Chest
Volume 99, Issue 1, January 1991, Pages 134-138
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Effects of Initial Flow Rate and Breath Termination Criteria on Pressure Support Ventilation

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To assess whether adjustments in the initial flow rate or breath termination criteria affected patient-ventilator synchrony, we studied the ventilatory pattern response to PS in 33 patients under two sets of circumstances: during seven different levels of delivered initial PS flow and during PS termination at 50 percent and at 25 percent of peak flow. In the study on initial PS flow, we found the following: (a) an optimal initial PS flow could be defined for a given level of PS that resulted in the patient obtaining maximal pressure and volume from the ventilator; (b) initial PS flows above and below this optimal flow were associated with faster breathing frequencies, shorter inspiratory times, smaller tidal volumes and a tendency for airway pressure to not reach the selected PS level; and (c) optimal initial PS flow was fastest in patients with the lowest compliances and the most active ventilatory drives. Changing PS termination criteria from 50 to 25 percent of peak flow had minimal effects on the ventilatory pattern or synchrony. We conclude that the initial PS flow to achieve the selected PS level is important in patient-ventilator synchrony but that termination criteria set between 25 and 50 percent of peak flow is not. (Chest 1991; 99:134–38)

Section snippets

PATIENTS AND METHODS

Thirty-three clinically stable, mechanically ventilated patients were studied. Initial data were collected while the patient was receiving his/her baseline mechanical ventilatory support (all patients were either on high levels of PS or on synchronized intermittent mandatory ventilation with lower levels of PS). Baseline data included: diagnosis, total minute ventilation determined by the ventilators exhaled volume monitor (MV), static respiratory system compliance (CRS, volume controlled tidal

RESULTS

Baseline characteristics of the 33 study patients are given in Table 1.

DISCUSSION

Several investigators have demonstrated that the spontaneous inspiratory effort which triggers a mechanically assisted breath does not cease with the delivery of that breath.7, 8, 9, 10 Thus, the potential for significant imposed ventilatory muscle loads exists if the mechanically assisted flows and volumes do not match the patient's desired flow pattern and tidal volume. Subjective dyspnea, as well as increased muscle energy demand, would appear to be a consequence of such imposed loads.8,11

ACKNOWLEDGMENT

The authors are indebted to Mrs. Janet Johns for her secretarial expertise.

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    A constant or square inspiratory flow waveform was applied with the IMV breaths. For PSV, the “% rise” setting on the ventilator (ie, the control to adjust the ventilator flow-rate output during inhalation and, thus, the rate of pressure rise, which results from the interaction of ventilator flow rate output and patient inspiratory flow rate demand10) ranged from 60 to 80%, and the expiratory sensitivity setting (“Esens”) or PSV breath-termination criteria was 25%. The levels of PSV, PEEP, fraction of inspiratory oxygen (Fio2), carbon dioxide elimination, and hemoglobin oxygen saturation were comparable for patients at all sites at the time of enrollment into the study (Table 1).

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