Fluids, Electrolytes, and Acid-Base Disorders Flashcards
What factors affect the percentage of total body water (TBW)?
Weight, age, sex, and relative amount of body fat
Equation for calculating Free Water Deficit
Free Water Deficit = TBW x [1 - (140/Serum Sodium in mEq/L)]
Which body tissue is the least hydrated?
Adipose tissue
What are the 3 compartments of TBW distribution?
Intracellular fluid, extracellular fluid, and transcellular fluid
Which fluid compartment is the most clinically important?
Extracellular fluid because it contains the intravascular and interstitial spaces
What is osmotic pressure and why is it of clinical importance?
The pressure required to maintain equilibrium with no net movement of solvent. Prime importance in determining the distribution of water between the extracellular and intracellular spaces
Equation for calculating lean body weight (LBW) for males and females
LBW (women) = 1.07 x weight (kg) - 148 x [weigh (kg)/height (cm)]^2
LBW (men) = 1.1 x weight (kg) - 128 x [weight (kg)/height (cm)]^2
Describe the composition of D5W (5% dextrose) including its tonicity
Provides 50 g dextrose per liter
Hypotonic
No electrolytes
1000 ml/L free water
Describe the composition of 0.225% NaCl (1/4 normal saline) including its tonicity
Hypotonic
Provides 38.5 mEq/L Na, 38.5 mEq/L Chloride
Describe the composition of 0.45% NaCl (1/2 normal saline) including its tonicity
Hypotonic
Provides 77 mEq/L Na, 77 mEq/L Chloride
500 ml/L free water
Describe the composition of 0.9% NaCl (normal saline) including its tonicity
Isotonic
Provides 154 mEq/L Na, 154 mEq/L Chloride
0 ml/L free water
Describe the composition of 3% NaCl (hypertonic saline) including its tonicity
Hypertonic
Provides 513 mEq/L Na, 513 mEq/L Chloride
-2331 ml/L free water
Describe the composition of Lactated Ringers (LR) including its tonicity
Isotonic
Provides 130 mEq/L Na, 109 mEq/L Chloride, 4 mEq/L K+, 3 mEq/L Calcium
0 ml/L free water
Describe the composition Albumin in 0.9% NaCl including its tonicity
Isotonic
Provides 154 mEq/L Na and 154 mEq/L Chloride
Describe the water distribution of 1 L IV Dextrose in water to extracellular fluid and intracellular fluid
333 mL ECF (250 mL interstitial fluid + 83 mL intravascular fluid)
667 mL ICF
Describe the water distribution of 1 L IV 0.9% NaCl (normal saline) to extracellular fluid and intracellular fluid
1000 mL ECF (750 mL interstitial fluid + 250 mL intravascular fluid)
0 mL intracellular fluid
What fluid requirement calculations are recommended in individuals age 65 years or older to prevent dehydration?
- An adjusted Holliday-Segar formula (1500 mL for the first 20 kg body weight + 15 mL/kg for remaining body weight)
- 30 mL/kg with a minimum of 1500 mL
- 1500-2000 mL/day
List some clinical conditions which would require the addition of fluid
Patients with severe diarrhea or emesis; large draining wounds; excessive diaphoresis; constant drooling; paracentesis losses; drains; high gastric fistula and ostomy outputs; persistent fevers; lactation
Explain the 2 equations some clinicians have success using for fluid requirements when the average between the 2 formulas is used
Equation 1 (based on body weight and age):
Age 18-55: 35 mL x body weight (kg)
Age 56-75: 30 mL x body weight (kg)
Age >75: 25 mL x body weight (kg)
Fluid-restricted adults (kidney/cardia disease or fluid overload states) </= 25 mL x body weight (kg)
Equation 2 (Holliday-Segar formula adjusted for age)
Age </=50 years: 1500 mL for first 20 kg body weight + [20 mL x remaining body weight (kg)]
Age >50 years: 1500 mL for first 20 kg body weight + [15 mL x remaining body weight (kg)]
What is the calculation for obesity-adjusted weight?
Obesity-Adjusted Body Weight (lb) = [(Actual Weight - IBW) x 0.25] + IBW
When is an obesity-adjusted weight often used?
