Effects of Flow Triggering on Breathing Effort

Abstract & Commentary

Aslanian and associates compared pressure and flow triggering of the mechanical ventilator during pressure support (PS) and volume assist/control (A/C) both in a lung model and in eight patients. In the lung model evaluation, the Puritan-Bennett 7200ae, Siemens 900C and 300, Bird 8400ST, Taema Cesar, Hamilton Veolar, Newport Wave, Draeger Evita 2, and Engstrom Erica ventilators were compared at pressure triggering sensitivities of –0.5, –1.0, and –2 cm H2O, and at flow sensitivities of 1, 2, 3, 4, and 5 L/min. In the clinical evaluation, the PB 7200ae was used to compare pressure triggering sensitivity –2 cm H2O and flow triggering sensitivity 2 L/min.

During both pressure triggering and flow triggering, there were significant differences observed among the different ventilators. The Newport Wave performed best during pressure triggering, followed by the Siemens 300, and the Siemens 300 and Draeger Evita 2 performed best during flow triggering. With the most sensitive settings, there were no differences between pressure and flow triggering in any ventilator. Pressure triggering was unavailable on the Draeger Evita 2 and Engstrom Erica, and flow triggering is unavailable on the Siemens 900C, Taema Cesar, Hamilton Veolar, and Newport Wave.

In the clinical arm of the study, flow triggering outperformed pressure triggering during pressure support, but during volume A/C there were no differences in patient effort. With PS, the esophageal pressure-time product per minute (193 ± 77 vs 168 ± 67 cm H2O s/min), the diaphragm pressure-time product/min (191 ± 80 vs 161 ± 68 cm H2O s/min), and the inspiratory work of breathing, both per min (12.2 ± 6.8 vs 10.5 ± 5.9 joules/min) and per liter (1.16 ± 0.53 vs 1.00 ± 0.49 joules/L) were all significantly lower with flow triggering. (Aslanian P, et al. Am J Respir Crit Care Med 1998;157:135-143.)

COMMENT BY ROBERT M. KACMAREK, PhD, RRT

This study clearly demonstrates that the triggering capabilities of today’s mechanical ventilators have improved dramatically compared to those of the previous generation of ventilators. In addition, based on the lung model data when set at consistent sensitivity settings, there were no differences in triggering effort between flow (FT) and pressure (PT) triggering during PS and A/C. This is contrary to the data on the 7200ae published by Sassoon et al (Crit Care Med 1987;17:1108 and ARRD 1992;145:1219) during CPAP. However, this difference is expected based on the way PT and FT function during CPAP. With PT and CPAP, the sensitivity setting is normally targeted (e.g., 1 cm H2O below baseline), whereas FT manufacturers have targeted actual baseline pressure or higher. With assisted modes of ventilation, once the ventilator senses patient effort, gas delivery is determined by the assisted breath algorithm rather than by the triggering algorithm.

The results of the clinical aspect of this study must be questioned for Aslanian et al to choose sensitivity settings that were not comparable in the laboratory analysis (-2 cm H2O PT; 2 L/min FT). If a more sensitive PT were selected (-0.5 or –1.0 cm H2O), it is unlikely that any differences would have been observed between the two types of triggers. Goulet et al (Chest 1997;111:164) observed no differences in triggering effort between PT and FT when PT of –0.5 and –1.0 cm H2O were compared to flow triggers of 2 and 3 L/min.

I would recommend the use of a flow trigger in preference to a pressure trigger whenever it is available. Since FT is clearly superior to pressure triggering during

CPAP and pressure triggers tend to be set with less sensitivity than flow triggers, and also because the accumulation of condensate in the ventilator tubing results in auto-cycling with PT set at -1.0 cm or less H2O, the new flow triggering systems should always be used.