Lower respiratory rates without decreases in oxygen consumption during neonatal synchronized intermittent mandatory ventilation

Objective: We tested the hypothesis that synchronization to patient effort during intermittent mandatory ventilation (SIMV), when compared to conventional unsynchronized intermittent mandatory ventilation (IMV), will decrease energy expenditure, as reflected by decreased oxygen consumption (VO₂). Design: We used a four-period crossover design. Each patient was studied over four 30-min continuous time intervals. Patients were randomized to receive initially IMV or SIMV, then crossed over such that each patient was treated twice with each modality. Data were analyzed using an analysis of variance technique. Setting: Patients were receiving treatment in the newborn intensive care unit of Children's Hospital, St. Paul. Patients: We studied 17 patients, who ranged from 23 to 37 weeks gestation, were ≤ 14 days old, and had study weights from 623 to 3015 g. All were mechanically ventilated for hyaline membrane disease. Measurements and results: We measured and compared VO₂, carbon dioxide consumption (VCO₂), minute ventilation (V(E)), total respiratory rate, heart rate, arterial blood pressure, and arterial oxygen saturation (SaO₂) values during IMV and SIMV. Total respiratory rate fell significantly during SIMV (73 ± 26 during IMV, 57 ± 17 during SIMV, p < 0.01) in spite of no significant change in VO₂ (0.6 ± 0.16% fall in VO₂ during SIMV) or VCO₂ (4.2 ± 0.19% increase in VCO₂ during SIMV) values. Moreover, there were no significant differences in heart rate, blood pressure, V(E), or SaO₂, values with either form of therapy. Conclusions: Though total respiratory rate fell, these data do not support the hypothesis that SIMV significantly reduces respiratory rate by decreasing oxygen consumption and carbon dioxide production during infant mechanical ventilation. Rather, the marked fall in respiratory rate may be due to a more efficient respiratory pattern.