两组还是三组

In a cross-over design with 19 obese patients who had an average BMI of 57 ± 12 kg/m2 they compared the hemodynamic effects of PEEP based on the standard ARDS network PEEP table versus higher PEEP determined by a lung recruitment procedure and PEEP titrated to respiratory system compliance as in the Alveolar Recruitment for Acute Respiratory Distress (ART) . In a subset they also compared changes in regional lung ventilation and perfusion by electrical impedance tomography (EIT) in these patients and selected non-obese patients from ART .

在一项19名肥胖患者（平均BMI为57±12 kg/m2）交叉设计研究中，他们比较了基于标准ARDS网络PEEP表设定的PEEP对血流动力学的效应与通过肺复张手法确定的更高的PEEP对血流动力学的效应以及如急性呼吸窘迫（ART）中肺复张中将PEEP滴定呼吸系统顺应性时的PEEP对血流动力学的效应。在亚组中，他们还通过电阻抗断层扫描（EIT）比较了这些患者和ART中选择的非肥胖患者的局部肺通气和灌注的变化。

各位老师，是这么理解吧：是三组的比较还是两组比较？谢谢

如果是两组比较，第二组就是：如急性呼吸窘迫（ART）中肺复张手法滴定呼吸系统顺应性时更高的PEEP对血流动力学的效应