• View in gallery
    Figure 1—

    Mean ± SD lung density in 6 anesthetized red-tailed hawks (Buteo jamaicensis) in dorsal recumbency (black bars), right lateral recumbency (white bars), and sternal recumbency (gray bars). Lung density was measured in CTUs; high (less negative) CTU values reflect high lung density. Lung density differed significantly (P < 0.05) between all pairs of body positions and was 7.0% lower in sternal versus dorsal recumbency.

  • View in gallery
    Figure 2—

    Mean ± SD lung volume in 6 anesthetized red-tailed hawks in dorsal recumbency (black bars), right lateral recumbency (white bars), and sternal recumbency (gray bars). Lung volume differed significantly (P < 0.05) between all pairs of body positions and was 5.6% higher in sternal versus dorsal recumbency.

  • View in gallery
    Figure 3—

    Mean ± SD air-sac volume in 6 anesthetized red-tailed hawks in dorsal recumbency (black bars), right lateral recumbency (white bars), and sternal recumbency (gray bars). The volume was measured caudal to T6 and does not include the clavicular air sac. Birds 3 and 4 had a markedly reduced air-sac volume in all 3 body positions; those birds were obese. Air-sac volume differed significantly (P < 0.05) between all pairs of body positions and was 28.5% higher in sternal versus dorsal recumbency.

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Effect of body position on respiratory system volumes in anesthetized red-tailed hawks (Buteo jamaicensis) as measured via computed tomography

Shachar MalkaWilliam R. Pritchard Veterinary Medical Teaching Hospital, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Michelle G. HawkinsDepartment of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616.

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James H. JonesDepartment of Surgery and Radiology, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Peter J. PascoeDepartment of Surgery and Radiology, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Philip H. KassDepartment of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Erik R. WisnerDepartment of Surgery and Radiology, School of Veterinary Medicine, University of California, Davis, CA 95616.

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Abstract

Objective—To determine the effects of body position on lung and air-sac volumes in anesthetized and spontaneously breathing red-tailed hawks (Buteo jamaicensis).

Animals—6 adult red-tailed hawks (sex unknown).

Procedures—A crossover study design was used for quantitative estimation of lung and air-sac volumes in anesthetized hawks in 3 body positions: dorsal, right lateral, and sternal recumbency. Lung volume, lung density, and air-sac volume were calculated from helical computed tomographic (CT) images by use of software designed for volumetric analysis of CT data. Effects of body position were compared by use of repeated-measures ANOVA and a paired Student t test.

Results—Results for all pairs of body positions were significantly different from each other. Mean ± SD lung density was lowest when hawks were in sternal recumbency (–677 ± 28 CT units), followed by right lateral (–647 ± 23 CT units) and dorsal (–630 ± 19 CT units) recumbency. Mean lung volume was largest in sternal recumbency (28.6 ± 1.5 mL), followed by right lateral (27.6 ± 1.7 mL) and dorsal (27.0 ± 1.5 mL) recumbency. Mean partial air-sac volume was largest in sternal recumbency (27.0 ± 19.3 mL), followed by right lateral (21.9 ± 16.1 mL) and dorsal (19.3 ± 16.9 mL) recumbency.

Conclusions and Clinical Relevance—In anesthetized red-tailed hawks, positioning in sternal recumbency resulted in the greatest lung and air-sac volumes and lowest lung density, compared with positioning in right lateral and dorsal recumbency. Additional studies are necessary to determine the physiologic effects of body position on the avian respiratory system.

Abstract

Objective—To determine the effects of body position on lung and air-sac volumes in anesthetized and spontaneously breathing red-tailed hawks (Buteo jamaicensis).

Animals—6 adult red-tailed hawks (sex unknown).

Procedures—A crossover study design was used for quantitative estimation of lung and air-sac volumes in anesthetized hawks in 3 body positions: dorsal, right lateral, and sternal recumbency. Lung volume, lung density, and air-sac volume were calculated from helical computed tomographic (CT) images by use of software designed for volumetric analysis of CT data. Effects of body position were compared by use of repeated-measures ANOVA and a paired Student t test.

