Fluid balance & Dehydration

Question 1

Total body water = ___% of total mass

Answer 1

60%

Question 2

3 components of fluid Tx

Answer 2

Maintenance
Deficit
Ongoing Losses

Question 3

definition of “maintenance fluids”

Answer 3

• Fluid requirement to maintain normal water balance

- Generally includes basic glucose, Na, Cl and K requirements as well

Question 4

where do we lose water? (4)

Answer 4

Insensible losses
Urine losses
Sweat (usually negligible)
Stool losses (usually negligible)

Question 5

What determines amount of insensible losses? (4)

What is the normal rate of loss for insensible losses?

Answer 5

Body surface area (per mass)
Temperature
Water barrier (skin)
Respiratory rate

Normal is ~400 cc/m2/day

Question 6

What are insensible losses composed of? (1)

Answer 6

free water

Question 7

What determines amount of urine losses?

Answer 7

Volume of fluid intake
Solute load
Renal Function (only in pathology)

[More you drink the more you pee…. But consider other end of the spectrum– what is the minimum urine output needed to maintain homeostasis? That depends on solute load.]

Question 8

________ and _________ determine urine water losses.
What is Max. urine concentration?
Eg. for solute intake of 500 mosm, what is obligate urine loss?

Answer 8

Maximum urinary concentrating ability and solute load
Max. urine concentration = 1000 mosm/L
Obligate urine losses= 500 / 1000 = 0.5 L

Question 9

• Equivalent of 4-2-1 rule in cc/kg/day

- when does this provide inadequate fluid? (2) Too much fluid? (1) Too much free water? (1)

Answer 9

100 cc/kg/day for the 1st 10 kg
50 cc/kg/day for the next 10 kg
20 cc/kg/day for each additional kg

The formula provides INADEQUATE fluid when:
Insensible losses exceed normal
Urine losses exceed normal (concentrating defect)

The formula provides TOO MUCH fluid when Urine losses are reduced or absent due to renal insufficiency (oliguria or anuria). DO NOT PRESCRIBE “MAINTENANCE FLUIDS” TO AN OLIGURIC OR ANURIC PATIENT!!!!

The formula provides TOO MUCH free water when Urine osmolality is high (ADH is high due to volume depletion) and solute intake is low (sick) - combination leads to HYPONATREMIA!

Question 10

What type of fluid should be used for maintenance? (2)

Answer 10

Controversial…
D5 0.9%NS for those at high risk of ADH secretion
D5 ½ NS if Na is normal and risk of ADH secretion is not high or if Na >145 mmol/L

Random points:

• Measure serum electrolytes before starting and at regular intervals.
• Maintenance IV fluids should provide ONLY the fluid required for maintenance.

Question 11

Diff betw dehydration and volume depletion

Answer 11

Dehydration= loss of water

Volume depletion= loss of fluid

Question 12

Estimating the deficit in dehydration: Hx (5) and phys exam (9).

Answer 12

History: Fluid intake, urine output (wet diapers), Vomiting, diarrhea, fever.
Phys exam: general appearance, radial pulse, respirations, anterior fontanelle, systolic BP, skin elasticity, eyes, tears, mucus membranes

Question 13

Estimating the defecit in dehydration: the 3 levels of dehydration and what you see on each of the following: general appearance, radial pulse, respirations, anterior fontanelle, systolic BP, skin elasticity, eyes, tears, mucus membranes

Answer 13

general appearance, radial pulse, respirations, anterior fontanelle, systolic BP, skin elasticity, eyes, tears, mucus membranes

MILD (4-5%; use 5%): Thirsty, restless, alert; Normal rate and strength; Normal, Normal, Normal, Pinch retracts immediately, Normal, Present, Moist

MODERATE (6-9%; use 8%): Pinch retracts immediately; Rapid and weak, Deep, may be rapid, Sunken, Normal or low, Pinch retracts slowly, Sunken, Absent, Dry

SEVERE (>=10%; use 10-12%): Drowsy, limp, cold, sweaty, cyanotic extremities; Rapid, thready, sometimes impalpable; Deep and Rapid; Very sunken; Low; Pinch retracts very slowly; Grossly sunken, Absent, Very dry.

Percentages are % of total body mass! (eg. 8% dehydrated in 10kg baby = 10*0.08 = 0.8 L = 800cc)

Question 14

5 things to consider before correcting water deficit

Answer 14

Get baseline electrolytes (Na, Cl, K), glucose
Urea and creatinine in some
Blood gas (HCO3) in many
Other tests as clinically indicated
Oral vs IV - oral is safe & effective - Use IV if unable to take (eg. mental status) or failure (eg. vomit). Use oral rehydration solution– offer small volumes (20-30 cc) every 10-15 min. Can try oral again after IV bolus.

Question 15

2 phases of correcting water deficit (and fluids used in each)

Answer 15

Phase 1: Rapidly restore moderately or severely compromised effective circulating volume; use crystalloid (Isotonic saline or Ringer’s lactate); give 10-20 ml/kg bolus.

Phase 2: Repair remaining deficit; use isotonic saline.
Total deficit-boluses given=remaining deficit.
Take serum Na into account.

Question 16

After giving bolus, how to correct the remaining deficit (IV) in each of iso-, hypo-, and hypernatremia

Answer 16

Remaining deficit = Total deficit - boluses given

Isonatremia (Na 135 - 146 mmol/L): Use NS. Method 1: Correct over ~12 - 24 hours, or Method 2: Give 20-40 cc/kg over 2-4 hr (ignore maintenance), then reassess.

Hyponatremia (Na~125); causes initial slow rise in Na followed by rapid increase. Correct 147 mmol/L): estimate total deficit clinically as per other cases, then estimate water deficit: TBW= weight X 0.6 (more in babies); 140 x TBW(desired) = Na(current) x TBW(current); free water deficit = TBW(desired) – TBW(current); Replace free water deficit with free water (D5W) or almost anything; Isotonic deficit = total fluid deficit – free water deficit; first correct isotonic deficit with 20cc/kg 0.9% saline bolus x 2, then calc remaining deficit = Total fluid deficit – boluses; Correct remaining deficit with [Na] below 0.5mmol/L/hr, use with D5W or D5 ½ NS or 0.9% saline; monitor closely.

Question 17

Fluid to use for ongoing losses

Answer 17

½ NS usually works well; Most logical to analyze the losses and replace with fluid of composition similar to losses

Question 18

when to prescribe fluid for ongoing losses

Answer 18

• diarrhea, vomit, etc

- Also prescribed when you can’t reliably predict a patient’s maintenance needs, eg. Oliguric acute renal failure