• View in gallery

    Representative transverse, lung window, sharp-algorithm CT image of a healthy cat that underwent anesthesia, 4 ventilatory protocols, and CT. Notice the typical appearance of atelectasis in the left caudal lung lobe (arrow).

  • View in gallery

    Representative CT images of a healthy cat that underwent anesthesia, 4 ventilatory protocols, and CT. The images depict 3-D segmentation and volume calculation tools that were used to determine lung volume in CT images. A—Three-dimensional image of the segmented lungs, with the calculated volume (170.8 cm3) displayed at the bottom left. B and C—Respective transverse and dorsal planar images indicating the regions of lung that were included in the volume calculation (shaded in purple). Notice that soft tissues, bones, and the trachea were excluded.

  • View in gallery

    Representative CT image of a healthy cat that underwent anesthesia, 4 ventilatory protocols, and CT. The image depicts an ROI that included all lung lobes. The ROI was generated by use of 3-D region grow and ROI export tools. Primary and secondary blood vessels, the trachea, and mainstem bronchi were excluded from the ROI.

  • View in gallery

    Mean ± SD number of lung lobes with ≥ 1 area of atelectasis (A), lung volume (B), and lung density (C) in 7 healthy cats during 4 ventilatory protocols on the basis of analysis of CT images. Ventilatory protocols included the following: ventilation immediately preceding acquisition of CT images via manual hyperventilation for 20 seconds with 10 breaths delivered at a PIP of 15 cm H2O, an inspiratory-to-expiratory ratio of 1:2, ZEEP, and acquisition of CT images during apnea (ventilatory protocol 1); ventilation with 1 deep breath by manual compression of the reservoir bag to achieve a PIP of 15 cm H2O, which was maintained during acquisition of CT images (ventilatory protocol 2); performance of a manual recruitment maneuver that consisted of sequential delivery of 2 breaths each at PIPs of 15, 20, 25, and 30 cm H2O, then 25, 20, and 15 cm H2O, with an inspiratory-to-expiratory ratio of 1:2, ZEEP, and acquisition of CT images during apnea (ventilatory protocol 3); and manual hyperventilation for 20 seconds with 10 breaths delivered at a PIP of 20 cm H2O, an inspiratory-to-expiratory ratio of 1:2, and a PEEP of 5 cm H2O, which was maintained during acquisition of CT images (ventilatory protocol 4). Ventilatory protocols were performed in random order in the cats. *Value differs significantly (P < 0.05) from the value obtained for ventilatory protocol 1. †Value differs significantly (P < 0.05) from the value obtained for ventilatory protocol 2. ‡Value differs significantly (P < 0.05) from the value obtained for ventilatory protocol 3.

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Comparison of four ventilatory protocols for computed tomography of the thorax in healthy cats

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  • 1 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061.
  • | 2 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061.
  • | 3 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061.
  • | 4 Department of Large Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061.
  • | 5 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061.

Abstract

Objective—To identify ventilatory protocols that yielded good image quality for thoracic CT and hemodynamic stability in cats.

Animals—7 healthy cats.

Procedures—Cats were anesthetized and ventilated via 4 randomized protocols (hyperventilation, 20 seconds [protocol 1]; single deep inspiration, positive inspiratory pressure of 15 cm H2O [protocol 2]; recruitment maneuver [protocol 3]; and hyperventilation, 20 seconds with a positive end-expiratory pressure of 5 cm H2O [protocol 4]). Thoracic CT was performed for each protocol; images were acquired during apnea for protocols 1 and 3 and during positive airway pressure for protocols 2 and 4. Heart rate; systolic, mean, and diastolic arterial blood pressures; blood gas values; end-tidal isoflurane concentration; rectal temperature; and measures of atelectasis, total lung volume (TLV), and lung density were determined before and after each protocol.

Results—None of the protocols eliminated atelectasis; the number of lung lobes with atelectasis was significantly greater during protocol 1 than during the other protocols. Lung density and TLV differed significantly among protocols, except between protocols 1 and 3. Protocol 2 TLV exceeded reference values. Arterial blood pressure after each protocol was lower than before the protocols. Mean and diastolic arterial blood pressure were higher after protocol 3 and diastolic arterial blood pressure was higher after protocol 4 than after protocol 2.

Conclusions and Clinical Relevance—Standardization of ventilatory protocols may minimize effects on thoracic CT images and hemodynamic variables. Although atelectasis was still present, ventilatory protocols 3 and 4 provided the best compromise between image quality and hemodynamic stability.

Abstract

Objective—To identify ventilatory protocols that yielded good image quality for thoracic CT and hemodynamic stability in cats.

Animals—7 healthy cats.