Results—Results for all pairs of body positions were significantly different from each other. Mean ± SD lung density was lowest when hawks were in sternal recumbency (–677 ± 28 CT units), followed by right lateral (–647 ± 23 CT units) and dorsal (–630 ± 19 CT units) recumbency. Mean lung volume was largest in sternal recumbency (28.6 ± 1.5 mL), followed by right lateral (27.6 ± 1.7 mL) and dorsal (27.0 ± 1.5 mL) recumbency. Mean partial air-sac volume was largest in sternal recumbency (27.0 ± 19.3 mL), followed by right lateral (21.9 ± 16.1 mL) and dorsal (19.3 ± 16.9 mL) recumbency.

Conclusions and Clinical Relevance—In anesthetized red-tailed hawks, positioning in sternal recumbency resulted in the greatest lung and air-sac volumes and lowest lung density, compared with positioning in right lateral and dorsal recumbency. Additional studies are necessary to determine the physiologic effects of body position on the avian respiratory system.

Inhalant anesthesia is an essential component of veterinary practice because many procedures require pain relief and immobilization. In avian medicine, inhalant anesthesia is of particular importance because of its usefulness for even simple, nonpainful procedures such as obtaining radiographs.1–5

The avian respiratory system has separate ventilatory and gas-exchange compartments, making it a highly efficient system, compared with that of mammals.3,6 Avian ventilatory components consist of the major airways, an air-sac system, the lungs, and the thoracic skeleton and associated muscles. The air sacs serve to ventilate the lungs but do not participate directly in gas exchange.6–10 Birds lack a muscular diaphragm, and the coelomic cavity behaves as 1 compartment. Ventilation results from active movement of the thoracic cage and the muscles of respiration during inspiration and expiration. During inspiration, the respiratory muscles expand the coelomic cavity, creating subatmospheric pressure in the air sacs allowing air to be inhaled into the respiratory system.6,10,11 During expiration, muscular contractions cause the air sacs to act as bellows, forcing air through the lungs.8,10,12 The ventilation-to-volume ratio affects the partial pressures of oxygen and carbon dioxide within the air sacs and thus within the lungs. Therefore, changes in air-sac volume and in muscle tone that accompany inhalation anesthesia in birds can alter air-sac gas composition.8,10 Physical restriction of the active ventilatory components is often reported as the most important condition leading to hypoventilation in anesthetized birds.2–4,13,14

Avian lungs adhere to the dorsal thoracic wall, are noncompliant, and are comprised of nonexpandable parabronchi and air capillaries.10,11,15–17 These factors have led to a widely accepted view that avian lungs are isovolumetric and not susceptible to physical restriction.2–5,11,18,19 One study20 in which contrast cinefluoroscopy was used revealed that lung volume changes of approximately 1.4% take place during the ventilatory cycle in anesthetized ducks.20 Little scientific evidence is available to support or refute whether physical restriction of lung volumes happens in anesthetized birds.

Bird positioning is also cited as a common cause of ventilatory restriction in anesthetized birds.2,4,5,18 The 2 most common body positions used during avian anesthesia are dorsal and right lateral recumbency.5,21 It has been hypothesized that dorsal recumbency may cause air sacs to be compressed by the internal organs, resulting in hypoventilation and hypoxemia that may worsen with increasing duration of anesthesia. In a study13 of domestic chickens, researchers detected through subjective observations that breathing changed in amplitude and frequency when birds were positioned in dorsal recumbency. This has led to widely accepted recommendations to avoid dorsal recumbency for anesthetic procedures in birds when possible.2,13,15,18 Sternal recumbency during anesthesia has been implicated as contributing even more to the respiratory compromise during anesthesia. Restriction of sternal movement when the bird's body weight is placed on the sternum has been suggested as a predominant cause of hypoventilation during anesthesia when positioned in sternal recumbency.3,4,17

