Calculations IV Flashcards
What is body mass index (BMI)?
Body mass index (BMI) is a measure of body fat based on height and weight that applies to adult men and women
What is BMI a useful measure of?
Body fat
What are the limitations of BMI?
BMI can over-estimate body fat in persons who are muscular, and can under-estimate body fat in frail elderly persons and others who have lost muscle mass
In terms of waist circumference, what waist size is associated with higher disease risk?
High risk is defined as a waist size > 35 inches for women or > 40 inches for men
What is the formula for BMI?
weight (kg)/[height (m)]^2 or weight (pounds)/[height (in)]^2 x 703
What BMI is considered underweight?
< 18.5 kg/m2
What BMI is considered normal weight?
18.5 - 24.9
What BMI is considered overweight?
25-29.9
What BMI is considered obese?
> 30
What are the three potential measures of body weight for a patient?
Actual (or total) body weight, ideal body weight and adjusted body weight
What is actual body weight or total body weight?
Actual body weight or total body weight is the weight of the patient when weighed on a scale
What is ideal body weight?
Ideal body weight is the healthy (ideal) weight for a person
How do you calculate IBW for males?
50 kg + (2.3 kg)(number of inches over 5 feet)
How do you calculate IBW for females?
IBW (females) = 45.5 kg + (2.3 kg)(number of inches over 5 feet)
What is adjusted body weight?
Adjusted body weight is calculated when patients are obese or overweight
What is the formula to calculate adjusted body weight?
IBW + 0.4(TBW - IBW)
If a person is underweight, what measure of body weight do you use?
Use total body weight for all medications
If a person is normal weight, what measure of body weight do you use?
- Use total body weight for most medications
- Use ideal body weight for aminophylline, theophylline, acyclovir and levothyroxine
If a person is obese, what measure of body weight do you use?
- Use ideal body weight for aminophylline, theophylline, acyclovir and levothyroxine
- Use total body weight for LMWHs, UFH and vancomycin
- Use adjusted body weight for aminoglycosides
What are flow rates?
Flow rates are used to specify the volume or amount of drug a patient will receive over a given period of time
What are examples of how flow rate can be expressed?
Milliters per hour, milligrams per hour, mcg/kg/min or as the total time to administer the entire volume of the infusion
What does flow rate depend on?
Flow rate depend on the dose of the medication and the concentration available
What is creatinine?
Creatinine is a breakdown product when muscle tissue makes energy
What is the normal range for serum creatinine?
The normal range for serum creatinine is approximately 0.6-1.3 mg/dL
What happens to creatinine when kidney function declines?
If kidney function declines and creatinine cannot be cleared, the creatinine levels will increase in the blood and the creatinine clearance with decrease
*Concentration of drugs that are renally cleared will also increase and a dose reduction may be required
What should be assessed when a patient is dehydrated?
Patients should be assessed for dehydration when the serum creatinine is elevated because dehydration can cause both the serum creatinine (SCr) and the blood urea nitrogen (BUN) to increase
What is the BUN:SCr ratio that indicates dehydration?
> 20:1
What are some signs of dehydration?
Signs of dehydration can include decreased urine output, tachycardia, tachypnea, dry skin/mouth/mucous membranes, skin tenting (skin does not bounce back when pinched into a fold) and possibly fever
What can usually cause dehydration?
Dehydration is usually caused by diarrhea, vomiting and/or a lack of adequate fluid intake
What is the Cockgroft-Gault equation?
CrCl (mL/min) = ([140 - (age of patient)]/[72 x SCr]) x weight in kg (x 0.85 if female)
*formula used to estimate renal function
When is the Cockcroft-Gault equation unreliable?
It is not reliable in very young children, ESRD patients or when renal function is fluctuating rapidly
If a patient underweight, what body weight measure should be used for calculating CrCL?
Total body weight
If a patient normal weight, what body weight measure should be used for calculating CrCL?
Ideal body weight
If a patient overweight or obese, what body weight measure should be used for calculating CrCL?
If BMI < 25, use ideal body weight. If BMI > 25, use adjusted body weight
What is pH?
The pH refers to the acidity or basicity of a solution
What does an increase or decrease in pH represent?
As a solution becomes more acidic (the concentration of proton increases), the pH decreases. When the concentration of protons decrease, the pH increases and the solution is more basic, or alkaline
How can the acid-base status of. patient be determined?
The acid-base status of a patient can be determined with an arterial blood gas (ABG)
What is the primary buffering system of the body?
The bicarbonate/carbonic acid
What organ helps maintain neutral pH?
The kidneys help to maintain a neutral pH by controlling bicarbonate reabsorption and elimination
What is the role of bicarbonate in the body?
Bicarbonate acts as a buffer and a base
How does the lungs help maintain a neutral pH?
The lungs help maintain a neutral pH by controlling carbonic acid (which is directly proportional to the partial pressure of carbon dioxide or pCO2) retain or released from the body
What is the role of carbon dioxide in the body?
Carbon dioxide acts as a buffer and an acid
What is acidosis?
An acid-base disorder that leads to a pH < 7.35
What is alkalosis?
If the disorder leads to a pH > 7.45
What is the primary disturbance in a metabolic acid-based disorder?
The primary disturbance in a metabolic acid-base disorder is the plasma HCO3 concentration
How is metabolic acidosis categorized?
A metabolic acidosis is characterized primarily by a decrease in plasma HCO3 concentration
How is metabolic alkalosis characterized?
In a metabolic alkalosis, the plasma HCO3 concentration is increased
What is metabolic acidosis associated with?
