Different modes of assisted ventilation in patients with acute respiratory failure

Abstract

The aim of the present study was to verify that the patient/ventilator interaction is similar, regardless of the mode of assisted mechanical ventilation (i.e. pressure-or volume-limited) used, if tidal volume (VT) and peak inspiratory flow (PIF) are matched. Therefore, the authors compared the effects of three different modes of assisted ventilation on the work of breathing (WOB) and gas exchange in patients with acute respiratory failure.For Protocol 1, in seven patients, the authors compared pressure support, assist pressure control and assist control (with square and decelerating wave inspiratory flow pattern) set to deliver the same VT and PIF. For Protocol 2, in another 10 patients, the authors compared pressure support and assist control with high (0.8 L?s -1 ) and low (0.6 L?s -1 ) PIFs set to deliver the same VT.In Protocol 1, there was no difference in WOB and gas exchange between the three modes of assisted ventilation tested. In Protocol 2, the decrease of PIFs during assist control significantly increased WOB.In conclusion, different modes of assisted ventilation similarly reduce work of breathing and provide adequate gas exchange at fixed tidal volume and peak inspiratory flow only. During assist control, tidal volume and peak inspiratory flow (set by the physician) are the main determinants of the patient/ventilator interaction. A primary goal of mechanical ventilation is to improve gas exchange and reduce the work of breathing (WOB) of patients with acute respiratory failure, without causing iatrogenic lung injury [1]. Assisted ventilation allows the patient to contribute to minute ventilation (V9E) and offers several advantages over controlled ventilation. It can reduce the need for sedation and paralysis, decrease the risk of barotrauma [2], improve intrapulmonary gas distribution [3], and prevent muscle atrophy [4,5]. During assisted ventilation, the WOB is dependent on both the ventilator settings and the patient9s ventilatory demand and mechanics.Pressure support ventilation (PSV) is a pressurelimited, flow-cycled mode of assisted ventilation, in which each breath is supported by a constant level of pressure at the airway (Paw), so that the tidal volume (VT) and inspiratory flow are more adaptable to the patient9s own ventilatory demand [6]. This manner of supporting the patient9s own ventilatory effort may be responsible for an improved comfort and synchrony with the ventilator, and has been shown to reduce the WOB and prevent diaphragmatic fatigue in patients with respiratory failure [7]. The main disadvantage of PSV is that it only works best in patients with stable respiratory conditions (i.e. an adequate sufficient ventilatory drive and somewhat preserved respiratory mechanics [8,9]).Assist control ventilation (ACV) is a volume-limited mode of assisted ventilation, in which the VT and the peak inspiratory flow (PIF) are set by the clinician and are not altered by the patient9s ventilatory demand. If the required VT and inspiratory flow are higher than what the ventilator suppli...

Effect of different inspiratory rise time and cycling off criteria during pressure support ventilation in patients recovering from acute lung injury

“…An alternative explanation for the flow-associated reduction in WOB I may arise from the possible effect of flow on the force velocity relationship of the respiratory muscles (19). For a given respiratory muscle output, an increase of peak inspiratory flow per se can contribute to a decrease in the pressure output of the inspiratory muscle in agree- With all the limitations of our approach, our data confirm the importance of an adequate match between the patient's need and the machine initial flow (6,8,15).…”

“…The WOB I per minute (WOB I min) and WOB per liter (WOB I L) were computed from a modified Campbell diagram, where the elastic recoil pressure of the chest wall was taken at the point of sharp inspiratory deflection of the Pes tracings, that is, the onset of inspiratory effort (14). The chest wall elastance was calculated by plotting Pes against tidal volume while the respiratory muscles were assumed to be in a state of relaxation achieved by a brief period (3-5 mins) of slight hyperventilation during which the patient's inspiratory activity was abolished (15). To exclude errors in computing WOB I and PEEP I due to expiratory muscle activity at the beginning of inspiration, the abrupt decay of Pga from its maximal end expiratory value to its minimal level was measured, and this value was subtracted from the onset of inspiratory effort (i.e., on the Pes tracings) (16).…”

Modulation and treatment of patient–ventilator dyssynchrony

“…Rules for setting an optimal rise time, however, are lacking. Notwithstanding this, we should note that studies have shown that both very high and very low rise time may be associated with increased work of breathing [25][26][27][28].…”

“…patient-ventilator asynchrony has been observed, mainly due to dissociation between the patient's respiratory effort and ventilator pressure both in terms of timing (uncoupling between the end of mechanical and neural inspiration) and inadequate or excessive assist [14,24]. In addition with pressure support the pressure rise time (defined as the time that pressure reaches the preselected threshold) may influence the synchronization between the patient and the ventilator [25][26][27][28]. In some of the new generation ventilators the rise time is adjustable and this represents a step forward in modulating patientventilator dyssynchrony.…”

Interpretation of ventilator curves in patients with acute respiratory failure

“…The latest (fourth) generation ventilators were introduced two decades later. The new devices are efficient in responding to patient demand, offer new ventilation modes, allow noninvasive ventilation, have started to incorporate automatic control systems (open lung tools, weaning tools, dual modes), 1 and have enormous monitorization capacities---including onscreen visualization of pressure, flow and volume curves. 2 In addition, the need has recently arisen to introduce telemedicine-based systems as online monitoring tools at the patient bedside.…”