The Control of Breathing during Weaning from Mechanical Ventilation (8)

Figure 2 shows the differences between C02RT and C02SB in each patient. Subjects 1 through 5 tolerated the full hour of weaning (group 1, dark shaded bars). In three of them, C02SB was ^2 mm Hg lower than C02RT. In group 1, the mean difference between C02RT and C02SB of 1.2 mm Hg did not achieve statistical significance (p^O.l). Patients 6 through 14 had symptom-limited weaning failure (group 2, light shaded bars). In these patients, the mean duration of the weaning trial was 13 min. Five of them developed asynchronous chest wall movements (detected by inductance plethysmography) near the end of the weaning trial, but only patient 8 became frankly acidemic (pH<7.35). In group 2, seven of nine patients retained C02 during unassisted breathing relative to C02RT (group mean difference C02SB — C02RT=5 ± 5 mm Hg, <0.01). C02RT and C02SB were equal in the remaining two patients.
Figure 3 shows the differences between C02MV and C02SB in each patient. In contrast to the findings depicted in Figure 2, there was no systematic difference between C02MV and C02SB in either group. In two patients of group 1, C02SB exceeded COaMV by 4 and 6 mm Hg, respectively. Both had been supported with conventional mechanical ventilation in the assist-control mode and were extubated uneventfully within 24 hours of our study.

Figure-2

Figure 2. The differences between arterial C02 tensions (in mm Hg) during recruitment testing (C02RT) and at the end of a weaning trial while breathing spontaneously (C02SB) are shown on the ordinate. Dark shaded bars (patients 1 through 5) represent observations in group 1; light shaded bars (patients 6 through 14) represent observations in group 2. Note that three of the five patients in group 1 had lower arterial C02 tensions during spontaneous breathing than predicted from C02RT. Also note that seven of the nine patients in group 2 had arterial C02 tensions that exceeded COaRT by 3 mm Hg or more.

Figure-3

Figure 3. The differences between arterial COs tensions (in mm Hg) between mechanical ventilation (COsMV) and spontaneous breathing (COsSB) are shown. Dark shaded bars (patients 1 through 5) represent observations in group 1. light shaded bars (patients 6 through 14) represent observations in group 2. Patients 4, 5, 9,10, and 11 were ventilated with volume preset ventilation in the assist control mode. The remaining patients were ventilated with intermittent mandatory ventilation or pressure support ventilation.

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