When an individual’s weight is equal to or greater than 125% IBW unless the excess weight is due to muscle mass
Calculate the fluid requirements for a 61 year old woman who is 5’4” tall and weighs 160 lb
BMI is 27.5, patient is 133% IBW
1. Calculate IBW: 100 lb + (5x4 lb) = 120 lb
2. Calculate obesity-adjusted weight: [(160-120)] x 0.25] + 120 = 130 lb (59 kg)
3. Calculate fluid requirements using Equation 1: 30 mL x 59 kg = 1770 mL/day
4. Calculate fluid requirements using Equation 2: 1500 mL + [15 x (59kg - 20kg)] = 2085 mL/day
5. Calculate the mean of the results from steps 3 and 4: (1770 + 2085)/2 = ~1900 mL/day
How would you manage maintenance fluids for a patient with heart failure?
Patient info: 65 y/o male, current weight 75 kg (IBW 66 kg), admit w/ 3+ pitting edema BLE receiving 8L/min O2, given 0.9% NaCl at 125 ml/hr, O2 requirements subsequently increased
Heart failure pt with evidence of fluid overload should be treated with loop diuretics and sodium and fluid restrictions. Start this patient on IV furosemide and change IV fluids to 0.45% NaCl at 10 mL/hr to maintain IV access. For pt with heart failure, fluid intake should be approximately 20-25 mL per kg estimated dry weight and clinical symptoms of fluid overload should be taken into account. Sodium intake restricted to <2000 mg/day (87 mEq). This patient received 3 L maintenance IV fluids which contributed to further respiratory decompensation requiring aggressive diuresis
List the volume and average electrolyte composition of saliva
1.5 L/day, 10 mEq/L Na, 26 mEq/L K+, 10 mEq/L Chloride, 30 mEq/L bicarbonate (HCO3-)
List the volume and average electrolyte composition of the stomach
1.5 L/day, 60 mEq/L Na, 10 mEq/L K+, 130 mEq/L Chloride, 0 bicarbonate
List the volume and average electrolyte composition of the duodenum
Variable volume, 140 mEq/L Na, 10 mEq/L K+, 80 mEq/L Chloride, 0 bicarbonate
List the volume and average electrolyte composition of the ileum
3 L/day, 140 mEq/L Na, 5 mEq/L K+, 104 mEq/L Chloride, 30 mEq/L bicarbonate
List the volume and average electrolyte composition of the colon
Variable volume, 60 mEq/L Na, 30 mEq/L K+, 40 mEq/L Chloride, 0 bicarbonate
List the volume and average electrolyte composition of the pancreas
Variable volume, 140 mEq/L Na, 5 mEq/L K+, 75 mEq/L Chloride, 115 mEq/L bicarbonate
List the volume and average electrolyte composition of bile
Variable volume, 145 mEq/L Na, 5 mEq/L K+, 100 mEq/L Chloride, 35 mEq/L bicarbonate
What are the treatment considerations in general if the electrolyte level is below normal range?
Consider available administration routes (IV access peripheral vs central), GI tract function, renal function, fluid status, concurrent electrolyte abnormalities, product availability
What are general potential treatments if an electrolyte level is above normal range?
Remove exogenous sources, discontinue offending agents or medications, facilitate elimination of electrolyte, treat other conditions that may be contributing to disorder
Normal sodium level?
Mild hyponatremia:
Moderate hyponatremia:
Severe hyponatremia:
Normal range 135-145 mEq/L
Mild: 130-135 mEq/L
Moderate: 125-129 mEq/L
Severe: <125 mEq/L
Difference between osmolality and osmolarity?
Osmolality is a measurement of concentration per weight (mOsm/kg H2O)
Osmolarity is a measurement of concentration per volume (mOsm/L)
Serum osmolality and osmolarity are used interchangeably since 1 L of H2O weighs 1 kg
Clinical manifestations of hyponatremia related to central nervous system dysfunction are more likely when the serum sodium concentration drops below ___, gradually or rapidly?
Below 120 mEq/L, rapidly
How does the rate of correction differ between acute vs chronic hyponatremia?
Acute hyponatremia correction can occur at the same rate of onset of hyponatremia (patients are often symptomatic)
Chronic hyponatremia requires slow correction because these patients have adapted to lower serum sodium levels
What is the recommended rate of correction for acute and chronic hyponatremia?
10-12 mEq/L per day for acute
6-8 mEq/L per day for chronic or unknown duration
Explain hypertonic hyponatremia
Caused by the presence of osmotically active substances other than sodium in the ECF (hyperglycemia and hypertonic infusions such as dextrose and mannitol)
What are the 3 main types of hypotonic hyponatremia?
Hypovolemic, euvolemic, and hypervolemic
What are the treatment goals for hypovolemic hyponatremia? Euvolemic? Hypervolemic?
