Gas exchange during mechanical ventilation and spontaneous breathing. Intermittent mandatory ventilation after open heart surgery
Pulmonary gas exchange rates in eight patients after open heart surgery were studied during weaning from the ventilator. We investigated continuous positive pressure ventilation (CPPV), intermittent mandatory ventilation (IMV) and spontaneous breathing with continuous positive airway pressure (CPAP)...
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Veröffentlicht in: | Chest 1986-07, Vol.90 (1), p.11-17 |
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Zusammenfassung: | Pulmonary gas exchange rates in eight patients after open heart surgery were studied during weaning from the ventilator. We
investigated continuous positive pressure ventilation (CPPV), intermittent mandatory ventilation (IMV) and spontaneous breathing
with continuous positive airway pressure (CPAP). During each mode of ventilation we measured: CO2 production (VCO2), O2 consumption
(VO2), cardiac output (CO), PaO2, Qs/QT and functional residual capacity (FRC). In addition, we analyzed in each single breath:
tidal volume (VT), series dead space volume (Vds), alveolar ventilation, alveolar efficiency for CO2 elimination (alv eff
CO2) and end-tidal CO2 concentration (FCO2et). We compared the results of CPPV, IMV and CPAP and the mandatory breaths (MB)
with the spontaneous breaths (SB) measured during IMV. CO was low during CPPV, when the patient still deeply sedated; it was
increased in IMV and remained constant in the following CPAP period. VCO2 and VO2 did not differ significantly when switching
from IMV to CPAP; therefore, work due to breathing seemed not to be reduced by the mandatory breath during IMV. Oxygenation
(PaO2, Qs/QT) did not change significantly when switching from one mode to the other. FRC was constant when changing from
CPPV to IMV, did not alter within the IMV-cycle and was reduced significantly when switching from IMV to CPAP. Dead space
ventilation was reduced in SB (compared to MB). The latter result is discussed on the basis of two mechanisms: Vds was reduced
and alv eff CO2 was increased. We conclude that compared to CPPV, IMV decreases mean alveolar pressure and reduces dead space
ventilation at constant FRC and with constant oxygenation. This may explain why, in the weaning process, IMV makes it possible
to start spontaneous breathing very early. |
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ISSN: | 0012-3692 1931-3543 |
DOI: | 10.1378/chest.90.1.11 |