What is ventilator asynchrony?

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The basic mechanism of patient-ventilator asynchrony is the mismatching between neural inspiratory and mechanical inspiratory time. Alterations in respiratory drive, timing, respiratory muscle pressure, and respiratory system mechanics influence the interaction between the patient and the ventilator. None of the currently available partial ventilatory support modes are exempt from problems with patient-ventilator asynchrony. Ventilator triggering design in the trigger phase and the set variables in the post-trigger phase contribute to patient-ventilator interaction. The set inspiratory flow rate in the post-trigger phase for assist-control volume cycled ventilation affects patient-ventilator asynchrony. Likewise, the initial pressure rise time, the pressure support level, and the flow-threshold for cycling off inspiration for pressure support ventilation are important factors affecting patient-ventilator asynchrony. Current investigations have advanced our understanding in this area; however, its prevalence and the extent to which patient-ventilator asynchrony affect the duration of mechanical ventilation remain unclear.

During pressure-support ventilation, tidal volume (VT) can vary according to the level of the patient's respiratory effort and modifications of the thoraco-pulmonary mechanics. To keep VT as constant as possible, the Siemens Servo 300 ventilator proposes an original modification of pressure-support ventilation, called volume-support ventilation (VSV). VSV is a pressure-limited mode of ventilation that uses VT as a feedback control: the pressure support level is continuously adjusted to deliver a preset VT. Thus, the ventilator adapts the inspiratory pressure level, breath by breath, to changes in the patient's inspiratory effort and the mechanical thoraco-pulmonary properties. The clinician sets VT and respiratory frequency, and the ventilator calculates a preset minute volume. It has been shown that ineffective respiratory efforts can occur during pressure-support ventilation. A mismatch between the neural (ie, patient) and mechanical (ie, ventilator) timings is the main cause of missing breaths occurring while the ventilator is in the inspiratory phase: the reason is that the ventilator does not cycle from inspiration to expiration until the inspiratory flow decreases to a threshold value (5% of the peak inspiratory flow). The patient's ineffective efforts can also occur during the expiratory phase of the ventilator: in that situation, the inspiratory effort occurs before complete lung emptying and is not high enough to trigger the ventilator. The risk of the patient making ineffective efforts is increased by the algorithm included in the VSV mode. If the patient makes numerous ineffective efforts, the frequency of effective efforts (recorded by the ventilator) can be lower than the set frequency, in which case a new target VT is calculated by the ventilator to achieve the preset minute volume. As VT increases, the mismatch between the neural and mechanical timings also increases. I report 3 clinical observations showing numerous patient respiratory efforts not sensed by the ventilator and inducing VT instability during VSV. These ineffective efforts can occur during inspiratory and expiratory phases. The mechanisms are discussed. [Respir Care 2001;46(3):255-262] Key words: asynchrony, dyssynchrony, patient-ventilator interactions, volume-support ventilation, pressure-support ventilation, tidal volume, inspiratory effort, ventilator triggering, mechanical ventilation.

Introduction

Pressure-support ventilation (PSV) is a patient-triggered, pressure-limited, flow-cycled mode of ventilation. PSV provides a constant level of positive pressure during spontaneous ventilation. Breaths are pressure-triggered or flow-triggered. During PSV, tidal volume (VT) can vary according to the patient's inspiratory efforts and thoraco-pulmonary mechanics. The Siemens Servo 300 ventilator offers a ventilation mode called volume-support ventilation (VSV). The goal of VSV is to ensure a constant, preset VT during PSV. Using a closed-loop control system, the Siemens Servo 300 adapts the level of inspiratory pressure support (PS) to deliver a preset VT. Delivered VT is used as a feedback control for continuous adjustment of the PS level.1 The ventilator automatically adapts the PS level to changes in the mechanical thoraco-pulmonary properties and the patient's inspiratory effort. To initiate VSV the clinician sets the target VT and respiratory frequency. VSV includes a specific algorithm. On the basis of the preset respiratory frequency and VT, the ventilator calculates a "minimum minute ventilation" (respiratory frequency multiplied by VT).1 If the patient's breathing frequency is lower than the preset frequency, the minimum minute volume cannot be reached. In that situation (minute volume lower than that calculated on the basis of the preset parameters), the ventilator calculates a new target VT as a reference for regulation of PS. The maximum new calculated VT may be up to 150% of the preset VT. For example, with a preset VT of 500 mL and a preset frequency of 10 breaths/min, the minimum minute volume is 5 L/min. If the patient's frequency drops below 10 breaths/min, the new calculated VT increases to reach the calculated minimum minute volume and the maximum new calculated VT will be 750 mL.

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During both pressure-triggered and flow-triggered VSV, a bias flow of 2 L/min is delivered into the circuit during expiration. When the ventilator is set to "flow-triggering," the ventilator can be triggered when a flow of 0.7-2 L/min is inhaled by the patient from this bias flow. Flow-triggering is set by moving the trigger button into the green area (lower sensitivity) or red area (higher sensitivity, with the risk of auto-triggering). In this study, we set the trigger button directly between the green area and the red area.

During VSV, the Siemens Servo 300 cycles from inspiration to expiration when the inspiratory flow reaches 5% of the peak inspiratory flow. A safety mechanism limits the inspiratory time (TI) to a value of 80% of the preset respiratory cycle duration.

Ineffective respiratory efforts may occur during the patient-triggered modes of ventilation, inducing VT variability.2 Nava et al3 and Jubran et al4 have shown that many patients suffering chronic obstructive pulmonary disease make ineffective efforts during PSV and appear to struggle against the ventilator. The potential for dyssynchronous interaction during VSV has not been evaluated in the literature.

Patient-ventilator asynchrony may escape routine clinical survey. Nevertheless, careful analysis of available respiratory waveforms provided by new ventilators allows the clinician to detect the wasted efforts occurring during the gas delivery phase of the ventilator or during the expiratory phase. Available graphic displays include scalars (waveform plotting pressure or flow or volume vs time) and loops (simultaneous plotting of two respiratory variables).5,6 Because active use of respiratory muscles may affect the patterns of the curves, patient and ventilator frequencies can be determined by examination of the displayed curves. "1:1 interaction" means that all the patient's breathing efforts trigger the ventilator and induce lung inflations applied by the ventilator. Conversely, "non-1:1 interaction" indicates the presence of active spontaneous inspirations that are not assisted by mechanical inflation.

This report illustrates some examples of patient-ventilator interaction during VSV with the Siemens Servo 300. We observe that patient-ventilator dyssynchrony during VSV can induce marked VT instability.

Basically it means that the patient's lungs and the ventilator's breaths are not working together in the most efficient manner. Most modern ventilators are equipped to sense the patient's breathing/lack of breathing and deliver breaths accordingly, reducing asynchrony.