■ Kidney
Several remarkable changes are observed in the urinary system as a result of pregnancy (Table 4-5). Kidney sizeeincreases approx- imately 1.5 cm (Bailey, 1971). Both theglomerular filtration rate (GFR) and renal plasma flow increase early in pregnancy.w The GFR increases as much as 25 percent by the second week after conception and 50 percent by the beginning of the second trimester. This hyperfiltration appears to result from two prin- cipal factors. First, hypervolemia-induced hemodilution low- ers the protein concentration and oncotic pressure of plasma entering the glomerular microcirculation. Second, renal plasma flow increases by approximately 80 percent before the end of the first trimester (Conrad, 2014; Cornelis, 2011). As shown in Figure 4-14, elevated GFR persists until term, even though renal plasma flow decreases during late pregnancy. Primarily as a consequence of this elevated GFR, approximately 60 percent of women report urinary frequency during pregnancy (Sandhu, 2009).
During the puerperium, a marked GFR persists during the first postpartum day principally from the reduced glo- merular capillary oncotic pressure. A reversal of the gestational hypervolemia and hemodilution, still evident on the first postpartum day, eventuates by the second week postpartum (Hladunewich, 2004).
Volume (LVV)
6 5 4 3
TLC FVC
RV
TLC FVC IC
RV FRC
FRC IC
IRV
Not pregnant
Pregnant (7–9 mos.)
IRV
VT
VT
ERV
ERV
RV RV
2 1 0
6 5 4 3 2 1 0
Volume (L)VV
FIGURE 4-13 Changes in lung volumes with pregnancy. The most significant changes are reduction in functional residual capacity (FRC) and its subcomponents, expiratory reserve volume (ERV) and residual volume (RV), as well as increases in inspiratory capacity (IC) and tidal volume (VT). (Redrawn from Hegewald, 2011, with permission.)
64 Maternal Anatomy and Physiology
SECTION 2
TABLE 4-5.Renal Changes in Normal Pregnancy
Parameter Alteration Clinical Relevance
Kidney size Approximately 1 cm longer on radiograph Size returns to normal postpartum Dilatation Resembles hydronephrosis on sonogram or
IVP (more marked on right)
Can be confused with obstructive uropathy; retained urine leads to collection errors; renal infections are more virulent; may be responsible for “distension syndrome”; elective pyelography should be deferred to at least 12 weeks postpartum Renal function Glomerular filtration rate and renal plasma
flow increase∼50%
Serum creatinine decreases during normal gestation;
> 0.8 mg/dL (>72μmol/L) creatinine already borderline; protein, amino acid, and glucose excretion all increase
Maintenance of acid-base
Decreased bicarbonate threshold;
progesterone stimulates respiratory center
Serum bicarbonate decreased by 4–5 mEq/L; Pco2
decreased 10 mm Hg; a Pco2of 40 mm Hg already represents CO2retention
Plasma osmolality Osmoregulation altered; osmotic thresholds for AVP release and thirst decrease;
hormonal disposal rates increase
Serum osmolality decreases 10 mOsm/L (serum Na ∼5 mEq/L) during normal gestation; increased placental metabolism of AVP may cause transient diabetes insipidus during pregnancy
AVP =vasopressin; IVP= intravenous pyelography; Pco2= partial pressure carbon dioxide.
Modified from Lindheimer, 2000.
Studies suggest that relaxin may be important for mediating both increased GFR and renal blood flow during pregnancy (Conrad, 2014; Helal, 2012). Relaxin increases endothelin and nitric oxide production in the renal circulation. This leads to renal vasodilation and decreased renal afferent and efferent arte- riolar resistance, with a resultant increase in renal blood flow and GFR. Relaxin may also increase vascular gelatinase activity
during pregnancy, which leads to renal vasodilation, glomerular hyperfiltration, and reduced myogenic reactivity of small renal arteries (Conrad, 2005).
As with blood pressure, maternal posture may have a con- siderable influence on several aspects of renal function. Late in pregnancy, for instance, urinary flow and sodium excretion aver- age less than half the excretion rate in the supine position com- pared with that in the lateral recumbent position. The impact of posture on GFR and renal plasma flow is more variable.
One unusual feature of the pregnancy-induced changes in renal excretion is the remarkably increased amounts of vari- ous nutrients lost in the urine. Amino acids and water-soluble vitamins are excreted in much greater amounts (Hytten, 1973;
Powers, 2004).
