The double trigger (DT) is the second asynchrony in frequency, after the failed cycles [1]. Compared to the latter, it can be more injurious since it usually increases tidal volume (Vt), transmural and transvascular pressure to levels that can cause ventilator-induced lung damage [2,3]; and self-inflicted by the patient (P_SILI) [4], as well diaphragmatic injury. DT and Reverse triggering (RT) may be related, although it is not always the case. However, the relationship between DT and a vigorous effort by the patient is a common feature.

We present three cases of critically ill patients admitted to the ICU who presented this asynchrony in the onset of the weaning period of the mechanical ventilation (MV). All the patients were sedated, RASS score 4.

The first case corresponds to a 54-year-old male patient, with a history of kidney transplantation and COVID-19 pneumonia, with 10 days MV, in volume-controlled assisted mode with a constant flow (ACV) (Figure 1). The second case was a 62-year-old male patient who was admitted for subarachnoid hemorrhage, complicated with an infraction due to vasospasm. After reduced sedation, monitoring was performed after four days during ACV mode (Figure 2). The third case is a 57-year-old male patient on the first postoperative day of double lung transplantation due to histiocytosis X; in this case, monitoring was performed during pressure support ventilation (PSV), (Figure 3). As a representative example of true RT, although without DT, we show the case four, the monitoring of a 69-year-old woman with COVID-19 pneumonia, during PSV, and after 19 days in MV (Figure 4).

In second and third patients; the flow signal was obtained using a Hans Rudolph 4700^® pneumotachograph connected to a Sensortechnics^® differential pressure transducer, LBAS025B. Airway pressure (Paw), Esophageal (Peso) and Gastric (Pgas) were obtained using a Sensortechnics^® SQ7500 pressure transducer. Vesical pressure (Pvesical) was obtained using a vascular sensor (Edwards TruWave^®). Peso and Pgas measurements were obtained using a specific probe, Nutrivent, Sindam^®. All signals were acquired by Teensy 3.5^® analogue-to-digital converter sampled at 1045 Hz/channel for 60 min.

For the first and fourth patients, both patients with COVID-19, the respiratory signals were obtained with connection to the Medibus protocol of the Evita ventilator using personal software, by the acquisition of Paw, flow and volume signals sampled at 125 Hz/channel. This methodology allowed us to preserve our usual monitoring system of contamination. We have replaced the parameters derived from esophageal monitoring with the muscular pressure deduced from the equation of motion (Figures 1 and 4).

In both monitoring systems, the parameters related to effort and respiratory works have been analyzed, as well as the delay between the commencement of the ventilator and the neural breath (dP). The dP divided by duration de mechanical cycle (Ttot) and multiplied by 360° provide the phase angle (θ). The frequency and ratio of the association of the two phenomena DT and RT, were also recording.

In the first and the second case, after assessing asynchrony, the ventilatory mode was changed to PSV, improving interaction with the ventilator. In the third case, it was necessary to increase the sedation. In case four, during PSV, inspiratory pressure was increased.

DT was initially described as a consequence of an intense and wide inspiratory muscular effort that caused double cycling of the ventilator [1].

False double triggering (Pseudo-double-triggering) was described as a double cycle that occurred after a cycle initially initiated by the ventilator followed by a subsequent inspiratory neuromuscular impulse that caused the second cycle of the ventilator [5].

At the same time, a very similar phenomenon was described in patients with respiratory distress consisting of the ventilator-triggered muscular efforts. This term was described as RT, and which could promote DT (6). This muscular effort, apparently triggered by the ventilator, has been attributed to a reflex phenomenon. In principle, the respiratory driver of the patient can be neuro-coupled or entrainment by this reflex mechanism [6].

Subsequently, DT has been classified into three variants [7]: DT with the initial impulse generated by the patient (DTP), which is equivalent to the original DT, and Double trigger with the initial impulse generated by the set rate in the ventilator followed by the neuromuscular impulse of the patient (DTV) that is equivalent to the false double trigger. That last has a variant: due to auto-triggering of the ventilator (DTA). The latter two variants are referred by other authors as DTs due to RT.

Note that RT may promote or not double cycling (DC) and that in both cases it will be accompanied by an increase in elevated transpulmonary pressure and volume, a good example is provided in the figures, and therefore potentially harmful both pulmonary (VILI, P-SILI and diaphragmatic due to polymeric contraction.

The existence of RT in the presence of a vigorous patient driver not related to a reflex or entrainment phenomenon has not been sufficiently elucidated so far, except for the description of the term of DTV [5], where a phenomenon equivalent to RT is not related to a reflex mechanism.

This type of double driver (one from the ventilator that precedes, the second one from the patient) that does not induce DT can be as frequent and potentially dangerous as the one that does induce it.

In all the previous studies they refer to patients ventilated in ACV mode with both volume control and pressure control. We also provide for the first time, to our knowledge, cases with pressure support ventilation (PSV), associated with auto-triggering, which resemble Pressure Control Ventilation (Figures 3-4).

Our opinion is that in the first three cases the double trigger and the Reverse are due to the simple mismatch of two different drivers, the neural being powerful enough to generate an increase in Vt. To demonstrate true RT we propose to cancel the mechanical cycle of the ventilator using an occlusion maneuver or to change to a brief cycle of interleaved CPAP, which would reveal whether or not the patient’s impulse is related to the mechanical cycle-, as shown in Figure 4.

We would prefer to call these asynchronies as Reverse Trigger in the broad sense or Dual DTV-type driver, where the mechanical cycle precedes the neural. In our experience, the most frequent double trigger in ACV mode is DTV and the DTA in support ventilation. We have rarely encountered classic DTP [8].

The authors declare that have no conflicts of interest.

BL JA, conducted and designed the study. SS JM collected and analyzed the patient data. Both authors have interpreted all data, written, and revised the manuscript.

Study approved by the Research Ethics Board, University Hospital Reina Sofía of Cordoba, Spain (RT-PSV 2017). Written Informed consent for publication was obtained from the patient’s family.

We thank Almudena Serrano Benítez, for their valuable assistance in reviewing the manuscript.