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Technical Developments |
1 From the Children's Radiological Institute (F.R.L., M.J.H.) and the Department of Pulmonary Medicine (R.G.C., R.L.F., D.A.F., K.S.M.), Columbus Children's Hospital, 700 Children's Dr, Columbus, OH 43205-2696, and the Department of Radiology, Cincinnati Children's Medical Center, Ohio (A.S.B.). Received April 17, 1998; revision requested May 28; final revision received October 22; accepted January 6, 1999. Supported in part by grant R01HL-54062 from the National Heart, Lung, and Blood Institute. Address reprint requests to F.R.L. (e-mail: flong@chi.osu.edu).
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Abstract |
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Index terms: Children, respiratory system • Computed tomography (CT), in infants and children, 60.12118 • Lung, CT, 60.12118 • Lung, function
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Introduction |
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In infants and toddlers, who cannot cooperate with breath holding, thin-section CT has been limited due to motion artifacts (3). Ultrafast electron-beam CT scanners, which are capable of obtaining single sections of the chest in as little as 0.1 second, decrease problems related to patient cooperation (3,6). However, ultrafast scanners do not address the need to obtain images at nearly full lung inflation and deflation and are available at only a very limited number of centers.
At present, motion-free inspiratory and expiratory thin-section CT in infants and young children requires general anesthesia and intubation to provide the necessary control of respiration. Ideally, thin-section CT in these children should be performed less invasively. Four years ago in our infant pulmonary function laboratory, we developed a noninvasive method for measuring full maximum expiratory flow-volume curves in sedated infants and young children by inducing controlled pauses in spontaneous respiration with use of a positive-pressure ventilation device (7). The purpose of this study was to describe the application of this technique to thin-section CT of the lungs in infants and young children at full lung inflation and at resting end expiration and to compare thin-section CT images obtained during tidal breathing with those obtained during controlled respiratory pauses.
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Materials and Methods |
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Controlled-Ventilation Technique
The positive-pressure ventilation device (designed by R.G.C.) consists of a face mask attached to a pneumotachometer and bias-flow apparatus with an adjustable pressure pop-off valve (Fig 1). With this device, air flow, volume, mask pressure, and end-tidal carbon dioxide can be monitored. The face mask was placed over the nose and mouth (Fig 2) while fresh air was supplied continuously via the inspiratory port. Positive pressure was applied at the face mask by occluding the expiratory port at the downstream end with a thumb. The pressure applied was adjusted by changing the setting of the pressure pop-off valve. Respiratory pauses were induced by means of a step increase in ventilation combined with rapid lung inflation. An inflation pressure of 25 cm of water was given synchronously with spontaneous tidal inspiration by observing the child's breathing. Lung inflations were repeated approximately three to six times until a respiratory pause occurred. During lung inflations, gentle digital pressure was applied over the cricoid cartilage (the Sellick maneuver), to prevent air from passing down the esophagus into the stomach (8).
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Results |
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Findings for infant pulmonary function tests, bronchoscopy with bronchoalveolar lavage, sweat testing, immunoglobulin measurements, and chest radiographs were nondiagnostic. A thin-section CT examination was requested to help detect and localize suspected small airway or interstitial lung disease before open lung biopsy.
With the infant sedated and breathing normally, a helical CT scan of the chest and three thin-section CT images at three levels were obtained. Controlled-ventilation thin-section CT images were then acquired at six anatomic levels with inflation pressures of 25 cm of water and at three levels at pressures of 7 cm of water. The induced respiratory pauses lasted approximately 10 seconds for the scans obtained at full inflation and end exhalation.
The thin-section CT images obtained during tidal breathing (Fig 5a) demonstrated multiple areas of increased and decreased lung parenchymal opacification that in this patient could be interpreted as areas of inhomogeneous aeration secondary to diffuse airways disease. The controlled-ventilation images (Fig 5b) revealed normal homogeneous lung attenuation and no evidence of airway or parenchymal disease. On the basis of the controlled-ventilation study, the biopsy was postponed, and the child is being followed up with a diagnosis of asthma.
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Infant pulmonary function tests revealed severe obstruction with moderate air trapping. An open lung biopsy revealed bronchiolitis obliterans. Findings at thin-section CT performed during quiet tidal breathing at the time of biopsy were consistent with the diagnosis (Fig 6a). Six months later, thin-section CT of the chest with controlled-ventilation technique was performed to further assess the extent of lung involvement. Full-inflation scans at mask pressure of 25 cm of water (Fig 6b) were acquired during respiratory pauses lasting approximately 12 seconds. Expiratory scans at mask pressure of 0 cm of water (Fig 6c) were obtained during pauses lasting approximately 8 seconds. The controlled-ventilation images (full inflation and end exhalation) revealed much more extensive central and peripheral airway damage (bronchiectasis and associated air trapping) secondary to adenoviral infection.
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-year-old girl with newly diagnosed cystic fibrosis. The child presented with failure to thrive and no history of major respiratory disease or symptoms. Findings at physical examination and chest radiography were unremarkable. Infant pulmonary function testing revealed mild air trapping but no clear reduction in flows. Nitrogen washout studies revealed evidence of trapped gas and nonhomogeneous airway disease. A thin-section CT examination of the chest was performed to further assess the extent of pulmonary disease.
Thin-section CT images were obtained during quiet tidal breathing and controlled ventilation at full inflation (mask pressure, 25 cm of water) and end exhalation (mask pressure, 0 cm of water). The induced pauses lasted approximately 12 seconds at full inflation and at end exhalation. On the thin-section CT images obtained during quiet breathing, bronchiectasis was not evident, whereas bronchiectasis and peribronchial thickening were identified on the controlled-ventilation images obtained at full inflation. Areas of air trapping suggested on the tidal breathing images were more clearly defined on the end-exhalation images. On the images obtained during tidal breathing at the level of the inferior pulmonary veins, pseudobronchiectasis was seen. After the images were reviewed, the parents agreed to treatment with intravenous antibiotics.