Procedures—Cats were anesthetized and ventilated via 4 randomized protocols (hyperventilation, 20 seconds [protocol 1]; single deep inspiration, positive inspiratory pressure of 15 cm H2O [protocol 2]; recruitment maneuver [protocol 3]; and hyperventilation, 20 seconds with a positive end-expiratory pressure of 5 cm H2O [protocol 4]). Thoracic CT was performed for each protocol; images were acquired during apnea for protocols 1 and 3 and during positive airway pressure for protocols 2 and 4. Heart rate; systolic, mean, and diastolic arterial blood pressures; blood gas values; end-tidal isoflurane concentration; rectal temperature; and measures of atelectasis, total lung volume (TLV), and lung density were determined before and after each protocol.

Results—None of the protocols eliminated atelectasis; the number of lung lobes with atelectasis was significantly greater during protocol 1 than during the other protocols. Lung density and TLV differed significantly among protocols, except between protocols 1 and 3. Protocol 2 TLV exceeded reference values. Arterial blood pressure after each protocol was lower than before the protocols. Mean and diastolic arterial blood pressure were higher after protocol 3 and diastolic arterial blood pressure was higher after protocol 4 than after protocol 2.

Conclusions and Clinical Relevance—Standardization of ventilatory protocols may minimize effects on thoracic CT images and hemodynamic variables. Although atelectasis was still present, ventilatory protocols 3 and 4 provided the best compromise between image quality and hemodynamic stability.

Computed tomography is a noninvasive imaging modality that can be rapidly performed and is a sensitive method for detection of diseases of the thorax in companion animals.1–5 Computed tomography is more sensitive than radiography for determination of the location and extent of lung diseases such as asthma, neoplasia, and pneumonia in cats.6–10 This information can aid veterinarians in determination of the severity of disease, development of a treatment plan, and monitoring of the responses of animals to treatments. Image-analysis software can be used to determine the values of pulmonary indices such as total lung volume and total mean lung density from thoracic CT images. In addition, CT image-analysis software can be used to generate 3-D images of thoracic vascular structures, airways, and nodules.11–15

Artifacts attributable to patient motion and pulmonary atelectasis often limit the diagnostic utility of CT images of the thorax for detection of diseases in dogs16 and humans.17 These artifacts can falsely increase the density of a CT image in areas of normal lung tissue and can decrease CT image resolution in areas of pathological change in the thorax. In humans, these problems have been addressed by giving patients specific instructions for breathing during CT, such as maintenance of full inspiration, full expiration, or a normal breathing pattern. In domestic animals, control of ventilation during CT requires general anesthesia. Unfortunately, atelectasis can develop during anesthesia because of compression of lung tissue, closure of airways, and absorption of alveolar gas.18,19 Common ventilatory strategies are designed to prevent this problem by recruitment of alveoli in anesthetized humans undergoing CT; these strategies include the use of PEEP or a single deep inspiration with a high PIP (> 20 cm H2O) for a brief (5- to 30-second) period.20

In animals, the PIP that minimizes atelectasis during anesthesia for CT of the thorax is unknown. The use of an excessive PIP in an anesthetized animal may cause barotrauma21 and adverse cardiovascular effects, such as decreased venous return and cardiac output and consequently decreased arterial blood pressure.20 Conversely, the use of an insufficient PIP may cause a decrease in the diagnostic quality of CT images because of the development of atelectasis. Other reports1–3 have provided a description of a variety of ventilatory protocols that minimize image artifacts during CT of animals. However, to the authors' knowledge, no controlled studies have been conducted to compare the effects of various ventilatory protocols on CT image quality and values of hemodynamic variables in cats.

We hypothesized that, in healthy anesthetized cats, the use of positive-pressure ventilation techniques could transiently eliminate anesthesia-induced atelectasis artifacts for CT imaging. In addition, we hypothesized that values of lung volume and density determined from CT images and values of hemodynamic variables in cats would be significantly different among various ventilatory protocols. The objective of the study was to compare CT findings and values of hemodynamic variables in healthy anesthetized cats ventilated via 4 protocols.

Materials and Methods

Animals—Seven healthy purpose-bred cats with a mean ± SD body weight of 4.3 ± 1.6 kg were included in the study. The cats were considered healthy on the basis of results of a physical examination, CBC, and assays to determine their FeLV and FIV status. Cats were housed together. Food was withheld from the cats for 12 hours before the study began. The cats had access to water ad libitum. The study was approved by the Institutional Laboratory Animal Care and Use Committee of Virginia Tech.

Study design—The cats were anesthetized and instrumented for determination of hemodynamic data. Computed tomographic images of each cat were acquired with a 16-slice CT scannera; the same CT scanner and scanning procedure were used for each cat. For each cat, CT was performed 4 times with each of 4 ventilatory protocols during the same anesthetic episode. The sequence in which the ventilatory protocols were performed was randomized for each cat via a randomized block design. The cats were allowed to breathe spontaneously for 15 minutes after each ventilatory protocol (ie, before performance of the next ventilatory protocol). Hemodynamic data were collected prior to performance of each ventilatory protocol and after completion of each CT. After the 4 ventilatory protocols were completed, the cats were allowed to recover from anesthesia in a routine manner. The CT images and associated data were archived for analysis at a later time.

