The effect of posture on renal function in late pregnancy

Kidney Physiology in Pregnancy

A woman's body undergoes a myriad of changes throughout the course of a normal gestation. The kidneys play a central role in driving adjustments that guarantee maternal and fetal well-being, including a dramatic increase in glomerular filtration rate, alterations in tubular function, and changes in electrolyte and acid/base handling. Early in gestation, systemic vasodilation, driven by both a change in quantity of and response to various hormones, leads to increased renal blood flow and glomerular filtration rate. Vasodilation also results in activation of the renin-angiotensin-aldosterone axis, which combined with changing tubular handling causes alterations in total body stores of electrolytes and total body water, resulting in a lower serum sodium concentration. In addition, mild proteinuria, glucosuria, and a decrease in serum calcium and magnesium are common. The primary acid/base change seen in pregnancy is a mild respiratory alkalosis due to progesterone effects. This article provides an overview of the current understanding of the healthy response of the kidneys to pregnancy, an understanding of which is key to caring for the pregnant patient, and early identification of alterations that may indicate underlying kidney pathology in pregnancy.

Assessment of renal function during pregnancy using a random urine protein to creatinine ratio and Cockcroft-Gault formula

The current standard for assessment of renal function in pregnant women is a 24-hour urine collection to determine creatinine clearance and proteinuria. It is easier to use the random urine protein to creatinine (P:C) ratio and the Cockcroft-Gault (CG) formula to estimate protein excretion and glomerular filtration rate, but the reliability of these formulae in combination for assessing renal function in pregnant women with renal disease is unknown. We compared the results of the P:C ratio with the 24-hour urinary protein excretion and the results of the CG clearance estimate with the 24-hour urine creatinine clearance in 34 pregnant women with underlying renal disease. Comparisons were made once in each trimester and postpartum. Prepregnancy weights were used in the CG formula: (140 - age x weight [kg] x 0.85)/72 x serum creatinine (mg/dL). Twenty-six first trimester, 33 second trimester, 21 third trimester, and 15 postpartum comparisons were made for creatinine clearance and 16 first trimester, 29 second trimester, 15 third trimester, and 15 postpartum comparisons were made for protein excretion. Measured creatinine clearance for the three trimesters combined (105 +/- 40 mL/min [mean +/- SD]) correlated significantly with CG clearances (113 +/- 52 mL/min; r = 0.87). The mean P:C values (2.03 +/- 3.15) for the three trimesters combined correlated significantly with 24-hour urine protein (2.25 +/- 4.21 g; r = 0.92). Our study demonstrates excellent correlations between the CG formula using prepregnancy weights and 24-hour creatinine clearance and between the P:C and 24-hour urinary protein in this population.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal haemodynamics and tubular function in human pregnancy

In human pregnancy, effective renal plasma flow and glomerular filtration rate increase to levels 50-80% above non-pregnant values. The increments occur shortly after conception, persist throughout the second trimester and reduce slightly in late pregnancy. The hyperfiltration of pregnancy does not seem to be a potentially damaging process, as intraglomerular pressure remains unchanged. The increased excretion of glucose and other nutrients, as well as uric acid and protein, is related in part to altered tubular function. Renal physiology is altered so much in pregnancy that non-pregnant norms cannot be used in antenatal care.

A prospective comparison of two endogenous creatinine clearance testing methods in hospitalized hypertensive gravid women

OBJECTIVE

Although 24-hour endogenous creatinine clearance testing is common in pregnancies complicated by hypertension, inaccuracies limit its usefulness. We controlled the conditions under which 4-hour endogenous creatinine clearance testing was performed and compared the results with outcomes of 24-hour tests from the same patients.

STUDY DESIGN

In 83 women hospitalized with mild hypertension in the third trimester, we measured endogenous creatinine clearance with a 4-hour urine collection during lateral recumbency and supervised oral hydration. This test was paired with a 24-hour test performed immediately thereafter. No restrictions or recommendations regarding ambulation or oral intake were imposed for the 24-hour test.

RESULTS

The 4-hour endogenous creatinine clearance value exceeded the 24-hour value in 133 of the 136 paired comparisons (p < 0.0001). Results of the tests from only the 29 patients with multiple paired tests showed more similarity (p < 0.005) among the 4-hour than among the 24-hour clearances.

CONCLUSION

The 4-hour endogenous creatinine clearance test, as described, provides a higher and less variable estimate of renal function in hypertensive pregnant women than does the 24-hour test.

The effect of pregnancy on renal function: physiology and pathophysiology

Marked changes in renal function occur with pregnancy. We present a summary of these changes in this review and give insight into possible mechanisms if they are known. Controversies exist regarding the therapy of pregnancy-induced hypertension and asymptomatic and recurrent bacteriuria. The current views on these topics are given. Specific renal diseases are summarized, including transplantation, and optimum management strategies and maternal and fetal prognosis during pregnancy are given.

