A 65-year-old woman with chronic obstructive pulmonary disorder is evaluated in the emergency department for fever, cough, and increased sputum production. Her symptoms began 48 hours ago and have progressively worsened. Her baseline arterial blood gas (ABG) results are pH 7.34, carbon dioxide 60 mm Hg, oxygen 81 mm Hg, and bicarbonate 32 mEq/L. A current ABG shows a partial pressure of carbon dioxide 72 mm Hg, partial arterial oxygen pressure 80 mm Hg.
Which of the following is closest to her expected pH?
A. 7.3
B. 7.25
C. 7.2
D. 7.15
投票结果：
该患者在基线时有代偿性慢性呼吸性酸中毒；其基线二氧化碳分压(pCO2)为60 mmHg，pH值(7.35-7.45)相对正常，重碳酸盐为32 mEq/L。这可以用以下公式来解释：预期碳酸氢盐= 24 + 4{（实际pCO2-40）/10}或32 = 24 + 4(60-40)/10）。患者出现慢性阻塞性肺疾病加重，很可能是由于基础呼吸道感染所致。这导致pCO2急剧升高12 mmHg，没有时间进行代谢代偿，需要48小时。她的pH值计算如下： pH变化 = 0.008 x（新pCO2-基线pCO2）或0.008 x(72-60)= 0.096，这是患者当前的pH 0.096 = 7.24。
References:
- Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances. N Engl J Med Med. 2014 Oct 9;371(15):1434-1445.
- Dorman PJ, Sullivan WJ, Pitts RF. The renal response to acute respiratory acidosis. J Clin Invest. 1954 Jan;33(1):82-90.
- Rose B, Post T. Clinical Physiology of Acid-Base and Electrolyte Disorders. 5th ed. New York, NY: McGraw-Hill; 2001.
- Nichols G Jr. Serial changes in tissue carbon dioxide content during acute respiratory acidosis. J Clin Invest. 1958 Aug;37(8):1111-1122
C
补充内容：
我们可以看到基础是慢性的呼酸合并代碱，呼吸性酸中毒为原发，因此碳酸氢根变化值=0.35*（60-40）±5.58=1.42-12.58，故碳酸氢根范围为25.2-36.58，因此实际碳酸氢根浓度32在此范围内；现在二氧化碳变到了72，改变了12mmHg，故碳酸氢根变化值因为-1.38到+9.78，故碳酸氢根为30.62-41.78，理论上不应该下降了所以范围32-41.78；因此计算H+=54-56.4这个范围内，所以答案B最可能。
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