The pattern of breathing during successful and unsuccessful trials of weaning from mechanical ventilation

We prospectively examined the pattern of breathing in patients being weaned from mechanical ventilation: one group (n = 10) underwent a successful weaning trial and were extubated, whereas another group (n = 7) developed respiratory failure and required the reinstitution of mechanical ventilation. During the period of ventilator support, minute ventilation (V̇I), tidal volume (VT), and respiratory frequency (f) were similar in the 2 groups. After discontinuation of the ventilator, V̇I remained similar in the 2 groups, but VT was lower and f was higher in the patients who failed the trial compared with those who were successful, 194 ± 23 and 398 ± 56 ml (p < 0.001), respectively, and 32.3 ± 2.3 and 20.9 ± 2.8 breaths/min (p < 0.001), respectively. The failure group displayed a significant increase in Pa(CO₂) (p < 0.005) during spontaneous breathing, without a concomitant increase in the alveolar-arterial P(O₂) difference. Eighty-one percent of the variance in Pa(CO₂) was accounted for by the pattern of rapid, shallow breathing. During weaning, resting respiratory drive (reflected by mean inspiratory flow, VT/TI) and fractional inspiratory time (TI/Ttot) were similar in the 2 groups. The patients in the failure group showed significant increases in VT/TI, 265 ± 27 to 328 ± 32 ml/s (p < 0.01), and V̇I, 5.82 ± 0.53 to 7.32 ± 0.52 L/min (p < 0.01), from the beginning to the end of the weaning trial; VT and f showed no further change. In the patients with a successful weaning outcome, extubation induced a transient increase in VT/TI and V̇I, which resolved during the following hour, with no further changes during the subsequent 24 h. In summary, (1) patients who failed a weaning trial had a well-maintained V̇I but a low VT and a high f, (2) these patients developed a further increase in respiratory drive from the beginning to the end of the weaning trial, and (3) patients with a successful weaning outcome showed only transient changes in breathing pattern after extubation. These results indicate that the development of hypercapnia in patients who fail a weaning trial is due to the development of rapid, shallow breathing with consequent inefficient gas-exchange, rather than the result of hypoventilation caused by a primary abnormality in respiratory center output.