Step 1: Calculate Preoperative Fluid Losses. Simply multiply the maintenance fluid requirements (cc/hr) times the amount of time since the patient took PO intake. Estimated maintenance requirements follow the 4/2/1 rule: 4 cc/kg/hr for the first 10 kg, 2 cc/kg/hr for the second 10 kg, and 1 cc/kg/hr for every kg above 20. Step 1: In the ER, the child is estimated as having 10% dehydration. The blood pressure is low and the heart rate is very high. This child is in shock. The goal is to rapidly stabilize the vital signs; maintenance fluid is not a consideration at this time. The child is given a 20 ml/kg bolus of 0.9% saline over 10-20 minutes.
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The 70 kg 'standard male' contains 42 liters of water - 60% of his body weight. The hypothetical adult female contains 55% of her body weight as water: this lower percent being due to a higher fat content. These figures are standard values which are quoted frequently and are average values.
2.1.1 Variations in Water Content
Variation due to Age
Neonates contain more water then adults: 75-80% water with proportionately more extracellular fluid (ECF) then adults. At birth, the amount of interstitial fluid is proportionally three times larger than in an adult. By the age of 12 months, this has decreased to 60% which is the adult value.
Total body water as a percentage of total body weight decreases progressively with increasing age. By the age of 60 years, total body water (TBW) has decreased to only 50% of total body weight in males mostly due to an increase in adipose tissue.
Variation between Tissues
Most tissues are water-rich and contain 70-80% water. The three major exceptions to this are:
- Plasma: 93% water (& 7% 'plasma solids')
- Fat: 10-15% water
- Bone: 20% water
Variation between Individuals
The variation between individuals in the ratio of TBW to total body weight is quite large but the majority of the variation is due to different amounts of adipose tissue as adipose has a low water content. Differences (between individuals) in the amount of bone and plasma are much smaller. Obese adults have a lower ratio because of the greater amount of adipose tissue. Differences in percent body water between males and females are primarily due to differences in amounts of adipose tissue. For any particular tissue of the body the variation is very much less but any variation that occurs is still mostly due to differences in amount of fat content.
2.1.2 Compartments
The water in the body is contained within the numerous organs and tissues of the body. These innumerable fluids can be lumped together into larger collections which can be discussed in a physiologically meaningful way. These collections are referred to as 'compartments'. The major division is into Intracellular Fluid (ICF: about 23 liters) and Extracellular Fluid (ECF: about 19 liters) based on which side of the cell membrane the fluid lies. Typical values for the size of the fluid compartments are listed in the table.
| Body Fluid Compartments (70 kg male) | |||||||||||||||||||||||||||||||||||
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2.1.3 Intracellular Fluid
The Intracellular Fluid is composed of at least 1014 separate tiny cellular packages. The concept of a single united 'compartment' called intracellular fluid is clearly artificial. The ICF compartment is really a 'virtual compartment' considered as the sum of this huge number of discontinuous small collections. How can the term ‘intracellular fluid' be used as though it was a single body of fluid? The reason is that though not united physically, the collections have extremely important unifying similarities which make the ICF concept of practical usefulness in physiology. In particular, similarities of location, composition and behaviour:
- Location: The distinction between ICF and ECF is clear and is easy to understand: they are separated by the cell membranes
- Composition: Intracellular fluids are high in potassium and magnesium and low in sodium and chloride ions
- Behaviour: Intracellular fluids behave similarly to tonicity changes in the ECF
Because of this physiological usefulness, it is convenient to talk of an idealised ICF as though it were a single real entity. The use of this convention allows predictions to be made about what will happen with various interventions and within limits these are physiologically meaningful.
2.1.4 Extracellular Fluid
A similar argument applies to the Extracellular Fluid. The ECF is divided into several smaller compartments (eg plasma, Interstitial fluid, fluid of bone and dense connective tissue and transcellular fluid). These compartments are distinguished by different locations and different kinetic characteristics. The ECF compositional similarity is in some ways, the opposite of that for the ICF (ie low in potassium & magnesium and high in sodium and chloride).
Interstitial fluid (ISF) consists of all the bits of fluid which lie in the interstices of all body tissues. This is also a ‘virtual' fluid (ie it exists in many separate small bits but is spoken about as though it was a pool of fluid of uniform composition in the one location). The ISF bathes all the cells in the body and is the link between the ICF and the intravascular compartment. Oxygen, nutrients, wastes and chemical messengers all pass through the ISF. ISF has the compositional characteristics of ECF (as mentioned above) but in addition it is distinguished by its usually low protein concentration (in comparison to plasma). Lymph is considered as a part of the ISF. The lymphatic system returns protein and excess ISF to the circulation. Lymph is more easily obtained for analysis than other parts of the ISF.
