Increase of Oxygen Consumption during a Progressive Decrease of Ventilatory Support Is Lower in Patients Failing the Trial in Comparison with Those Who Succeed

Abstract

Background:The aim of this study was to test the hypothesis that, during weaning from mechanical ventilation, when the pressure support level is reduced, oxygen consumption increases more in patients unable to sustain the decrease in ventilatory assistance (weaning failure). Methods: Patients judged eligible for weaning were enrolled. Starting from 20 cm H 2 O, pressure support was decreased in 4-cm H 2 O steps, lasting 10 min each, until 0 cm H 2 O; this level was kept for 1 h. The average oxygen consumption from the last 3 min of each step, along with other ventilatory variables, was measured by indirect calorimetry (M-CAiOVX "metabolic module," Engstrom Carestation; GE Healthcare, Madison, WI) and recorded. Patients were defined as belonging to the failure group if, at any moment, they developed signs of respiratory distress according to standard criteria, or to the success group otherwise. Results: Twenty-eight patients were studied. In most patients, the minimum oxygen consumption was not recorded at the highest pressure support applied. Sixteen patients were able to complete the weaning trial successfully, whereas 12 failed it; the success group had a minimum oxygen consumption lower than failure group (mean Ϯ SD: 174 Ϯ 44 vs. 215 Ϯ 53 ml/min, P Ͻ 0.05). Moreover, although respiratory drive (assessed by P0.1) increased more in the failure group, this group had a lower increase in oxygen consumption, contradicting our hypothesis. Conclusions: Patients failing a decremental pressure support trial, in comparison with those who succeed, had an higher baseline oxygen consumption and were not able to increase their oxygen consumption in response to an increased demand.

The Ratio of Inspiratory Pressure Over Electrical Activity of the Diaphragm Remains Stable During ICU Stay and Is Not Related to Clinical Outcome

“…Whereas overassistance is associated with the risk of elevated tidal volumes and asynchronies, 19 an insufficient assist level results in large negative pressure swings that can severely injure the lungs 18,20 or the diaphragm itself 21 and cause increased oxygen consumption. 22 Moreover, had the P mus /EA di index value been relatively constant among all different subjects, one could calculate the P mus just from the EA di , without the need of an occluded airway pressure measurement (eg, in non-intubated subjects). However, the value proved to be quite variable in the subjects included in this study, with a median of 1 but an interquartile range varying from 0.59 to 1.34 cm H 2 O/V.…”

Metabolic Requirement of Septic Shock Patients Before and After Liberation From Mechanical Ventilation

“…A common technique in the intensive care unit (ICU) is to gradually liberate patients from mechanical ventilation using a repeated “work” and “rest” cycle. This cycle allows respiratory muscles to rest adequately with partial pressure support from the ventilator so that atrophied respiratory muscles may be strengthened and self‐breathing can begin in a less fatigued state 11 . However, there is evidence that energy expenditure may gradually elevate through each “work” and “rest” cycle 11 .…”

“…This cycle allows respiratory muscles to rest adequately with partial pressure support from the ventilator so that atrophied respiratory muscles may be strengthened and self‐breathing can begin in a less fatigued state 11 . However, there is evidence that energy expenditure may gradually elevate through each “work” and “rest” cycle 11 . Based on the available evidence, we hypothesized that energy expenditure would vary over time in the process of weaning from mechanical ventilation to the fully liberated state when pressure support from the ventilator is entirely withdrawn.…”

Estimation of Patient’s Inspiratory Effort From the Electrical Activity of the Diaphragm*

“…All the assisted ventilatory modalities are based on the concept that the patient and the ventilator share the work of breathing (WOB): a variable amount of work is generated by the patient's respiratory muscles and the remaining part is provided by the ventilator (1). In the clinical practice, it would be important to quantitate the amount of inspiratory pressure generated by the patient and the level of unloading provided by the ventilator because underassistance can lead to patient's exhaustion and discomfort, whereas excessive ventilatory assist can significantly increase oxygen consumption and the risk of patientventilator asynchrony (2,3), which in turn is associated with prolonged duration of mechanical ventilation (4). The standard reference for the measurement of the pressure developed by respiratory muscles (Pmusc) is based on esophageal pressure (Pes) measurement (5), which is still not very frequent in the clinical practice.…”