Renal Function Tests
The physiological changes in renal hemodynamics induced dur- ing normal pregnancy have several implications for the inter- pretation of renal function tests (Appendix, p. 1292). Serum creatininee levels decrease during normal pregnancy from a mean of 0.7 to 0.5 mg/dL. Values of 0.9 mg/dL or greater sug-gg gest underlying renal disease and should prompt further evaluation.
Creatinine clearance in pregnancy averages 30 percent e higher than the 100 to 115 mL/min in nonpregnant women (Lindheimer, 2000). This is a useful test to estimate renal function, provided that complete urine collection is made during an accurately timed period. If either is done incor- rectly, results are misleading (Lindheimer, 2010). During the day, pregnant women tend to accumulate water as dependent edema, and at night, while recumbent, they mobilize this fluid with diuresis. This reversal of the usual nonpregnant diurnal pattern of urinary flow causes nocturia, and urine is more dilute than in nonpregnant women. Failure of a pregnant
–20 0
Weeks’ gestation
Glomerular filtration rate Effective renal plasma flow
Filtration fraction 30
40 60 80 100
Percentage change
0–20 30–40
Nonpregnant 20–30
FIGURE 4-14 Relative changes in measures of glomerular filtra- tion rate (GFR), effective renal plasma flow (ERPF), and filtration fraction during normal pregnancy. (Redrawn from Davison, 1980, with permission.)
CHAPTER 4
woman to excrete concentrated urine after withholding fluids for approximately 18 hours does not necessarily signify renal damage. In fact, the kidney in these circumstances functions perfectly normally by excreting mobilized extracellular fluid of relatively low osmolality.
Urinalysis
Glucosuriaaduring pregnancy may not be abnormal. The appre- ciable increase in GFR, together with impaired tubular reab- sorptive capacity for filtered glucose, accounts for most cases of glucosuria (Davison, 1974). For these reasons alone, Chesley (1963) calculated that about a sixth of pregnant women should spill glucose in the urine. That said, although common during pregnancy, when glucosuria is identified, the possibility of dia- betes mellitus should not be ignored.
Hematuria is often the result of contamination during cola - lection. If not, it most often suggests urinary tract disease.
Hematuria is common after difficult labor and delivery because of trauma to the bladder and urethra.
Proteinuria is typically defined in nonpregnant patients as a a protein excretion rate of more than 150 mg/day. Because of the aforementioned hyperfiltration and possible reduction of tubu- lar reabsorption, significant proteinuria during pregnancy is usually defined as a protein excretion rate of at least 300 mg/day (Hladunewich, 2011). Higby and coworkers (1994) measured protein excretion in 270 normal women throughout pregnancy (Fig. 4-15). Their mean 24-hour excretion for all three trimes- ters was 115 mg, and the upper 95-percent confidence limit was 260 mg/day without significant differences by trimester.
These investigators also showed that albumin excretion is mini- mal and ranges from 5 to 30 mg/day. Interestingly, however, Cornelis and colleagues (2011) noted that proteinuria is greater in the second half of pregnancy, which does not correspond precisely to the earlier peak in GFR (see Fig. 4-14). Alternative explanations might include alterations in tubular reabsorptive capacity or the presence of other proteinaceous material that might be detected in the urine of pregnant women. A recent study in normal gravidas also showed proteinuria levels greater than established thresholds (Phillips, 2014).
Measuring Urine Protein
The three most commonly employed approaches for assessing proteinuria are the qualitative classic dipstick, the quantitative 24-hour collection, and the albumin/creatinine or protein/cre- atinine ratio of a single voided urine specimen. The pitfalls of each approach have recently been reviewed by Conrad and col- leagues (2014). The principal problem with dipstick assessment is that renal concentration or dilution of urine is not accounted for. For example, with polyuria and extremely dilute urine, a negative or trace dipstick could actually be associated with excessive protein excretion.
The 24-hour urine collection is affected by urinary tract dilata- tion, which is discussed subsequently. The dilated tract may lead to errors related both to retention—hundreds of milliliters of urine remaining in the dilated tract—and to timing—the remaining urine may have formed hours before the collection. To minimize these pitfalls, Lindheimer and Kanter (2010) recommend that the patient first be hydrated and positioned in lateral recumbency—
the definitive nonobstructive posture—for 45 to 60 minutes.
After this, she is asked to void, and this specimen is discarded.