Anesthesia—On the day of the study, acepromazine maleateb (0.1 mg/kg), butorphanol tartratec (0.4 mg/kg), dexmedetomidined (0.01 mg/kg), and ketamine hydrochloridee (5 mg/kg) were administered IM to the cats. Twenty minutes later, a 22-gauge catheter was aseptically placed in a cephalic vein of each cat. Prior to intubation, 0.2 mL of lidocainef was topically applied to the larynx of each cat to prevent laryngospasm. If additional drugs were needed to accomplish intubation, propofolg (3 mg/kg, IV) was administered to effect. After placement of an endotracheal tube, anesthesia was maintained with isofluraneh in oxygen by means of a Bain nonrebreathing system. Atipemazolei (0.05 mg/kg, IM) was administered to each cat immediately after intubation.

Measurement of hemodynamic variables—The time from placement of instrumentation for measurement of hemodynamic variables to the start of the ventilatory protocols was standardized at 60 minutes for each cat (ie, ventilatory protocols were not started until 60 minutes after investigators started instrumentation). All cats were positioned in sternal recumbency. For each cat, a 24-gauge catheter was placed in a dorsal pedal artery and connected to a transducer, which was zeroed at the height of the right atrium. Transducers were connected to a patient monitorj that had been calibrated, which was used to continuously record SAP, MAP, and DAP. Arterial blood samples (1 mL) were obtained and analyzed to determine values of blood gas variables (pH, Pao2, hemoglobin oxygen saturation, Paco2, HCO3 concentration, and base excess) for each cat. Lead II ECGs were continuously recordedj and used to determine heart rates. End-tidal isoflurane concentrations were determined by use of a gas analyzer.j For each cat, end-tidal gas samples were continuously aspirated from an 8F polypropylene catheter that had been inserted via a side port into the endotracheal tube, with the tip of the catheter positioned at the distal end of the endotracheal tube. Rectal temperatures were continuously monitoredj and maintained between 36.6° and 38.3°C with a forced-air patient warmerk placed over the cats when it would not interfere with study procedures. An airway manometerl that had been calibrated was connected to a plastic elbow adaptor and placed between the endotracheal tube and the breathing system. The manometer was used to measure PIP and end-expiratory pressure during each ventilatory protocol.

Ventilatory protocols—During ventilatory protocol 1, the cats were manually hyperventilated for 20 seconds immediately before performance of CT; 10 breaths were delivered at a PIP of 15 cm H2O with an inspiratory-to-expiratory ratio of 1:2 and ZEEP. The CT images were acquired while the cats were apneic. During ventilatory protocol 2, the cats received 1 deep breath by manual compression of the reservoir bag at a PIP of 15 cm H2O; this inspiratory pressure was maintained during acquisition of CT images. During ventilatory protocol 3, a manual recruitment maneuver was performed on the cats before performance of CT. During the recruitment maneuver, 2 breaths were delivered sequentially at PIPs of 15, 20, 25, and 30 cm H2O, then at PIPs of 25, 20, and 15 cm H2O, which were delivered with an inspiratory-to-expiratory ratio of 1:2 and ZEEP. The CT images were acquired while the cats were apneic. During ventilatory protocol 4, the cats were manually hyperventilated for 20 seconds; 10 breaths were delivered at a PIP of 20 cm H2O, an inspiratory-to-expiratory ratio of 1:2, and a PEEP of 5 cm H2O; this PEEP was maintained during acquisition of CT images.

CT procedure—Cats were positioned in sternal recumbency with their heads entering the CT gantry first. An air-filled tuberculin syringe was placed next to the thorax of each cat and included in each CT image as a calibration phantom. The CT scanner settings were as follows: sharp algorithm; collimation, 2 mm; standard volume, 1.0 mm3; helical mode; 120 kVp; detection diameter, 240 mm; exposure time, 500 milliseconds; 300 mA; focal spot, 0.9 mm; CT dose index, 37.7; 512 × 512 matrix; and spacing, 0.348 pixels. The total CT scan time during each ventilatory protocol was 11 seconds.

Measurement of hemodynamic data—Hemodynamic data were obtained for each cat at 2 times (immediately prior to initiation of each ventilatory protocol and after completion of each CT scan) for each ventilatory protocol. Values of heart rate, SAP, MAP, DAP, rectal temperature, end-tidal isoflurane concentration, end-tidal CO2 concentration, and arterial blood gas variables (pH, Pao2, Paco2, HCO3 concentration, and base excess) were recorded.

