Sir: We read with great interest the paper by A.W. Thille et al. [1] reporting the incidence and patterns of patient-ventilator asynchrony during assisted mechanical ventilation. Double triggering, one of the major types of patient-ventilator asynchrony, was defined by the authors as two consecutive ventilator cycles separated by a very short expiratory time (< 50% of the mean inspiratory time) with the first cycle being patient triggered. Interestingly, double triggering is second to ineffective triggering in incidence and may account for 28% of the asynchrony occurrences in patients under assisted control ventilation. When we viewed the illustrative recordings showing double triggering (see Fig. 3 in [1]), we could not find evidence of pressure deflection in the triggering phase of the first breath. Minimal airway pressure changes during flow triggering may be one explanation [2]; however, we suspect that some of the first breaths in double triggering may not be patient triggered, but are instead controlled or autotriggered because the ventilator set rate is higher in patients having a greater number of double-triggering occurrences. A good example is given in Fig. 1, which depicts a recording from a mechanically ventilated patient in our hospital who had heart failure and pneumonia. The patient was ventilated in pressure-controlled mode, with a ventilator set rate of 25/min and a trigger sensitivity of −2 cmH2O. Esophageal pressure was used as a reference for the triggered breath. After the first breath, the subsequent double-triggering-like breaths were not a real double triggering because inspiratory activity occurred only after the machine delivered breath.

Fig. 1
figure 1

Recording from a mechanically ventilated patient with heart failure and pneumonia

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Double triggering occurs as a result of the combination of premature termination of mechanical ventilation and continued inspiratory effort, which decreases the expiratory flow to zero and initiates the triggering process [3]. Although the use of flow/airway pressure waveform alone may be enough for the detection of ineffective triggering [4], the use of flow/airway pressure waveform to detect double triggering should be further validated with esophageal pressure signals or other objective measures.