Tuesday, August 3, 2010

Board Review; Pregnancy answer

Each of the following choices is expected to occur during pregnancy except for A and D. Let’s go through each of the possible choices.

A. This in partially incorrect. Hyperventilation (with resultant hypocapnea, not hypercapnea) occurs during pregnancy and is largely a consequence of stimulation of the respiratory center via progesterone. Therefore, in pregnancy you typically see a chronic respiratory alkalosis which is compensated by renal excretion of bicarbonate. Luckily, in pregnancy the oxygen-dissociation curve is not dramatically altered due to increased 2,3 DPG levels in pregnancy and normalization of blood pH through renal compensation.

B. This is correct. During pregnancy the intestinal absorption of calcium nearly doubles as early as week 12 of pregnancy and is likely the major maternal adaptation to meet the fetal need for calcium. As intestinal calcium absorption increases, extra calcium is either deposited in the fetal skeleton, the maternal skeleton or is excreted by the kidneys. Renal calcium excretion is increased as early as the 12th week of gestation and 24 hour urine values (corrected for creatinine excretion) can exceed the normal range. Conversely, fasting urine calcium values are normal or low, confirming that the hypercalciuria is a consequence of the enhanced intestinal calcium absorption. Pregnancy is recognized as a risk factor for kidney stones and the absorptive hypercalciuria of pregnancy is one reason for this.

C. This is also correct. During pregnancy kidney size can increase by 20%. This increase in kidney size also corresponds to an increase in GFR and renal blood flow.

D. This is incorrect. Remember that filtration fraction (FF) is the ratio of glomerular filtration rate (GFR) to renal blood flow (RBF). The filtration fraction represents the proportion of fluid reaching the kidneys which eventually passes into the tubular lumen. In pregnancy, although the GFR is increased, RBF is increased to a greater extent. Therefore, FF actually decreases in pregnancy. Since the increase in GFR and RBF is not associated with an increase in FF, there is no risk of secondary FSGS in pregnancy. This is supported by small studies.

E. This is correct. During pregnancy, urinary protein excretion increases from normal nonpregnant levels of 60–90 mg/24 hours to 180–250 mg/24 hours in the third trimester. Even if slightly increased during pregnancy, urine protein excretion rarely reaches levels that are detected by usual screening methods, such as 1+ on urinary dipstick (i.e., 30 mg/dL, which is roughly equivalent to 300 mg in 24 hours). Although the mechanism for this possible increase has not been established, it seems likely that absorption of filtered protein in the proximal tubule is reduced.

F. This is correct as well. During pregnancy, physiologic changes occur in volume- and osmoregulation that effect plasma osmolality and sodium concentration. During a normal pregnancy, the average plasma osmolality decreases by 5-10 mmol, and the sodium concentration is decreased by 5 mmol/l. This drop in plasma osmolality has been explained by a phenomenon termed ‘reset osmostat’. A ‘reset osmostat’ is when the osmotic threshold above which ADH-release and a thirst stimulus occur changes. This threshold is decreased to a lower steady state value during normal pregnancy. The mechanism that causes the reset-osmostat phenomenon is unknown.

On a lighter note, the “Gross Clinic” has completed its restoration and is currently on exhibit at the Philadelphia Museum of Art. It is a must-see for any devotee of medicine.

Michael Lattanzio DO

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