Pathophysiology of fluid redistribution and mechanism of action
In critical situations such as acute blood loss, shock development, severe intoxications, and marked dehydration, the body triggers a pathological redistribution of water resources. Water actively shifts from intercellular spaces into the vascular bed. The consequence of this process is the development of an acute fluid deficit directly in the interstitial space.
Electrolyte (salt) solutions have a low molecular weight. This physico-chemical parameter allows them to easily penetrate through capillary walls back into the interstitium, thereby restoring the required fluid volume in the extravascular sector.
Pharmacological effects of such agents include:
- Improvement of blood rheological properties.
- Restoration of microcirculation processes in tissues.
The main disadvantage of saline solutions is that they leave the vascular bed quite rapidly. Therefore, the main tactic of infusion therapy is their combined use with colloidal solutions — this is necessary to effectively prolong their circulation time in vessels.
Isotonic sodium chloride solution
The most common drug in this group is the 0.9% aqueous sodium chloride solution. It is manufactured either in ready-to-use sealed industrial containers or can be prepared extemporaneously (immediately before use) in a pharmacy.
Clinical characteristics:
- Indications: significant fluid losses accompanied by extracellular dehydration syndrome.
- Dosage: administration of up to 2 liters of the drug per day is permitted.
- Application features: due to the rapid departure of fluid from the vascular bed, the drug demonstrates extremely low efficacy in shock and blood loss when used as monotherapy. It should only be used in a rational combination with blood component transfusions, as well as with special anti-shock blood-substituting solutions.
Ringer-Locke solution
Ringer-Locke solution is a multi-component agent. In its chemical and electrolyte composition, it is significantly more physiological than a plain isotonic sodium chloride solution.
The preparation is made on the basis of double-distilled water. 1000 ml of the finished solution contains a strictly calculated amount of components:
- Sodium chloride: 9 g
- Sodium bicarbonate: 0.2 g
- Calcium chloride: 0.2 g
- Potassium chloride: 0.2 g
- Dextrose: 1 g
This composition is indicated for the complex treatment of shock and the replenishment of acute blood loss. To achieve the maximum therapeutic effect, it is used exclusively in combination with transfusions of whole blood, donor plasma, and various hemodynamic blood-substituting fluids.
Regulation of acid-base balance and energy capacity
Infusion therapy is also aimed at stabilizing the internal environment and providing nutritional support.
For targeted correction of the acid-base balance (ABB), specialized regulators are used in clinical practice:
- 5–7% sodium bicarbonate solution.
- 3.66% trometamol solution.
When evaluating the energy capacity of blood substitutes, it is necessary to take into account the carbohydrate content. The calculation of energy value is based on the fact that 1 gram of dextrose releases 4 kcal of energy. Accordingly, upon the infusion of 1 liter of 20% dextrose solution, the patient receives 800 kcal.
Briefly: answers to topical questions
Which specific colloidal solutions are used in combination with crystalloids?
Natural and synthetic colloidal solutions are used in combination with crystalloids. The infusion therapy regimen includes the following agents:
- Human albumin — a natural colloidal blood preparation.
- Hydroxyethyl starches — synthetic colloids (including HES).
- Dextrans — solutions with an average molecular weight of 30,000–40,000 and 50,000–70,000.
- Modified gelatin — a synthetic colloidal solution.
At the same time, the proportion of colloids should not exceed one-third of the total calculated volume of the ongoing infusion.
What types of dehydration are distinguished in surgery besides extracellular?
In addition to extracellular dehydration, the presented materials distinguish:
- Cellular dehydration — develops when the ratio between the extra- and intracellular water sectors is disturbed.
- Isotonic dehydration — occurs with equivalent (proportional) loss of water and electrolytes; plasma osmolarity and sodium levels remain within normal limits.
- Hypertonic dehydration — associated with predominant water loss with lesser or no salt loss; accompanied by an increase in extracellular fluid osmolarity and cellular dehydration.
The composition of Ringer-Locke solution is easy to remember using the rule "Base 9 and three 0.2 salts": the base is 9 grams of sodium chloride, and potassium, calcium, and sodium bicarbonate salts are taken at exactly 0.2 grams each (plus 1 g of dextrose per liter of water).
Class and exam questions
Why cannot saline solutions be used as the sole treatment in severe shock?
Due to their low molecular weight, they very quickly leave the vascular bed and pass into the interstitium. Therefore, to maintain vascular pressure, they must be combined with colloidal solutions or blood products.
Where does water shift during severe dehydration and blood loss?
In these pathological conditions, water rapidly shifts from intercellular spaces directly into the vascular bed, leading to a fluid deficit in the interstitial space.
What is the maximum daily dose of 0.9% sodium chloride solution?
In case of significant fluid losses accompanied by extracellular dehydration, the dosage can be up to 2 liters per day.
What else to study on this topic
- Principles of balanced infusion therapy
- Parenteral nutrition: lipids and carbohydrates
- Hemodynamic colloidal solutions
- Pathophysiology of extracellular dehydration
- Anti-shock blood-substituting solutions
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