Increase of oxygen consumption during a progressive decrease of ventilatory support is lower in patients failing the trial in comparison with those who succeed.
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 H2O, pressure support was decreased in 4-cm H2O steps, lasting 10 min each, until 0 cm H2O; 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 +/- SD: 174 +/- 44 vs. 215 +/- 53 ml/min, P < 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.
Bellani
Giacomo
G
Department of Experimental Medicine, University of Milan-Bicocca, Monza, Italy.
Foti
Giuseppe
G
Spagnolli
Ester
E
Milan
Manuela
M
Zanella
Alberto
A
Greco
Massimilano
M
Patroniti
Nicolò
N
Pesenti
Antonio
A
D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010101:Oxygen Consumption / Q000502:physiology*; D011669:Pulmonary Wedge Pressure / Q000502:physiology; D012121:Respiration, Artificial / Q000379:methods; D012128:Respiratory Distress Syndrome / Q000175:diagnosis* / Q000503:physiopathology; D012129:Respiratory Function Tests / Q000379:methods; D012131:Respiratory Insufficiency / Q000175:diagnosis* / Q000503:physiopathology; D013997:Time Factors; D016896:Treatment Outcome; D015300:Ventilator Weaning / Q000379:methods*