Detection of lung atelectasis—Two board-certified radiologists (JCJ and GBD) who were unaware of the ventilatory protocol evaluated each CT image in random order and developed a consensus opinion of the number of locations of atelectasis in each lung lobe. Atelectasis was defined as a focal area of increased soft tissue opacity within the pulmonary parenchyma. For atelectasis to be identified in a given location, the observers had to agree and have high diagnostic certainty. Observers primarily evaluated sharp algorithm 2-mm transverse images, which were displayed in a lung window. Atelectasis was confirmed on the basis of evaluation of 1-mm3 multiplanar reformatted images (Figure 1). For each lung lobe on each CT image, observers assigned a score of 0 (no atelectasis) or ≥ 1 (atelectasis; the number indicated the number of locations of atelectasis within a lung lobe). Observers also recorded their subjective assessments of the appearance of the areas of atelectasis.

Determination of lung volume—An observer (JCJ) who was unaware of the ventilatory protocol for each image performed all measurements of lung volume. The volume of aerated lung tissue was calculated for each CT image with the CT scanner's image-analysis workstation and software tools for segmentation and volume measurement. Digital data for each CT scan were loaded into the workstation's 3-D software. A setting for a solid lung display was selected, and the default threshold density range (−1,280 to −110 HU) was used. Other air-filled structures (eg, trachea, mainstem bronchi, esophagus, and stomach) were removed from the image by use of a segmentation tool. The margin of the remaining 3-D lung display was selected with an electronic cursor, and a volume measurement tool was used to calculate the lung volume, which was expressed in cubic centimeters (Figure 2).

Figure 1—
Figure 1—

Representative transverse, lung window, sharp-algorithm CT image of a healthy cat that underwent anesthesia, 4 ventilatory protocols, and CT. Notice the typical appearance of atelectasis in the left caudal lung lobe (arrow).

Citation: American Journal of Veterinary Research 73, 5; 10.2460/ajvr.73.5.646

Figure 2—
Figure 2—

Representative CT images of a healthy cat that underwent anesthesia, 4 ventilatory protocols, and CT. The images depict 3-D segmentation and volume calculation tools that were used to determine lung volume in CT images. A—Three-dimensional image of the segmented lungs, with the calculated volume (170.8 cm3) displayed at the bottom left. B and C—Respective transverse and dorsal planar images indicating the regions of lung that were included in the volume calculation (shaded in purple). Notice that soft tissues, bones, and the trachea were excluded.

Citation: American Journal of Veterinary Research 73, 5; 10.2460/ajvr.73.5.646

Determination of lung density—An observer (JCJ) who was unaware of the ventilatory protocol for each image performed all measurements of lung density. Digital data for each CT scan were transferred to a personal computer and imported into image analysis software.m An ROI that included all lung lobes in each image was drawn by use of 3-D region grow and ROI export tools. Primary and secondary blood vessels, the trachea, and mainstem bronchi were excluded from ROIs prior to export (Figure 3). The ROIs for the air-filled tuberculin syringe phantoms were generated with an oval ROI tool. The oval ROI was copied and pasted into each image and then propagated over at least 10 CT image slices with a maximum-intensity thick slab tool. The CT density values for lungs and phantoms were calculated as the mean of the ROI values for all CT image slices by means of spreadsheet software.n

Figure 3—
Figure 3—

Representative CT image of a healthy cat that underwent anesthesia, 4 ventilatory protocols, and CT. The image depicts an ROI that included all lung lobes. The ROI was generated by use of 3-D region grow and ROI export tools. Primary and secondary blood vessels, the trachea, and mainstem bronchi were excluded from the ROI.

Citation: American Journal of Veterinary Research 73, 5; 10.2460/ajvr.73.5.646

Statistical analysis—Statistical tests were selected and performed by a statistician. The number of locations of atelectasis in each lung lobe was compared for each ventilatory protocol by use of a Friedman test. The numbers of lung lobes that had atelectasis were compared for each ventilatory protocol via a linear generalized estimating equations analysis. Lung densities were compared for each ventilatory protocol by use of a mixed-model ANCOVA. Lung volumes; values of hemodynamic, respiratory, and blood gas variables; temperatures; and end-tidal isoflurane concentrations were compared by use of a mixed-model ANOVA. All analyses were performed with statistical software.o Values of P < 0.05 were considered significant.

Results

None of the ventilatory protocols eliminated atelectasis in the cats. However, the least squares mean ± SE number of lobes with atelectasis in each cat during ventilatory protocol 1 (2.42 ± 0.4) was significantly (P < 0.003) higher than it was during the other ventilatory protocols (Figure 4). No significant differences were found in the number of locations of atelectasis in each lung lobe among the ventilatory protocols. Mean ± SD lung volume calculated from CT images was 154.4 ± 53.8 mL, 321.0 ± 87.5 mL, 161.6 ± 41.6 mL, and 247.2 ± 81.6 mL for protocols 1, 2, 3, and 4, respectively. These values differed significantly (P < 0.002) for comparisons among all ventilatory protocols except for comparisons between ventilatory protocols 1 and 3. Mean ± SD lung density calculated from CT images was −754.42 ± 32.81 HUs, −879.37 ± 18.74 HUs, −762.66 ± 35.41 HUs, and −835.02 ± 26.41 HUs for protocols 1,2, 3, and 4, respectively. These values differed significantly (P < 0.018) for comparisons among all ventilatory protocols except for comparisons between ventilatory protocols 1 and 3.

