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Decreased respiratory complexity during apnea in acute respiratory distress syndrome


Respiratory complexity (RC) as assessed by sample entropy (SampEn) is lower in patients failing spontaneous breathing trials. We evaluated the role of RC in monitoring subjects with severe acute respiratory distress syndrome (ARDS) treated with the Cardiohelp (Maquet, Rastatt, Germany) extracorporeal life-support system (ECLS). We hypothesized that RC is reduced during apnea.


Six sheep sedated with midazolam were connected to the Cardiohelp via a 23F Avalon catheter. Blood flow was ~2 l/minute and FiO2 was 0.5. Sheep were on CPAP of 8 cmH2O with FiO2 of 1 via tracheostomy. After ~6 hours of ECLS, ARDS was induced by injection of 0.1 ml/kg oleic acid. Apneas (defined as >60% reduction in minute ventilation) developed both before (Preinjury) and after ARDS (Postinjury). Heart rate (HR), mean arterial pressure (MAP), respiratory rate (RR) and tidal volume (TV) were recorded, and the PaO2:FiO2 ratio (PFR) calculated corresponding to sections of respiratory pressure waveforms containing 200 breaths during Baseline, during Preinjury and Postinjury apnea periods. Similar periods were selected before and after apneas. Inter-breath interval (IBI) and its deviation, SampEn, percentage of forbidden words (FW), bit per-word entropy (BPWEn), symbol distribution entropy (DisNEn), and stationarity (StatAv) were calculated using software. Statistics by one-way ANOVA with adjustment for multiple comparisons.


See Table 1. SampEn was associated with PFR (r 2 = 0.31, P < 0.001).

Table 1 (abstract P93)


Respiratory complexity during apnea and ECLS remains unchanged in a healthy state but decreases after ARDS. It has a significant association with PFR and may serve as an index of injury severity.

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Correspondence to A Batchinsky.

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Batchinsky, A., Necsoiu, C., Langer, T. et al. Decreased respiratory complexity during apnea in acute respiratory distress syndrome. Crit Care 17, P93 (2013).

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  • Oleic Acid
  • Mean Arterial Pressure
  • Acute Respiratory Distress Syndrome
  • Pressure Waveform
  • Spontaneous Breathing Trial