Figure 2.

Example, in one subject (#4), of the course of the twitch transdiaphragmatic pressure (Pdi,tw) elicited by cervical magnetic stimulation as a function of the intensity of the underlying diaphragm contraction. (A): voluntary effort; (B): non-volitional “single-breath” effort in response to the first presentation of an infinite resistance. (C): non-volitional “continuous” effort, 10 breathing cycles after application of an inspiratory threshold load (ITL). The intensity of the underlying diaphragmatic contractions (abscissa) is expressed as a percentage of the maximal transdiaphragmatic pressure measured by static manoeuvres (Pdi,max). Pdi,tw (ordinate) are expressed as a percentage of their maximal values measured during the procedure considered. Solid lines depict the linear regression, dotted lines depict the corresponding 95% confidence interval. Note that the “continuous ITL” scales differ from the other scales because of the threshold load range (0 to 45 cm H₂O). In the subject that this figure depicts, panel B shows that a diaphragm contraction producing a Pdi amounting to about 50% of the maximal Pdi recorded during voluntary contractions corresponded to near maximal diaphragm activation (phrenic stimulation triggered by higher pressure did not produce further pressure increments). In other words, the recruitment of phrenic motoneurons required to produce a given pressure was greater during the “single breath” procedure than during the voluntary procedure. During the “continuous ITL procedure” (C), we needed the subjects to be able to breathe in sustainedly, and therefore the inspiratory limb of the breathing circuit could not be occluded. We chose inspiratory threshold values in a reasonable range regarding the induced respiratory discomfort. What (C) shows is that the Pdi,tw value corresponding to about 35% of Pdi,max is twice higher than the Pdi,tw value corresponding to the same percentage of Pdi,max during the “single breath” procedure (B). This supports our hypothesis of a differrent dynamica of phrenic motoneurons recruitment between single breath loading and sustained loading.