Figure 1.

A-C, Demonstration of expiratory muscle relaxation-induced ventilator triggering (ERIT). Flow, Paw, Pes,total (ie, uncorrected Pes signal), Pes,insp (ie, Pes,total – Pga), Pga, and EAdi waveforms recorded during pressure support ventilation with 6 cm H₂O (A), 10 cm H₂O (B), and 2 cm H₂O (C) of support above a positive end-expiratory pressure of 16 cm H₂O. Pao₂ to Fio₂ ratio was 132 mm Hg, and the patient was receiving continuous sedation (Richmond Agitation Sedation Scale score of –4, with propofol 6.6 mg/kg predicted body weight/h and fentanyl 0.66 μg/kg predicted body weight/h). Before recording, adequate position of the double-balloon catheter and EAdi catheter was confirmed using the Baydur maneuver and esophageal spasms present in the Pes tracing, but not in the Pga tracing, and by using the EAdi positioning tool provided on the Servo-U ventilator (Getinge), respectively. Ventilator flow, Paw, and EAdi waveforms were obtained from the ventilator using Servo-tracker software (release 4.2). Simultaneously, Paw, Pes, and Pga were acquired with a dedicated measurement setup (Biopac MP160; BIOPAC, Inc.). Data were synchronized based on Paw tracings that were acquired simultaneously using both acquisition devices and were processed using a software routine for MATLAB 2020b (Mathworks). The onset for different signals was defined as follows (A, B): (1) onset ventilator triggering (Vent,trigg; black dashed line) was defined as start of inspiratory flow (ie, nadir in Paw); (2) onset expiratory muscle relaxation (orange solid line) was defined as start of continuous decrease in Pga; and (3) onset EAdi (light blue solid line) was defined as start of inspiratory EAdi increase of > 0.5 μV, provided that EAdi peak is > 2 μV (to qualify as a breath). Ttot was defined based on flow zero-crossings (A, blue arrow in flow signal). Total Pes decrease was calculated from the onset in Pga drop (concomitant with start of decrease in Pes,total) to Pes,total nadir. The decrease in Pes,insp (ie, reflecting the patient's true inspiratory effort) was calculated from the onset in Pga drop (concomitant with start of decrease in Pes,insp) to Pes,insp nadir. The Pga drop from expiratory muscle relaxation was calculated from the onset in Pga drop to Pga nadir. A, EAdi onset started after ventilator triggering, whereas the drop in Pga and Pes occurred before ventilator triggering. The start of drop in Pga and Pes was very close to the first time point of the sudden decrease in Paw below set positive end-expiratory pressure (start of triggering phase). B, Example showing that not all ERIT breaths were followed by inspiratory effort (arrow): EAdi < 2 μV and the total Pes decrease was approximately equal to the Pga drop from expiratory muscle relaxation. C, Example of double breaths (gray area) with ERIT in a 2:1 pattern. The first ventilator pressurization was the result of some degree of expiratory muscle relaxation not followed by patient inspiratory effort (absent EAdi indicated by arrow in EAdi signal and the total Pes decrease approximately equal to the Pga drop). Then, complete expiratory muscle relaxation resulted in a ventilator insufflation followed by an inspiratory effort. EAdi = electrical activity of the diaphragm; Paw = airway pressure; Pes = esophageal pressure; Pes,insp = esophageal pressure related to inspiratory effort; Pes,total = total esophageal pressure; Pga = gastric pressure; Ttot = total ventilator cycle duration; Vent,trigg = ventilator triggering.