 were accepted as significant.

Results

Lung characteristics and density measurements—The mean, SD, and range for measurements of the thorax as well as lungs and other thoracic structures were summarized (Table 1). In 9 of 10 alpacas, the right lung extended farther cranially and caudally than did the left lung. In 1 animal, the left lung extended 8 mm farther cranially than did the right. Cranially, the right lung dominated the thorax, especially cranioventrally. The cranial portion of the left lung did not extend ventrally to the thoracic body wall. The right lung extended cranially as far as the cranial aspect of C7 and caudally as far as the caudal aspect of L2. The left lung extended cranially as far as the caudal aspect of C7 and caudally as far as the caudal aspect of L1. Additionally, in all alpacas, the medial aspect of both lungs extended farther caudally than did the lateral aspect (Figure 1). There was no clear separation between individual lung lobes in any animal with the exception of the accessory lung lobe. The accessory lung lobe was visualized caudal to the heart and ventral to the esophagus, and its medial surface encircled approximately half of the caudal vena cava.

Figure 1—
Figure 1—

Dorsal (A), right lateral (B), and left lateral (C) reconstructed 3-D CT views of the lungs of a healthy adult alpaca in a study to characterize and quantitatively assess the typical pulmonary anatomy in this species. The right lung extends farther in the cranial and caudal directions, compared with the left lung. The caudal aspects of the lungs extended farther caudally at the medial aspect than at the lateral aspect. No lobation of the lungs is evident in these views. L = Left. R = Right.

Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.572

Table 1—

Mean ± SD and range of measurements of thoracic structures determined via multidetector row CT in 10 healthy female alpacas.

MeasurementMeanRange
Thorax
 Measured at thoracic inlet (cm)
  Height11.0 ± 0.810.1 to 12.6
  Width5.3 ± 0.64.5 to 6.3
 Measured at the level of T4 (cm)
  Height18.3 ± 1.617.0 to 21.4
  Width11.6 ± 1.410.1 to 13.7
 Measured at the level of T8 (cm)
  Height23.2 ± 1.421.6 to 25.8
  Width21.6 ± 2.518.7 to 25.8
Lungs*
 Difference between extents of left and right lungs (cm)
  Cranial extents2.4 ± 0.5−0.8 to 3.2
  Caudal extents4.1 ± 0.63.3 to 5.0
 Difference between caudomedial and caudolateral extents of the lung (cm)
  Right lung5.0 ± 1.03.6 to 6.5
  Left lung3.8 ± 1.12.2 to 4.9
Trachea
 Luminal diameter at thoracic inlet (cm)
  Minimum1.5 ± 0.11.3 to 1.7
  Maximum1.9 ± 0.21.4 to 2.2
 Luminal height at thoracic inlet (cm)1.7 ± 0.11.5 to 1.9
 Trachea-to-thoracic inlet ratio (%)15 ± 0.0213 to 18
  Luminal diameter cranial to bifurcation (cm)  
   Minimum1.9 ± 0.21.8 to 2.1
   Maximum2.2 ± 0.21.7 to 2.5
  Angle at heart base (°)11.8 ± 2.29 to 16
Body of T4
 Height (cm)
  Minimum1.6 ± 0.11.5 to 1.7
  Maximum2.1 ± 0.12.0 to 2.1
 Maximum width (cm)3.0 ± 0.12.8 to 3.1

Measurements were made with dedicated software. Cranial and caudal extents of the lungs were determined in transverse and sagittal images viewed side by side; caudomedial and caudolateral extents of the lungs were determined in dorsal plane images. Dimensions of T4 and the angle of the trachea to the thoracic vertebral column at the level of the base of the heart were determined in the sagittal plane.

Differences between right and left lungs were calculated as right minus left; differences between caudomedial and caudolateral extents of the lungs were determined as medial minus lateral.

There was no significant difference in mean lung parenchyma density between right and left lungs or between the dorsal and ventral aspects of the lung before or after IV contrast medium injection (Figure 2). Mean ± SD precontrast parenchyma density for all lungs was −869.4 ± 40.3 HU (range, −742 to −960 HU), and the postcontrast value was −824.9 ± 51.1 HU (range, −665 to −931 HU). The range of the differences between paired samples was large (1 to 153 HU); although only 10 of 180 (5.6%) paired samples had a < 10 HU difference, 122 of 180 (67.8%) values fell within the mean ± SD difference of 44.4 ± 27.5 HU between pre- and postcontrast lung parenchyma densities.