Evaluation of lung and air-sac volumes in anesthetized birds positioned in dorsal, sternal, or lateral recumbency would provide important information regarding anatomic changes associated with specific positioning during anesthesia. Lung and air-sac volumes are difficult to quantitatively estimate by use of conventional radiography for clinical purposes. There are few studies20,22 in which lung volume and air-sac anatomy has been compared by means of radiography or CT. Helical CT has been used in human and veterinary medicine to evaluate volume and density of organ systems such as the lungs and liver.23,24 However, to our knowledge, no studies have been conducted to evaluate whether changes in respiratory volume take place with different body positions during anesthesia in any species of bird.

The purpose of the study reported here was to examine the effect of body positions (dorsal, right lateral, and sternal recumbency) on lung and air-sac volumes and on lung densities in anesthetized red-tailed hawks (Buteo jamaicensis) by use of quantitative CT image analysis. We hypothesized that air-sac or lung density and volume would vary among the 3 positions.

Materials and Methods

Animals—Six adult 1.0- to 1.6-kg red-tailed hawks of unknown sex were used in the study. The study protocol was approved by the Institutional Animal Care and Use Committee of the University of California, Davis. The birds were permanent residents of the California Raptor Center at the University of California and deemed unfit for release because of previously diagnosed orthopedic conditions. The birds were housed in outdoor mews and fed a diet of killed mice and day-old chicks daily; water was provided ad libitum. All birds were judged healthy, with the exception of the previously diagnosed orthopedic problems, on the basis of results of physical examination, CBC and serum biochemical analysis, and fecal evaluation for endoparasites 10 to 14 days prior to the start of the study. Food was withheld from the birds for 12 hours prior to the beginning of each assessment phase.

Study design—The birds were randomly assigned via coin toss to 1 of 2 assessment groups in a crossover design. Each bird was assessed twice, with the order of positioning changed each time. For 1 assessment, birds were anesthetized in dorsal, right lateral, and then sternal recumbency. For the other, birds were anesthetized in sternal, right lateral, and then dorsal recumbency. A period of no less than 1 week was allowed between assessments.

Anesthesia was induced in each bird with 3% to 4% isofluranea delivered in 95% to 100% oxygen via a face mask. The birds were then intubated with a 3.5-mm uncuffed endotracheal tube and maintained on 2% isoflurane in 98% to 100% oxygen at a flow rate of 1 L/min with spontaneous ventilation. The respiratory rate was monitored throughout each assessment. Body temperature was monitored by use of an esophageal thermometer probe,b and a warm air blanket was placed under each bird to maintain esophageal temperature between 39.5° and 41°C. Heart rate was monitored with a Doppler probec placed under the maxillary beak over the palatine artery.

Instrumentation and positioning were completed approximately 10 minutes after induction of anesthesia, after which each bird was maintained in the first recumbency position (dorsal or sternal recumbency, depending on the assessment group) for an additional 15 minutes before CT evaluation began. A whole-body helical CT scan was then performed (excluding the head, neck, and extremities). Anesthesia was maintained for 15 minutes in each body position before helical CT scans were performed. After one set of scans was obtained, the body position was changed in accordance with the assessment group to which birds had been assigned. The second assessment in which the order of body positions was changed was performed in each bird 1 to 2 weeks after the end of the first assessment to allow time for recovery from the previous assessment.

