ICU cornerstone: 'triggering effort'

COPD = chronic obstructive pulmonary disease; PEEP = positive end-expiratory pressure. The most frequent reason for admission to an intensive care unit is need for ventilatory assistance. At least, this is my experience over the past 20 years from working in university and county hospitals. Regardless of the illness that provokes respiratory insufficiency, the majority of patients with respiratory failure eventually require mechanical ventilation. If there is an improvement in oxygenation after a few days, then we are forced to consider reducing the intensity of ventilatory support. In the early years of my critical care training, when we checked ventilated patients under the effects of low-level sedation and were attempting to wean them off ventilation, such patients normally had to make an effort to trigger the ventilator. At that time, our teachers told us that this was normal; 'it is a kind of exercise training' said one, and 'it is our way of finding out which patients can work harder' said another. It was not clear why the 'trigger knob' had always to be in the standard position (not too difficult or too easy). One of my mentors told me to take advantage of the triggering effort because he thought that this effort could be related to a sophisticated parameter used in respiratory physiology, namely the occlusion pressure, or P0.1. After that, I started conducting research by obtaining many recordings of airflow and airway pressure in intubated, ventilated patients while they attempted to breathe. My mentor was right, and we published some stimulating papers about the 'triggering effort' [1]. Nevertheless, patients continued having to work to breathe while they were on the ventilator. When I reviewed our experimental tracings, I realized that patient response was not so predictable. Each patient had his or her own pattern of response, and most of them started inspiration before finishing expiration. At that time, not much was known about this type of patient response. We looked at these results again and made new tracings in different patients. Among those patients who showed abrupt changes in expiratory flow when they started inspiration early before the end of expiration, we realized that inspiratory efforts were not immediately rewarded by the sound of ventilator's insufflation. There was as much delay in the response of the ventilator as there was abnormality in the flow tracings. At the same time, the description of auto-PEEP (positive end-expiratory pressure) by Pepe …