Hypovolemic: volume expansion (for both urine Na < or > 20 mEq/L) with isotonic fluids to expand the ECF volume
Euvolemic: water restriction (500-1000 ml/day)
Hypervolemic: sodium and water restriction
Explain hypovolemic, euvolemic, and hypervolemic hyponatremia
Hypovolemic: losing more sodium than water
Euvolemic: urine osmolality is always greater than serum osmolality and urine sodium is >20 mEq/L, stable sodium intake/output but retain additional amounts of water, kidneys are inappropriately concentrating urine and volume status is adequate
Hypervolemic: have some element of end-organ damage (renal failure, liver failure with ascites, heart failure), resulting in fluid retention or third spacing; retain more water than sodium
What are the extrarenal loss causes of hypovolemic hypotonic hyponatremia?
Fluid losses from excessive sweating, GI loss (vomiting, diarrhea, fistula drainage, NG suction, ostomy drainage), open wounds, fluid drains or third spacing/sequestration (burns, effusions, peritonitis, ascites, pancreatitis, intestinal obstruction)
What are the renal loss causes of hypovolemic hypotonic hyponatremia?
Fluid loss from the use of diuretics, osmosis diuresis, salt-wasting nephropathy, mineralocorticoid deficiency, pseudohypoaldosteronism, bicarbonaturia
What are the causes of euvolemic hypotonic hyponatremia?
SIAD (syndrome of inappropriate antidiuresis), hypothyroidism, drug induced, reset osmostat, hypopituitarism, psychogenic polydipsia
What are the edema-forming state causes of hypervolemic hypotonic hyponatremia? What is the other cause?
Disorders associated with edema: heart failure/CHF, nephrotic syndrome, hepatic cirrhosis
Acute and chronic renal failure
Describe hypovolemic hypernatremia and the treatment
Deficit of both sodium and water but water losses exceed sodium losses; need to determine source of fluid loss; treatment involves volume expansion by replacing hypotonic fluids (isotonic saline) via enteral or parenteral route
What are the extrarenal and renal fluid loss causes of hypovolemic hypernatremia?
Extrarenal: profuse sweating, severe diarrhea, respiratory losses
Renal: diuretics, glycosuria, obstructive uropathy, acute/chronic renal failure
Describe euvolemic hypernatremia, common causes, and treatment
Decrease in total body water, but total body sodium remains normal; commonly caused by diabetes insipidus (central or nephrogenic); treated by replacing water deficit and removing and/or treating the underlying cause; desmopressin challenge to determine if central or nephrogenic
What is the difference between central and nephrogenic diabetes insipidus?
Central is an impairment of ADH secretion; nephrogenic occurs when kidneys cannot respons to ADH circulating in the serum
Describe hypervolemic hypernatremia, common causes, and treatment
An increase in total body sodium and total body water may be normal or increased. Common causes are iatrogenic (overadministration of sodium-containing IV fluids) and mineralocorticoid excess (Cushing’s syndrome, adrenal hyperplasia). Treated by correcting underlying disorder, administering diuretics (furosemide), and replacing water deficit
How do you calculate free water deficit for the initial replacement volume?
Free water deficit = TBW (total body water in L) x [1 - (140/serum Na)]
Correction rate for chronic and acute hypernatremia
Due to risk of cerebral edema. Should not exceed 10 mEq/L/day in chronic or unknown duration; may correct at a rate of 2 mEq sodium per L per hour until serum sodium reaches 145 mEq/L
Calculate free water deficit for a 78 year old woman (60 kg) with serum sodium level 165 mEq/L
TBW for women = LBW x 0.5
LBW for women = 1.07 x weight in kg - 148 x (weight in kg/height in cm)^2
Free water deficit = TBW x [1 - (140/serum sodium)]
Mainstay treatment for SIAD?
Restrict fluids to 500-1000 ml/d. If symptomatic, administer exogenous salt. If refractory to conventional treatment, may require pharmacologic therapy with loop diuretics and/or vasopressin-2 receptor antagonists (conivaptan, tolvaptan)
Normal potassium concentration?
3.5-5 mEq/L
Where is most of the body’s potassium located?
Inside the cells
What are normal daily potassium requirements?
0.5-2 mEq/kg
1 gm of potassium = 25 mEq of potassium
adequate potassium intake is 40-50 mEq (1600-2000 mg/day)
Which factors are the most important in the influence of the regulation of the internal distribution of potassium?