CT imaging—Transverse CT images of the coelom were acquired for each body position. Birds were scanned by means of a single-slice helical CT scannerd with the following imaging parameters: 2-mm image collimation, helical acquisition with a 1-second acquisition time and a pitch of 1.0, 12-cm field of view, 120 kVp, 100mA, and moderately edge-enhancing reconstruction algorithm. All images were then processed at a remote, dedicated image analysis workstation.e Volumetric and density estimates of the lungs and air sacs were calculated by use of a software package designed for volumetric analysis of CT data.f Operator-defined ROIs were drawn to separately outline the margins of the lungs and air sacs. These ROIs were used to create a 3-D rendering of the lungs and air sacs for each bird for each position during the 2 assessments. Briefly, the analysis software estimates volume by defining the cross-sectional area of a given ROI, defining a partial volume for a given structure of interest by multiplying area by the collimation width, then summing the partial volumes from all images incorporating the structure of interest. The total volume of the 3-D image of the lungs or air sacs was determined.

The volume of the air sacs (partial air-sac volume) was calculated only for the caudal aspect of the coelom, including the cranial thoracic, caudal thoracic, and abdominal air sacs. For consistency in calculating areas in different positions, a landmark in the CT images that was used to standardize cross-sectional areas, and hence volumes, was T6, which is the free vertebra between the notarium and the synsacrum in red-tailed hawks. Mean lung density was determined in a similar manner by use of the same software. Density was calculated in CTUs, which represent a linear density scale.

Three images were selected for analysis when the bird was in each position during each phase of the study. The first image was chosen at the level of the syrinx, the second image was chosen at the level of the pulmonary veins, and the third image was chosen at the level of the gastric isthmus, between the proventriculus and ventriculus. For each image, 4 ROIs of identical size were manually drawn and mean density in CTUs was recorded. The anatomic landmark was consistently identified from the CT images and confirmed with multiple views of axial, sagittal, and coronal sections.

Respiratory rate during image acquisition was measured by counting breaths during a timed interval. To determine whether position affected ventilatory duty cycle (the ratio of inspiratory to expiratory duration), inspiratory and expiratory durations were measured in a separate experiment involving 6 red-tailed hawks anesthetized with isoflurane. These birds breathed through a Fleisch pneumotachometer in line with the anesthesia circuit while lying in dorsal and lateral recumbency. Ventilatory flows were recorded from which inspiratory and expiratory duration were measured for 10 breaths after the birds had been anesthetized for 30 minutes.

Statistical analysis—Mean ± SD values were calculated for lung density, lung volumes, and air-sac volumes for each bird and all 6 birds. A 2-way repeated-measures ANOVA was used to compare the effect of position while controlling for replicates. When differences were significant in the ANOVA analyses, post hoc pairwise comparisons of results for the various positions were made by use of a paired Student t test and a sequentially rejective Bonferroni test procedure to preserve a nominal A value of 0.05.

Differences in respiratory rate at the 3 positions and at the first and second assessments during the CT measurements were also evaluated with 2-way repeated-measures ANOVA. Differences in respiratory rate among positions during the pneumotachometer measurements were evaluated by means of 1-way repeated-measures ANOVA. Differences in respiratory rate between the CT and pneumotachometer measurements, with position as a factor, were analyzed by means of 2-way ANOVA. Differences in ventilatory duty cycle between positions during the pneumotachograph measurements were analyzed by means of 1-way repeated-measures ANOVA. A value of P < 0.05 was considered significant for all analyses.

Results

The mean number of images per scan for each body position (dorsal, right lateral, and sternal recumbency) was 54 (range, 49 to 59), and acquisition time for each image was 1 second. Recovery from anesthesia was unremarkable for all birds after both assessments.

Results of all pairwise comparisons for lung density, lung volume, and air-sac volume with respect to the 3 body positions were significant; all values changed with every position. Mean ± SD lung density was lowest in sternal recumbency (−677 ± 28 CTUs), followed by right lateral (−647 ± 23 CTUs) and dorsal (−630 ± 19 CTUs) recumbency (Figure 1). Lung density was 7.0% lower in sternal versus dorsal recumbency. Mean lung volume was largest in sternal recumbency (28.6 ± 1.5 mL), followed by right lateral (27.6 ± 1.7 mL) and dorsal (27.0 ± 1.5 mL) recumbency (Figure 2). Lung volume was 5.6% higher in sternal versus dorsal recumbency. Mean partial air-sac volume was largest in sternal recumbency (27.0 ± 19.3 mL), followed by right lateral (21.9 ± 16.1 mL) and dorsal (19.3 ± 16.9 mL) recumbency (Figure 3). Air-sac volume was 28% higher in sternal versus dorsal recumbency. For all 3 variables, the values for sternal recumbency differed most from those of dorsal recumbency.

