不同呼吸频率机械通气对ARDS病人二氧化碳清除的影响

目的比较常规呼吸频率和高呼吸频率持续正压机械通气对急性呼吸窘迫综合征(ARDS)病人二氧化碳清除的影响。方法对16例ARDS病人入科后第一天给予有创机械通气,设置呼吸机模式为压力控制(PCV)。前后两次的控制压力及PEEPe设置相同(≤30cmH2O)且吸呼比(I∶E)不变。比较不同呼吸频率机械通气后的分钟通气量(MV);二氧化碳分压(PaCO2)、氧分压(PaO2);PEEPi;心脏指数(CI)、平均动脉血压(MBP)、中心静脉压(CVP)。结果与使用常规呼吸频率时比较,高呼吸频率通气时的分钟通气量虽然明显上升(P<0.05),但二氧化碳分压和氧分压无明显变化;内源性呼气末正压及中心静脉压有显著增高(P<0.05);在心脏指数、平均动脉血压方面有显著下降(P<0.05)。结论ARDS机械通气病人应用高呼吸频率并不能增加机体二氧化碳的清除,但导致内源性呼气末正压升高;同时降低心脏射血指数,影响血液动力学。

Effect of different respirtory rates on CO2 clearance during mechanical ventilation in acute respiratory distress syndrome

Objective To compare the clinical efficacy of low-rate conventional mechanical ventilation with high-rate strategy in clearing CO2 in patients of acute respiratory failure who are receiving continuous positive pressure mechanical ventilation.Methods We studied 16 patients with acute respiratory distress syndrome during the adjustment of ventilator settings on the first day of invasive mechanical ventilation in pressure-controlled mode. At first we determined the positive end-expiratory pressure that suppressed any intrinsic positive end-expiratory pressure at a respiratory rate of 15 breaths/min for 45 minutes, then changed to a high rate of 30 breaths/min for 45 minutes with the same airway pressure limitation (control pressure<=30cmH2O), positive end-expiratory pressure and the same inspiratory expiratory time ratio (I:E). We compared blood gas analysis, respiratory measurements, and evaluation of hemodynamics by using two different respiratory strategies.Results Compared with the low-rate mechanical ventilation, the high-rate strategy neither significantly reduced PaCO2 nor significantly improved PaCO2. The latter significantly increased dynamic hyperinflation resulting in a substantial intrinsic positive end-expiratory pressure (P<0.05). Right ventricular outflow impedance increased, leading to a significant increase in central venous pressure and drop in the cardiac index (P<0.05).Conclusion A high respiratory rate, during mechanical ventilation in patients with acute respiratory distress syndrome does not improve CO2 clearance, but it produces dynamic hyperinflation, impaires right ventricular ejection and reduces cardiac index.