Patient-ventilator interaction during synchronized intermittent mandatory ventilation. Effects of flow triggering.

Abstract

Synchronized intermittent mandatory ventilation (SIMV) intermixes assisted and spontaneous breaths. Its ability as a weaning technique has been questioned on the basis that patients show little adaptation to ventilator assistance. We studied inspiratory effort and patient-ventilator interaction at different levels (SIMV, 100, 50, and 0%) of flow-triggered SIMV versus pressure-triggered SIMV in patients during the weaning period. The two triggering systems were evaluated during constant flow and constant pressure mandatory SIMV breaths. Inspiratory effort was estimated as the esophageal pressure time product (PTP) per breath (PTP/b) and per minute (PTP/min). The PTP/b and PTP/min of both mandatory and spontaneous breaths were significantly lower during flow triggering than during pressure triggering SIMV, irrespective of the ventilatory mode. During pressure-triggered SIMV PTP/b and PTP/min were identical for mandatory and spontaneous breaths, whereas during flow-triggered SIMV PTP/b and PTP/min were significantly lower for mandatory than for spontaneous breaths. This difference was greatest when flow triggering and constant pressure ventilation were associated. These data show that flow triggering reduces inspiratory effort during both mandatory and spontaneous SIMV breaths and obtains a better patient-ventilator interaction.

Control of breathing in mechanically ventilated patients

“…The degree of downregulation should depend on the sensitivity to CO 2 and the magnitude of PCO 2 reduction. Notwithstanding that the response to unloading might be related, to some extent, to baseline mechanical load or to the mode of mechanical ventilation [37], these results emphasize the importance of chemical feedback during mechanical ventilation. Paradoxically, the role of chemical feedback has been largely ignored by studies dealing with the effect of mechanical ventilation on respiratory muscle activity.…”

“…Chemical feedback could be a critical factor for this breath programming. These results have recently been challenged by GIULIANI et al [37], who showed that the mode of mechanical ventilation is important to show an effect of unloading on respiratory effort. They demonstrated that inspiratory effort was smaller in mandatory breaths than in spontaneous only if SIMV is applied with flow-triggering and constant pressure.…”

“…Currently, in mechanically-ventilated patients inspiratory flow rates are adjusted mainly for the purpose of enhancing patient-ventilator interaction and of changing inspiratory time, and, thus, affecting airway pressures, dynamic hyperinflation, haemodynamic status and distribution of ventilation [37,58,59]. However, inspiratory flow rates may affect respiratory output in a way that has been largely ignored in patient management.…”

Equipment needs for noninvasive mechanical ventilation

“…Some ventilators have a fixed trigger, whereas in others the sensitivity can be varied. In the past, ventilators were usually pressure triggered, but it has been shown that flow-triggered devices are more sensitive than pressure-triggered devices [9][10][11]. The sensitivity of the inspiratory trigger needs to balance two extremes: 1) an insensitive trigger increases work of breathing; and 2) a trigger that is too sensitive leads to auto triggering.…”

Weaning from mechanical ventilation

“…These "flow-by" or flow-triggered mechanisms have been compared to conventional demand-valve systems, and recent studies [58][59][60] have reported that flow-by systems, used either in CPAP or SIMV, are able to produce a significant decrease in respiratory work and other indices of inspiratory muscle effort when compared to the conventional demand-valve.…”