Figure 1—
Figure 1—

Mean ± SD lung density in 6 anesthetized red-tailed hawks (Buteo jamaicensis) in dorsal recumbency (black bars), right lateral recumbency (white bars), and sternal recumbency (gray bars). Lung density was measured in CTUs; high (less negative) CTU values reflect high lung density. Lung density differed significantly (P < 0.05) between all pairs of body positions and was 7.0% lower in sternal versus dorsal recumbency.

Citation: American Journal of Veterinary Research 70, 9; 10.2460/ajvr.70.9.1155

Figure 2—
Figure 2—

Mean ± SD lung volume in 6 anesthetized red-tailed hawks in dorsal recumbency (black bars), right lateral recumbency (white bars), and sternal recumbency (gray bars). Lung volume differed significantly (P < 0.05) between all pairs of body positions and was 5.6% higher in sternal versus dorsal recumbency.

Citation: American Journal of Veterinary Research 70, 9; 10.2460/ajvr.70.9.1155

Figure 3—
Figure 3—

Mean ± SD air-sac volume in 6 anesthetized red-tailed hawks in dorsal recumbency (black bars), right lateral recumbency (white bars), and sternal recumbency (gray bars). The volume was measured caudal to T6 and does not include the clavicular air sac. Birds 3 and 4 had a markedly reduced air-sac volume in all 3 body positions; those birds were obese. Air-sac volume differed significantly (P < 0.05) between all pairs of body positions and was 28.5% higher in sternal versus dorsal recumbency.

Citation: American Journal of Veterinary Research 70, 9; 10.2460/ajvr.70.9.1155

The mean respiratory rate did not differ significantly (P = 0.78) among positions during the CT scans (dorsal recumbency, 29 ± 4 breaths/min; right lateral recumbency, 27 ± 3 breaths/min; and sternal recumbency, 28 ± 4 breaths/min), nor did it vary significantly (P = 0.69) between assessments (first assessment, 28 ± 4 breaths/min; second assessment, 29 ± 4 breaths/min). There was no significant (P = 0.65) interaction between order and body position. Similarly, body position did not significantly (P = 0.25) affect respiratory rate during the pneumotachometer measurements at the 30-minute measurement point (right lateral recumbency, 27 ± 10 breaths/min; dorsal recumbency, 26 ± 7 breaths/min), and there was no difference between values obtained during the CT and pneumotachometer measurements in either position (right lateral vs dorsal recumbency position effect, P = 0.91; CT vs pneumotachometer values, P = 0.63; interaction between measurements and position, P = 0.56). Duty cycle did not differ significantly (P = 0.50) when the birds were in dorsal versus right lateral recumbency as compared by means of repeated-measures ANOVA (right lateral recumbency, 1.59 ± 0.87; dorsal recumbency, 1.36 ± 0.41).

Discussion

In avian anesthesia, dorsal and lateral recumbency are the most commonly recommended body positions and it is considered critical that unrestricted movement of the thoracic cage be maintained.1,2,4,5,18 The results of the present study indicated that of the 3 body positions commonly used during avian anesthesia (dorsal, right lateral, and sternal recumbency), sternal recumbency resulted in the greatest lung and air-sac volumes and lowest lung density in red-tailed hawks, compared with respective values in lateral or dorsal recumbency. It appeared that avian lungs might be able to undergo larger volume excursions during the ventilatory cycle as a result of positional changes than has been reported elsewhere.10,11,15–17,20

