Lessons 5 & 6 Flashcards
Most important goal of fluid therapy
To maintain hemodynamic stability and protect vital organs from hypoperfusion
True/False: blood loss is better tolerated in a euvolemic patient
True
Total Body Water
Males vs Females
TBW is…
60% body weight in males
55% body weight in females
- Infants have up to 80%*
- Obese = less TBW (fat contains little water)*
Intracellular vs Extracellular Water
Intracellular = 2/3 of TBW
Extracellular = 1/3 TBW
*Note: the picture makes it look like extracellular takes up more space
How is extracellular water further subtyped?
Remember extracellular = 1/3 TBW
Extravascular water = 2/3 of Extracellular
Intravascular water = 1/3 of Extracellular
Define orthostatic hypotension
SBP decrease of >20 mmHg from supine to standing
Indicates fluid deficit of 6-8% body weight
Perioperative Fluid Requirement Factors to account for
- Maintenance Fluid
- Compensatory Fluid Bolus
- NPO and other deficits
- Third Space Losses
- Blood Loss
- Special additional losses
How to calculate maintenance fluid requirements?
4-2-1 Rule
4 ml/kg/hr for the 1st 10 kg of body weight
2 ml/kg/hr for the 2nd 10 kg of body weight
1 ml/kg/hr for every kg thereafter
OR
1.5 ml/kr/hr for adults
What is maintenance fluid replacement meant to account for?
Meant to replace insensible water losses from respiratory tract, sweat, feces, and urine
Why do we give a compensatory fluid bolus?
Potent inhalational agents decrease BP by vasodilation and, to a lesser extent, myocardial depression (propofol and barbiturates have similar action)
Fluid bolus (500 ml in a 70kg patient) early in anesthetic care is customary to mitigate severe decreases in BP
How to calculate NPO deficits
NPO = number of hours NPO x maintenance fluid replacement
Usually start in the am (healthy people do not wake up and drink their entire overnight fasting deficit)
Bowel prep may result in how much fluid loss?
up to 1 L
How do we replace “third space” losses?
Superficial surgical trauma: 1 ml/kg/hr
Minimal surgical trauma (hernia, knee surgery): 2 ml/kg/hr
Moderate surgical trauma (pelvic surgery): 3 ml/kg/hr
Major trauma (open abd surgery): 4 ml/kg/hr
Replacement of blood loss
The 4:1 rule
For every ml of isotonic cystalloid that stays intravascular, about 3-4 ml will be lost to extravascular space (replacement volume needs to be 4x blood loss)
Limitations of the 4:1 Rule
For patient’s with significant blood loss you are putting large volumes of fluid into their interstitial space intentionally…
…Edema disrupts normal function of extravascular space
When would you use colloids (6% hetastarch, 5% albumin) over crystalloids?
When blood loss exceeds 5 ml/kg
About 70% of colloid remains in intravascular space compared to 20% of crystalloid
Note, using colloids is controversial and some studies show increased mortality when used in trauma patients
Combination of water and electrolytes
Crystalloids
Balanced salt solutions = LR, Plasmalyte, Normosol
Hypotonic salt solution = D5W
Fluids containing molecules sufficiently large enough to prevent transfer across capillary membranes
Colloids
6% Hetastarch (Hespan) and 5% Albumin
Problems with D5W
Can result in hyperglycemia
Glucose is metabolized and taken up by cells…
…only 95% of pure water left behind remains in the intravascular space)
Large quantities of normal saline results in
a mild metabolic acidosis because the high levels of chloride ions exchange with bicarbonate ions, encouraging its elimination
“Problems” with LR
- Need to monitor sodium levels (identify declining sodium concentration)
- LR contains calcium and should not be mixed with banked blood (inactives the EDTA anticoagulant)*
- Hyperkalemia (especially in patients with renal failure)**
- *Unless 1 unit of packed red blood cells takes more than 2 hours to transfuse, this is not an issue*
- **In patients w/o RF, LR will typically on raise K levels if the person is below 4 (…which means they should have K replaced regardless)*
Does LR cause lactic acidosis?
NO
This has been proven by many studies…
BLUF: LR is replacing NS as the “go to” resuscitative fluid (exception = brain injuries requiring diuresis w/ hypertonic saline)
What is plasmalyte?
A product that tries to deliver the best aspects of LR and NS
It has no free water or calcium so it should not cause dilutional hyponatremia and can be mixed with blood. It does have potassium (dangerous with RF patients). It is rarely used in the OR since it is expensive and the real-life benefit is minimal.
Clinical Evaluation of Fluid Replacement
Urine Output: 0.5 ml/kg/hr
Vital Signs: BP and HR
MMM
Arm and Neck veins
Invasive monitoring (CVP, PCWP, echo)
Lab tests (Hct, BUN, pH, serum bicarb)
*Note: this is a very simplistic way to look at “volume responsiveness”
