Fluid & Electrolytes

Transcription

1 Fluid & Electrolytes A physiologic acid-base balance is maintained by integrated function of renal neuroendocrine and pulmonary systems. Pathological conditions can alter the normal homeostatic mechanisms leading to serious fluid and electrolyte abnormalities. Fluid & electrolyte disorders are commonly seen in hospitalized patients. Approximately 60-70% of an adult s body weight is water. Body water content varies inversely with body fat content. Body water is distributed into two compartments: intracellular volume, which is 40% of total body weight or 28 L in a 70 kg patient and extracellular volume, which is 20% of total body weight or 14L in a 70 kg. The extracellular fluid compartment is further divided into the following: interstitial compartment with 75% of the extra cellular volume, which would be 15% of total body weight or 10.5 L in a 70 kg patient and the plasma volume with 25% of the extra cellular volume, which would be 5-10%of total body weight or 3.5 L in a 70 kg patient. The intracellular fluid contains large amounts of potassium, magnesium, phosphates and sulfates, but its sodium content is very low. Both the plasma and interstitial fluid contain large amounts of sodium and chloride. Water, sodium, and potassium are lost in the urine. Daily water loss in the absence of surgical stress usually falls in the range of 0.5 ml per kg per hour to 1 ml per kg per hour in normal adults. Insensible water loss from skin and lungs averages 600 ml per day to 1,00 ml per day. Respiratory rate, ambient temperature humidity and body temperature significantly influence the amount fluid loss. The water deficit can increase by 100 ml per day to 200 ml to day for each degree of temperature above 37 C. For a patient with a tracheostomy breathing non humidified air, this liter can be up to 1.5 liters per day. For patients sweating excessively, another 500 ml to 3,000 ml may be lost. Edema fluid is sequestered in potential spaces within the body in special situations. The fluid is lost from the plasma and if severe, it can result in hypovolemia. This process is often referred to as third spacing. Potential causes of third spacing include peritonitis, pancreatitis, burns, trauma, sepsis, nephrotic syndrome, cirrhosis, and abdominal surgery. Specific examples of third spacing include ascites, lymphedema, dermal sequestration in burns and bowel edema. Isotonic saline or other extra cellular volume expanders may be needed to correct the fluid deficit.

2 In summary, an average adult requires 2,000 ml to 3,000 ml fluid daily. A practical fluid maintenance guideline for patients of any age is 4 cc per kg per hour for the first ten kg of body weight, 2 cc per hour for the next 10 kg of body weight. For example, a patient weighing 70 kg would need maintenance fluids of forty plus twenty plus 50 ml per hour to equal 110 ml per hour. A variation of this formula is to administer 100 cc per kg for the first 10 kg of body weight, 50 cc per kg for the next 10 kg of body weight and 20 cc per kg for the remaining body weight over a period of 24 hours. Daily water requirements can also be calculated based on body surface area as 1,500 cc per M 2 per 24 hours. Sodium and potassium electrolytes are usually administered in maintenance IV fluids. The average daily requirements of sodium and potassium are 2 to 3 ml equivalents per day for sodium and 1 ml equivalent to 2 ml equivalents per kg per day for potassium. Potassium supplementation is especially important because of the obligate lost through the urine. To minimize protein metabolism and to prevent ketosis 100g/day to 150g/day of carbohydrates should also be administered. REPLACEMENT STRATEGIES Crystalloids There are two main categories of replacement IV fluids: crystalloids and colloids. Crystalloid fluids are mixtures of physiologically active solutes. Sodium is the major component of most crystalloid fluids. Its extra cellular distribution determines the distribution and equilibration of the infused crystalloid solutions. Since the extravascular fluid compartment, infused sodium and its accompanying water will reside primarily outside of the vascular compartment. Consequently, in replacing blood loss with crystalloid, one commonly replaces three times the volume of blood loss with crystalloid solution. Types of crystalloid solution include the following: Normal or isotonic saline, Ringer s lactate, or dextrose-containing solutions. Isotonic saline or normal saline, which contains 9 gm of sodium chloride per liter or 0.9% solution. Isotonic saline is used to expand the extra cellular fluid compartment. When used

