Renal Physiology and Pathology in Pregnancy

Normal Physiology

Pregnancy is associated with several structural and functional changes within the kidneys.

Structural Changes

• The kidneys themselves are expected to increase in size by 1-1.5cm, likely due to increased maternal blood, which doubles by 7 months of gestation.¹
• Compression from the uterus and hormone-driven smooth muscle relaxation cause dilation of the renal collecting ducts, renal pelvis, and ureters.^1,2
• The resulting urinary stasis increases the risk of urinary tract infections during pregnancy.¹

Changes in Renal Function

• Most changes in renal function are mediated by progesterone and relaxin.¹
• GFR increases by about 50%.¹
• Plasma Cr and BUN decrease by around 40%.1 Notably, this means a creatinine level deemed “normal” by laboratory values may in fact be abnormal in the setting of pregnancy.
• Increased GFR alongside decreased renal tubular reabsorption leads to a mild glycosuria and proteinuria; proteinuria less than 300mg in 24 hours is considered normal in pregnancy.^1,2

AKI During Pregnancy

Diagnosis of AKI

• There are no standardized criteria to diagnose AKI in pregnant women.
• In general, patients’ kidney function should be considered alongside their vital signs, pregnancy course, and comorbidities. Reference values concerning AKI include serum Cr above 0.8mg/dl, doubling of Cr levels, or oliguria.^3,4

Classifying AKI

• Pregnancy-related AKIs can be characterized into prerenal, intrarenal, or postrenal AKIs, as they are in nonpregnant patients.⁴
• Pregnancy-related AKIs can also be thought of in terms of when in the pregnancy they arise.³
  • AKIs during the first trimester are typically associated with hypovolemia from hyperemesis gravidarum or septic abortions.
  • Later in pregnancy, the differential expands to include hypertensive disorders of pregnancy such as preeclampsia/HELLP (Hemolysis, Elevated Liver enzymes and Low Platelets) syndrome or acute fatty liver disease of pregnancy, as well as exacerbations of intrinsic kidney diseases, such as lupus nephritis or thrombotic microangiopathies like thrombotic thrombocytopenic purpura and atypical hemolytic uremic syndrome (aHUS).
    • Several of these disorders share clinical features beyond their impact on renal function, such as anemia, thrombocytopenia, peripheral edema, and proteinuria, and require careful evaluation for diagnosis.
  • Preeclampsia, HELLP, and aHUS can also occur during the postpartum period.
    • These, as well as hemorrhage and sepsis, can result in bilateral renal cortical necrosis, a rare progression to severe renal dysfunction which occurs in 1.5-4.5% of AKIs during pregnancy.⁵ It presents with anuria and often requires initiation of dialysis until renal recovery.

ESRD and Dialysis During Pregnancy

• Women with chronic kidney disease are at higher risk of preterm delivery, preeclampsia, and worsening renal function during pregnancy.
• ESRD requiring dialysis is associated with decreased fertility, and the resultant pregnancies remain at higher risk of preterm delivery, preeclampsia, and fetal complications such as intrauterine growth restriction and mortality.^3,4
• Therefore, patients with ESRD require close monitoring of renal function and fetal development throughout pregnancy.

Dialysis During Pregnancy

• Longer, more frequent dialysis sessions, at least 20 hours per week, are associated with better maternal and fetal outcomes.³
• Maintaining baseline BUN levels, or at least a serum BUN less than 50mg/dL, is favorable.³
• Intermittent hemodialysis is associated with large fluid and electrolyte shifts and the potential for hypotension; this may decrease uterine blood flow, so fetal heart rate monitoring is recommended during dialysis sessions.⁴
• Peritoneal dialysis has less abrupt hemodynamic swings but is not associated with improved fetal outcomes compared to hemodialysis, and may be associated with an increased incidence of small-for-gestational-age births.^3,4

Anesthetic Management of Renal Disease⁴

Both spinal and epidural anesthesia are safe to perform in women with ESRD or AKI, although several considerations must be considered when creating an anesthetic plan.

• Electrolyte disturbances, anemia, coagulopathy, and renal function should be assessed preoperatively.
  • Neuraxial is likely contraindicated in cases with thrombocytopenia, coagulopathy, and severe hypovolemia.
• Intravascular volume assessment must be performed, especially if the patient is on dialysis, as volume status can vary significantly depending on the timing since the last dialysis session.
  • Volume-depleted patients may require close hemodynamic monitoring and management of hypotension when it occurs after neuraxial blockade.
• Women with coexisting hypertension should have a plan for blood pressure control during labor. Patients with chronic hypertension may have left ventricular dysfunction.
• If a patient must undergo general anesthesia, anesthesiologists should take the same precautions as they would in nonpregnant patients with ESRD.

Renal Transplant

• Pregnancy is considered safe one year after transplant if the patient has stable renal function, adequate graft function, and an established immunosuppressive regimen.⁶
• Immunosuppressive medications must be continued during pregnancy to preserve graft function. While all immunosuppressives cross the placenta to varying degrees, most are not associated with increased risk of congenital anomalies, with the notable exception of mycophenolate mofetil.⁶
• About 50% of renal transplant recipients will have preterm delivery, and around 30% will develop preeclampsia.⁶
• Neuraxial procedures are safe in posttransplant patients, barring any hematologic concerns or coagulopathy that may arise from immunosuppressive medications or preeclampsia.⁶