Results of image quality scoring are summarized in the Table. For each of the three children, the average scores for the three levels were markedly better with the controlled-ventilation method. This was true for all categories and for controlled-ventilation images obtained at both full lung inflation and end exhalation.
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Discussion |
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The physiologic basis for capturing ventilation depends on a combination of a step increase in ventilation, which produces mild hypocarbia and thus decreases respiratory drive (9), and rapid chest expansion, which produces a vagally mediated pause in spontaneous respiratory effort, the Hering-Breuer response (10,11). Over the past 4 years, we have applied this technique for obtaining pulmonary function tests in more than 500 children aged from 1 month to 4 years. We have been successful in producing controlled pauses in respiration in more than 98% of the other children tested and in the three children whose cases are presented herein. In newborns and children younger than 2 months, respiratory pauses are more difficult to induce because of their more rapid respiratory rates. We have not studied children older than 4 years.
The theoretic complications of the procedure relate to the known complications of sedation and to the potential problems associated with hyperventilation (hypocarbia, seizures) and lung inflation (air leaks). In our pulmonary function laboratory and clinical CT cases, we have not experienced any complications. During controlled-ventilation procedures, monitored oxygen saturation levels have remained in the normal range and monitored end-tidal carbon dioxide levels have dropped approximately 2–4 torr and remained constant during repeated deep inspirations. This small drop in carbon dioxide level is not known to have untoward effects. In addition, as pressures used for lung inflations are no greater than those that occur during normal crying or spontaneous sighing, air leaks are unlikely to occur.
We would not recommend this procedure in patients with a known seizure disorder or a known or potential air leak. None of our patients has experienced prolonged apnea after the procedure, and we would not expect this to occur as end-tidal carbon dioxide levels drop only 2–4 torr and do not drop further after repeated controlled-ventilation procedures. We have not studied patients with such severely compromised pulmonary reserve that the deep sedation required for the procedure would be contraindicated. Patients have undergone infant pulmonary function testing while receiving supplementary oxygen with a similar method. It should be reemphasized that this procedure is performed in a very controlled manner with a physician at the bedside already using ventilatory equipment and with continuous monitoring of vital signs.
Another potential complication is gastric air distention during positive-pressure ventilation, which is not likely to produce major consequences at mask pressures of less than or equal to 25 cm of water (12,13). Because gastric air distention could increase the risk of gastroesophageal reflux and aspiration, we recommend gentle cricoid pressure during lung inflations (the Sellick maneuver), to close the communication between the airway and esophagus. In one of our patients (case 3), mild gastric air distention occurred despite use of the Sellick maneuver, but this had no ill effect.
On the basis of our previous experience in the infant pulmonary function laboratory, the duration of spontaneously induced respiratory pauses varies between children but lasts approximately 5–15 seconds for full inflation and 3–10 seconds for end exhalation. In the three cases presented herein, pauses ranged from 8 to 12 seconds. This provides enough time for thin-section CT images to be acquired at three to six anatomic levels.
Findings in our three clinical cases demonstrate the improved image quality of thin-section CT of the lungs in infants and young children with the controlled-ventilation technique. This technique successfully eliminated or minimized blurring and artifacts related to breathing motion, which were widespread on thin-section CT images acquired during quiet tidal breathing. Artifacts included pseudobronchiectasis (case 3), which has been described as an imaging pitfall in the diagnosis of bronchiectasis (14).
The ability to acquire thin-section CT images at full lung inflation and end exhalation has considerable advantages. Images obtained during quiet tidal breathing in a sedated child are images of the lungs very near end exhalation (Fig 3, top). These images at low lung volume depict the effects of crowding of the pulmonary vessels and bronchi, as well as poor distention of the lumen of the airways (Figs 5a, 6a). With full lung inflation, the depiction of airways and vessels is improved so a true assessment of bronchial lumen diameter, bronchial wall thickness, and vessel size becomes possible. At low lung volumes, bronchiectasis can be obscured (Fig 6a), but it becomes obvious when the airways are inflated (Fig 6b).
On thin-section CT images acquired during tidal breathing, nonhomogeneous lung parenchymal opacification, which is probably secondary to low lung volumes and motion artifact (Fig 5a), cannot be differentiated from a mosaic pattern of lung attenuation secondary to parenchymal, vascular, or small airways disease. The end-exhalation images obtained during a respiratory pause (Fig 6c) allowed us to be confident that the mosaic pattern of lung attenuation was secondary to air trapping rather than vascular or lung parenchymal disease, as has been described by others in adult patients (4).
Because the equipment needed to produce controlled pauses in infant ventilation is inexpensive and easy to obtain, the controlled-ventilation method we describe should be transferable to the majority of centers in which children receive care. However, use of this method requires an additional investment in time and preparation, exposes the practitioner to a low level of radiation, and at first should be undertaken with the assistance of members of the pulmonary or anesthesia department, who are more experienced in providing patient ventilation. Although general acceptance of the technique awaits further studies with larger numbers of patients that demonstrate its clinical utility and applications, we believe that the controlled-ventilation technique, when applied in a selective manner, will prove valuable in infants and young children.
By applying physiologic responses to augmented ventilation, motion-free thin-section CT images of the chest can be acquired noninvasively in the sedated infant or young child at both full lung inflation and deflation. The technique is simple, reproducible, rapid, and safe.
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Acknowledgments |
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Footnotes |
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