INTRAVENOUS FLUID THERAPY DURING ANESTHESIA
- Water, Electrolytes, Glucose requirement, Disposition
The intravascular compartment consists of blood cells, colloids, and solutes. Each one of them plays a specific role in the homeostasis. In the perioperative period there are losses and shifts of ECF between compartments. Injury, surgery, endocrine pathology contribute to those shifts and ultimately influence outcome. It is generally accepted that the total body water of a 70kg adult patient is approximately 60-70% of the weight and approximately two-thirds of it is intracellular. The focus of this chapter is the intravascular volume which consists of extracellular volume, plasma, and intracellular volume attributable to erythrocytes, leukocytes, platelets.
The plasma, constituting approximately 3 L, consists of inorganic ions, albumin and small molecules. The inorganic ions are found on both sides of the cellular membranes and their concentration is maintained due to an energy consuming process. The Na+/K+ ATP-dependent pump maintains a higher N+ and Cl– concentration in the extracellular space while K+ concentration is higher intracellularly. The albumin and other larger molecules are kept in the intravascular compartment by the endothelium cells due to their size. Smaller molecules, however, can cross freely this barrier. The endothelium cells and thus the barrier they provide can be disrupted by injury, surgery, or inflammatory processes. The result Is a disruption of homeostasis with significant deleterious effects on the body. Additionally, disease states can cause disruption of the inorganic ion homeostasis and leading to fluids shifts between compartments leading to edema, poor perfusion, lactate buildup, poor excretion of harmful metabolites and causing additional injury.
Starling’s Equation underscores the important forces (hydrostatic and oncotic) affecting fluid distribution between capillary and interstitial space:
Jv = Kf [(Pc – Pi) – Ï (πc – πi)]
Jv – net filtration or net fluid movement
Kf – filtration coefficient
Pc and Pi – the hydrostatic pressures in the capillaries and interstitial space respectively
Ï¬ – reflection coefficient
πc and πi – capillary and interstitial oncotic pressure
The natural driving force and thus fluid movement is from capillary to interstitial space, where the excess fluid is cleared by the lymphatics. Diseases and trauma, whether due to surgery or otherwise induced and leading to inflammation and release of toxic substrates, disrupts the balance and the function of the endothelium and reducing the reflection coefficient. The increased permeability can lead to changes in the interstitial fluid composition which changes the oncotic pressure difference leading to further extravasation of fluid and resulting in tissue edema. This edema compromises local perfusion and accumulation of toxic byproducts causing a vicious cycle and ultimately death.
The osmotic pressure is due to semipermeable membranes. Solutes which freely traverse a membrane don’t build an osmotic pressure gradient across the membrane.
Glucose is present in the intracellular fluid and serves to provide energy substrate. It is regulated through insulin and maintained at a level between 70 and 90 mmol/L in healthy adults. Increase in the glucose concentration can change the osmotic pressure across the endothelium and cause fluid shifts leading to
Our goal as anesthesiologist is to maintain the intravascular compartment and assure adequate delivery of oxygen and nutrients to the organs while maintain good clearance of metabolic byproducts.
The following classification of the perioperatively used fluids is ubiquitous: crystalloids and colloids. Crystalloids with ionic solution and osmolality close to that of plasma are deemed balanced solutions. The glucose is used to provide energy substrate and used in hypoglycemic patients or in combination with insulin. Once the glucose is metabolized, the reminder of the free water can be easily distributed along all compartments. Colloids consist of dissolved large molecular substances. They are generally described by their molecular weight or MWw. This property contributes to the oncotic pressure created intravascularly with intact endothelium and glycocalyx. Naturally occurring colloids encompass albumin, immunoglobulins, fresh frozen plasma, and plasma protein fraction. Semisynthetic ones are: gelatins, dextrans, and hydroxyethyl starches (HES). Semisynthetic and naturally occurring colloids have raised the concern of viral and prion transmission, particularly those from bovine origin. While most of the colloids have variable size of molecules, human albumin is more uniform.
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Gelatins are bovine collagen derivatives. Some preparations can contain Ca or other inorganic ions and those need to be taken into consideration. Dextrans are biosynthesized sucrose derivatives. They are best described by their molecular weight, i.e. Dextran 40 has a molecular size of 40,000 Daltons (Da) and Dextran 70 – 70,000 Da. Their clearance is highly dependent on their molecular size with smaller molecules freely filtered through the renal glomerulum and larger sizes are metabolized by the reticular endothelial system first and then excreted through the gut.
Hetastarches are derivatives of amylopectine. They are divided into high-molecular weight, medium molecular weight and low molecular weight. They can be dissolved into normal saline or balanced solution.
All semisynthetic colloids are known to exert an effect on kidneys and coagulation. Thus, there is a maximum dose recommended by the manufacturers.
- Normal Salinevs.LactatedRingers‘vs. Plasmalytevs.D5W
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