The avian respiratory system is unique among vertebrates and is more efficient at gas exchange than the mammalian respiratory system. Airflow through lungs is primarily unidirectional during inspiration and expiration for most bird species, and the ventilatory component, which is comprised of the air sacs, is separate from the gas-exchange component, which is comprised of a meshwork of air and blood capillaries surrounding the parabronchi.7,11,25,26 The diameter of the air capillaries (10 μm) is much smaller than that of mammalian alveoli, making atelectasis of air capillaries impossible in physiologic conditions.27 Were an air capillary to collapse, surface tension would render it impossible to reinflate.7,28,29 As a result, avian lungs have been perceived as rigid and noncompliant, relative to those of mammals.

In a study20 of anesthetized ducks, cineradiography of markers placed on pulmonary aponeuroses revealed that during the respiratory cycle when ducks were upright, lung volume changed by approximately 1.4%. The results of our study suggested an increase in lung volume of 5.6% when hawks were positioned in sternal versus dorsal recumbency, and that increase corresponded to a lung density decrease of 7.0%. When hawks were positioned in right lateral recumbency, the changes in respiratory values from those of sternal or dorsal recumbency were intermediate. Because collapse of the small airways in avian lungs is nearly impossible in vivo, these changes in volume and density may have been attributable to gravity and changes in hydrostatic pressure that altered the volume of pulmonary blood capillaries and increased interstitial fluid volume consequent to changes in body position. If the air capillaries do not change in volume enough to affect lung volume, then the total lung volume could hypothetically change between these positions because of changes in pulmonary vascular volume or by changes in the volumes of larger airways (eg, parabronchi, secondary bronchi, or primary bronchi). The radii of the larger airways would only need to change by a few percentage points to result in the 5.6% change in lung volume that was detected when body position was changed in our study. Any of these changes could hypothetically have important effects on ventilation-perfusion mismatching within avian lungs and subsequently on gas exchange.26,30–33

Partial air-sac volume differed among the 3 body positions. This was most likely attributable to the differential distribution of viscera and fat tissue in each body position, which again was influenced by gravity. Air-sac volume was greatest when birds were positioned in sternal recumbency and smallest when they were positioned in dorsal recumbency. Our data indicated that air-sac volume in dorsal recumbency was 28.5% smaller than that in sternal positioning, which does not support the supposition that sternal recumbency restricts the sternum and impairs ventilation. The air sacs were largely compressed by the ventral viscera and fat tissue that were redistributed by gravity during dorsal recumbency. Additional research is needed to determine the physiologic effects, if any, of the various body positions on ventilation and gas exchange in red-tailed hawks.

We measured only partial air-sac volume that included the cranial thoracic, caudal thoracic, and abdominal air sacs. The decision to use this approach was a pragmatic one to simplify the calculations because the clavicular air sac is variable in conformation and difficult to delineate accurately by use of CT. We did not believe that this approach unduly biased our interpretation of the data because the caudal air sacs are the main ventilatory reservoir during the respiratory cycle, whereas the clavicular air sac functions as a reservoir for gas after it leaves the lungs.7,11,18,27

Variance associated with the air-sac volume measurements was high among red-tailed hawks for all 3 body positions. This variation was most likely attributable to sizable differences in body weight among the 6 birds. In 1 bird, the integrated air-sac volume was low and nearly obliterated by fat tissue surrounding viscera (No. 3; Figure 3). The CT images revealed that the birds generally had large quantities of fatty tissue within the coelom, which was difficult to assess during physical examinations conducted prior to the study. Elimination of data for the aforementioned bird from the analyses did not significantly change the variability in our findings; therefore, these data were retained. In such birds, compromise of air-sac volumes might result in the birds having to generate greater pressure excursions when positioned in lateral or dorsal recumbency versus sternal recumbency.

