Relationship between inspiratory drive and perceived inspiratory effort in normal man.

1. To examine the relationship between the inspiratory effort sensation (IES) and respiratory drive as reflected by mouth occlusion pressure (P0.1) we have studied loaded and unloaded ventilatory responses to CO2 in 12 normal subjects. 2. The individual coefficient of variation of the effort sensation response to CO2 (IES/PCO2) between replicate studies was 21% and was similar to the variability of the ventilatory response (VE/PCO2) (18%) and the occlusion pressure response (P0.1/PCO2) (22%). 3. IES was well correlated with P0.1 (r greater than 0.9) for both free-breathing and loaded runs. 4. Resistive loading reduced the ventilatory response to hypercapnia from 19.3 l min-1 kPa-1 (SD 7.5) to 12.6 l min-1 kPa-1 (SD 3.9) (P less than 0.01). IES and P0.1 responses increased with resistive loading from 2.28 (SD 0.9) to 3.15 (SD 1.1) units/kPa and 2.8 (SD 1.2) to 3.73 (SD 1.5) cmH2O/kPa, respectively (P less than 0.01). 5. Experimentally induced changes in PCO2 and respiratory impedance were accompanied by increases in IES and P0.1. We found no evidence that CO2 increased IES independently of its effect on respiratory drive.