Figure 2—
Figure 2—

Computed tomographic images of the thorax in a healthy adult alpaca. A—Transverse image obtained at the level of T7 prior to administration of contrast medium, depicting regions of interest where right and left ventral (RV and LV) and dorsal (RD and LD) lung parenchyma density measurements were obtained. B—Transverse image obtained at the same location as in panel A 60 seconds after IV administration of iodinated contrast medium (iopamidol [1 mL/kg, IV]). C—Transverse image obtained at the level of the thoracic inlet. Lines represent the measurements for minimum (Min) and maximum (Max) diameters and height (H) of the tracheal lumen. The half moon–shaped structure to the right of the trachea is the air-filled esophagus. D—Midsagittal image illustrating the measurement of the angle between the thoracic vertebral column and trachea at the base of the heart (white lines). E—Transverse image obtained at the level of T6 depicting luminal diameter measurements of the right and left first caudal bronchi (RCd1 and LCd1, respectively). F—Transverse image obtained at the level of T7 showing luminal diameter measurements of a bronchus (B) and its associated artery (A) and vein (V). The artery is in closer proximity to the bronchus than is the vein. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.572

Bronchial branching pattern—The most consistent branching pattern and areas of lung supplied by each bronchus were summarized (Figure 3). In all animals, the cranioventral aspect of the right lung was supplied by a bronchus that originated from the right lateral aspect of the trachea cranial to the tracheal bifurcation. The bifurcation of the trachea was located at the level of the cranial border of T4 (n = 2 alpacas), the caudal border of T4 (3), the middle of T5 (3), or the caudal border of T5 (2). The luminal diameter of the right cranial bronchus was the most variable of all bronchi with a range of 3 to 9 mm (Table 2). This bronchus divided almost immediately into a dorsal and larger ventral branch, both of which extended cranially. The cranial aspect of the left lung was supplied by a bronchus that originated from the left mainstem bronchus just caudal to the bifurcation of the trachea. The left cranial bronchus was smaller than the right cranial bronchus in 8 of 10 alpacas. The left cranial bronchus also divided into a dorsal and larger ventral branch, which extended cranially. The accessory bronchus was found to originate caudal to the bifurcation of the trachea from the ventromedial aspect of the right mainstem bronchus. The accessory bronchus coursed ventromedially to supply the accessory lung lobe.

Figure 3—
Figure 3—

Typical branching patterns of the trachea and bronchi in alpacas. A—Schematic illustration of the most common branching pattern detected in alpacas of the present study. The cranial aspect of the right lung is supplied by a bronchus originating from the trachea cranial to the bifurcation (R cranial). The trachea divides into a right and left mainstem bronchus; the left mainstem bronchus gives off a cranial bronchus just caudal to the bifurcation (L cranial), and both mainstem bronchi divide further into right or left first, second, and third caudal bronchi (R caudal 1 to 3 and L caudal 1 to 3, respectively). The accessory bronchus originates from the right mainstem bronchus caudal to the bifurcation of the trachea and supplies the accessory lung lobe. Both mainstem bronchi taper and end in a terminal bronchus. B—Reconstructed sagittal plane CT image of bronchial branching in the left lung. The left cranial bronchus supplies the cranioventral aspect of the lung. The first, second, and third left caudal bronchi supply the ventral portion, midportion, and dorsal portion of the caudal aspect of the lung, respectively. The left mainstem bronchus tapers into the left terminal bronchus after giving off the caudal bronchi.

Citation: American Journal of Veterinary Research 74, 4; 10.2460/ajvr.74.4.572

Table 2—

Mean ± SD and range of luminal diameters at various locations within the bronchial tree as measured via CT in 10 healthy female alpacas.

VariableMean ± SD (mm)Range (mm)
Cranial bronchi
 Right7 ± 23–9
  Dorsal branch3 ± 12–5
  Ventral branch6 ± 14–7
 Left5 ± 13–7
  Dorsal branch2 ± 11–3
  Ventral branch4 ± 13–5
Mainstem bronchi
 Right12 ± 210–16
 Left12 ± 29–14
Accessory bronchus4 ± 13–5
First caudal bronchi
 Right7 ± 16–8
  Dorsal branch5 ± 13–6
  Ventral branch4 ± 13–5
 Left6 ± 15–8
  Dorsal branch5 ± 13–6
  Ventral branch3 ± 13–4
Second caudal bronchi
 Right6 ± 14–7
  Dorsal branch4 ± 12–5
  Ventral branch3 ± 12–4
 Left5 ± 13–7
  Dorsal branch3 ± 12–5
  Ventral branch3 ± 12–4
Third caudal bronchi
 Right4 ± 13–7
  Dorsal branch2 ± 11–4
  Ventral branch2 ± 11–3
 Left4 ± 13–6
  Dorsal branch2 ± 11–5
  Ventral branch2 ± 11–4
Terminal bronchi
 Right6 ± 24–9
  Dorsal branch3 ± 12–5
  Ventral branch2 ± 11–3
 Left6 ± 14–9
  Dorsal branch3 ± 11–6
  Ventral branch2 ± 11–6

Measurements were made in transverse CT images with dedicated software. Mainstem bronchi were measured just caudal to the bifurcation of the trachea. Minimum cross-sectional diameters of bronchi were determined at the level of division from the trachea or mainstem bronchus. Additional measurements were obtained in the cranial aspect of the thorax at the level of the body of T3 or T4 and in the caudal aspect of the thorax at the levels of T7, T8, and T9.

