CAPITULO III DOS NUEVOS ENFOQUES EN LA EXPLICACIÓN DE LOS PROCESOS INTELECTUALES DE
1.2. La Teoría Implícita Pentagonal: una nueva vía hacia la superdotación
1300 mL
INTAKE
water content of solid food...1,000 water from food metabolism...300
OUTPUT
skin loss...600 respiratory loss...400 GI tract loss...200
1200 mL
Extra-renal fluid loss, sensible and insensible, is explained further in the next chapter, Hypernatremia, starting on page 186.
In chronic SIADH, patients have to follow a strict fluid restriction to maintain a normal sodium concentration. The restriction of water in-take is designed to balance fluid inin-take and urine output. Why do these calculations ignore water intake from solid food and extra-renal fluid loss?
The reason these other sources of water intake and loss can be ig-nored is that they roughly balance each other out, as shown above. Thus, adjusting water intake based on urine output is enough to make a change in net water balance.
Treatment! ! ! ! !Euvolemic hyponatremia! ! ! ! !Chronic, asymptomatic! ! ! ! ! The management of chronic SIADH employs strategies to increase the maximum daily urine volume.
The long-term management of chronic SIADH focuses on making sure the patient does not ingest more water than the kidney can excrete. While the primary method is simple fluid restriction, other methods rely on in-creasing the maximum daily urine volume. Inin-creasing urine output allows liberalization of fluid restriction, improving quality of life. There are two methods of increasing the maximum daily urine volume.
• Increase the daily solute load. Since maximum daily urine vol-ume is the solute load divided by the minimum urine osmolality, increasing the solute load increases urine volume. Urea tablets or increased salt intake can be used to increase the daily solute load.
• Decrease the minimum urine osmolality. Inhibiting ADH activ-ity decreases the minimum urinary osmolalactiv-ity and increases the daily urine volume. ADH can be disabled through the use of drugs. Deme-clocycline, a tetracycline derivative, is used for this purpose.
maximum daily urine volume
daily solute load minimum urinary
osmolality
Increase daily solute load
• oral urea tablets
• increased sodium intake
STRATEGIESTO INCREASE MAXIMUM DAILYURINE VOLUME
Decrease minimum urine osmolality
• demeclocycline
The maximum urinary volume equals the daily _________ load divided by the average _______ osmolality.
solute urine Lithium is a psychiatric drug used in the treatment of bipolar disorder. One of its side effects is nephrogenic diabetes insipidus (increased urine output due to a lack of renal response to ADH, lowering the minimum urine osmolality). Because of this side effect, lithium was for-merly used in the treatment of SIADH. Unfortunately, lithium has many side effects and is less effective than demeclocycline, the drug of choice for treating chronic SIADH.
Treatment!Euvolemic hyponatremia! ! ! ! !Acute, symptomatic! ! ! ! ! When symptoms are present, rapid correction is necessary.
In acute symptomatic hyponatremia, a rapid drop in sodium causes cere-bral edema and increased intracranial pressure. The most severe neuro-logic complications include seizure, coma and death. In acute symptomatic hyponatremia, the risks of untreated cerebral edema outweigh the risks of rapid sodium correction. Rapid (but regulated) correction of hyponatremia must be done to prevent permanent disability.
The goal of correction is to increase the sodium concentration to 125 mEq/L.
The rate of sodium correction which is considered safe is 1–2 mEq/L per hour.
Acute hyponatremia causes cerebral edema.
Returning the sodium to normal reverses the cerebral edema.
Ahh!
When rapid correction is needed, there is insufficient time for fluid re-striction to raise the sodium concentration. To achieve the appropriate rate of sodium correction in acute, symptomatic hyponatremia, a step-wise ap-proach should be followed.
Step one. Calculate the water excess.
Step two. Calculate the time of correction in hours. By dividing the change in sodium by the desired rate of correction (1 to 2 mEq/ Lper hour), the number of hours required to correct the sodium concentration can be determined.
Step three. Calculate the hourly water loss by dividing the water excess by the time of correction in hours.
water excess
HOURLY WATERLOSS
time of correction
125 – [Na+]current 1-2 mEq/L per hour change in [Na+]
TIMEOFCORRECTIONINHOURS
rate of correction
Treatment!Euvolemic hyponatremia! ! ! ! !Acute, symptomatic! ! ! ! ! Calculate the amount and rate of sodium correction.
WATEREXCESS RATEOFCORRECTION
John Doe is brought to the E.R. after being found down. On arrival, he be-gins to seize. His initial sodium con-centration is 114 mEq/L. Calculate the hourly water loss necessary to correct his sodium concentration to a safe lev-el, he weighs 42 kg.
Step 1:...42 × 0.6 × (1 – (114 ÷ 125)) water excess = 2.21 L or 2210 mL Step 2:...125 – 114 ÷ (1 or 2) time of correction = 5.5 to 11 hrs Step 3:...2210 mL ÷ (5.5 to 11 hrs) hourly water loss = 201 to 403 mL
WATEREXCESS
men 60%
women 50%
infants 70%
weight (kg)
1
[Na+] 125
Treatment!Euvolemic hyponatremia! ! ! ! !Acute, symptomatic! ! ! ! ! Diuretics and hypertonic saline are used to remove excess free water.
3%
NaCl Lasix
Once all the calculations have been done, the treatment of acute symp-tomatic hyponatremia continues as follows:
Step four. Administer IV furosemide (Lasix®) to increase urine output (adults 40 mg IV, children 1 mg/kg IV).
Step five. Monitor urine output, urine sodium and plasma sodium every hour. Sodium lost in the urine needs to be replaced every hour. In addition, dextrose or extra doses of furosemide may be needed, depending on urine output.
Replace urine sodium. Calculate the sodium lost in the urine (urine volume × urine [Na+]). Replace the urine sodium with 3%
NaCl; one mL of 3% saline contains 0.5 mEq of sodium. Since the 3% saline is only supposed to replace salt and not add water, the urine output for the next hour must match the target urine output plus the volume of 3% saline given to replace sodium.
Urine output too high. If the urine output is too high, the amount of output in excess of the target urine output must be replaced with D5W.
Urine output too low. If the urine output is too low, an additional dose of furosemide should be given.
After one hour, John Doe has a urine sodium of 80 mEq/L, and a urine volume of 500 mL. This means he has lost ____ mEq/L of sodium. To replace the sodium, _______ mL of 3% saline must be given.
Since the urine volume is 500 mL, which is greater than the target of
aaa 40 mEq 80 mL
Hypervolemic hyponatremia can be due to CHF, liver failure and renal failure. The fall in sodium concentration in these patients is typically chronic and asymptomatic. In these disorders, the focus is treatment of the under-lying disorder. In CHF, for example, instituting measures which improve cardiac output and increase effective circulating volume can eliminate the signal for ADH release, causing the sodium concentration to return toward normal.
Specific treatment for hyponatremia is limited to water restriction. The amount of water ingested must be less than the maximum daily urine vol-ume. As in chronic SIADH, the urine osmolality is relatively constant in hypervolemic hyponatremia, and the minimum urine osmolality needed to calculate the maximum daily urine volume can be determined by checking a urine osmolality.