Respiratory volume-timing relationship during sustained elevation of functional residual capacity

In 7 spontaneously breathing dial-urethane anesthetized cats a negative pressure was produced around the thorax and abdomen to increase the functional residual capacity (FRC) by about 1 tidal volume for up to 60 min. A tracheal cannula was connected to a resistive manifold for selective loading of inspiration or expiration. Two resistive loads and tracheal occlusion were presented six times each at control FRC (FRCc), after 60 min at elevated FRC (FRCe) and 30 min after return to FRCc. Inspiratory and expiratory durations (TI and TE) were measured from diaphragmatic EMG. We observed that TI at FRCe (0.88 +/- 0.11 sec) was not significantly shorter than TI at FRCc (1.06 +/- 0.14 sec). Tracheal occlusion at FRCe caused a shorter TI (1.37 +/- 0.15 sec) than at FRCc (1.79 +/- 0.21 sec) (P less than 0.05). The slope (m) of the VI-TI relationship generated by the resistive loads at FRCe was steeper (m = -65 +/- 7 ml X sec-1) and shifted upward from the VI-TI curve at FRCc (-50 +/- 6 ml X sec-1) (P less than 0.05). The VE-TE relationship at FRCe was not significantly changed from control. Thirty minutes following return to FRCc, TI was still slightly shorter (0.96 +/- 0.11 sec) than the initial TI at FRCc. We conclude: (1) The slope of the VI-TI relationship is determined to a great extent by the total lung volume. However, under the conditions of sustained elevation of FRC, this relationship is influenced by the partial adaptation of slowly adapting pulmonary receptors SARs. (2) The increased SAR activity at end expiration during FRCe may not influence the control of TE.