Bilaterally, the remaining ventral portions of the lungs were supplied primarily by 3 bronchi that branched from the ventrolateral aspects of each mainstem bronchus (Figure 3). These bronchi, denoted as right or left first, second, and third caudal bronchi, supplied the caudoventral, midcaudal, and caudodorsal aspects of each lung, respectively. The branching patterns of these bronchi were highly consistent, and in only 2 alpacas was an additional left caudal bronchus of an appreciable size seen. The first caudal bronchi supplied the ventral portions of the caudal aspects of the lungs and typically extended 1 to 2 cm caudoventrally before bifurcating into dorsal and ventral branches, which also continued in a primarily caudoventral direction. The second and third caudal bronchi supplied the midportions and dorsal portions of caudal aspects of the lungs, respectively. The mainstem bronchi continued axially in a caudal direction and terminated gradually, supplying the most medial portions of the caudodorsal aspects of the lungs. The terminal bronchi were identified as the caudal aspects of each mainstem bronchus after the division of the ventral branch of the third caudal bronchus. These terminal bronchi generally continued axially in a caudal direction and gave off dorsal, lateral, and medial branches typically 1 to 2 mm in diameter before bifurcating in the periphery as 2 to 3 small final branches. Mean ± SD and range measurements for these first- and second-generation bronchi were summarized (Table 2). There was no significant difference between measurements of any right and left bronchi at the same level.

Bronchi and associated arteries and veins—A total of 136 bronchi and their associated blood vessels (74 and 62 in right and left lungs, respectively) met the inclusion criteria for analysis. One hundred eleven complete sets of artery, bronchus, and vein measurements were available, 7 sets were missing the artery measurement, and 18 sets were missing the vein measurements. Arteries were anatomically more closely associated with the bronchi than were veins. The mean ± SD difference in diameter between bronchus and artery, bronchus and vein, and artery and vein was 0.8 ± 0.9 mm (range, 0 to 4 mm). The largest difference in diameter identified between a bronchus and a corresponding artery or vein was a single measurement of 4 mm. There was no apparent trend for either a vessel or the related bronchus being larger. Overall, of 136 sets of measurements, the bronchi were larger than the corresponding vessels in 34; the vessels were larger than the bronchus in 45; and the bronchus and vessels were the same size in 57. There was no significant difference between right and left lung in any of the artery, vein, or bronchus measurements.

Discussion

Results of this study provided qualitative and quantitative data characterizing the pulmonary parenchyma, bronchi, and associated blood vessels in healthy adult alpacas. Over the last decade, more information regarding imaging findings in healthy camelids has become available; however, such studies9,19 are mostly limited to radiographic findings with general emphasis on cardiovascular structures rather than the pulmonary system. Additionally, a recent report19 described limitations in fully evaluating the caudal pulmonary vasculature due to summation of structures. Given the difficulties in adequately assessing camelid patients without causing stress, CT may provide a practical means of completely examining the cardiovascular and respiratory systems. Computed tomography is a newer modality in veterinary medicine but is becoming increasingly available in teaching hospitals and private practices. Thus, baseline values established by examination of clinically normal alpacas will serve as a resource for veterinarians when evaluating CT images of these patients.

In all alpacas, the medial portion of the caudal aspects of the right and left lungs extended farther caudally than the lateral portion of the caudal aspects of the lungs; thus, a notable portion of the caudal aspect of the lungs did not extend to the thoracic wall. Not only can gastrointestinal sounds interfere with auscultation in caudal regions of the thorax, this finding indicates that even under the best circumstances, lungs would not be readily auscultated in these areas. Therefore, even in cases of unremarkable physical examination findings, CT of the thorax may be warranted if any respiratory disease is suspected to fully assess all portions of the lungs.

The tracheal bifurcation in all alpacas was located at the level of T4 to T5. We found no previous information about the position of the bifurcation of the trachea in the literature on camelids. In small animals, displacement of the bifurcation can be detected when cranial mediastinal masses are present, and it is expected that similar alterations would be seen in camelids with masses in the cranial mediastinum.

