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

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Abstract
Measurement of ventilation heterogeneity with the multiple breath N2 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 to those using controlled breathing. Eight normal subjects (age range 23-50 yrs, 5F/3M) twice underwent MRI-based specific ventilation imaging consisting of 5 repeated cycles with the inspired gas switching between 21% and 100% O2 every ~2 minutes (total imaging time ~20 minutes). In each session tests were performed free-breathing (FB, no constraints), and with controlled breathing (CB) at a respiratory rate of 12 breaths/min and no tidal volume control. The specific ventilation distribution in a mid-sagittal slice of the right lung was calculated and the heterogeneity 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 bpm (mean 11.5 ± 2.2, P=0.62 compared to 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 SVI as typically performed provide an accurate measure of ventilatory heterogeneity.
Funding Information
  • HHS | NIH | National Heart, Lung, and Blood Institute (HL-119263)
  • HHS | NIH | National Heart, Lung, and Blood Institute (HL-127980)
  • HHS | NIH | National Heart, Lung, and Blood Institute (HL-119201)
  • HHS | NIH | National Heart, Lung, and Blood Institute (HL-129990)

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