Plasma is the only major fluid compartment that exists as a real fluid collection all in one location. It differs from ISF in its much higher protein content and its high bulk flow (transport function). Blood contains suspended red and white cells so plasma has been called the ‘interstitial fluid of the blood'. The fluid compartment called the blood volume is interesting in that it is a composite compartment containing ECF (plasma) and ICF (red cell water).
The fluid of bone & dense connective tissue is significant because it contains about 15% of the total body water. This fluid is mobilised only very slowly and this lessens its importance when considering the effects of acute fluid interventions.
Transcellular fluid is a small compartment that represents all those body fluids which are formed from the transport activities of cells. It is contained within epithelial lined spaces. It includes CSF, GIT fluids, bladder urine, aqueous humour and joint fluid. It is important because of the specialised functions involved. The fluid fluxes involved with GIT fluids can be quite significant. The electrolyte composition of the various transcellular fluids are quite dissimilar and typical values or ranges for some of these fluids are listed in the Table.
The total body water is divided into compartments and useful physiological insight and some measure of clinical predictability can be gained from this approach even though most of these fluid compartments do not exist as discrete real fluid collections.
2.1.5 Functional ECF
The water in bone and dense connective tissue and the transcellular fluids is significant in amount but is mobilised much more slowly then the other components of the ECF. The remaining parts of the ECF are called the functional ECF. The ratio of ICF to ECF is 55:45.
The functional ECF is more important when considering the effects of acute fluid interventions and the ratio of ICF to functional ECF is 55:27.5 (which is 2:1). (See Section 8.1 for discussion of acute fluid infusions).
| Typical Electrolyte Concentrations in Some Transcellular Fluids(in mmol/l) | ||||||||||||||||||||||||||||||||||||||||||||
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'Fluid Physiology' by Kerry Brandis -from http://www.AnaesthesiaMCQ.com
The initial goal of treating dehydration is to restore intravascular volume. The simplest approach is to replace dehydration losses with 0.9% saline. This ensures that the administered fluid remains in the extracellular (intravascular) compartment, where it will do the most good to support blood pressure and peripheral perfusion.
Therapy may be started with a rapid bolus of 0.9% saline to combat incipient shock. But correction of dehydration has to be accompanied by provision of maintenance fluid- after all, the child is breathing, losing free water through the skin, and is urinating! As discussed earlier, maintenance fluid is provided as D5 0.18% or D5 0.3% saline. The combination of 0.9% saline (dehydration correction) and 0.18% saline (maintenance fluid) averages to approximately 0.45% (half-normal) saline. This approximation is acceptable because the kidneys will sort out what to keep and what to excrete.
A typical sequence of events in the management of a child with 10% dehydration AND A NORMAL SERUM Na LEVEL is given below. Management of children with a serum Na level of < 135 or> 145 mEq/L is beyond the scope of this discussion.
Step 1: In the ER, the child is estimated as having 10% dehydration. The blood pressure is low and the heart rate is very high. This child is in shock. The goal is to rapidly stabilize the vital signs; maintenance fluid is not a consideration at this time.
Fluid 4 2 1
The child is given a 20 ml/kg bolus of 0.9% saline over 10-20 minutes. The vital signs stabilize (the bolus can be repeated if necessary).
Step 2: the patient is transferred to the inpatient unit. By this time, serum electrolytes levels are available and the serum sodium concentration is within the normal range. Subsequent fluid therapy is calculated as follows:
This child's total fluid loss was 10% of 10 kg, or 1000 ml. Of this, 200 ml has already been infused in the ER, so the remaining deficit is 800 ml.
Fluid Ounces 1 2 Of A Pint
Typically, half the total deficit is replaced in the first eight hours after admission and the remaining fluid is given over the next 16 hours. So, this child needs 300 ml of 0.9% saline in the next eight hours (for a total of 500 ml) and another 500 ml in the next 16 hours.
However, maintenance fluid also has to be administered. The volume of maintenance fluid for 24 hours is 1000 ml (100 ml/kg X 10 kg). This needs to be given as D5 0.33% saline.
