Respiratory Muscle Performance, Pulmonary Mechanics, and Gas Exchange Between the BiPAP S/T-D System and the Servo Ventilator 900C With Bilevel Positive Airway Pressure Ventilation Following Gradual Pressure Support Weaning

¹ From the University of Mississippi Medical Center, G. V. (Sonny) Montgomery VA Medical Center, Division of Pulmonary and Critical Care Medicine, Department of Medicine, Jackson, MS.

Rajesh G. Patel, MD, FCCP, G. V. (Sonny) Montgomery VA Medical Center, 1500 East Woodrow Wilson Drive, Jackson, MS 39216

Study objective: Our objective was to compare respiratory muscle performance, pulmonary mechanics, and gas exchange between the BiPAP S/T-D ventilation system (Respironics Inc; Murrysville, PA) and the Servo Ventilator 900C (Siemens-Elma AB; Sweden) with similar inspiratory and expiratory airway pressure in patients who are recovering from acute respiratory failure.

Study design: A prospective, randomized, clinical trial.

Setting: Medical ICU.

Patients and methods: We studied 27 medical patients on mechanical ventilators following gradual pressure support weaning. Each patient breathed while in the following equivalent modes: (a) an inspiratory pressure preset (pressure support mode) of 5 cm H₂O with an external positive end-expiratory pressure (PEEP) of 5 cm H₂O on the Servo Ventilator 900C and (b) an inspiratory pressure preset of 10 cm H₂O with an expiratory pressure preset of 5 cm H₂O on the BiPAP S/T-D. Using the CP-100 pulmonary monitor, we compared the total work of breathing (WOB), the pressure-time index (PTP), and other pulmonary mechanics and gas exchange parameters between the two modes.

Results: The WOB in joules per liter (mean ± SE) (0.76 ± 0.08 vs 0.73 ± 0.08, p = 0.70), the WOB in joules per minute (8.62 ± 1.06 vs 8.11 ± 0.96, p = 0.60), and the PTP in cm H₂O/s/min (187 ± 18 vs 167 ± 18, p = 0.21) between the BiPAP S/T-D and the Servo Ventilator 900C were not statistically different. There were statistically significant differences between the two ventilators in auto-PEEP (1.34 ± 0.37 vs 0.88 ± 0.30 cm H₂O, p = 0.03), duty cycle (0.44 ± 0.01 vs 0.37 ± 0.01, p<0.001), and expiratory airway resistance (11.81 ± 1.53 vs 8.75 ± 1.22 cm H₂O/L/s, p < 0.001), but not in respiratory rate (27.48 ± 1.54 vs 28.06 ± 1.61 breaths/min, p = 0.40) or in minute ventilation (10.43 ± 0.59 vs 10.27 ± 0.37 L/min, p = 0.66). There was a statistically significant difference in the ratio of PaO₂ to the fraction of inspired oxygen (FIO₂) (333 ± 21 vs 300 ± 22, p < 0.03) but not in PaCO₂ (48 ± 2 vs 47 ± 2 mm Hg, p = 0.59) between the BiPAP S/T-D and the Servo Ventilator 900C.

Conclusions: Despite differences in initiating and maintaining the inspiratory and expiratory phases, in breathing circuits, and in ventilator circuits between the two ventilators, the performance of the BiPAP S/T-D is equally efficacious to that of a conventional mechanical ventilator in supporting respiratory muscles. Thus, the BiPAP S/T-D is safe and effective when used in mechanically ventilated patients recovering from acute respiratory failure who do not require total ventilatory support.

Key Words: bilevel positive airway pressure • CO₂ • flow-triggered pressure support • pressure-time product • pressure-triggered • rebreathing • work of breathing

Submitted on January 23, 1998
Accepted on May 8, 1998

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