Similar to the brief anatomic descriptions in other studies, the only lung lobe that was clearly delineated by CT in the present study was the accessory lung lobe. This finding indicates that the level of detail seen in these CT images was comparable to that of gross anatomic inspection of this area. Similarly, the small third-generation bronchi and associated vessels were also easily identified in most alpacas, indicating that CT provides a valuable tool for assessing small airways, vessels, and pulmonary parenchyma. In humans, parenchymal contrast enhancement is an indicator for diffuse interstitial disease such as pulmonary fibrosis or interstitial pneumonia10 and can be used in evaluation for pulmonary thromboembolic disease.20 In all alpacas, the mean contrast enhancement in the lung parenchyma after IV iodinated contrast was 44.4 HU (range, 4 to 153 HU). To our knowledge, similar studies evaluating contrast enhancement of pulmonary parenchyma have not been performed in other veterinary species, but this may provide additional information for evaluating the lung parenchyma in alpacas. In the present study, there was no significant difference in parenchymal density between dorsal and ventral aspects of the lung before or after IV administration of contrast medium, indicating these healthy animals were well ventilated without the use of positive pressure ventilation.

Results of the present study provide baseline information regarding normal values for dimensions of various thoracic structures, similar to studies in other species.21 We found that the mean angle between the trachea and the thoracic vertebral column (11.8°) was similar to the value identified radiographically in adult llamas (14.4°).9 Although, to our knowledge, there are currently no radiographic studies of healthy adult alpacas, presumably these findings could be reliably used in conventional radiography. Alterations in this angle have been shown to be an indicator of cardiomegaly in dogs and cats and may also indicate the presence of a cranial mediastinal mass.9 However, one of the shortcomings of the present study was that only 10 healthy adult alpacas were evaluated; the results should be considered with this in mind, and it is not known whether the reported variables would be substantially altered in diseased animals.

To our knowledge, hypoplasia of the trachea has not been described in camelids; however, this species is predisposed to several congenital abnormalities.22–24 Tracheal diameter was relatively uniform in the study population, and the ratio of tracheal height to that of the thoracic inlet (15%) was slightly smaller than that identified in nonbrachycephalic dogs (20%)25 and slightly larger than that recently identified in goats (13%).21 Diameter of the trachea in the alpacas in our study was similar throughout its length before it enlarged slightly and elongated in a left-to-right direction at the level of the thoracic inlet, similar to findings described for adult llamas.9 However, whether the addition of positive pressure ventilation would affect tracheal or bronchial measurements in alpacas, as has been reported in goats,21 is currently unknown.

In camelids, the mainstem bronchi have been described as dividing into an apical bronchus, a cardiac bronchus, and a larger diaphragmatic bronchus.3 However, in the CT evaluation of alpacas in the present study, we identified 3 caudal bronchi of similar diameter in both lungs, and we have referred to these as first, second, and third caudal bronchi of the right and left lungs. We believe this labeling system is more representative of the consistently identified anatomy in these adult alpacas.

An important contribution of the study reported here is the measurement of bronchi and their associated vessels. We found that the mean difference between the bronchi and their parallel vessels was < 1 mm. Clinicians may find this data useful when evaluating the thorax of adult alpacas for diseases in which changes of the diameters of bronchi or blood vessels would be anticipated. Examples include bronchiectasis, congenital strictures, heart failure, shunting, or hypovolemia.

Results of the present study indicated that CT can be used for rapid analysis of the thorax in sedated alpacas. On the basis of our findings, we suggest CT should be considered as an early diagnostic tool in alpacas and potentially in llamas because it provides excellent qualitative and quantitative information with minimal handling. Future studies should investigate the use of CT to evaluate a larger population of healthy camelids as well as those with tachypnea, abnormal lung sounds, or suspicious pulmonary changes detected via conventional radiography to compare and validate our findings. Sedation was adequate for evaluation of our healthy patient population but may prove insufficient in animals with pulmonary compromise in which substantial respiratory motion may limit findings.

ABBREVIATIONS

DICOM

Digital Imaging and Communications in Medicine

HU

Hounsfield units

a.

Hospira Inc, Lake Forest, Ill.

b.

Ketaset, Fort Dodge Animal Health, Fort Dodge, Iowa.

c.

Toshiba Aquilion, Toshiba America Medical Systems Inc, Tustin, Calif.

d.

eFilm, version 3.3.0, Merge Healthcare, Heartland, Wis.

e.

GraphPad Prism, version 5.01, GraphPad Software Inc, La Jolla, Calif.

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Contributor Notes

Presented in abstract form at the 2011 American College of Veterinary Radiology Scientific Meeting, Albuquerque, October 2011.

The authors thank Jason Wiest for technical assistance.

Address correspondence to Dr. Stieger-Vanegas (Susanne.Stieger@oregonstate.edu).