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When it comes to physiology, “common sense” and “conventional wisdom” may not be

      To the Editors:

      I thank Goldkrand and Jackson (Goldkrand JW, Jackson MJ. Blood pressure measurement in pregnant women in the left lateral recumbent position. Am J Obstet Gynecol 1997; 176: 642-3) for presenting information which should be, but has not been, obvious to physicians and nurses who care for pregnant women. I have fought the battle with rare success for >20 years.
      Obstetricians seem to be the one group that will change a variable (alter position) to achieve a desired result. Measuring blood pressure by either direct or indirect methods in the upper extremity of a patient in the lateral decubitus position is equivalent to raising a transducer above the level of the aortic bulb in a catheterized vascular tree. The practitioner simply raises the transducer until happy, albeit incorrect.
      The error of this logic should be apparent to any student of freshman physiology or physics. The components of blood pressure are cardiac output and systemic vascular resistence. Other than those women with supine hypotension syndrome in pregnancy, alterations of position change neither cardiac output nor systemic vascular resistance. To be sure, in the excepted group, the cardiac output and blood pressure go up, not down.
      The measurement of blood pressure must be compensated by the height of the hydrostatic column of blood relative to the aortic bulb (millimeters/13, mercury being 13 times the weight of water or blood). I tell students and residents that this is why we take all those “irrelevant courses” in undergraduate school and the preclinical years. Perhaps admissions committees should require mathematic or philosophic logic also.