Figure 4—
Figure 4—

Mean ± SD number of lung lobes with ≥ 1 area of atelectasis (A), lung volume (B), and lung density (C) in 7 healthy cats during 4 ventilatory protocols on the basis of analysis of CT images. Ventilatory protocols included the following: ventilation immediately preceding acquisition of CT images via manual hyperventilation for 20 seconds with 10 breaths delivered at a PIP of 15 cm H2O, an inspiratory-to-expiratory ratio of 1:2, ZEEP, and acquisition of CT images during apnea (ventilatory protocol 1); ventilation with 1 deep breath by manual compression of the reservoir bag to achieve a PIP of 15 cm H2O, which was maintained during acquisition of CT images (ventilatory protocol 2); performance of a manual recruitment maneuver that consisted of sequential delivery of 2 breaths each at PIPs of 15, 20, 25, and 30 cm H2O, then 25, 20, and 15 cm H2O, with an inspiratory-to-expiratory ratio of 1:2, ZEEP, and acquisition of CT images during apnea (ventilatory protocol 3); and manual hyperventilation for 20 seconds with 10 breaths delivered at a PIP of 20 cm H2O, an inspiratory-to-expiratory ratio of 1:2, and a PEEP of 5 cm H2O, which was maintained during acquisition of CT images (ventilatory protocol 4). Ventilatory protocols were performed in random order in the cats. *Value differs significantly (P < 0.05) from the value obtained for ventilatory protocol 1. †Value differs significantly (P < 0.05) from the value obtained for ventilatory protocol 2. ‡Value differs significantly (P < 0.05) from the value obtained for ventilatory protocol 3.

Citation: American Journal of Veterinary Research 73, 5; 10.2460/ajvr.73.5.646

Values of SAP, MAP, and DAP in the cats were significantly (P < 0.020) lower immediately after completion of CT scans, compared with values immediately prior to performance of each ventilatory protocol (Table 1). Comparison of values of hemodynamic variables obtained immediately after completion of CT scans revealed that ventilatory protocol 2 resulted in significantly (P < 0.030) lower MAP and DAP than did protocol 3 and resulted in significantly (P = 0.040) lower DAP than did protocol 4. The difference in values between SAP, MAP, and DAP obtained immediately prior to performance of the ventilatory protocols and those values obtained immediately after completion of CT scans was not significant for any of the ventilatory protocols. Ventilatory protocol 3 was associated with higher pH and lower Paco2 values immediately prior to performance of the protocol than immediately after completion of the CT scan. None of the ventilatory protocols caused hypercapnea or hypocapnea in the cats. No significant differences were detected for comparisons among values of other variables.

Table 1—

Mean ± SD SAP, MAP, and DAP and the difference between values (value of a variable obtained after performance of a ventilatory protocol minus the value of a variable obtained before performance of a ventilatory protocol) for 4 ventilatory protocols used during CT examination of the thoraxes of 7 healthy cats.

ProtocolVariableBeforeAfterDifference
1SAP89.8 ± 13.079.4 ± 13.3*−10.4
 MAP72.7 ± 9.862.8 ± 8.7*−9.8
 DAP63.1 ± 10.054.4 ± 8.0*−8.7
2SAP93.7 ± 15.373.5 ± 14.7*−20.2
 MAP76.2 ± 12.655.0 ± 11.6*−21.2
 DAP64.8 ± 13.746.5 ± 11.7*−18.2
3SAP98.1 ± 16.085.2 ± 13.9*−12.9
 MAP82.2 ± 11.268.7 ± 11.8*−13.5
 DAP72.4 ± 11.659.5 ± 12.0*−12.8
4SAP94.7 ± 17.180.5 ± 18.5*−14.2
 MAP77.8 ± 14.966.5 ± 13.3*−11.2
 DAP68.0 ± 14.758.2 ± 12.6*−9.7

Values reported are in mm Hg. Ventilatory protocols included the following: ventilation immediately preceding acquisition of CT images via manual hyperventilation for 20 seconds with 10 breaths delivered at a PIP of 15 cm H2O, an inspiratory-to-expiratory ratio of 1:2, ZEEP, and acquisition of CT images during apnea (ventilatory protocol 1); ventilation with 1 deep breath by manual squeezing of the reservoir bag to achieve a PIP of 15 cm H2O, which was maintained during acquisition of CT images (ventilatory protocol 2); performance of a manual recruitment maneuver that consisted of sequential delivery of 2 breaths each at PIPs of 15, 20,25, and 30 cm H2O, then 25, 20, and 15 cm H2O, with an inspiratory-to-expiratory ratio of 1:2, ZEEP, and acquisition of CT images during apnea (ventilatory protocol 3); and manual hyperventilation for 20 seconds with 10 breaths delivered at a PIP of 20 cm H2O, an inspiratory-to-expiratory ratio of 1:2, and a PEEP of 5 cm H2O, which was maintained during acquisition of CT images (ventilatory protocol 4). Ventilatory protocols were performed in random order in the cats.

