The effect of the supine position on renal function in the near-term pregnant woman

Maternal posture-physiology interactions in human pregnancy: a narrative review

There are several well-known medical conditions in which posture and gravity interact with natural history, including pregnancy. In this review, we provide a comprehensive overview of interactions between maternal posture and maternal physiology and pathophysiology at rest during pregnancy. We conducted a systematic literature search of the MEDLINE database and identified 644 studies from 1991 through 2021, inclusive, that met our inclusion criteria. We present a narrative review of the resulting literature and highlight discrepancies, research gaps, and potential clinical implications. We organize the results by organ system and, commencing with the neurological system, proceed in our synthesis generally in the craniocaudal direction, concluding with the skin. The circulatory system warranted our greatest and closest consideration-literature concerning the dynamic interplay between physiology (heart rate, stroke volume, cardiac output, blood pressure, and systemic vascular resistance), pathophysiology (e.g., hypertension in pregnancy), and postural changes provide an intricate and fascinating example of the importance of the subject of this review. Other organ systems discussed include respiratory, renal, genitourinary, gastrointestinal, abdominal, and endocrine. In addition to summarizing the existing literature on maternal posture-physiology interactions, we also point out gaps and opportunities for further research and clinical developments in this area. Overall, our review provides both insight into and relevance of maternal posture-physiology interactions vis à vis healthcare's mission to improve health and wellness during pregnancy and beyond.

Mechanisms of Renal Vasodilation and Hyperfiltration During Pregnancy

Glomerular filtration rate (GFR) and renal plasma flow (RPF) increase by 40-65% and 50-85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration appears to be almost completely due to the increase in RPF, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The major pregnancy hormone involved is relaxin. The mediators downstream from relaxin include endothelin (ET) and nitric oxide (NO). New evidence indicates that relaxin increases vascular gelatinase activity during pregnancy, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and NO. Whether the chronic volume expansion characteristic of pregnancy contributes to the maintenance of gestational renal changes requires further investigation. Additional studies are also needed to further delineate the molecular basis of these mechanisms and, importantly, to investigate whether they apply to women.

New vascular system in reptiles: anatomy and postural hemodynamics of the vertebral venous plexus in snakes

Using corrosion casting, we demonstrate and describe a new vascular system--the vertebral venous plexus--in eight snake species representing three families. The plexus consists of a network of spinal veins coursing within and around the vertebral column and was previously documented only in mammals. The spinal veins of snakes originate anteriorly from the posterior cerebral veins and form a lozenge-shaped plexus that extends to the tip of the tail. Numerous anastomoses connect the plexus with the caval and portal veins along the length of the vertebral column. We also reveal a posture-induced differential flow between the plexus and the jugular veins in two snake species with arboreal proclivities. When these snakes are horizontal, the jugulars are observed fluoroscopically to be the primary route for cephalic drainage and the plexus is inactive. However, head-up tilting induces partial jugular collapse and shunting of cephalic efflux into the plexus. This postural discrepancy is caused by structural differences in the two venous systems. The compliant jugular veins are incapable of sustaining the negative intraluminal pressures induced by upright posture. The plexus, however, with the structural support of the surrounding bone, remains patent and provides a low-pressure route for venous return. Interactions with the cerebrospinal fluid both allow and enhance the role of the plexus, driving perfusion and compensating for a posture-induced drop in arterial pressure. The vertebral venous plexus is thus an important and overlooked element in the maintenance of cerebral blood supply in climbing snakes and other upright animals.

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.

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.

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.

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.

Studies of the Inferior Vena Cava in Late Pregnancy

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