[单选] 怀孕33周的一名23岁孕妇gravida 2第1段就医 因为呼吸急促 到目前为止，妊娠一直很简单。 她 开始注意到人们需要大约20周的呼吸时间， 自那时以来一直保持这种状态，最近的情况只是稍有恶化。 她没有胸痛，恶心， 呕吐，发汗或相关症状，并且具有正常的运动耐力。 她有 没有医疗问题，也从未做过手术。 她过去的产科史对 足月分娩2年前。 她的血压是100/60毫米汞柱，脉搏是80 /分钟， 温度为37．0 C（98．6 F），呼吸为16 / min。 她的身体检查是 完全在正常范围内。 胸部X线检查也正常。 实验室研究，包括 动脉血气，显示：A 23-year-old woman， gravida 2， para 1， at 33 weeks" gestation comes to the physician because of shortness of breath． The pregnancy has been uncomplicated up to this point． She started noticing the awareness of the need to breathe at about 20 weeks and this has remained since that time with only a slight recent worsening． She has no chest pain， nausea， vomiting， diaphoresis， or associated symptoms， and has normal exercise tolerance． She has no medical problems and has never had surgery． Her past obstetric history is significant for a term vaginal delivery 2 years ago． Her blood pressure is 100/60 mm Hg， pulse is 80/min， temperature is 37．0 C （98．6 F）， and respirations are 16/min． Her physical examination is entirely within normal limits． Chest x-ray is also normal． Laboratory studies， including an arterial blood gas， show: pH: 7．45 PCO2: 28 mm Hg PO2: 96 mm Hg Sodium: 135 mEq/L Potassium: 4．0 mEq/L Chloride: 105 mEq/L Bicarbonate: 22 mEq/L 这些实验室值趋向于以下哪个？These laboratory values are trending toward which of the following？