Renin-angiotensin-aldosterone system in normal and hypertensive pregnancy. Response to postural stimuli

Most studies that have attempted to distinguish pregnancy-induced hypertension from chronic hypertension in pregnancy include arbitrary clinical definitions and morphological reports based on renal biopsy. To evaluate whether these conditions have different responses to stimuli to the renin-angiotensin-aldosterone system, we studied four normal nonpregnant women, eight normal pregnant women, 10 women with pregnancy-induced hypertension, and 14 with chronic hypertension in pregnancy, in the third trimester of pregnancy, after they had sequentially adopted the supine, the left lateral recumbent, and the orthostatic positions for 90 minutes each. Postural maneuvers did not significantly change mean arterial pressure in pregnancy-induced hypertensive or in normal pregnant women, although in chronic hypertensive women, a significant reduction in this parameter was observed in left lateral recumbency. The renin-angiotensin-aldosterone system was significantly less activated with women in the supine position in pregnancy-induced hypertensive and chronic hypertensive women; however, as opposed to pregnancy-induced hypertensive women, those with chronic hypertension reassumed their humoral response to upright posture, which was accompanied by a significant reduction in sodium excretion. The parallelism between plasma renin activity and aldosterone levels, absent in normal pregnancy, returned in pregnancy-induced hypertensive and chronic hypertensive women in the erect posture (r = 0.73, p less than 0.01; r = 0.68, p less than 0.01, respectively). These data suggest that the adoption of the left lateral recumbent position in pregnancy reduces mean arterial pressure only in chronic hypertensive women. Moreover, in chronic hypertension, the upright position provoked a significant response of the renin-angiotensin-aldosterone system. This effect was not observed in women with pregnancy-induced hypertension.

Renal haemodynamics and tubular function in human pregnancy

In human pregnancy, effective renal plasma flow and glomerular filtration rate increase to levels 50-80% above non-pregnant values. The increments occur shortly after conception, persist throughout the second trimester and reduce slightly in late pregnancy. The hyperfiltration of pregnancy does not seem to be a potentially damaging process. The increased excretion of glucose and other nutrients, as well as uric acid and protein, is related in part to altered tubular function. Renal physiology is altered so much in pregnancy that non-pregnant norms cannot be used in antenatal care.

Hypertension in pregnant women

Although hypertension is the most common and significant risk factor in pregnancy, the risks can be minimized with careful supervision and therapy. The mere presence of hypertension, even of severe hypertension, is neither a contraindication to pregnancy nor an indication for the termination of pregnancy, provided that appropriate management principles are applied. For optimum management, the skills of the obstetrician, the physician and, later in pregnancy, of the neonatologist should all be employed to the full. The ability to monitor the fetus closely, and to offer intensive care to the premature newborn, is critical in making decisions about the timing and mode of delivery. These women, as all other women with high-risk pregnancies, should be receiving treatment in centres which can offer adequate care both to the mother and to the baby.

Non-postural serial changes in renal function during the third trimester of normal human pregnancy

Seventeen healthy women were investigated near the beginning and again near the end of the third trimester of their normal pregnancies. Infusion studies were performed in the left lateral position. There was a highly significant decrease in effective renal plasma flow but not in glomerular filtration rate, measured as inulin clearance. Plasma creatinine concentration increased significantly, but the renal handling of creatinine was unchanged; simultaneous 24-hour creatinine clearance showed a tendency to decrease. Serum urate concentration also increased significantly, apparently due to an increase in net tubular reabsorption of urate.

Serial changes in renal haemodynamics during normal human pregnancy

Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were determined at constant intervals during and after the normal pregnancies in 25 healthy women. Compared with non-pregnant values, ERPF increased by 80 percent during early pregnancy but fell significantly from this new level during the third trimester. GFR, however, remained at a level 50 per cent above the non-pregnant throughout pregnancy. Filtration fraction (GFR/ERPF) was significantly reduced during early pregnancy but rose to a value equivalent to the non-pregnant during the third trimester. Comparable data of previous workers are re-interpreted.

24-hour creatinine clearance during the third trimester of normal pregnancy

Serial changes in 24-hour creatinine clearance were determined in 10 healthy women during late pregnancy and after the puerperium. A decrease occurred in all subjects during the six weeks preceding delivery, so that mean values for the last three weeks of pregnancy did not differ from the non-pregnant mean. Possible reasons for this phenomenon and its clinical implications are discussed.

Renal physiology in pregnancy

The changes which occur in renal excretory function during pregnancy are discussed. The relationship between glomerular filtration rate and effective renal plasma flow is examined in the light of new serial data obtained under conditions of saline diuresis. Changes in the renal handling of nutrients and in the excretion of waste products during pregnancy are reviewed.

