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Objective evaluation of patient–ventilator interactions during noninvasive ventilation (NIV)

^* CORIA UMR 6614, University of Rouen, ^# ADIR Association, Hôpital de Bois guillaume, Rouen, ^¶ Pulmonary and Respiratory Intensive Care Department, Rouen University Hospital & UPRES EA 3830 (IFR MP23), Institute for Biomedical Research, University of Rouen, France

CORRESPONDENCE: Herinaina Rabarimanantsoa, Research Laboratory CORIA UMR 6614 & Rouen University Hospital, Rouen, France

The success of NIV depends on patient–ventilator interactions. These interactions are evaluated with the subjective comfort score which is not always reliable. An objective evaluation is thus required. To evaluate these interactions, we use a statistical measure of the variability of a physiological signal, i.e the Shannon entropy. Our purpose is to show whether estimating Shannon entropy from airway pressure (S_P) and from the total duration of ventilatory cycles (S_T) may evaluate objectively the patient–ventilator interactions during NIV.

Pressure support NIV was applied to 4 COPD patients, 4 OHS patients in stable state and 4 healthy subjects during six successive 10-min periods with various inspiratory pressure. The flow and the airway pressure signals were recorded with sensors located near the mask. Good patient–ventilator interactions were assumed to correspond to patient well synchronized (low ineffective efforts) and with low ventilatory variability. All the subjects were awaked and both Shannon entropies S_P and S_T were computed for each ventilatory tracing.

The incidence of ineffective efforts (IE) varied from 0 to 64.7 %. S_P appeared to be strongly correlated to this incidence (r = 0.91). S_T quantified precisely the ventilatory variability. When S_P was plotted versus S_T, 4 distinct groups of patients were distinguished as follows:

S_P<1 and S_T<1: IE<10% but no ventilatory variability

S_P<1 and S_T>1: IE <10% with high ventilatory variability

S_P>1 and S_T<1: IE >10% but no ventilatory variability

S_P>1 and S_T>1: IE > 10% with high ventilatory variability

Shannon entropies objectively evaluate the patient–ventilator interactions in terms of ineffective efforts and ventilatory variability. Good patient–ventilator interactions occur when there is only few ineffective efforts (S_P<1) and a low ventilatory variability (S_T<1).

KEYWORDS: Chronic respiratory failure, noninvasive ventilation, patient–ventilator interactions, Shannon entropy