Sensation of inspired volume in normal subjects and quadriplegic patients

To investigate the influence of respiratory muscle tension and feedback from rib cage receptors, the sensation of inspired volume was compared in normal subjects and quadriplegic patients during active breathing, with and without the addition of an inspiratory resistive load, and during passive ventilation produced by a tank respirator. In separate trials, volume sensation was assessed using tests of magnitude estimation and volume reproduction. The mean exponents and standard errors for the magnitude estimation of inspired volume in normal subjects were 1.32 +/- 0.08, 1.24 +/- 0.06, and 1.23 +/- 0.09 during passive, active, and loaded breathing, respectively. These values were not significantly different from one another, nor were there any differences between normal subjects and quadriplegics. During volume reproduction trials where the mechanical conditions were different between control and test breaths, the inspired volumes during active unloaded breathing were significantly smaller than during passive ventilation but greater than the inspired volumes during loaded breathing. Errors in volume reproduction were no different in normal subjects and quadriplegics, suggesting that inputs from rib cage receptors are not essential for the sensation of inspired volume. The sensation of inspired volume in both normal subjects and quadriplegics was found to be unaffected by inhalation of a 2% lidocaine solution. This suggests that upper airway receptors are also not essential for volume sensation. The intensity of the sensation of a given inspired volume may depend on the level of the central nervous system motor command and/or on the tension developed by the diaphragm.