Fluid & Electrolytes Flashcards
What 2 factors are manipulated by intravascular fluid therapy
Preload and hemoglobin
Total body water (TBW) percentage of body and compartments
60%, ~40L
- ICF (intracellular fluid) = 2/3 of TBW, ~25L
- ECF (extracellular fluid) = 1/3 of TBW ~15L
ECF compartments
IVF (intravascular fluid) -Blood and plasma -20%, ~3L ISF (interstitial fluid space) -Lymph and transcellular fluid -80%, ~12L ISF and IVF are separated by a capillary membrane
Fluid distribution percentages to remember
60/40/20 (15/5) 60% TBW 40L TBW 20L ICF 15L ECF 5L IVF
Major ICF Cations
Potassium (most abundant)
Magnesium
Major ICF Anions
Proteins
Phosphates
Organic ions
Major ECF Cations
Sodium (most abundant)
Calcium
Major ECF Anions
Chloride
Bicarbonate
How is intracellular osmolality maintained
Na/K/ATPase pump
- Exchanges 3 Na for 2 K, offsets tendency for Na to diffuse into the ICF
- In ischemia or hypoxia this is disrupted, causing swelling of cells
4 ways fluid moves between compartments
Diffusion
-Particles down concentration gradient through lipid bilayer or protein channels (facilitated)
Osmosis
-Water moving between cell compartments (intracellular and interstitial space)
Active Transport
-Water, ions, molecules pumped against concentration gradient on cell wall, requires enzymes and energy
-Controls cell volume and concentration
Filtration
-Pressure causes water, ions, molecules to move to a lower pressure area (edema)
Starlings Forces
Determines amount and direction of filtration
- Governed by differences in hydrostatic pressure (interstitial, capillary) in addition to osmotic forces (interstitial, plasma colloid)
- Fluid tends to move out of capillaries at arterial end (BP>osmotic pressure) and back in at venous end (BP
Hydrostatic pressure
Pressure that fluid exerts on the walls
-Interstitial or capillary
Osmotic pressure
Pressure against a semipermeable membrane that prevents water from diffusing across the membrane
- Interstitial or plasma colloid
- Based on osmotic particles not molecular weight
Plasma colloid osmotic pressure
Significant to anesthetic
- Can be increased or decreased depending on what we give to maintain circulating fluid volume in intravascular space
- Crystalloid increases loss of fluid from intravascular to interstitial space
- Replace 3:1 crystalloid to colloid
Osmolarity
Expression of the number of osmoles of solute in a liter of solution
Osmolality (and normal serum and urine osmo)
Number of osmoles of solute in a kilogram of solvent
- Serum osmo: 285-295 mOsm/kg
- Urine osmo: 500-800 mOsm/kg
Tonicity
How a solution affects cell volume
-Compares the osmolarity of solution to the plasma
How to estimate osmolarity
(Na x 2) + (Glucose / 18) + (BUN / 2.8)
-Na is the most important osmotically active substance influencing water in the brain tissue
Control of plasma osmolality
Closely regulated by osmoreceptors in the hypothalamus
- Control secretion of ADH and the thirst mechanism
- ADH is the most important regulator of serum osmolality
Surgery effects on fluid status (3)
Hemorrhage
Evaporative loss
-From exposed viscera (all water, no electrolytes)
Third spacing
-From manipulation of tissues
-Fluid redistributed from intravascular space to interstitial space
-Replacement best done with fluid composition close to ECF
Hypovolemia vs dehydration
Hypovolemia = ECF water deficit, blood volume Dehydration = low water relative to sodium level, TBW
Hypovolemia can result from ____ or ____ (and how to replace)
Absolute loss of fluid
-From GI tract, polyuria, diaphoresis, decreased intake
Relative loss of fluid
-Water redistributed in the body = reduced circulating volume
-Burns, third spacing form surgery
Weight doesn’t decrease
Replace with isotonic crystalloids
Anesthetic considerations for hyponatremia
Hypovolemia emphasizes vasodilation or cardiac depression from anesthetic agents, predisposing to hypotension and hypoperfusion of tissues
Anesthetic considerations for hypernatremia
Elective surgery should be postponed in patients with significant hypernatremia (>150) until cause is established and fluid deficit is corrected
Anesthetic considerations for hypokalemia
Cancelling surgery for mild-severe (<2.6) isn’t warranted
- Avoid hyperventilation, use glucose free solutions
- Increased sensitivity to neuromuscular blocking agents may be seen
Anesthetic considerations for hyperkalemia
Elective surgery shouldn’t be done in patients with hyperkalemia
- Vigilent ECG monitoring
- Succinylcholine and LR are contraindicated
- Mild hyperventilation may be desirable (avoid acidosis) (10mmHg decrease in PaCO2 = 0.5mmol/L decrease in K)
- Can accentuate effects of neuromuscular blocking agents
Calcium roles (2) and what form is active
2nd messenger
-Couples cell membrane receptors to cellular responses
-Muscle contraction, release of hormones and neurotransmitters
Coagulation of blood
Ionized is most clinically significant
Only unbound ionized calcium is active (normal level 4.6-5.6)
