American journal of respiratory and critical care medicine
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Am. J. Respir. Crit. Care Med. · Jan 1995
Comparative StudyThe effect of mechanical ventilation on oxygen consumption in critically ill patients.
We measured oxygen consumption (VO2) during spontaneous breathing with continuous positive airway pressure (CPAP), assist control ventilation (AC), and control ventilation during muscle relaxation (AC-MR) in eight patients undergoing resuscitation from cardiopulmonary failure. VO2 decreased in all eight patients between CPAP and AC-MR; mean VO2 (255 +/- 92 ml/min) on CPAP exceeded that on AC-MR (209 +/- 79 ml/min) (p < 0.005). Compared with CPAP, AC without MR reduced VO2 in five of eight patients and mean VO2 (227 +/- 59 ml/min) tended to decrease (p = 0.14); clinical examination did not distinguish patients requiring MR to reduce VO2 further. ⋯ Both VO2resp and the mechanical work performed by the ventilator on the respiratory system were increased to about five times the efficiencies reported for normal patients, but VO2resp did not correlate with the mechanical work because of a wide range of respiratory muscle efficiencies. These efficiencies are less than those reported in normal patients, which may reflect the effect of sepsis, acidemia, hypoxia, or other conditions in these patients. We conclude that mechanical ventilation with muscle relaxation reduces VO2 by more than 20%; beyond stabilizing pulmonary gas exchange, these interventions preserve limited O2 delivery (QO2) for other vital organs.
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Am. J. Respir. Crit. Care Med. · Jan 1995
Comparative StudyPatient-ventilator interaction during synchronized intermittent mandatory ventilation. Effects of flow triggering.
Synchronized intermittent mandatory ventilation (SIMV) intermixes assisted and spontaneous breaths. Its ability as a weaning technique has been questioned on the basis that patients show little adaptation to ventilator assistance. We studied inspiratory effort and patient-ventilator interaction at different levels (SIMV, 100, 50, and 0%) of flow-triggered SIMV versus pressure-triggered SIMV in patients during the weaning period. ⋯ During pressure-triggered SIMV PTP/b and PTP/min were identical for mandatory and spontaneous breaths, whereas during flow-triggered SIMV PTP/b and PTP/min were significantly lower for mandatory than for spontaneous breaths. This difference was greatest when flow triggering and constant pressure ventilation were associated. These data show that flow triggering reduces inspiratory effort during both mandatory and spontaneous SIMV breaths and obtains a better patient-ventilator interaction.