3 as the sole IV fluid for resuscitation, it may result in a hyperchloremic metabolic acidosis when given in large amounts. Ringers lactate is a balanced electrolyte solution that replaces potassium, calcium and lactate in isotonic saline for part of sodium and chloride with the addition of lactate as a buffer. There is no convincing evidence that this fluid is superior to other crystalloids. Some are concerned that the calcium in the solution may cause worsening of the no reflow phenomenon which consists of hyper confusion following resuscitation from ischemia. No reflow is believed to be caused by ischemic vasoconstriction mediated by calcium influx into vascular smooth muscle, which takes place during the ischemic period in a patient hemorrhagic shock or other ischemic events. Another concern with Ringers lactate is that the calcium can cause precipitation of the agents used to preserve red blood cells. Dextrose is available in concentrations of 5%, 10%, 25% and 50% solution in water. Five percent dextrose in saline and in Ringers lactate is also available. Dextrose provides 170 kilo calories per liter in the 5% mixture and each 50g contributes 278 ml osbald to a solution. Although it provides a very small amount of calories, 150g of dextrose per day can minimize ketosis and is useful to spare protein in short periods of fasting. However, dextrose has been shown to increase lactic acid production in ischemic organs, particularly in the central nervous system. Based on the status, some authors do not recommend the routine use of glucose infusions in patients at risk for cerebral ischemia. Normosol is a highly buffered crystalloid which has 27 ml equivalence per liter of acetate and 23 ml per liter of gluconate. The ph of normosol is 7.4 which is similar to plasma. Most crystalloids have a ph ranging from 5-7. Normosol also contains magnesium instead of calcium. The major advantage of normosol is its adjusted ph. Magnesium may beneficial in a solution phenomenon after shock resuscitation. Both normosol and plasmalyte are more expensive compared to commonly used crystalloids and have not been shown to be superior. Finally, hypertonic saline solutions of 3%, 5% and 7.5% ( Hot Salt ) have attracted interest as a possible alternative fluids for shock resuscitation. When hypertonic solutions are infused, fluid is translocated from the interstitial and cellular compartments to the intravascular volume. The result is an expansion of plasma volume that is substantially greater than the actual volume infused with accompanying improvement in cardiac output and mean arterial pressure. Since a small amount of fluid is needed for clinical response, this form of therapy seems particularly well suited resuscitation is the short duration of response(1-2 hours). Other side effects include hypernatremia, hyperchloremia, and hyperosmolality. Comparison of Crystalloid Solutions Dextrose (mg dl -1 ) Na Cl K Mg Ca Lactate Approximate ph mosm L -1 (calculated) ECF % dextrose in water 5% dextrose in % NaCl 5% dextrose in % NaCl 0.9% NaCl Lactated Ringers Solution 5% dextrose in lactated Ringer s solution Normosol-R α % NaCl Contains acetate 27 meq L -1 and gluconate 23 meq L -1

