Ten intubated patients receiving mechanical ventilation were studied (Tables 1 and 2). At the time of study, the patients were clinically stable, capable of some spontaneous ventilation, and without positive end-expiratory pressure. The patients were judged ready for weaning from the ventilator by their attending physicians, and mechanical ventilation was discontinued shortly after the study except in patients 1 and 7. These two patients had evidence of paradoxical abdominal motion during spontaneous breathing and therefore may have had diaphragmatic fatigue.
Airway pressure, inspiratory flow rate, diaphragmatic EMG, and inspiratory volume were measured during each experiment. The diaphragmatic EMG was recorded from two silver/silver chloride adherent cutaneous leads placed in each nipple line at the costal arch with a common lead in the right axillary line at the tenth rib. The electromyographic signal was processed through a differential isolation amplifier and was bandpass filtered between 30 and 500 Hz. Electrocardiographic interference was reduced using a gating device similar to one described by Prechtyl et al. Airflow was measured by a pneumotachograph (Fleisch No. 3) and differential pressure transducer (Validyne MP45) in the inspiratory limb of the ventilator circuit near the endotracheal tube. Flow was electronically integrated to provide inspired tidal volume. The signals were amplified, displayed, and recorded on an oscillographic recorder for later analysis (Electronics for Medicine VR6). fully
The study protocol began with a period of spontaneous breathing followed by ventilator-delivered assist/control breaths and then ventilator-controlled breaths. Ventilator-delivered inspiratory tidal volume and flow rate were varied in multiples of the observed spontaneous values during the assisted and controlled breath portion of the study. Patients initiated assisted breaths by developing a small negative airway pressure, which was followed by ventilator-assisted inspiration at the selected flow rate and tidal volume. Controlled breaths were similar to the assisted breaths but initiated by the ventilator. Spontaneous breaths were drawn through the ventilator but not assisted. The sensitivity of the demand valve was 0.5 cm HgO for both assisted and spontaneous breaths.
Table 1—Characteristics of Patients
|4||73||65||Flail chest; CHF|
|9||47||81||Resolving ARDS; MI|
|10||62||54||Restricted chest wall|
Table 2—Spontaneous Ventilation
|P&tient1||Tidal Volume, ml140 ±26||Flow Rate, L/min27 ±1.5||RespiratoryFrequency15|
|2||244 ±10||19 ±.3||18|
|4||261 ±4||12 ± .1||24|
|6||282 ±20||24 ± .9||15|
|8||452 ±84||32 ±3||20|
|9||406 ±21||34± 1||24|