A mismatch within the patient-ventilator interface is a phenomenon which commonly occurs with invasively and noninvasively mechanically ventilated patients. The term “dyssynchrony” implies an abnormality of the expected synchrony between patient and ventilator.
- Mismatches between the patient and ventilator, also known as a- or dyssynchronies, are a frequent occurrence in mechanically ventilated patients.
- One of the most prevalent forms is double triggering, which is usually due to improper matching of mechanical breath I-times to neural I-times and of particular concern in ARDS patients as it may lead to excessive tidal volume delivery.
- Diagnosing double triggering and differentiating between the three different types can represent a considerable challenge, and requires close observation and analysis of the ventilator’s scalar waveforms.
- New technology may assist clinicians in avoiding double triggering by automatically adjusting the inspiratory time in accordance with the patient’s needs.
The frequency of dyssynchronies has been studied and they are estimated to occur at least one time in no less than 50% of patients who receive mechanical ventilation (MV) for more than 24 hours. The two most common dyssynchronies are ineffective (missed) triggering and double triggering (DT) (1). Double triggering is defined as two ventilator insufflations delivered within one patient inspiratory effort (2). The root cause for this dyssynchrony is a disproportionately shorter inspiratory time (I-time) of the mechanical breath in comparison to patient neural I-time. The resultant premature cycling of the first breath can result in inadvertent delivery of a subsequent second breath during a single inspiratory activation. This is of specific concern in patients with acute respiratory distress syndrome and most commonly occurs in fixed flow volume-targeted ventilation, because it can lead to excessive tidal volume delivery resulting from breath stacking (3). Although perceived simple in concept, the recognition of this problem is often overlooked and undiagnosed by the end user (4).
Diagnosis and resolution
The primary method of DT diagnosis is the observation and evaluation of ventilator scalar waveforms. A scalar waveform is any variable displayed over time. Most mechanical ventilators commonly allow the display of pressure, flow and/or volume over time. To further facilitate the analysis of those waveforms, some ventilators allow the display of esophageal (approximated pleural) pressure over time. To demonstrate the steps toward proper identification of DT, screenshots of ventilator waveforms are provided below. Figure 1 displays common pressure, flow, and volume waveforms revealing the DT phenomenon during invasive ventilation. Initially, the untrained eye may not be able to diagnose this phenomenon, nor may it correctly determine the origin of the problem. Commonly mistaken for the patient actively generating a second breath (Breath 2) after delivery of a mechanically timed breath (Breath 1) or air hunger, this problem, if it continues, can lead to severe adverse effects related to mechanical ventilation. Therefore, closer analysis is recommended and can be performed by utilizing esophageal manometry to compare and contrast pleural pressure and the ventilator’s airway pressure and flow changes. Another example below, which shows a ventilator displaying pressure and flow time scalars, provides a subtle hint of possible DT, but may also be mistaken for an additional active inspiratory effort (Figure 2). The addition of the esophageal pressure scalar waveform (Pes-Paux waveform) reveals that in fact a double trigger is present because of the subsequent delivery of breaths during a single active inspiratory effort (see the decrease in pleural pressure in Figure 3).
Differentiating and classifying the type of DT is also challenging at the bedside. Current research suggests that DT can be classified into three different types (2):
- Patient-triggered (DT-P): First triggered breath has an esophageal pressure decrease of >1 cmH2O and may be associated with a strong inspiratory effort
- Auto-triggered (DT-A): First triggered breath occurs earlier than the ventilator-set time trigger, without a concomitant drop in esophageal pressure
- Ventilator-triggered (DT-V): First breath occurs at the ventilator-set time trigger, without a concomitant drop in esophageal pressure
Data has shown there is often a delay in triggering in the pre-inspiratory phase of between 0.07-0.13 seconds (5). Evaluation of the airway pressure decrease is more powerful than the flow change at the 0.13 seconds trigger delay phase (2). Therefore, the pressure decrease of > .49 cm H2O at this point can distinguish DT-P breaths from DT-A and DT-V breaths (2). Additional data revealed that the neural inspiratory time, which can be calculated as the onset of a rapid decline of esophageal pressure to the nadir, was significantly longer in the first DT-P breath than in previous breaths (6). Therefore, airway pressure decreases coupled with neural inspiratory time calculations can assist in identifying patient double triggering.
The most common causes for DT are the improper matching of mechanical breath I-times to neural I-times, and an insufficient level of pressure support with high respiratory drives (7). Specifically, the mechanical breath I-time is too short in comparison to the longer neural I-time. Therefore, lengthening the mechanical breath inspiratory time to match the patient's neural inspiratory time or increasing the ventilator output pressure and tidal volume may minimize or eliminate DT (8). However, this requires the end user to be present to observe this phenomenon, as well as manual manipulation of the ventilator. Partial automation of this adjustment is made possible by the IntelliSync+ function* on Hamilton Medical ventilators. IntelliSync+ pays close attention to the cycling criteria of each breath and adjusts the inspiratory time in accordance with patient need**. This option reduces the number of asychronies, thus providing improved patient comfort, and may also have a beneficial effect on patient outcomes.
* Not yet available in all markets.
** This does not apply to controlled mandatory breaths as TI is set by the user.
- Thille AW, Rodriguez P, Cabello P, Lellouche F, Brochard L. Patient-ventilator asynchrony during assisted mechanical ventilation. Intensive Care Med 2006;2(10):1515-1522
- Liao KM, Ou CY, Chen CW. Classifying different types of double-triggering based on airway pressure and flow deflection in mechanically ventilated patients. Resp Care 2011;56 (4):460-466
- Pohlman MC, McCallister KE, Schweickert WD et al. Excessive tidal volume from breath stacking during lung protective ventilation for acute lung injury. Crit Care Med 2008;36(11):3019-3023
- Colombo D, Cammarota G, Alemani M, et al. Efficacy of ventilator waveforms observation in detecting patient-ventilator asynchrony. Crit Care Med 2011; 39:2452–7.
- Takeuchi M, Williams P, Hess D et al. Continuous positive airway pressure in new-generation mechanical ventilators: a lung model study. Anesthesiology 2002;96(1):162-172
- Parthasarathy S, Jubran A, Tobin M. Assessment of neural inspiratory time in ventilator-supported patients. AJRCCM 2000;162:546-552
- Kallet RH, Campbell AR, Dicker RA, et al. Effects of tidal volume on work of breathing during lung-protective ventilation in patients with acute lung injury and acute respiratory distress syndrome. Crit Care Med 2006; 34:8–14
- Vignaux L, Vargas F, Roeseler J et al. Patient-ventilator asynchrony during non-invasive ventilation for acute respiratory failure: a multicenter study. Intensive Care Medicine 2009;35:840-846
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