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

Abstract

Proton magnetic resonance (MR) imaging to quantify regional ventilation-perfusion ( ${\dot{V}}_A/\dot{Q}$ ) 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 F_I 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 F_I O₂ does not affect ${\dot{V}}_A/\dot{Q}$ heterogeneity, but this is unproven. We tested the hypothesis that alternating F_I O₂ exposure increases ${\dot{V}}_A/\dot{Q}$ mismatch in nine patients with abnormal pulmonary gas exchange and increased ${\dot{V}}_A/\dot{Q}$ 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 ${\dot{V}}_A/\dot{Q}$ ratio, LogSD $\dot{V}$ , and perfusion versus ${\dot{V}}_A/\dot{Q}$ ratio, LogSD $\dot{Q}$ were calculated. LogSD $\dot{V}$ 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 $\dot{Q}$ 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 ${\dot{V}}_A/\dot{Q}$ matching and if SVI measurements are made after perfusion measurements, any potential effects will be minimized.

Keywords: hyperoxia; magnetic resonance imaging; pulmonary perfusion distribution; pulmonary ventilation distribution; specific ventilation imaging; ventilation-perfusion ratio.