EFFECTS OF FLOW TRIGGERING ON BREATHING EFFORT DURING PARTIAL VENTILATORY SUPPORT

The effects of flow triggering (FT) as compared with pressure triggeri ng (PT) on breathing effort have been the focus of several studies, an d discrepant results have been reported. In the initial part of our st udy, a lung model was used to quantify triggering effort (airway press ure-time product, PTPaw) for a range of sensitivity settings in nine n ew-generation ventilators. A ventilator providing both FT and PT was t hen used to compare these systems during pressure-support (PSV) and vo lume-targeted assist-control ventilation (ACV) in eight ventilator-dep endent patients, using sensitivity settings (2 L/min for FT and -2 cm H2O for PT) that had proven significantly different in the initial ben ch study. indexes of effort included the esophageal and transdiaphragm atic pressure-time products and inspiratory work of breathing per minu te (PTPes/min, PTPdi/min, and Wi/min, respectively). The experimental study revealed significant differences between ventilators in PTPaw at commonly used settings. In two of three ventilators featuring both sy stems, PTPaw was significantly lower with FT than PT (p < 0.001). In t he clinical study, FT as compared with PT,was associated with reductio ns in all indexes of breathing effort during PSV: 16 +/- 6% (p < 0.001 ), 13 +/- 10% (p < 0.01), and 14 +/- 12% (p < 0.05) for PTPdi/min, PTP es/min, and Wi/min, respectively. By contrast, no differences were fou nd when FT was used during ACV. Although FT reduced triggering effort in both modes (p < 0.001), the effects observed during the post-trigge r phase differed, and explained the discrepant results between the two modes. We conclude that FT more effectively reduces breathing effort when used in conjunction with a pressure-targeted mode than with a vol ume-targeted mode, especially when flow delivery is close to or below demand.