Values for Pdi,tw or Pmo,tw in normal subjects can be found in more than 40 published studies, irrespective of the technique used (10, 56, 65, 83-85, 88-90, 100, 107, 110, 115, 116, 118-120, 129, 131, 132, 134, 145, 146, 155, 160, 161, 165-167,
170-173, 175-190) (Table 4). Normal young (20-35 years) male individuals provide the vast majority of the data.
Bearing in mind that the technique used and various methodological factors can exert a major influence on the results, it seems possible to propose the following, for bilateral PNS.
Amplitude of bilateral twitch transdiaphragmatic pressure. Assuming a correct technique, at FRC, Pdi,tw with ES should be above 15 cm H2O and Pdi,tw with CMS should be above 20 cm H2O.
A Pdi,tw below 15 cm H2O, whatever the PNS technique, should raise high suspicion of diaphragm dysfunction (110, 114, 131, 179, 187, 190-196).
Influence of age, sex, and other characteristics. As with voluntary pressures (26, 128, 197), normal Pdi,tw tends to be lower in women than in men and tends to decrease with age (120, 131, 182, 190). The influence of other factors such as height, race, fitness, etc., is unknown, although data suggest that baseline Pdi,tw in highly fit subjects is not different from normal subjects (185), but with a better resistance to maximal exercise-induced fatigue.
Other characteristics of the twitch transdiaphragmatic pressure response to phrenic nerve stimulation. The ratio of Pes,tw to Pdi,tw in normal individuals is usually between 0.35 and 0.55.
Amplitude of twitch transdiaphragmatic pressure. Available data are still scarce, but a value of Pmo,tw during CMS less negative than —11 cm H2O should probably prompt more detailed investigation (110). Pmo,tw during ES tends to be lower, and a value more negative than — 8 cm H2O with this technique is probably normal (119).
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