Effect of timing, flow, lung volume, and threshold pressures on resistive load detection

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Abstract
The acuity of human resistive load detection (RLD) was tested by comparing threshold detection (.DELTA.R50) under control conditions with that obtained when loads were applied at different times in inspiration, with different inspiratory flows, at different lung volumes and with different background loads. For loads applied suddenly during established inspiratory flow, the mean .DELTA.R50 was 0.94 .+-. 0.04 (.+-. SEE [standard error of the estimate]) cm H2O .cntdot. 1-1 .cntdot. s compared with 0.42 .+-. 0.13 cm H2O .cntdot. 1-1 .cntdot. s (P < 0.01) for loads applied before inspiration, suggesting that information generated early in the breath is important and the RLD is not subserved by a simple peripheral proprioceptive mechanism. Rapid voluntary inspiration through nonlinear resistances did not improve RLD (mean .DELTA.R50 0.86 cm H2O .cntdot. 1-1 .cntdot. s compared to a control of 0.56 cm H2O .cntdot. 1-1 .cntdot. s; P < 0.05) despite the much greater pressures and resistances at high flow rates, suggesting that detection of a resistive load occurs early in inspiration. Alteration of the background load by breathing at increased lung volume (1 l above FRC [functional residual capacity]) or by breathing against a standing pressure of 1 or 3 cm H2O (i.e., pressure independent of volume or flow) did not significantly reduce RLD. The change in pressure due to the added load, expressed as a fraction of the total driving pressure, evidently cannot subserve RLD. Apparently, external resistive load detection is subserved by the relation between pressure and flow over the early part of inspiration.