Within a protocol, value differs significantly (P < 0.05) from the value obtained immediately before performance of the ventilatory protocol.

For values obtained immediately after performance of a ventilatory protocol, value differs significantly (P < 0.05) from the value obtained immediately after performance of ventilatory protocol 2.

Discussion

In the study reported here, an attempt was made to minimize the effects of confounding factors by inclusion of only healthy cats and by use of the same anesthesia, instrumentation, and CT scanning procedures in all cats. Propofol was administered to 3 of the cats to allow completion of anesthetic induction and intubation. Dexmedetomidine was reversed (by administration of atipamezole) in the cats after they were intubated to prevent the drug from having an effect on values of the measured variables. The time between induction of anesthesia and the start of data collection was standardized at 60 minutes to allow investigators time to perform instrumentation of all cats and to minimize effects of anesthetic drugs on measured variables. If investigators completed instrumentation of a cat in < 60 minutes, the ventilatory protocols and CT scans were not started until the 60 minutes had elapsed. This period also allowed enough time for the effects of propofol to dissipate in the 3 cats to which it was administered.22 The end-tidal isoflurane concentration was maintained at approximately 0.89% in all cats in the present study to prevent movement and to minimize the cardiovascular effects of the inhalation anesthetic. The depressant effects of isoflurane are dose dependent, and the dose that was used in the present study was below the minimum alveolar concentration described for cats.23

Other authors have investigated the use of hyperventilation to induce apnea as a standard ventilatory protocol before performance of CT scans in dogs7 and calves.24 Therefore, a ventilatory protocol that included hyperventilation with a PIP of 15 cm H2O (ventilatory protocol 1) was used as a control protocol in the present study. This protocol caused apnea in the cats and minimized movement artifacts on CT images; these artifacts would likely have been present on CT images if a ventilatory protocol that included spontaneous ventilation had been used. Results of the present study indicated that there were more lung lobes that had atelectasis in cats during ventilatory protocol 1, compared with results for the other ventilatory protocols. This finding could have been related to the PIP that was used during protocol 1, which seemed to be adequate to ventilate most lung airways but was probably not high enough to reopen areas of lung that were already atelectatic. To the authors' knowledge, there have been no studies in which the inspiratory pressure needed to reopen atelectatic areas in lungs of cats has been investigated. Authors of another report25 determined that a mean PIP of 14 cm H2O is required to open the smallest airways in excised, unperfused dog lungs. Distal airway pressure is lower than proximal airway pressure because of airway resistance to airflow26; therefore, it would be reasonable to assume that airway pressures > 14 cm H2O are necessary to reopen collapsed airways. It is unknown whether this airway pressure value could be extrapolated to use in clinical situations because other factors, such as surfactant function, may be altered during anesthesia.27

Similar to findings for ventilatory protocol 1, atelectasis was detected in CT images acquired during ventilatory protocols 2, 3, and 4 in the present study. Thus, we were unsuccessful in achieving our objective of identifying a ventilatory protocol that could eliminate atelectasis in anesthetized cats undergoing CT of the thorax. Factors that can be correlated with development of atelectasis include compression of lung tissue (compression atelectasis), absorption of alveolar gas (absorption atelectasis), and loss of surfactant.20 Compression atelectasis is caused by relaxation of respiratory muscles and cranial displacement of the diaphragm, which causes compression of lung tissue.20 To minimize the potential for development of atelectasis in a dependent lung, all cats in the present study were kept in sternal recumbency from induction of anesthesia through completion of CT. Absorption atelectasis is caused by airway occlusion or a high fraction of inspired oxygen and uptake of a large volume of gas from alveoli into the blood, both of which lead to collapse of alveoli and development of atelectasis.20,28,29 Cats in the present study received 100% inspired oxygen during ventilation so that the protocols were similar to a common clinical scenario in which cats receive a high fraction of inspired oxygen and develop atelectasis. Once atelectasis has developed, surfactant function may decrease, which causes recurrence of alveolar collapse. Recurrence of atelectasis has been reported to develop as soon as 5 minutes after performance of a reexpansion maneuver.30 Therefore, the time between each protocol was standardized at 15 minutes, and the sequence in which the protocols were performed was randomized. This was intended to decrease effects of other protocols and duration of anesthesia on measures of lung inflation and values of hemodynamic variables in the present study.

