Ventilation heterogeneity measured by multiple breath inert gas testing is not affected by inspired oxygen concentration in healthy humans

Ventilatory heterogeneity in the normal human lung is unchanged by controlled breathing

Measurement of ventilation heterogeneity with the multiple-breath nitrogen washout (MBW) is usually performed using controlled breathing with a fixed tidal volume and breathing frequency. However, it is unclear whether controlled breathing alters the underlying ventilatory heterogeneity. We hypothesized that the width of the specific ventilation distribution (a measure of heterogeneity) would be greater in tests performed during free breathing compared with those performed using controlled breathing. Eight normal subjects (age range = 23-50 yr, 5 female/3 male) twice underwent MRI-based specific ventilation imaging consisting of five repeated cycles with the inspired gas switching between 21% and 100% O₂ every ~2 min (total imaging time = ~20 min). In each session, tests were performed with free breathing (FB, no constraints) and controlled breathing (CB) at a respiratory rate of 12 breaths/min and no tidal volume control. The specific ventilation (SV) distribution in a mid-sagittal slice of the right lung was calculated, and the heterogeneity was calculated as the full width at half max of a Gaussian distribution fitted on a log scale (SV width). Free breathing resulted in a range of breathing frequencies from 8.7 to 15.9 breaths/min (mean = 11.5 ± 2.2, P = 0.62, compared with CB). Heterogeneity (SV width) was unchanged by controlled breathing (FB: 0.38 ± 0.12; CB: 0.34 ± 0.09, P = 0.18, repeated-measures ANOVA). The imposition of a controlled breathing frequency did not significantly affect the heterogeneity of ventilation in the normal lung, suggesting that MBW and specific ventilation imaging as typically performed provide an unperturbed measure of ventilatory heterogeneity.NEW & NOTEWORTHY By using MRI-based specific ventilation imaging (SVI), we showed that the heterogeneity of specific ventilation was not different comparing free breathing and breathing with the imposition of a fixed breathing frequency of 12 breaths/min. Thus, multiple-breath washout and SVI as typically performed provide an unperturbed measure of ventilatory heterogeneity.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Ventilation-perfusion heterogeneity measured by the multiple inert gas elimination technique is minimally affected by intermittent breathing of 100% O2

Proton magnetic resonance (MR) imaging to quantify regional ventilation-perfusion ( V ˙ A / Q ˙ ) ratios combines specific ventilation imaging (SVI) and separate proton density and perfusion measures into a composite map. Specific ventilation imaging exploits the paramagnetic properties of O2 , which alters the local MR signal intensity, in an FI O2 -dependent manner. Specific ventilation imaging data are acquired during five wash-in/wash-out cycles of breathing 21% O2 alternating with 100% O2 over ~20 min. This technique assumes that alternating FI O2 does not affect V ˙ A / Q ˙ heterogeneity, but this is unproven. We tested the hypothesis that alternating FI O2 exposure increases V ˙ A / Q ˙ mismatch in nine patients with abnormal pulmonary gas exchange and increased V ˙ A / Q ˙ mismatch using the multiple inert gas elimination technique (MIGET).The following data were acquired (a) breathing air (baseline), (b) breathing alternating air/100% O2 during an emulated-SVI protocol (eSVI), and (c) 20 min after ambient air breathing (recovery). MIGET heterogeneity indices of shunt, deadspace, ventilation versus V ˙ A / Q ˙ ratio, LogSD V ˙ , and perfusion versus V ˙ A / Q ˙ ratio, LogSD Q ˙ were calculated. LogSD V ˙ was not different between eSVI and baseline (1.04 ± 0.39 baseline, 1.05 ± 0.38 eSVI, p = .84); but was reduced compared to baseline during recovery (0.97 ± 0.39, p = .04). There was no significant difference in LogSD Q ˙ across conditions (0.81 ± 0.30 baseline, 0.79 ± 0.15 eSVI, 0.79 ± 0.20 recovery; p = .54); Deadspace was not significantly different (p = .54) but shunt showed a borderline increase during eSVI (1.0% ± 1.0 baseline, 2.6% ± 2.9 eSVI; p = .052) likely from altered hypoxic pulmonary vasoconstriction and/or absorption atelectasis. Intermittent breathing of 100% O2 does not substantially alter V ˙ A / Q ˙ matching and if SVI measurements are made after perfusion measurements, any potential effects will be minimized.