The Na-K-ATPase pump and the plasma potassium concentration
Range for mild, moderate, and severe hypokalemia
Mild: 3-3.5 mEq/L
Moderate: 2.5-2.9 mEq/L
Severe: <2.5 mEq/L
Drug-induced causes of hypokalemia occur in what 3 ways?
Increased renal potassium loss/excretion, excess/increased loss in stool, and transcellular shift (potassium shift from ECF to ICF)
When are patients with hypokalemia more likely to be symptomatic?
Commonly asymptomatic in mild disorders, symptoms occur with more severe cases
Symptoms of moderate and severe hypokalemia?
Nausea, vomiting, lassitude, constipation, generalized weakness, cardiac arrhythmias, rhabdomyolysis, paralysis leading to respiratory compromise, death
Nonmedication causes of hypokalemia from loss in stool?
Infectious diarrhea, tumors, jejunoileal bypass, enteric fistula, malaborption, intestinal ion-transport defects, cancer therapy, geophagia
Nonmedication causes of hypokalemia from loss in urine?
Mineralocorticoid excess, primary hyperaldosteronism, congenital adrenal hyperplasia, renin-secreting tumors, extopic corticotrophin syndrome, Cushing’s syndome, glucocorticoid-responsive aldosteronism, renovascular hypertension, malignant hypertension, vasculitis, apparent mineralocorticoid excess, Liddle’s syndome, 11beta-hydroxysteroid hydrogenase deficiency, impaired chloride-associated sodium transport, Bartter’s syndrome, Gitelman’s syndrome
Describe the treatment options for the following scenario: 78 year old woman (60 kg) w/ h/o uncontrolled hypertension presents with 5 day history of N/V, fever, fatigue. BP low, unable to keep food down, decreased skin turgor and dry mucous membranes. Na level 165 mEq/L, BUN 26, creatinine 0.86, urine sodium <5 mEq/L, all other labs WNL
Vital signs reflect hypovolemia, urine sodium level is consistent with sodium conservation and hypovolemia, differential diagnosis includes extrarenal losses from history of vomiting and high fevers. Treatment options include correction of water deficit slowly over a period of at least several days bc hypernatremia is likely chronic based on the onset of vomiting. Calculate water deficit (= 5.3 L). Administer hypotonic fluids IV to correct half of the water deficit (2.65 L) in the first 24 hours.
0.45% NaCl (1/2 normal saline): 1L only contributes 500 mL toward the water deficit so approximately 5.3L will be needed in the first 24 hours.
Dextrose 5% in water: 1L contributes 1000mL toward the water deficit so approximately 2.65L will be needed in the first 24 hours
Potential causes of transcellular shifts of potassium into the cells?
Metbolic alkalosis and increases in insulin and catecholamiones (epinephrine or norepinephrine)
Hypokalemia is often refractory to treatment in the setting of what other electrolyte deficiency?
Hypomagnesemia
What are the goals of therapy for hypokalemia?
Avoidance or resolution of symptoms, restoring the serum potassium concentration to normal, and preventing hyperkalemia
IV replacement dosing guidelines for hypokalemia in patients with normal renal function
Serum potassium 3-3.5 mEq/L: IV not recommended 20-40 mEq
Serum potassium 2.5-2.9 mEq/L: IV potassium 40-80 mEq
Serum potassium <2.5 mEq/L: IV potassium 80-120 mE
What are the available variations of IV potassium supplements?
Available as chloride, acetate, and phosphate salts
When is potassium acetate use as an alternative to potassium chloride?
In the presense of a metabolic acidosis because acetate is converted to bicarbonate by a normally functioning liver
Why is oral correction of hypokalemia generally safer than IV correction?
Reduces the risk of overcorrection and rebound hyperkalemia
What dosage of oral potassium is generally sufficient to prevent hypokalemia? What dosage may be required to treat hypokalemia?
10-30 mEq/day for prevention, 40-100 mEq/day for treatment
How are potassium supplements best administered orally?
In a moderate dosage over a period of several fays to 1 week to achieve complete potassium repletion
When is IV potassium supplementation reserved for?
For the treatment of severe hypokalemia or when the condition of the GI tract precludes the use of oral agents
What precaution should be taken if potassium infusion exceeds 10 mEq/hour?
Continuous cardiac monitoring to detect any signs of hyperkalemia
Total daily potassium supplementation in most cases should not exceed:
40-100 mEq/day (or 0.5 to 1.2 mEq/kg/day)