Cats in the present study had significantly higher lung density (ie, lower overall aeration of lung tissue) during ventilatory protocols 1 and 3 than during ventilatory protocols 2 and 4. During protocol 2, CT images were acquired at the end of inspiration, which caused higher lung volume than was detected during apnea in ventilatory protocols 1 and 3. Values of lung density determined from CT images acquired during protocol 4 were significantly higher than those determined from images acquired during protocols 1 and 3. This was most likely because PEEP was used during ventilatory protocol 4; use of PEEP allows small airways to remain open during expiration and increases functional residual capacity.26 Although evaluation of CT images acquired during ventilatory protocol 1 revealed a significantly higher number of lung lobes that had atelectasis than was found for the other protocols, the overall lung density and lung volume were similar to those for protocol 3 (recruitment maneuver and ZEEP). This similarity was possibly attributable to a balance between the number of atelectatic areas and the size of those areas in lung lobes. This finding suggested that the overall degree of atelectasis was similar for protocols 1 and 3 and use of these protocols did not severely affect overall aeration of lung tissue in cats in the present study. Another indication that lung aeration was adequate in these cats was the Pao2 value; for all ventilatory protocols, the Pao2 value was > 395 mm Hg at every time it was measured.

The use of a single deep inspiratory breath (ventilatory protocol 2) resulted in the highest mean ± SD lung volume (321.0 ± 87.5 mL), compared with results for the other ventilatory protocols. This result was expected because lung inflation was maintained during performance of the CT scan. That lung volume value is higher than the total lung volume value (287.3 ± 34.6 mL) reported in another study of cats.31 Therefore, caution should be used when ventilating cats that have pulmonary disease because their inspiratory and expiratory reserve volumes may be reduced and their lungs can be easily overdistended.

An increase in intrathoracic pressure causes a reduction in central vascular blood volume by displacing blood to the periphery of the vasculature, which reduces venous return of blood to the heart. An increase in intrathoracic pressure also causes decreased ventricular transmural end-diastolic pressure, ventricular end-diastolic volume, and, ultimately, stroke volume, stroke work, and arterial blood pressure.32 In the present study, SAP, MAP, and DAP decreased significantly after all ventilatory protocols, compared with values obtained before they were performed. Measurements obtained immediately after completion of CT indicated that ventilatory protocol 2 (use of a single deep inspiration) resulted in significantly lower MAP and DAP than did ventilatory protocol 3 (use of a recruitment maneuver). Ventilatory protocol 2 caused a significantly lower DAP than did ventilatory protocol 4 (use of hyperventilation and PEEP). The PIP that was used in ventilatory protocol 2 was 15 cm H2O, which was considerably lower than the PIP (40 cm H2O) used in humans undergoing a single deep inspiration protocol.20 However, a PIP of 15 cm H2O was sustained for the duration of the CT scan (11 seconds) in the present study, and the next hemodynamic measurements were obtained as soon as this airway pressure was released. Therefore, compared with the other ventilatory protocols, there may not have been enough time for MAP and DAP to return to their baseline values immediately after ventilatory protocol 2. Depending on the hemodynamic stability of a patient, a transient decrease in MAP could be adequately compensated for in a clinical setting. However, patients that have clinical signs of hypovolemia are hemodynamically unstable and are at a high risk for cardiovascular collapse during an extended insufflation ventilatory maneuver. The SAP, MAP, and DAP values obtained immediately prior to performance of the ventilatory protocols in the present study were compared to detect carryover effects from the other ventilatory protocols previously performed. No significant differences were found among the values of these variables regardless of the anesthetic time at which they were measured, which confirmed that cardiovascular effects of ventilatory protocols were short lasting and did not influence measurements obtained during the subsequently performed protocols in the present study.

The present study had several limitations. The volume of atelectasis in the lungs of the cats was not evaluated because focal areas of atelectasis typically had ill-defined margins and the observers were concerned that volumes of these areas could not be accurately and consistently determined. Nonetheless, the observers' subjective assessments of CT images did not reveal substantial discrepancies in the size of focal atelectatic areas among lung lobes or ventilatory protocols. Considering that atelectasis appears similar to disease on CT images, which can influence the diagnosis that is made, a goal of the present study was to identify a ventilation protocol that eliminated anesthesia-induced atelectasis. Therefore, it would have been valuable to identify the ventilatory protocol that caused the least amount of atelectasis. Additionally, evaluation of hemodynamic variables did not include measurement of cardiac output. Because blood pressure is influenced by vascular tone, the effects of ventilatory protocols on cardiac output could not be directly evaluated. At the time this study was conducted, we did not have access to a monitor that could continuously measure cardiac output. A dynamic monitor capable of determining cardiac output for every heart contraction would have been needed to evaluate acute and short-duration changes in cardiac output. However, the values of arterial blood pressure that were obtained in the present study provided valuable information, and it was assumed that changes in arterial blood pressure reflected overall cardiovascular system performance in the cats. Finally, measurement of hemodynamic variables was performed at only 2 times (before each ventilatory protocol and after completion of the CT scans) in the cats. It would have been valuable to have measured hemodynamic variables during performance of ventilatory protocols because hemodynamic effects of the protocols were short lived and values may not have been obtained during the peak of cardiovascular depression. As the present study progressed, we observed that cats had severe hypotension during ventilatory protocols 2 and 3, but the degree of hypotension may not have been completely detected. Although the healthy cats that were used in the present study recovered quickly from cardiovascular depression, hemodynamically unstable patients may not have responded the same way while exposed to these ventilatory protocols.