4 Colloids In addition to crystalloids, the other category of replacement IV fluids is colloids. Colloids are substances of large molecular weight that are used for fluid resuscitation in patients in shock. The large particles are retained in the vascular space where they exert an osmotic force called colloid oncotic pressure (COP) that keeps fluid in the blood vessels. Colloids commonly used in clinical practice include: albumin, plasmanate, dexstran, hydroxyethyl starch or Hetastarch. Albumin is the colloid by which all others are measured. It is commercially prepared from pooled venous plasma, sterilized and adjusted to a ph of 5.9. Albumin is available in 5% and 25% hyperoncotic preparation with colloid oncotic pressures of 20 ml of mercury and 70 ml mercury respectively. Albumin normally accounts for 80% of the plasma colloid oncotic pressure and has a serum half life of approximately three weeks. The oncotic effect of therapeutically administered albumin infusion include: transient abnormalities in the prothrombin or (PT), partial thromboplastin time (PTT), and platelet counts. Negative inotropic effects secondary to lower ionized calcium levels can occur, and increased risk for adrenal insufficiency. Partial extravascular distribution can worsen pulmonary edema, especially in patients with increased vascular permeability. Albumin has rarely been associated with hepatitis transmission. Dextrans are prepared from glucose polymers. Two preparations are commonly available: dexstran 40 and dexstran 70, both of which have colloid oncotic pressure of 40 mg of mercury. Dex strand 40 has been used to decrease clotting based on its rheological effects. Both dexstran 40 and dexstran 70 have also been used for volume expansion. Their initial intravascular expansion is greater than the volume infused, but volume expansion is short-lived because more than 50% of the dexstran is cleared within 6 hours. Side effects associated with the use of dexstrans include bleeding that is secondary to inhibition of platelet aggregation and factor VIII deactivation and anaphylactoid reactions which can occur in up to 1% of patients in acute renal failure. Dexstrans are not commonly used for fluid resuscitation because of their short half lives and the potential side effects. Hetastarch is a natural starch derived from amylopectin which is composed of highly branched glucose polymers. A 6% solution in.9% saline is available for clinical use. Hetastarch has colloid oncotic pressure of 30 ml of mercury, resulting in a volume expansion similar to albumin. However, its oncotic effects are more prolonged with up to 50% persisting after more than 24 hours. Unfortunately, hetastarch has also been reported to produce prolongation of PT and PTT and bleeding time and a anaphylactoid reactions and hyperamylasemia. Amylase elevations up to three times the normal value have been observed and can last about for about five days, but clinical significance of this is unclear. Regardless of early concerns about hetastarch induced coagulopathy, the clinical data available suggests that it is not a common problem. Which is better? Volume resuscitation from shock, whether it is associated with the major operative procedure, trauma or sepsis, is one of the most controversial topics in medicine. Some authors maintain that resuscitation with crystalloids efffectively restores and maintains extra cellular fluid volume. Others strongly believe that the addition of some form of colloid is necessary to replenish and maintain plasma volume. This group cites the importance of maintaining plasma on colloid pressure while restoring plasma volume to minimize interstitial edema, particularly in the lungs where it can adversely effect the gas exchange. It is difficult to interpret data comparing crystalloids and colloids because many studies have been poorly designed for this purpose. No studies have shown the superiority of one over the other in volume resuscitation.

5 There is no doubt that the colloids re-expand the intravascular volume more efficiently than crystalloids. Resuscitation with crystalloids requires 2 to 4 times more than the required volume of colloids and can take twice as long depending upon IV access. Colloids are also superior in their ability to improve cardiac output and oxygen delivery. Although studies show that the appropriate amounts of crystalloid can effectively re expand the intravascular volume, some authors maintain that in the more severely injured patient with cardiovascular compromise, a regimen containing colloids, transfusion as necessary and crystalloids is best for quickly increasing cardiac output, oxygen content, and as a result oxygen delivery. This is an area which remains unresolved. Physicians who advocate the use of colloids maintain that because crystalloids distribute through the extravascular space, patients are predisposed to developing pulmonary edema. Crystalloid supporters claim that when vascular permeability is increased colloids leak and are trapped in the pulmonary interstitial space. The available data suggests when pulmonary artery wedge pressure is kept below 15mm of mercury, there is no increased risk of pulmonary edema regardless of the type of fluid used. Even in case of increased pulmonary capillary permeability in patients who have preexisting heart failure, the base line pulmonary artery wedge pressure may be higher than normal and values as high as 20 mm of mercury can be tolerated. There is widespread evidence that the outcome of patients who are resuscitated from any king of shock is not different, whether colloids or crystalloids are used for resuscitation. There may be a subset of patients with severe shock who would benefit from rapid volume expansion with colloids, but this has yet to be shown. Colloids are much more expensive than crystalloids and this is one of the strongest arguments against routine use. In summary, there theoretical advantages for colloid resuscitation. However, costs and side effects, including anaphylaxis and bleeding complications outweigh the benefits of routine use of colloids. RISKS OF IV THERAPY IV fluid therapy is generally perceived as a low risk intervention. However, there are complications to this therapy. Infusion related phlebitis is the most common complication of IV therapy, with an incidence as high as 70% in some studies. Risk factors for development of phlebitis include the use of plastic cannulas, long catheters, insertion over joints or mobile areas, prolonged cannulation and infusion of irritating solutions, such as fluids containing hypertonic glucose and solutions containing potassium. Clinical manifestations of phlebitis include pain, tenderness, erythema, and swelling around the area of insertion. Treatment consists mainly of removing the catheter, pain relief and warm pads to alleviate local discomfort. The second most common complication of IV therapy and perhaps the most dangerous is infection. Some studies report that as much as 20% of hospital acquired bacteremia originates from vascular catheters. Staph aureus and staph epidermitis are the most commonly involved pathogens, but gram negative bacilli enterococci and candida have also been implicated in IV line related sepsis. Risk factors associated with catheter infections include poor insertion technique and cannula contamination. Electrolyte disturbances can complicate IV fluid therapy. Administration of hypotonic fluids can result in severe acute hyponatremia. Less frequently, hypertonic solutions may induce hypernatremia. Potassium administration can lead to life threatening hyperkalemia, especially in patients with renal dysfunction. Diabetic patients receiving dextrose-containing solutions commonly develop hyperglycemia. Finally, fluid overload and pulmonary edema from IV fluids can cause significant morbidity and mortality. BLOOD THERAPY

