HHE Emergency and critical care 2019 | Page 11

FIGURE 1 Patient’s inspiration: pressure wave 35 30 25 20 15 10 5 Negative defl ection in pressure wave due to patient’s inspiratory eff ort (red arrow). The two dotted lines indicate the beginning of the patient’s inspiratory eff ort (1) and the delayed ventilator support (2) – inspiratory delay 1 FIGURE 2 Patient’s inspiration: fl ow wave 1500 1000 500 0 -500 -1000 -1500 The blue arrow shows the change in fl ow slope due to a patient’s inspiratory eff ort. The initial part of the expiration phase is characterised by a certain slope, representing the passive defl ation of the lungs. When any inspiratory muscle activity starts, it produces a change in fl ow shape, making the slope steeper towards zero fl ow. Note that the ventilator is able to detect patient’s activity only if it reaches the set inspiratory trigger level (always at positive values of fl ow). Thus, delayed triggering will always be present if patient’s inspiration starts at negative values of fl ow (for example in presence of hyperinfl ation). The two dotted lines indicate the inspiratory delay 1 . technologies able to optimise the issue (mainly diaphragmatic electrical activity, other automatic triggering systems available on a few modern ICU ventilators). However, these tools are not always available for every ICU patient. The second approach is based on the observation of ventilator waveforms, the direct recognition of asynchronies and the optimisation of the ventilator settings. Obviously this method is more applicable in the ICU because it does not require special technologies; however, a good knowledge of the most frequent patterns, and of the underlying pathophysiology, is essential to make it effi cient. The waveform method is based on the observation of the standard curves displayed on the ventilator screen (fl ow and airway pressure), because they are as sensitive and specifi c as oesophageal pressure, which thus far is considered the gold standard for detection of asynchronies. The method is centred on the identifi cation of the patient’s spontaneous activity. Patient’s inspiration Typically, when a patient starts a breath, this causes a negative defl ection on the pressure curve (Fig.1); on the fl ow curve, a positive defl ection can be detected, even if fl ow is still negative (Fig.2). Flow and pressure changes correlate with oesophageal pressure, thus they are suffi cient to detect a patient’s respiratory activity in most cases. 6,7,13 With these simple rules, a patient’s inspiratory activity can be detected even when it is neither detected nor assisted by the ventilator: in other words, ventilator waveforms can reveal a patient’s attempt to trigger the ventilator that does not reach its aim, namely an ineffective effort (Fig.3). The patient’s start of expiration can also be detected on fl ow and pressure waves; physiologically, it corresponds to a time point between the nadir of muscular pressure curve and its return to the baseline. This time point varies from patient to patient depending on respiratory mechanics and breathing pattern, but can be conveniently approximated at half relaxation. 8 If a muscular pressure curve is not available, indirect signs of relaxation can be detected on fl ow wave and their appearance varies depending on the assistance given from the ventilator. FIGURE 3 1250 Delayed cycling to expiration 1000 750 35 30 500 25 250 0 20 -250 -500 15 -750 -1000 10 (A) Change in fl ow shape similar to the one previously observed in case of delayed triggering; here no ventilator assistance follows, that is in case of ineff ective eff ort. (B) Wasted eff ort causes a typical depression in the expiratory phase on the pressure curve (red circle) 1 11 HHE 2019 | hospitalhealthcare.com