Immediately following this void, her 24-hour collection begins.
During the final hour of collection, the patient is again placed in the lateral recumbent position. But, at the end of this hour, the final collected urine is incorporated into the total collected volume.
The protein/creatinine ratio is a promising approach because data can be obtained quickly and collection errors are avoided.
Disadvantageously, the amount of protein per unit of creati- nine excreted during a 24-hour period is not constant, and there are various thresholds that have been promulgated to define abnormal. Nomograms for urinary microalbumin and creatinine ratios during uncomplicated pregnancies have been developed by Waugh and coworkers (2003).
■ Ureters
After the uterus completely rises out of the pelvis, it rests on the ureters, which laterally displaces and compresses them at the pel- vic brim. Above this level, increased intraureteral tonus results (Rubi, 1968). Ureteral dilatation is impressive, and Schulman and Herlinger (1975) found it to be greater on the right side in 86 percent of women (Fig. 4-16). Unequal dilatation may result from cushioning provided the left ureter by the sigmoid colon and perhaps from greater right ureteral compression exerted by the dextrorotated uterus. The right ovarian vein complex, which is remarkably dilated during pregnancy, lies obliquely over the right ureter and may contribute significantly to right ureteral dilatation.
Progesterone likely also has some effect. Van Wagenen and Jenkins (1939) described continued ureteral dilatation after removal of the monkey fetus but with the placenta left in situ. The relatively abrupt onset of dilatation in women at midpregnancy, however, seems more consistent with ureteral compression.
Ureteral elongation accompanies distention, and the ureter is frequently thrown into curves of varying size, the smaller of which may be sharply angulated. These so-called kinks are poorly named, because the term connotes obstruction. They are usually single or double curves that, when viewed in a radiograph taken in the same plane as the curve, may appear as acute angulations.
Another exposure at right angles nearly always identifies them 300
200
Protein (mg/24 hr)
100
0 0
Gestational age (weeks) 30
95%
1st Trimester 2nd Trimester 3rd Trimester
Mean
40 20
10
FIGURE 4-15 Scatter plot of women showing 24-hour urinary total protein excretion. Mean and 95-percent confidence limits are outlined. (Redrawn from Higby, 1994, with permission.)
66 Maternal Anatomy and Physiology
SECTION 2
to be more gentle curves. Despite these anatomical changes, Semins and associates (2009) concluded, based on their review, that complication rates associated with ureteroscopy in pregnant and nonpregnant patients do not differ significantly.
■ Bladder
There are few significant anatomical changes in the bladder before 12 weeks. From that time onward, however, increased
A
B
FIGURE 4-16 Hydronephrosis. A.Plain film from the 15-minute image of an intravenous pyelogram (IVP). Moderate hydrone- phrosis on the right (arrows) and mild hydronephrosis on the left (arrowheads) are both normal for this 35-week gestation. B.Axial magnetic resonance (MR) image from a study performed for a fetal indication. Moderate hydronephrosis on the right (white arrow) and mild on the left (black arrow) are incidental findings.
uterine size, the hyperemia that affects all pelvic organs, and the hyperplasia of bladder muscle and connective tissues elevate the trigone and cause thickening of its posterior, or intraureteric, margin. Continuation of this process to the end of pregnancy produces marked deepening and widening of the trigone. There are no mucosal changes other than an increase in the size and tortuosity of its blood vessels.
Using urethrocystometry, Iosif and colleagues (1980) reported that bladder pressure in primigravidas increased from 8 cm H2O early in pregnancy to 20 cm H2O at term. To com- pensate for reduced bladder capacity, absolute and functional urethral lengths increased by 6.7 and 4.8 mm, respectively. At the same time, maximal intraurethral pressure increased from 70 to 93 cm H2O, and thus continence is maintained. Still, at least half of women experience some degree of urinary incon- tinence by the third trimester (van Brummen, 2006; Wesnes, 2009). Indeed, this is always considered in the differential diag- nosis of ruptured membranes.
Toward the end of pregnancy, particularly in nulliparas in whom the presenting part often engages before labor, the entire base of the bladder is pushed forward and upward, converting the normal convex surface into a concavity. As a result, dif-ff ficulties in diagnostic and therapeutic procedures are greatly increased. In addition, pressure from the presenting part impairs blood and lymph drainage from the bladder base, often rendering the area edematous, easily traumatized, and possibly more susceptible to infection.