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Volumetric Xenon-CT Imaging of Conventional and High-frequency Oscillatory Ventilation1

Posted May 31, 2009 Updated Apr 7, 2023
By Daniel G. Mulreany BS , Brett A. Simon MD PhD , Kieran J. Murphy MD , R. Blaine Easley MD
9 min read

Rationale and Objectives

For mechanical ventilation of patients with pulmonary injuries, it has been proposed that high-frequency oscillatory ventilation (HFOV) offers advantages over conventional ventilation (CV); however, these advantages have been difficult to quantify. We used volumetric, dynamic imaging of Xenon (Xe) washout of the canine lung during both HFOV and CV to compare regional ventilation in the two modalities.

Materials and Methods

Three anesthetized, mechanically ventilated animals were studied, each at three different ventilator settings. Imaging was performed on an experimental Toshiba 256-slice scanner at 80 kV, 250 mAs, and 0.5-second scans, yielding 12.8 cm of Z-axis coverage. Repeated images were acquired at increasing intervals between 1 and 10 seconds for 90 seconds during HFOV and using retrospective respiratory gating to end-expiration for 60 seconds during CV. Image series were analyzed to quantify regional specific ventilation (s V˙ V

˙ ) from the regional density washout time constants.

Results

High-quality, high-resolution regional ventilation maps were obtained during both CV and HFOV. Overall ventilation decreased at smaller tidal volume, as expected. Regional s V˙ V

˙ was more uniform during HFOV compared to CV, but the underlying distribution of lung aeration was similar.

Conclusions

High-resolution volumetric ventilation maps of the lung may be obtained with the 256-slice multidetector computed tomographic scanner. There is a marked difference in the distribution of regional ventilation between CV and HFOV, with a significant gravitational ventilation gradient in CV that was not present during HFOV. This technique may be useful in exploring the mechanisms by which HFOV improves gas exchange.

Functional lung imaging with x-ray computed tomography facilitates the study of pulmonary structure–function relationships by providing both detailed anatomic information as well as co-localized physiologic information . As the speed and volumetric coverage of multidetector computed tomographic (CT) scanners has increased over time, so has the ability to rapidly and efficiently characterize variation in regional lung function across the entire lung. Recently, volumetric CT image acquisition took a giant leap with the introduction of the Toshiba Aquilion One scanner, a multidetector CT scanner capable of acquiring 320 contiguous 0.5-mm thick axial slices in a single 0.33-second rotation, making true volumetric CT imaging commercially available. We had an opportunity to test a preproduction 256-slice version of this scanner to evaluate its potential for studying the distribution of pulmonary gas exchange during high-frequency oscillatory ventilation (HFOV), a novel form of mechanical ventilation with application in management of patients with refractory acute lung injury .

For mechanical ventilation of patients with pulmonary injuries, it has been proposed that HFOV offers advantages over conventional ventilation (CV); however, the physiology of HFOV remains poorly characterized . In contrast to CV, which delivers breaths that mimic normal breathing with volumes of 400–900 mL at 6–30 breaths/minute, HFOV delivers breaths of 40–150 mL at 180–900 breaths/minute (3–15 Hz). The rationale behind this approach is that by ventilating with smaller breaths at an elevated mean lung volume, it may be possible to avoid the injurious extremes of lung overdistention and collapse that can arise from the large volume excursions applied to the stiff, injured lung during CV . Because the tidal volume in HFOV is smaller than the anatomic dead space (the volume of the conducting airways leading to the gas-exchanging alveolar region), the methods of gas exchange and, further, the factors governing the distribution of ventilation, are distinctly different from those in CV. It has been difficult to directly measure these differences in intact subjects.

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Materials and methods

Animal Preparation

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Experimental Protocol

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Table 1

Ventilator Settings

Subject Conventional Ventilation High-Frequency Ventilation RR (breaths/min) Vt (mL) PEEP (cm H 2 O) ƒ (Hz) Vt (mL) MAP (cm H 2 O) 1 10 60 10 10 80 10 10 100 10 2 30 220 7.5 30 400 7.5 30 400 0 3 30 250 7.5 10 80 10 10 100 10

MAP, mean airway pressure; PEEP, positive end-expiratory pressure; RR, respiratory rate; Vt, tidal volume.

Each animal was imaged at a total of three high-frequency oscillatory ventilation and/or conventional ventilator settings.

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Imaging

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Figure 1, Coronal view of the lung demonstrating the increased axial coverage of a single image acquisition with increasing number of detectors from 16 to 64 to 256.

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Image Analysis

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Figure 2, Representative density-time curves and exponential fits for two regions of interest from the multibreath washout of xenon from the lung during high-frequency oscillatory ventilation. CT, computed tomography; HU, Hounsfield units.

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Figure 3, Representation of three-dimensional data. Each region of interest in the lung volume was separately curve fit to determine its specific ventilation and the values color-coded. For display purposes, data within each 1.8 cm of z-coverage were averaged into one two-dimensional (2D) colormap, creating six 2D maps that represent the entire scanned volume from apex to base.

Figure 4, Colormaps showing the apex to base distribution of specific ventilation in subject 2 at tidal volumes of 220 and 400 mL, rate 30 breaths/min, and 7.5 cm H 2 O positive end-expiratory pressure (PEEP). Note the overall increase in ventilation at the higher tidal volume (Vt) as well as the strong vertical gradient in specific ventilation seen during conventional ventilation.

Table 2

Measures of Ventilation Heterogeneity

Stepwise Linear Regression r 2 HFOV CV Y 0.028 ± 0.03 0.294 ± 0.12 Y/Z 0.040 ± 0.02 0.296 ± 0.12 Y/Z/S 0.042 ± 0.02 0.298 ± 0.12

Coefficients of Variation HFOV CV Total: COV t 0.41 ± 0.06 0.46 ± 0.12 Other: COV i 0.40 ± 0.06 0.38 ± 0.12 Y only: COV y 0.09 ± 0.03 0.25 ± 0.04 ∗

CV, conventional ventilation; HFOV, high-frequency oscillatory ventilation; S , side (left vs. right); SD, standard deviation; Y , dorsal-ventral (vertical) height; Z , coronal location (apex-base).

Means ± SD; n = 5 HFOV, n = 4 CV.

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Results

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Figure 5, Colormaps showing the apex to base distribution of specific ventilation in subject 3 during conventional ventilation (CV) at 30 breaths/min, tidal volume (Vt) = 250 mL, and positive end-expiratory pressure (PEEP) 7.5 cm H 2 O, and during high-frequency oscillatory ventilation (HFOV) at frequency 10 Hz, mean airway pressure (MAP) 10 cm H 2 O, and Vt of 80 and 100 mL. There is an overall increase in specific ventilation during HFOV as Vt increases. As in Figure 4 , there is a steep vertical ventilation gradient during CV, but ventilation is extremely uniform during HFOV.

Figure 6, Colormaps of lung density, expressed as % air content, for the same lung slices depicted in Figure 5 . The vertical gradient in lung aeration or regional expansion is very similar during conventional ventilation and high-frequency oscillatory ventilation (HFOV).

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Discussion

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Conclusions

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Acknowledgments

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Academic Radiology, Volume 16, Issue 6
Journals General Radiology
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