What is St Marks fluid and how does it work?
St Marks fluid, also known as St Mark's electrolyte mix or E-mix, is a homemade oral rehydration solution (ORS) developed at St Mark's Hospital in London specifically for patients with intestinal failure, such as those with short bowel syndrome or a high-output stoma. Unlike other commercially available ORS products, its key characteristic is a high sodium concentration (approximately 90 mmol/L) and the absence of potassium, which is critical for patients who cannot tolerate additional potassium.
The fluid's mechanism of action relies on the principle of coupled glucose and sodium transport in the intestines. In a healthy gut, this process efficiently pulls water and electrolytes from the intestinal lumen into the bloodstream. In patients with conditions like short bowel syndrome, the reduced intestinal surface area impairs this natural absorption. St Marks fluid is an isotonic solution, meaning it has a solute concentration similar to blood plasma. By delivering a high concentration of glucose and sodium together, it maximizes the passive absorption of salt and water, which effectively helps to restore fluid balance and prevent dehydration. In contrast, drinking large quantities of low-sodium or 'hypotonic' fluids like plain water can actually worsen fluid loss in these patients, as it draws fluid and sodium from the body into the bowel.
Clinical uses of St Marks fluid
St Marks fluid is an important therapeutic tool for managing fluid and electrolyte balance in specific patient populations. Its primary indications include:
- Short Bowel Syndrome (SBS): This condition, caused by surgical removal or congenital defects of the small intestine, leads to malabsorption and a reduced ability to absorb fluids and nutrients. The ORS helps manage the resulting high-volume diarrhea or stoma output and decreases the need for intravenous fluid or nutritional support.
- High-Output Stoma: Patients with an ostomy (such as an ileostomy) that produces more than 1,500 mL of fluid per day are at significant risk of dehydration. St Marks fluid provides a high-sodium solution to help the body absorb more water from the intestine, reducing output and maintaining hydration.
- Enterocutaneous Fistula: A fistula is an abnormal connection between the intestine and the skin. Patients with high-output fistulas can experience substantial fluid and electrolyte losses, and St Marks fluid is used to counteract this.
Preparation and administration
Since St Marks fluid is not commercially pre-mixed, it must be prepared daily at home following a precise recipe to ensure the correct electrolyte concentration.
Here are the typical ingredients for a one-liter batch:
- Glucose powder
- Sodium chloride (table salt)
- Sodium bicarbonate (baking soda), which can be substituted with sodium citrate if the taste is intolerable
- 1 litre of cold tap water
Specific quantities for each ingredient are crucial for effective rehydration and should be obtained from a healthcare professional or reliable clinical source.
All ingredients must be dissolved completely in the water. The solution should be consumed by sipping it slowly throughout the day, rather than drinking it all at once, to maximize absorption. Any unused portion should be discarded after 24 hours. The taste, which can be salty or bitter, can be improved by chilling the fluid, freezing it into a slush, or adding a small splash of flavoring, such as squash or fruit juice, ensuring not to alter the concentration significantly.
St Marks fluid vs. other oral rehydration solutions
St Marks fluid has key differences from other common oral rehydration solutions, particularly its electrolyte profile, which makes it uniquely suited for specific clinical needs.
| Feature | St Marks Fluid | WHO Standard ORS | Commercial Sports Drinks |
|---|---|---|---|
| Sodium Content | High (approx. 90 mmol/L) | Adequate (approx. 75 mmol/L) | Lower (Varies, but generally lower than clinical ORS) |
| Potassium Content | Potassium-free | Contains potassium (approx. 20 mmol/L) | Contains potassium (Varies) |
| Primary Use | Management of high-output stoma and short bowel syndrome | General dehydration due to conditions like diarrhea | Mild dehydration during exercise and fluid replacement |
| Availability | Prepared at home from basic ingredients | Sold as pre-packaged sachets | Widely available in supermarkets |
| Risk Factor | High sodium requires careful monitoring to prevent hypernatremia | Standardized formula is generally safe, but potassium needs monitoring in renal patients | Often high in sugar and lower in sodium, unsuitable for severe clinical dehydration |
Potential side effects and precautions
While highly effective, St Marks fluid must be used under medical supervision, as its high sodium content can pose risks if improperly managed.
- Hypernatremia (High Blood Sodium): If the solution is incorrectly prepared with too much salt or if intake is not monitored, it can lead to dangerous levels of sodium in the blood. Symptoms can include severe thirst, confusion, weakness, and vomiting.
- Accurate Measurement: The home preparation requires precise measurements to maintain the correct sodium-to-glucose ratio. Inaccurate mixing can make the solution ineffective or unsafe.
- Monitoring by Healthcare Team: Patients must work closely with their doctor or dietitian to determine the appropriate volume and to manage overall fluid intake, which may include restricting other hypotonic fluids.
In conclusion, St Marks fluid is a vital, non-commercial oral rehydration solution for managing severe fluid and electrolyte imbalances in specific patient groups, particularly those with short bowel syndrome or a high-output stoma. Its unique potassium-free, high-sodium formulation is designed to maximize intestinal absorption and prevent dehydration, but its use requires careful adherence to preparation instructions and medical oversight due to the potential for side effects related to its high sodium content. For comprehensive information on managing these conditions, patients can consult the guidance published by St Mark's Academic Institute at stmarksacademicinstitute.org.uk.