Effects of homogeneous and heterogeneous changes in the lung periphery on spirometry results

BACKGROUND AND OBJECTIVES

The most widespread chronic pulmonary disorders are associated with heterogeneous changes in the lung periphery and spirometry is the most commonly used test to monitor these diseases. So far only a few attempts have been undertaken to investigate the effects of lung inhomogeneity on spirometry results. The aim of this work was to evaluate whether the spirometric curve and indexes are sensitive to parallel peripheral inhomogeneities, and if the level of heterogeneity can be deduced from this test.

METHODS

To this end, an enhanced computational model for forced expiration, taking into account a heterogeneous structure and properties of the respiratory system, was used. Two main phenomena were mimicked: small airways narrowing and the loss of tissue elastic recoil. Numerical simulations were performed with the model having 76 separate peripheral compartments. For a given degree of mean change, three heterogeneity levels were investigated and compared to the effects of homogeneous alterations.

RESULTS

All spirometric curves representing different patterns of inhomogeneous constriction, computed for each of the investigated cases, almost coincided with the curve originating from homogeneous changes, regardless of the heterogeneity level. Also the differences between the spirometric indexes obtained for heterogeneous and homogeneous alterations were negligible in comparison to their values.

CONCLUSION

The main finding is that the spirometry results are insensitive to the level of heterogeneity in the lung periphery and that it is practically impossible to distinguish between the homogeneous or heterogeneous nature of pathological processes occurring in this lung region.

Comparison of quantitative multiple-breath specific ventilation imaging using colocalized 2D oxygen-enhanced MRI and hyperpolarized 3He MRI

Two magnetic resonance specific ventilation imaging (SVI) techniques, namely, oxygen-enhanced proton (OE-1H) and hyperpolarized 3He (HP-3He), were compared in eight healthy supine subjects [age 32 (6) yr]. An in-house radio frequency coil array for 1H configured with the 3He transmit-receive coil in situ enabled acquisition of SVI data from two nuclei from the same slice without repositioning the subjects. After 3 × 3 voxel downsampling to account for spatial registration errors between the two SV images, the voxel-by-voxel correlation coefficient of two SV maps ranged from 0.11 to 0.63 [0.46 mean (0.17 SD); P < 0.05]. Several indexes were analyzed and compared from the tidal volume-matched SV maps: the mean of SV log-normal distribution (SVmean), the standard deviation of the distribution as a measure of SV heterogeneity (SVwidth), and the gravitational gradient (SVslope). There were no significant differences in SVmean [OE-1H: 0.28 (0.08) and HP-3He: 0.32 (0.14)], SVwidths [OE-1H: 0.28 (0.08) and HP-3He: 0.27 (0.10)], and SVslopes [OE-1H: -0.016 (0.006) cm-1 and HP-3He: -0.013 (0.007) cm-1]. Despite the statistical similarities of the population averages, Bland-Altman analysis demonstrated large individual intertechnique variability. SDs of differences in these indexes were 42% (SVmean), 46% (SVwidths), and 62% (SVslopes) of their corresponding overall mean values. The present study showed that two independent, spatially coregistered, SVI techniques presented a moderate positive voxel-by-voxel correlation. Population averages of SVmean, SVwidth, and SVslope were in close agreement. However, the lack of agreement when the data sets were analyzed individually might indicate some fundamental mechanistic differences between the techniques. NEW & NOTEWORTHY To the best of our knowledge, this is the first cross-comparison of two different specific ventilation (SV) MRI techniques in the human lung (i.e., oxygen-enhanced proton and hyperpolarized 3He). The present study showed that two types of spatially coregistered SV images presented a modest positive correlation. The two techniques also yielded similar population averages of SV indexes such as log-normal mean, SV heterogeneity, and the gravitational slope, albeit with some intersubject variability.