Now the fluid calculation looks like this: Etrecheck pro 6 2 2018.
| 0-8 hours | 9-24 hours | |
|---|---|---|
| Deficit | 300 ml of 0.9% saline | 500 ml of 0.9% saline |
| Maintenance | 333 ml of D5 0.33% saline | 666 ml of D5 0.18% saline |
| Averaged total | 663 ml of D5 0.45%normal saline | 1166 ml of D5 0.45% normal saline |
Note #1:Once the child has started urinating, KCl should be added to the intravenous fluids at a concentration of 20 mEq/L.
Note #2:If the child continues to vomit or have significant diarrhea, the volume of ongoing fluid loss should be estimated and added to the deficit every few hours as 0.9% saline. Ideally, the diapers should be weighed. If this is not possible, then a volume of 50-100 ml should be used for each stool in an infant and 100-200 ml for the older child.
Note #3:The dehydration component of fluid replacement MUST be provided as 0.9% saline. NEVER use a hypotonic saline, such asD5 0.18%(fifth-normal saline),D5 0.3%(third-normal saline) or evenD5 0.45%(half-normal saline) to correct dehydration.Dehydration and hypovolemia result in secretion of anti-diuretic hormone, which causes retention of free water, and provision of hypotonic replacement fluid can lead to potentially life-threatening hyponatremia.
Step 3: Suppose the child is well hydrated by the second hospital day, but is still feeling queasy and does not want to drink. Maintenance fluids can now be continued as D5 0.33% or D5 0.50% saline with 20 mEq/L of KCl.
The moral of the story:
Fluid 2.1 Channel Tv Soundbar
- If you are correcting only dehydration (as when giving a bolus in the ER), use 0.9% saline.
- If you are correcting dehydration and providing maintenance fluids at the same time, add both volumes and use D5 0.45% saline.
- If you are providing fluid only, may use D5 0.18% saline or D5 0.33% saline.
- Once the child starts urinating, add KCl at a concentration of 20 mEq/L.
- Estimate and replace ongoing losses, if significant.
Some more words of caution:
The blood brain barrier prevents rapid movement of solutes out of, or into, the brain. On the other hand, water can move freely across the blood brain barrier. Rapidly developing hyponatremia causes a shift of water into the brain; conversely, hypernatremia can lead to brain dehydration and shrinkage.
Severe, acute hyponatremia may result in brain edema with neurological symptoms such as a change in sensorium, seizures, and respiratory arrest. This is a life-threatening medical emergency and requires infusion of hypertonic saline.
Acute hypernatremia results in a reduction in brain volume. This can lead to subdural bleeding from stretching and rupture of the bridging veins that extend from the dura to the surface of the brain.
Given time, the brain can alter intracellular osmotic pressure to better match plasma osmolality.
With persistent or slowly developing hyponatremia, brain cells extrude electrolytes and organic osmoles and the increase in brain volume is blunted or avoided. Neurologic symptoms are absent or subtle.
With persistent hypernatremia, brain cells generate organic osmoles (also known as idiogenic osmoles) to compensate for the increase in plasma osmolality. Again, the change in brain volume is partially blunted. These processes take 24-48 hours to become effective and leave the brain with a decreased (in hyponatremia) or increased (hypernatremia) osmolar content.
Just as the adaptation takes 24 hours or more, un-adaptation also takes time. Rapid correction of long-standing hypo- or hypernatremia has the potential for severe neurological consequences because of sudden changes in brain volume in the opposite direction. The neurologic manifestations associated with overly rapid correction of hyponatremia is called osmotic demyelination syndrome.
So, hyper- or hyponatremia of long duration should be corrected slowly.
In the past decade, there have been a number of case reports of patients developing dangerous hyponatremia during intravenous fluid therapy. To avoid this,
- As discussed above, use ONLY normal saline for volume replacement. Never use hypotonic saline; these patients are secreting ADH which can lead to water retention. The appropriate volume of normal saline can be combined with the hypotonic saline being used for provision of maintenance fluid requirements so that the final solution is D5 0.45% normal saline.
- NEVER use excessive volumes of hypotonic saline as a maintenance fluid. Calculate the requirement, and don't exceed it!
- If the serum sodium is dropping below 138 mEq/L, switch to normal saline for rehydration and maintenance.
- If a patient is suspected to have the syndrome of inappropriate secretion of ADH (SIADH), use only normal saline for rehydration and maintenance.
- Post-operative patients have a tendency for SIADH. These patients should receive only normal saline, even for maintenance.
Photo by Javier Correa from Photospin
Fluid Calculation 4 2 1
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