ICM ARTICLE REVIEW
Background
Breath dyssynchrony stacking (BDS) refers to the unintended high tidal volumes that occur as a consequence of incomplete exhalation between consecutive inspiratory cycles delivered by the ventilator. This can commonly occur during volume-preset assist control modes during lung protective ventilation for ARDS. BDS typically occurs during either double or reverse triggering in assist control modes. Double triggering refers to the sustained inspiratory effort that lasts longer than the patient triggered respiratory cycle. This results in triggering the subsequent inspiratory cycle with incomplete interval exhalation. Reverse triggering occurs when rhythmic passive ventilator-induced insufflations result in diaphragm entrainment out of phase with the machine. The patient’s inspiratory muscle effort determines the number and timing of ventilator inspiratory cycles, allowing the patient to determine the true tidal volume (Vt) delivered irrespective of the assist mode used.
BSD can be assessed by the bedside by assessing the airway flow and pressure waveforms. Breath stacking can be identified by ‘consecutive breaths occurring in close proximity with incomplete exhalation between them’. The authors of this study recently published in Intensive Care Medicine (Sept 2016) developed the BREATHE (Breathing Recognised Expected vs. Actual Tidal volume for lung Health Enhancement) criteria to evaluate BSD [1]. Five domains of patient- ventilator interaction were considered; ventilator cycling, interval expiratory volume, cumulative inspiratory volume, expiratory time, and inspiratory time. This allowed assessment of BSD frequency, BSD tidal volume, and BSD minute ventilation. The Thielle method 2 or visual waveform inspection exists as methods of assessing BSD.
Objectives
The main objectives of this study was to ascertain the incidence of BSD in ventilated patients with ARDS, compare the BREATHE criteria to other BSD criteria, to demonstrate that BSD increases Vt in volume preset assist control modes, and to assess the effect of neuromuscular blockers (NMBs) on BSD- driven high Vt.
BSD using the BREATHE criteria – Key findings
Thirty-three patents with ARDS were enrolled over an 8-month period. 27% of patients died prior to hospital discharge or within 28 days. Waveform recording was initiated at 15.9±6.7hrs following starting ventilation. The observed BSD frequency was 27 (7-59) breaths per hour without neuromuscular blockers (NMBs). There was significant heterogeneity in BSD between patients. The observed BSD Vt was 11.3 (9.7- 13.3) ml/kg which was significantly higher than the preset Vt (6.3 (6.0- 6.8)ml/kg. BSD minute volume was 5.4 (1.3- 12.6) ml/kg/min. Peak airway pressures did not increase significantly with BSD. The use of NMBs significantly reduced the frequency of BSDs (0 (0-1) breath/ hour) and BSD minute volume (0.0 (0.0-.0.2)ml/kg/min).
Comparison between BREATHE, Thielle, and manual inspection of the waveform
Compared to the existing method of assessing BSD (Thille method), the BREATHE method recorded a lower frequency of BSD. However, the BREATHE criteria picked more high tidal volume breaths. Furthermore, the calculated Vt using the BREATHE criteria was higher than that using the Thielle criteria. The net BSD minute volume was similar between the Thielle and BREATHE methods. Compared to visual waveform inspection, there was slightly higher agreement with the BREATHE criteria compared to the Thielle criteria. BREATHE is therefore more sensitive and specific than the Thielle method for detecting high volume inspiratory cycle pairs. BREATHE was also better than manual inspection in that occult high volume insufflations are more likely to be detected.
Summary
Frequent BSDs occur which lead to potentially injurious tidal volumes despite the ventilator being set with lung-protective settings. NMBs significantly reduce the amount of BSD. Future work is necessary to ascertain if occult BSD should be integrated into protective lung strategies in ARDS trials and eventually clinical care.
This article review was prepared by Nishkantha Arulkumaran on behalf of the NEXT Committee.
References
1. Beitler JR, Sands SA, Loring SH, et al. Quantifying unintended exposure to high tidal volumes from breath stacking dyssynchrony in ARDS: the BREATHE criteria. Intensive care medicine. Sep 2016;42(9):1427-1436.
2. Thille AW, Rodriguez P, Cabello B, Lellouche F, Brochard L. Patient-ventilator asynchrony during assisted mechanical ventilation. Intensive care medicine. Oct 2006;32(10):1515-1522.
