Chronic renal insufficiency occurs in 0.03–0.2% of all pregnancies. Although rare, these parturients have a significantly increased maternal and fetal morbidity and mortality and can present a significant challenge to the anaesthetist. The management of this high-risk group of patients is often complicated and is supported by little evidence. It requires an understanding of the physiological changes of pregnancy, the optimization of existing management, close monitoring of both mother and fetus, and early involvement of obstetricians, nephrologists, neonatologists and anaesthetists, with early intervention as required.
Renal insufficiency in pregnancy may be classified into three categories. These include known pre-existing renal disease diagnosed prior to the pregnancy, sub-clinical chronic renal disease unveiled by the pregnancy or new-onset disease that develops during the pregnancy. Although general principles can be applied to the management of these patients, the causes, associated co-morbidities and prognosis of these patients may differ greatly.
It is important to consider both the impact of renal insufficiency upon maternal and fetal outcome, and the impact of a pregnancy upon the short- and long-term course of chronic kidney disease. These patients have an increased risk of pregnancy-induced hypertension, pre-eclampsia, preterm delivery, intrauterine growth restriction and perinatal mortality. The demands of pregnancy may result in a transient, and sometimes a permanent decline in renal function.
The assessment of renal function in pregnancy
Pregnancy itself results in anatomical and physiological changes in the renal system (See Chapter 1). Changes in water metabolism, extracellular fluid volume regulation, acid–base regulation and the renal handling of electrolytes, protein and glucose result in altered ‘normal ranges’ of typical laboratory values. These values continue to change from the first to the second and third trimesters. This complicates the diagnosis, assessment and surveillance of renal insufficiency. The gold standard for measuring and therefore monitoring renal function remains inulin clearance, but it is difficult to perform. Creatinine clearance measured with a 24-hour urine collection is the most well-validated method for approximating glomerular filtration rate (GFR). Formulaic estimations of GFR lack validity in pregnancy.
GFR and creatinine clearance typically increase by 40–65% and serum creatinine concentrations fall. It has been suggested that the upper limit of serum creatinine concentration should approximate 85, 80 and 90 mol/L in the first, second and third trimesters, respectively. A blood urea nitrogen concentration of >13 mg/dL may also indicate renal insufficiency in pregnancy.