Prospective study of angiotensin II response to positional change in pregnancy-induced hypertension

The response of endogenous angiotensin II levels to positional change, lateral to supine recumbency, was investigated in a prospective study of 55 primigravid patients during the last half of pregnancy. Blood samples were obtained in the lateral and supine recumbent positions. The mean supine angiotensin II level was significantly higher between 29 and 34 weeks' gestation in those patients destined to develop pregnancy-induced hypertension than in those who remained normotensive (P less than 0.05). As gestation advanced, the mean per cent relative change of angiotensin II from the lateral to the supine position altered from negative to positive in those patients destined to develop pregnancy-induced hypertension, whereas it remained negative in those patients who remained normotensive. These findings are discussed in relation to pathophysiologic alterations in the development of pregnancy-induced hypertension.

The outcome of pregnancy after gonadotrophin therapy

One hundred and ten women conceived 143 times following induction of ovulation by gonadotrophins. The abortion rate was 21 per cent. Some bleeding occurred in 38-7 per cent of pregnancies and 54 per cent of them ended in abortion. For the 112 pregnancies reaching 20 weeks, the multiple pregnancy rate was 26-8 per cent (21 twins, 5 triplets, 3 quadruplets and 1 sextuplet). Hypertension was present in 8-9 per cent of patients and in 3-3 per cent of those with multiple pregnancy. The length of gestation was related to the number of fetuses at birth and postmaturity did not occur. The Caesarean section rate was 32-1 per cent. The birth weight of the infants was normal and the male to female sex ratio was 0-64 for singleton births and 0-78 for twins. The fetal loss was 15-9 per cent (7-1 per cent for pregnancies of over 28 weeks). Growth and development of the children were apparently normal. The incidence of all congenital malformations was 7 per cent.

Renal clearances of estriol conjugates in normal human pregnancy at term

In late human pregnancy more than 90 per cent of the total estriol (E3) in body fluids consists of four conjugates, estriol-3-sulfate (E3-3S), estriol-16-glucosiduronate (E3-16G), estriol-3-glucuronide (E3-3G), and estriol-3-sulfate-16-glucosiduronate (E3-3S-16G). Since the relative amounts of E3 in blood and urine would be determined by the kidney, the renal clearance of each conjugate was determined and compared with inulin and p-aminohippuric acid (PAH) clearance, as measures of glomerular filtration rate (GFR) and the effective renal plasma flow. Five women were studied in the lateral decubitus position with inulin and PAH infusion. Samples of blood and urine were collected at 40 minute intervals and analyzed. The method for E3 conjugates involved separation of the four conjugates on Sephadex LH-20, enzyme hydrolysis, and radioimmunoassay. Renal clearances for E3-3S and E3-3S-16G were below inulin. E3-3G approximated inulin; E3-16G exceeded inulin and approached PAH. In plasma E3-3S-16G represented 48.4 +/- 7.2 per cent; in urine E3-16G represented 69.5 +/- 7.3 per cent of total E3. Thus, different conjugates predominate in blood and urine.

Investigations into the influence of posture on renal plasma flow and glomerular filtration rate during late pregnancy

Renal plasma flow and glomerular filtration rate were investigated in three positions (supine, left lateral and sitting) in a group of eighteen healthy women during late pregnancy and again after the puerperium. No difference in renal function could be attributed to the position adopted. However, an unexplained decrease in both of the indices occurred during the course of prolonged infusion in any position.

Effect of hypotonic expansion on sodium, water, and urea excretion in late pregnancy: the influence of posture on these results

Mineralocorticoid-treated, normotensive third trimester subjects positioned in lateral recumbency were studied before and during the infusion of 300 mEq of hypotonic saline. Urinary sodium excretion increased in all subjects from a mean value of 199 to 416 muEq/min. In 12 maximally hydrated subjects free water clearance (C(H2O)) and urine flow (V) increased from means of 7.54 and 9.50 to 11.6 and 14.5 ml/100 ml of glomerular filtrate (GFR) Also the ratio of urea to inulin clearance (C(urea)/C(inulin)) increased from 0.59 to 0.64. The changes in the renal handling of water and urea suggest that fractional sodium reabsorption decreased at proximal nephron sites. The subjects then assumed a supine position, and the results were compared to those obtained during the lateral recumbent control periods. Filtered sodium decreased in 11 experiments, but in five studies it remained up to 2.6 mEq/min above control values. There was only one instance in which a significant increase in sodium excretion occurred. It was concluded that supine recumbency blunts natriuresis despite volume expansion or an increase in the filtered load of sodium.Finally, in the 12 hydrated subjects supine recumbency reduced C(H2O) and V from a mean of 11.6 and 14.5 to 6.2 and 8.2 ml/100 ml of GFR. In eight of these experiments urine osmolality fell or did not change. Simultaneously, C(urea)/C(inulin) fell from 0.64 to 0.57. These data suggest that the antinatriuresis, which occurred when the volume-expanded subjects were positioned in supine recumbency, was accompanied by a decrease in the fractional reabsorption of sodium at proximal nephron sites.