Force-frequency relationships of in vivo human and in vitro rat diaphragm using paired stimuli

Supramaximal stimuli, with time intervals of 100 ms (10 Hz) to 10 ms (100 Hz), were delivered in pairs to the phrenic nerves, bilaterally, in five seated normal subjects, while transdiaphragmatic pressure swings (Pdi,s) were recorded at relaxed end-expiratory lung volume with airways closed. In fresh diaphragms, Pdi,s increased between 10-20 Hz and reached a plateau between 20-30 Hz. Diaphragmatic fatigue decreased Pdi,s at all frequencies. Pdi,s was assumed to be the sum of two successive responses (T1+T2), T1 being constant at any frequency and equal to a single twitch, T2 being obtained by subtraction. We found that T2 amplitude, which was significantly reduced after fatigue, was fully returned to normal after 15 min rest at high, not at low, stimulation frequencies. The ratio of T2 at 10 Hz over 100 Hz (T2(10/100)) thus decreased from 1.33 +/- 0.05 before fatigue to 0.97 +/- 0.12 after fatigue, and to 0.81 +/- 0.06 after 15 min rest. Similar results were obtained in isolated rat diaphragmatic strips stimulated and fatigued in vitro, from which we found a highly linear relationship (r = 0.94, p < 0.001) between the ratio of T2(10/100) and that of tetanic force at 10 Hz over 100 Hz (P10/100). We conclude that phrenic nerve paired twitches provide similar information when obtained from phrenic tetanic stimulation in terms of diaphragmatic contractility, and the decrease in T2(10/100) ratio indicates diaphragm low frequency fatigue.