6 Transfusions are given to increase oxygen-carrying capacity, to improve coagulation function or to increase intravascular volume. The decision to transfuse depends on the patient s underlying medical problems, risk factors which may affect the patient s reserve, the type of surgery and rate of blood loss experienced intraoperatively, as well as the timing or likelihood of surgical control of bleeding. Allowable blood loss (ABL) is the amount of blood the anesthesiologist is willing to allow the patient to lose before accepting the risks of transfusion. There are several methods for determining the ABL, but all rely upon the patient s starting hematocrit and a determination of the lowest hematocrit the anesthesiologist is willing to tolerate in that particular patient. For a healthy young person, that lowest number may be a hematocrit of 21 or hemoglobin of 7 (based upon data obtained from Jehovah s Witnesses who refused transfusion and allowed observation of their complications and eventual deaths from exsanguination). For a patient with limited reserve (coronary artery disease, for example) that number may be 30 (based upon data showing that optimal rheologic flow conditions and oxygencarrying capacity balance at a crit of 30 or hemoglobin of 10). Based on these numbers then, one way to estimate ABL is: Starting crit- Lowest acceptable crit X TBW X 10% (plasma volume) Average of starting and lowest crit TBW Total Body Water Note: Hematocrit is % of plasma volume taken up by rbc s Example 1 (Healthy college student): X 70 kg (0.70) x.1 =.645 x 49 x.1 = 3.16 Liters ABL 31 Example 2 (65 year old businessman with CAD): X 70 kg (0.70) x.1 =.309 x 49 x.1 = 1.51 Liters ABL 35.5 BLOOD COMPONENTS Whole blood is no longer commonly given in the US, as component therapy is felt to be a more efficient use of blood resources. Whole blood is felt by some to be the best replacement for acute hemorrhage. If stored less than 24 hours, clotting factors and platelets will be functional; if older, function diminishes. Obviously, more volume is given than if one administers packed red blood cells(rbc s). PACKED RED BLOOD CELLS Units generally contain mls, with a hematocrit of 60-80%. One unit generally increased an adult s hemoglobin about 1 gm/dl (increases hematocrit 3-4%). Rbc s should not be administered with glucose-containing solutions (which can cause hemolysis) or Lactated Ringers (which contains calcium and can cause micro-aggregation). Administration can be facilitated by reconstituting with ml of normal saline. Complications of transfusion include major and minor transfusion reactions. PLATELET CONCENTRATES

7 Indicated in the treatment of thrombocytopenia caused by consumption, sequestration or destruction. One unit of platelets will generally increase an adult s platelet count between 5,000 and 10,000 mm 3. Complications of transfusion include HLA antigen sensitization and transmission of viral diseases. FRESH FROZEN PLASMA Fresh frozen plasma (FFP) consists of the fluid portion of one unit of whole blood, frozen within 6 hours of collection. All coagulation factors are functional except platelets. Requires crossmatching. Complications of transfusion include allergic reactions and transmission of viral diseases. May not be superior to specific factor therapy. CRYOPRECIPITATE Cryoprecipitate (cryo) contains that fraction of plasma that precipitates when FFP is thawed, including high concentrations of Factor VIII. A standard adult dose includes precipitate from about 10 units, so the patient is exposed to 10 donors with a single transfusion. COMPLICATIONS OF BLOOD THERAPY Complication of blood therapy are not uncommon and include the following: Transfusion reactions - febrile, allergic or hemolytic reactions can occur. Metabolic abnormalities - acidosis, hyperkalemia, decreased 2,3, DPG Citrate intoxication alkalosis, hypocalcemia Transmission of viral disease HIV, hepatitis, CMV Hypothermia Coagulation disorders - Dilutional thrombocytopenia, dilution of Factors V and VIII and disseminated intravascular coagulation (DIC).

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