The effect of changing ventilator settings on indices of ventilation inhomogeneity in small ventilated lungs

BACKGROUND: In ventilated newborns the use of multiple breath washout (MBW) techniques for measuring both lung volume and ventilation inhomogeneity (VI) is hampered by the comparatively high dead space fraction. We studied how changes in ventilator settings affected VI indices in this particular population. METHODS: Using a computer simulation of a uniformly ventilated volume the interaction between VI indices (lung clearance index (LCI), moment ratios (M(1)/M(0), M(2)/M(0), AMDN(1), AMDN(2)) of the washout curve) and tidal volume (V(T)), dead space (V(D)) and functional residual capacity (FRC) were calculated. The theoretical results were compared with measurements in 15 ventilated piglets (age <12 h, median weight 1135 g) by increasing the peak inspiratory pressure (PIP). FRC and VI indices were measured by MBW using 0.8% heptafluoropropane as tracer gas. RESULTS: The computer simulation showed that the sensitivity of most VI indices to changes in V(D)/V(T )and V(T)/FRC increase, in particular for V(D)/V(T )> 0.5. In piglets, the raised PIP caused a significant increase of V(T )from 15.4 ± 9.5 to 21.9 ± 14.7 (p = 0.003) and of the FRC from 31.6 ± 14.7 mL to 35.0 ± 15.9 mL (p = 0.006), whereas LCI (9.15 ± 0.75 to 8.55 ± 0.74, p = 0.019) and the moment ratios M(1)/M(0), M(2)/M(0 )(p < 0.02) decreased significantly. No significant changes were seen in AMDN(1 )and AMDN(2). The within-subject variability of the VI indices (coefficient of variation in brackets) was distinctly higher (LCI (9.8%), M(1)/M(0 )(6.6%), M(2)/M(0 )(14.6%), AMDN(1 )(9.1%), AMDN(2 )(16.3%)) compared to FRC measurements (5.6%). Computer simulations showed that significant changes in VI indices were exclusively caused by changes in V(T )and FRC and not by an improvement of the homogeneity of alveolar ventilation. CONCLUSION: In small ventilated lungs with a high dead space fraction, indices of VI may be misinterpreted if the changes in ventilator settings are not considered. Computer simulations can help to prevent this misinterpretation.