Two approaches were used to decide if there was an effect of delivered flow rate or tidal volume on the Voff. Data were grouped by flow rate and then by tidal volume. First, for all patients, regardless of delivered volume, Voff values at the lowest flow rates (^1.0 times spontaneous) were compared to Voff values at highest flows (^2.0 times spontaneous). Similarly, for all patients, regardless of delivered flow, Voff values at lowest delivered tidal volumes were compared to Voff values at highest volumes. Second, the mean slope of all of the Voff vs tidal volume and then Voff vs flow rate regressions were tested for significant difference from zero using Students t-test; a significant difference (p<0.05) supported an effect of flow rate or tidal volume on Voff.
Diaphragmatic EMG persisted after the ventilator was triggered for assisted breaths despite alterations in flow rate and tidal volume (Fig 2). The mean Voff for the entire group during assisted breaths at all ventilator settings was 93 ±34 (SD) percent of the spontaneous tidal volume, with a range of 25 to 195 percent. Comparison of Voff at low flow rates to that at high flow rates demonstrates that alterations in delivered flow rate significantly affected Voff (p = 0.0024). This was confirmed by examining the Voff vs flow rate regression line slopes (p<0.00001). An effect of delivered tidal volume on Voff was not clearly demonstrated (p = 0.71 for low tidal volume vs high tidal volume comparison; p = 0.017 for slope comparison).
During controlled breaths, there was no phasic diaphragmatic electromyographic activity until the ventilator-delivered tidal volume was close to the patients spontaneous tidal volume (Fig 3). Electromyographic activity and a reduced peak inspiratory pressure were seen late in inspiration at lower tidal volumes, indicating some diaphragmatic activity during these ventilator-initiated breaths. The lowest Voff values were in two patients with suspected diaphragmatic fatigue (mean, 58 ±29 percent of spontaneous tidal volume); this was significantly different from the mean Voff of the entire group (p<0.0001). cheap-asthma-inhalers.com
Figure 2 A (left). Effect of variation in ventilators flow rate in patient 4. B (right). Effect of variation in ventilators flow rate and tidal volume in patient 8. Note persistence of phasic EMG in assisted breaths.
Figure 3. Controlled breaths from three patients. Note absence of phasic electromyographic activity, except last breath in patient 8, which shows some activity late in breath. Tidal volume is close to spontaneous tidal volume in this instance.