Diaphragmatic electrical activity: a new tool to assess lung hyperinflation?

447 September 2014 “ EXPIRATION and slow decrease of light together reaching minimum together ... in about 10 s and immediately cry as before. Silence and hold for about 5 s.” In the script for his late play “Breath,” Samuel Beckett conveys in sound and light his awareness of long and vital expiratory times. His personal experience with emphysema might have provided some of the inspiration. In the current issue of ANESTHESIOlOgy, Bellani et al.1 propose a new method to quantify the respiratory mechanical consequences of these required long exhalations in ventilated patients and to assess their response to positive end-expiratory pressure (PEEP). Auto-PEEP is the positive alveolar pressure in excess of any applied PEEP present in some patients’ lungs at the end of a passive exhalation. It results from incomplete pulmonary emptying due mainly to increased airway resistance and reduced lung elastic recoil, typical of chronic obstructive pulmonary disease, and potentially worsened postoperatively.2 Auto-PEEP is related to dynamic lung hyperinflation, that is, end-expiratory volumes above the volume of lung elastic relaxation, with onset of spontaneous or mechanical inspiration characteristically during an ongoing exhalation. The main determinants of auto-PEEP are minute ventilation, expiratory time, and time constant of the respiratory system (i.e., the product of the respiratory system resistance and compliance). Increased dyspnea, muscle fatigue, barotrauma, hemodynamic compromise, and even cardiac arrest are some of the important clinical consequences.3 In patients under assisted modes of mechanical ventilation and during ventilator weaning, auto-PEEP accounts for a substantial increase in patient’s muscle energy expenditure4 and is associated with ineffective efforts, that is, inspiratory muscle contraction insufficient to trigger a ventilator breath. This is due to the mechanical load added by autoPEEP to the respiratory system, which hinders the achievement by the patient of usual triggers of ventilatory support, pressures or flows. Despite its relevance, use of auto-PEEP as a variable to guide clinical practice in spontaneously breathing ventilated patients is limited due to the difficulty of its measurement which requires absence of expiratory muscle activity and placement of an esophageal balloon. Neurally adjusted ventilatory assist (NAVA) is a ventilatory mode that uses the electrical activity of the diaphragm (EAdi-derived from electrodes from a special esophageal catheter) to trigger assisted support. Because the trigger, cycling, and assistance level are all based on diaphragm electrical activity and not on pressures or flows, it has been shown to improve patient– ventilator synchrony.5 Bellani et al.1 studied whether that electrical activity of the diaphragm could also be used to estimate auto-PEEP during two spontaneous modes of mechanical ventilation: pressure support ventilation (PSV) and NAVA. The authors measured the EAdi at the onset of inspiratory flow (denominated auto-EAdi) in 10 patients with suspected auto-PEEP and showed that auto-EAdi followed closely auto-PEEP in individual patient analysis. The variable relation between EAdi and the pressure generated by the respiratory muscles among patients was elegantly addressed by calibrating the EAdi to the negative deflection in the proximal airway pressure during an inspiratory effort against closed inspiratory and expiratory valves.6 The authors concluded that the auto-EAdi provides a simple and reliable tool for continuously monitoring the presence of dynamic auto-PEEP at the bedside. Diaphragmatic Electrical Activity