None of the ventilatory protocols used in the present study eliminated atelectasis in the cats. Thus, persistence of atelectasis may be unavoidable in cats exposed to conditions similar to those used in the present study. This possibility should be considered when CT images of cats that have clinical signs of disease are interpreted. The ventilatory protocols used in the present study also had significant effects on lung volume and density as determined from CT images. Thus, ventilatory protocols should be standardized in future studies in which CT of the thorax of cats is evaluated. Use of hyperventilation followed by apnea and ZEEP (ventilatory protocol 1) resulted in the greatest number of lung lobes with atelectasis. Therefore, that ventilatory protocol cannot be recommended for CT of the thorax of cats. Use of a single deep inspiration at a PIP of 15 cm H2O (ventilatory protocol 2) resulted in the lowest number of lung lobes with atelectasis, although the difference was only significant for comparisons with protocol 1. Ventilatory protocol 2 also resulted in more severe cardiovascular depression than was detected during protocols 3 and 4. However, cardiovascular depression was short lasting in the healthy cats that were included in the present study. Use of ventilatory protocol 2 also resulted in a lung volume that exceeded previously published reference values for lung volume in cats. Therefore, caution should be used when this protocol is used for hemodynamically unstable cats or for cats suspected to have restrictive lung disease. Use of a recruitment maneuver with ZEEP (ventilatory protocol 3) and use of hyperventilation with a PEEP of 5 cm H2O (ventilatory protocol 4) resulted in the best compromise between CT image quality and hemodynamic stability in the cats in the present study.

ABBREVIATIONS

DAP

Diastolic arterial blood pressure

HU

Hounsfield unit

MAP

Mean arterial blood pressure

PEEP

Positive end-expiratory pressure

PIP

Peak inspiratory pressure

ROI

Region of interest

SAP

Systolic arterial blood pressure

ZEEP

Zero positive end-expiratory pressure

a.

Aquilion 16, Toshiba America Inc, Irvine, Calif.

b.

Acepromazine maleate (1 mg/mL), Vedco Inc, St Joseph, Mo.

c.

Torbugesic (10 mg/mL), Fort Dodge Animal Health, Fort Dodge, Iowa.

d.

Dexdomitor (0.5 mg/mL), Pfizer Animal Health, Exton, Pa.

e.

Ketaset (100 mg/mL), Fort Dodge Animal Health, Fort Dodge, Iowa.

f.

Lidocaine HCl (20 mg/mL), Agrilabs, St Joseph, Mo.

g.

PropoFlo (10 mg/mL), Abbot Laboratories, North Chicago, Ill.

h.

Isoflo, Abbott Laboratories, North Chicago, Ill.

i.

Antisedan atipemazole HCl (5 mg/mL), Pfizer Animal Health, Exton, Pa.

j.

Cardiocap S5, Datex Ohmeda, Helsinki, Finland.

k.

Bair hugger, Arizant Inc, Eden Prairie, Minn.

l.

Anesthesia Associates Inc, San Marcos, Calif.

m.

OsiriX Imaging Software, version 3.3.2, 32 bit, OsiriX Foundation, Los Angeles, Calif.

n.

Microsoft Excel 2008 for Mac, version 12.1.0, Microsoft Corp, Redmond, Wash.

o.

SAS, version 9.2, SAS Institute Inc, Cary, NC.

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Contributor Notes

Dr. Jones' present address is Division of Animal and Nutritional Sciences, Davis College of Agriculture, Natural Resources and Design, West Virginia University, Morgantown, WV 26506.

Supported by the Veterinary Memorial Fund from the Virginia-Maryland Regional College of Veterinary Medicine, Virginia Veterinary Medical Association, and Maryland Veterinary Medical Association.

Presented in abstract form as an oral presentation at the 10th World Congress of Veterinary Anaesthesia, Glasgow, Scotland, September 2009.

Address correspondence to Dr. Henao-Guerrero (nguerrer@vt.edu).