Monday, March 2, 2009

Physiological Changes During Pregnancy

Physiological Changes During Pregnancy
The hormonal effects on the gastrointestinal tract are an issue of debate among anaesthetists. Relaxation of the lower oesophageal sphincter has been described, but there have been differing views about the effect on motility of the gastrointestinal tract and the times at which it is most prominent. Many believe that there is also retardation of gastrointestinal motility and gastric emptying, producing increased gastric volume with decreased pH, beginning as early as 8-10 weeks of gestation.

Recent studies, however, have shed a different light on the subject. Measuring peak plasma concentrations of drugs absorbed exclusively in the duodenum in both non-pregnant and pregnant volunteers, at different times of gestation, it was shown that peak absorption occurred at the same interval in all women with the exception those in labour. This suggests that gastric emptying is delayed only at the time of delivery.

Mechanical Changes the enlarging uterus causes a gradual cephalad displacement of stomach and intestines. At term the stomach has attained a vertical position rather than its normal horizontal one. These mechanical forces lead to increased intragastric pressures as well as a change in the angle of the gastroesophageal junction, which in turn tends toward greater oesophageal reflux.
Renal plasma flow and glomerular filtration rate begin to increase progressively during the first trimester. At term, both are 50-60% higher than in the non-pregnant state.

This parallels the increases in blood volume and cardiac output. The elevations in plasma flow and glomerular filtration result in an elevation in creatinine clearance. Blood urea and serum creatinine are reduced by 40%. The increase in glomerular filtration may overwhelm the ability of the renal tubules to reabsorb leading to glucose and protein losses in the urine. Thus, mild glycosuria (1-10 gm/day) and/or proteinuria (to 300 mg/day) can occur in normal pregnancy.

This is a measure of the osmotic activity of a substance in solution and is defined as the number of osmoles in a kilogram of solvent. In practice it indicates that the plasma concentrations of electrolytes, glucose and urea, fall if more water than sodium, for example, is retained. Over the whole period of gestation there is retention of 7.5L of water and 900 mmol of sodium. After the 12th week of gestation, progesterone can induce dilation and atony of the renal calyses and ureters. With advancing gestation, the enlarging uterus can compress the ureters as they cross the pelvic brim and cause further dilatation by obstructing flow. These changes may contribute to the frequency of urinary tract infections during pregnancy.

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