Am. J. Respir. Crit. Care Med., Vol 153, No. 4, Apr 1996, 1322-1327.
The tension-time index of the diaphragm revisited in quadriplegic patients with diaphragm pacing
S Nava, F Rubini, E Zanotti and D Caldiroli
Department of Pneumology, Centro Medico di Montescano, Italy.
The fatigue threshold of the human diaphragm, index of its endurance and
fatigability, corresponds, during spontaneous breathing, to a tension-time
index (TTdi = Pdi/Pdimax x T1/Ttot (i.e., the inspiratory time over the
total breath duration) of 0.15 to 0.18. We studied three quadriplegic
patients with diaphragm pacing in order to reassess this threshold in
patients in whom the contribution of the other respiratory muscles is
lacking. Transdiaphragmatic pressure (Pdi) was obtained from the difference
between gastric (Pga) and esophageal (Pes) pressures while the
electromyograms (EMG) of both hemidiaphragms were recorded with surface
electrodes. Four runs at different TTdi were performed on different days in
each subject, varying either the Pdi developed per breath, changing the
frequency of stimulation, or the T1/Ttot. The time of fatigue was defined
when Pdi decreased during the trials by 35% from baseline. No evidence of
transmission fatigue (i.e., decline in action potential amplitude) was
observed. The TTdi at which fatigue occurred in all the quadriplegic
patients was around 0.10 to 0.12, well below the values previously
described. After fatigue had occurred, the force recovery during the
force-frequency curve was not complete after 2 h at low frequencies,
whereas at high frequencies it was fully complete at 30 min. We conclude
that when respiration is accomplished only by the diaphragm, without the
contribution of the other respiratory muscles, the fatigue threshold is
lower than previously reported.
