Incidence of Hyponatremia in PEG-SD Compared to PEG-ELS

Inclusion Criteria: * Adults age 18 years or older scheduled for elective outpatient colonoscopy: 8am - noon. Exclusion Criteria: * Unable or unwilling to consent * Pregnant * Breast feeding * Significant psychiatric illness -\> 50% colon resection * Bowel obstruction * History of hyponatremia (Serum sodium \<135 mmol/L) * End stage renal disease on dialysis * History of chronic kidney disease (other than kidney stones) * Decompensated cirrhosis, including: * History of bleeding due to portal hypertension (varices, gastropathy, etc) within 3 months * Hepatic encephalopathy (not controlled with medications) within 3 months * Clinical presence of ascites * Active cardiac disease * Recent myocardial infarction (\<4weeks) * Unstable angina * Congestive heart failure NYHA Functional Class Stage III or IV * Stage III: Marked limitation of activity. Less than ordinary activity (e.g. walking short distances, 20-100 m) causes fatigue, palpitations, dyspnea. Comfortable at rest. * Stage IV: Severe limitations. Experiences symptoms even while at rest. Mostly bedbound patients. Exclusion Criteria (post-enrollment), from baseline labs: * Serum creatinine \> 1.5 mg/dL * Serum potassium \< 3.3 or \> 5.5 mmol/L * Serum sodium \< 135 mmol/L or \>150 mmol/L * Serum calcium \< 8.0 or \> 11.0 mg/dL