Anesthetic considerations of hypocalcemia
Controlled ventilation drives Ca and K in the cells
-Avoid hyperventilation (lowers Ca and K)
Be careful with NMBAs, barbiturates, and volatile anesthetics
Anesthetic considerations in hypomagnesemia
Frequently overlooked, especially in critically ill patients Coexistent electrolyte disturbances -Hypokalemia -Hypophosphatemia -Hypocalcemia
Anesthetic considerations of hypermagnesemia
Requires close monitoring of ECG, BP, neuromuscular function
Potentiation of vasodilation and negative inotropic properties of anesthetics
Reduce muscle relaxants by 25-50%
Serially measure Ca and Mg
Crystalloids (and 3 most common used in surgery)
Isotonic, closely matches electrolyte concentration of ECF (water and electrolytes)
- 0.9% NS
- LR
- P-Lyte
NS excess, benefits
Excess: Can cause hyperchloremic metabolic acidosis
Benefit: No potassium, good for renal failure patients and cardiopulmonary bypass
LR
- Slightly hypotonic, high volume will lower Na to 130
- Has K, can cause hyperkalemia in renal failure patients
- Liver converts lactate to bicarb, can cause metabolic alkalosis
- Ca can bind citrated anticoagulants and clot donor blood
P-Lyte
- More expensive
- Balanced
- Contains a small amount of K
Colloid parenteral fluids
Contain osmotically active substances of high molecular weight that don’t easily cross the capillary membrane, so draw fluid into intravascular space and expand the circulating volume
Hetastarch
Synthetic colloid
- Has coagulopathy risk with dose >20mL/kg
- Prolonged PTT from decreased factor VIII
Dextran
Synthetic colloid
-Used in vascular surgery to prevent thrombosis (promotes anticoagulation, decreased platelet adhesiveness with dose >20mL/kg/day)
Crystalloid advantages and disadvantages
Advantages
-Inexpensive
-No side effects
-Maintains IVF and COP as well as colloids
-No coagulation abnormalities
Disadvantages
-Larger volume needed, dilutes plasma proteins
-Peripheral edema, possibly impaired wound healing and O2 transport
-Potential for pulmonary edema
-Transient effect, intravascular half life is 20-30 minutes
Colloid advantages and disadvantages
Advantages
-Maintains plasma oncotic pressure
-Less redistribution than crystalloids, requires smaller volume for resuscitation
-Less peripheral edema
-Rapid resuscitation
Disadvantages
-Expensive
-Linked to coagulopathy (Dextran>hetastarch>hextend)
-More frequent/serious side effects: Anaphylaxis (dextran), decreased calcium (albumin), renal failure (dextran)
-Intravascular half life is 3-6 hours
Crystalloid or colloid = superior for periop?
No research has proven the superiority of one over another
Calculating hourly maintenance fluid
4-2-1 plan First 10kg: 4mL/kg/hr Next 10kg: 2mL/kg/hr Each kg after 20: 1mL/kg/hr Shortcut: add 40 to weight in kg
Calculating NPO deficit
Hours of NPO x hourly maintenance fluid
Intraop replacement of fluid deficit
1st hr: 1/2 deficit
2nd hr: 1/4 deficit
3rd hr: 1/4 deficit
Calculating 3rd spacing/evaporated surgical fluid loss
Minimal tissue trauma (hernia repair): 2-4mL/kg/hr
Moderate tissue trauma (cholecystectomy): 4-6mL/kg/hr
Severe tissue trauma (bowel resection): 6-8mL/kg/hr
Calculating other losses and blood loss
Account for fluids lost through NG, excess urine output, bowel prep, and lipo suction output 1:1
-Replace blood loss with crystalloid 3:1 or colloid 1:1
Maximum allowable blood loss equation
MABL = (EBV x (Starting Hct-Target Hct)) / Starting Hct
Goal Hct>30 Hgb> 10 (many exceptions)
Estimated blood volume (EBV) in neonates, infants, and adults
Neonates -Premature: 90-100mL/kg -Full term: 80-90mL/kg Infants: 80mL/kg Adults: 70mL/kg
How much blood is in a soaked sponge and laparotomy pad?
4x4 sponge: 10mL
Lap pad: 100-150mL
What from PRBCs shifts the oxygen hemoglobin dissociation curve to the left and what does this promote
Hypothermia, low levels of 2, 3 DPG
Promotes tissue hypoxia
Emergency blood product transfusion options
1: Give type-specific partially cross-matched blood
2: Give type-specific non-cross-matched blood
3: Give type O negative blood
- Least desirable
- After 2 or more units of this continue with it even if pts blood type becomes available
How much will one unit of PRBCs increase hgb and hct? 1 unit plts?
Hgb: 1gm/dl
Hct: 3%
Plts: 5-10mm^3
Risk of bacterial infection with what blood product? Why?
Platelets
-Stored at room temperature, also decreased shelf life
Dose of FFP
5-8mL/kg to reverse warfarin
10-15mL/kg for all other purposes
Cryoprecipitate (factors contained, indications)
I (Fibrinogen), VIII, vWF
Fibrinogen <80-100mg/dL
Low antibodies, ABO compatibility isn’t necessary
Blood transmission risk of infection highest for ____
Hepatitis B: 1 in 220,000
Treatment for acute hemolytic reaction
Stop transfusion, send to blood bank Support BP with IVF and pressors If anaphylactic: consider antihistamine *Maintain UOP with IVF, diuretics, renal dose dopamine Monitor for lung injury