Computed tomography is the most accurate and reproducible imaging technique available for in vivo measurement of organ volume in humans.23,24,34–37 The development of helical CT technology has resulted in faster acquisition of images and higher-quality scans. Shorter scanning times reduce the respiratory motion artifact. Accuracy of volumetric estimations is considered to be > 90% based on results of other studies24,34,38–40 in which estimates of known volumes were compared with volumetric measurements by use of the same technique against a known phantom volume. In our study, birds breathed spontaneously, so the phase of respiration during acquisition could have increased the measurement error. Measurements of air-sac (and lung) volume could have been biased by 2 factors with respect to timing of CT image acquisition: entrainment of image acquisition with ventilatory cycle or a marked difference in duty cycle among positions. The former could have influenced the results by systematically measuring volume during the same phase of inflation or deflation during the cycle. The latter could have influenced the results by acquiring a greater portion of the image signal during inspiration or expiration in one position versus the others if differences in duty cycle existed among positions.

The respiratory rates of the birds ranged from 25 to 37 breaths/min, with a mean of 28 breaths/min during CT scanning. The mean number of images per scan for each position was 54, and acquisition time for each image was 1 second. Therefore, the typical CT scan took 54 seconds to complete and included data from all inspirations and expirations during that period. In only 1 of the 36 scans included in the study was a bird's respiratory rate 30 breaths/min, which is a harmonic of the image acquisition rate of the CT scanner, so entrainment of the 2 frequencies did not take place. In that bird, acquisition of areas of ROIs measured in consecutive slices should have been at different phases of the respiratory cycle, shifting phase by a mean of 3.3% with each slice. Final calculations of areas of ROIs would have averaged out any effect of respiratory cycle.

If ventilatory duty cycle differed significantly among positions, then a greater number of CT slices would have been acquired while lungs were inflated versus deflated, depending on the probability of acquiring an image during inspiration or expiration. However, by measuring ventilatory flows with a pneumotachograph, our research group has found that duty cycle does not differ with respect to body position in red-tailed hawks anesthetized by use of an anesthetic protocol similar to the one in the present study. We do not have any data regarding duty cycle when birds are positioned in sternal recumbency, but respiratory rate in sternal recumbency in the study reported here did not differ from that of the other body positions.

Results of the present study indicated that small but potentially physiologically important changes in avian lung and air-sac volumes and lung density took place when body position was changed in anesthetized red-tailed hawks. Anesthetic recommendations for positioning birds in dorsal or lateral rather than sternal recumbency may need to be reconsidered because these recommendations were based on the assumption that ventilation is compromised when birds are positioned in sternal recumbency. However, it is possible that species of birds with anatomic conformations other than the hawks in the present study may have a different response to changes in body position. Correlations between CT measurements and clinical and physiologic features have been established for humans but not for birds.41 Additional investigation of the physiologic importance of the reported findings is needed to determine whether the morphologic changes are important to gas exchange and oxygen transport in red-tailed hawks.

ABBREVIATIONS

CT

Computed tomography

CTU

Computed tomography unit

ROI

Region of interest

a.

IsoFlo, Abbott Laboratories, North Chicago, Ill.

b.

Telethermometer YSI-2100, Yellow Springs Instruments, Yellow Springs, Ohio.

c.

Ultrasonic Doppler Flow Detector, Park Medical Electronics Inc, Aloha, Ore.

d.

General Electric Medical Systems, Milwaukee, Wis.

e.

Advantage Window Workstation 4.27, General Electric Medical Systems, Milwaukee, Wis.

f.

Voxtool 5.4.46, General Electric Medical Systems, Milwaukee, Wis.

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Contributor Notes

Dr. Malka's present address is Avian and Exotic Pet Service, VCA-Sacramento Animal Medical Group, 4990 Manzanita Ave, Carmichael, CA 95618.

Supported by a Resident's Grant from the William R. Pritchard Veterinary Medical Teaching Hospital, School of Veterinary Medicine, University of California.

The authors thank Richard Larson, Jason Peters, and Patrick Nicholas for technical support.

Address correspondence to Dr. Hawkins.