Indices of Inspiratory Effort

The diaphragm electromyogram (EMGdi) signal was recorded (DISA type 15 COl Amplifiers, Denmark) in all subjects with surface electrodes (Graphics Control Ltd, Gananoque, Canada) placed over the right sixth and seventh intercostal spaces near the costal margin. In six subjects (nos. 1,3,4,5,8,11), EMGdi was also recorded with a bipolar esophageal electrode which was anchored at the gastroesophageal junction with an inflatable balloon. In three subjects (nos. 6,10,11), prominent accessory muscle activity was visibly evident during spontaneous breathing and electromyo­gram s were also recorded from the parasternal or sternocleidomas­toid muscles. All EMG signals were processed by a moving time averager (CWE Inc, Ardmore, PA) with a time constant of 100 ms after band pass filtering (.03-10 KHz) and rectification.

Respiratory intrathoracic pressure was obtained with an esopha­geal balloon-catheter system (Pes) in all subjects from the normal and obstructive groups. Since three of the subjects in the restrictive group (nos. 5,6,7) had tracheostomies, tracheal pressure (Ptrach) was readily accessible and was monitored via an 18-gauge needle inserted through a cork which occluded the tracheostomy port. Intrathoracic pressure was not recorded in subject no. 4. Esophageal and tracheal pressures were measured with a differential transducer (model MP-45, ±50 сшНД Validyne Corp, Northridge, CA). viagra plus

All signals were recorded on an 8-channel strip chart recorder (7758B System Recorder, Hewlett-Packard Co, Waltham, MA).

Study Protocol

Subjects were studied in the semi-recumbent position. Efforts were made to reduce unnecessary noise and distractions and to encourage maximal relaxation. No specific instructions were given regarding mouth opening or closing. Control measurements of electromyograms, intrathoracic pressure, oxygen saturation and Pco2 were made with the mask in place and the ventilator cycling beside the subject but not connected to the mask. The ventilator rate was set to slightly exceed the subjects’ spontaneous breathing frequency. The ventilator was then connected to the nasal mask and was gradually adjusted to achieve 2-3 cmHaO increases in Pmask until inspection of the tracings revealed no further decreases in phasic activity of the surface EMGdi. Several minutes were spent at each pressure level and subjects were coached to enhance relaxation. The initial measurements were repeated after 20 minutes using the NPPV settings which resulted in the maximum reduction of phasic surface diaphragm EMG activity. Experiments lasted approximately one to two hours.

To determine the effect of mouth position on phasic activity of the diaphragm during NPPV, one normal (no. 3), two restrictive (nos. 4,5) and two obstructive (nos. 10,11) subjects were instructed to open their mouths during NPPV once optimal mask pressures had been determined. Ventilator settings were not adjusted when the mouth was opened. Measurements of EMGdi and intrathoracic pressure were repeated after two minutes with a stable breathing pattern.

Analysis

Measurements of the phasic amplitude of the processed inspira­tory muscle EMGs and intrathoracic pressure were averaged over 15 consecutive breaths during spontaneous breathing, as well as with NPPV at the various Pmask levels. The mean value of the phasic amplitude for each EMG signal during quiet breathing was defined as the control value. The mean amplitude of the EMG during NPPV was expressed as a percentage of the initial value.
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Significance of differences between control and NPPV values were analyzed by paired t test. P values <0.05 were considered to be statistically significant. All results are reported as mean ± standard error of the mean (SEM).