Metabolic acidosis may be associated with an increase in the anion gap
What is the primary disturbance in a respiratory acid-base disorder?
The primary disturbance in a respiratory acid-base disorder is pCO2
How is respiratory acidosis characterized?
In respiratory acidosis, the pCO2 is elevated
How is respiratory alkalosis characterized?
In respiratory alkalosis, the pCO2 is decreased
What is the reference range of pH?
7.35 - 7.45
What is the reference range for pCO2?
35 - 45 mmHg
What is the reference range for pO2?
80 - 100 mmHg
What is the reference range for HCO3?
22 - 26 mEq/L
What is the reference range for O2 sat?
> 95%
How are ABGs presented in a written chart note?
ABG: pH/pCO2/pO2/HCO3/O2 sat
What is the anion gap?
The anion gap is the difference in the measured cations and the measured anions in the blood
What are the possible causes of metabolic acidosis?
Cyanide, uremia, toluene, ethanol (alcoholic ketoacidosis), diabetic ketoacidosis, isoniazid, methanol, propylene glycol, lactic acidosis, ethylene glycol, salicylates
When is the anion gap considered high?
> 12 mEq/L
How is the anion gap calculated?
Anion gap (AG) = Na - Cl - HCO3
What is the role of buffer systems?
Buffer systems help to reduce the impact of too few or too many hydrogen ions in body fluids. Buffers minimize fluctuations in the pH so that harm is avoided
What could the under or overproduction of hydrogen ions do?
These hydrogen ions could cause harm, including degrading some drugs, destabilizing proteins, inhibiting proteins, inhibiting cellular functions and, with too much of a change outside of the narrow change, cells die and death can occur
What are buffer systems composed of?
Buffer systems are composed of either a weak acid and salt of the acid or a weak base and a salt of the base
What is an acid?
An acid is a compound that dissociates, releasing (donating) protons into solution
*Once the proton is released, the compound is now a conjugate base, or its salt form
What is a base?
A base picks up, or binds, the proton
How are acid-base reactions equilibrium reactions?
There is drug moving back and forth between the acid and base state
What can be used to determine if the drug is acting as an acid or base?
The pH and pKa
What does it mean when pH = pKa?
When the pH = pKa, the molar concentration of the salt form and 50% in acid form. When the pH = pKa, this is the point at which half of the compound is not protonated (ionized), and half is protonated (un-ionized)
What does it mean when there is a strong acid or base?
A strong acid or base means 100% dissociation
What does it mean when there is a weak acid or base?
A weak acid or base means very limited dissociation
What does it mean when pH > pKa?
If the pH > pKa, more of the acid is ionized, and more of the conjugate base is un-ionized
What does it mean when pH = pKa?
If the pH = pKa, the ionized and un-ionized forms are equal
What does it mean when pH < pKa?
If the pH < pKa, more of the acid is un-ionized, and more of the conjugate base is ionized
Why is the percentage of drug in the ionized vs. un-ionized state important?
An ionized drug is soluble but cannot easily cross lipid membranes. An un-ionized drug is not soluble but can cross the membranes and reach the proper receptor site. Most drugs are weak acids so they are soluble and can pick up a proton to cross the lipid bilayer
What is the purpose of Henderson-Hasselbalch equation?
It is used to solve for the pH
What is the weak acid formula?
What is the weak acid formula?
pH = pKa + log [salt/acid]
What is the weak base formulas?
pH = (14 - pKb) + log [base/salt] = pKa + log [base/salt]
How do you calculate the percent ionization of a weak acid?
% ionization = 100 / [1 + 10^(pKa - pH)]
How do you calculate the percent ionization of a weak base?
% ionization = 100 / [1 + 10^(pH - pKa)]
Describe calcium carbonate
Calcium carbonate has acid-dependent absorption and should be taken with meals. It is a dense form of calcium and contains 40% elemental calcium
Describe calcium citrate.
Calcium citrate has acid-independent absorption and can be taken with or without food. It is less dense and contains 21% elemental calcium
What is calcium acetate used for?
Calcium acetate is used as a phosphate binder and not for calcium replacement
*Though the capsules contain 25% elemental calcium, absorption from this formulation is poor
How is aminophylline and theophylline dosed?
They are dosed using IBW in normal weight and obese patients for safety
How do you convert from aminophylline to theophylline?
Multiply by 0.8
How do you convert from theophylline to aminophylline?
Divide by 0.8
What happens when a patient’s neutrophil count is low?
The lower a neutrophil’s count, the more susceptible that patient is to infection
What is the Clozapine REMS programs designed for?
The Clozapine REMS Program is designed to reduce the risk of severe clozapine-induced neutropenia; clozapine cannot be refilled if the ANC is < 1000 cells/mm^3
If a patient is neutropenic, what are some monitoring parameters?
A neutropenic patient should be monitored for signs of infections, including fever, shaking, general weakness or flu-like symptoms. Precautions to reduce infection risk, such as proper hand-washing and avoiding others with infection, should be followed
What is normal ANC levels?
2200 - 8000
What is neutropenia levels?
< 1000 (at risk for infection)
What is severe neutropenia levels?
< 500
What is profound neutropenia levels?
< 100
How do you calculate ANC?
Multiply the WBC by the percentage of neutrophils (the segs plus the bands) and divide by 100
ANC (cells/mm^3) = WBC x [(% segs + % bands)/100]
How can neutrophils be labeled on a lab report?
Neutrophils can be labeled polymorphonuclear cells (PMNs or polys) or segmented neutrophils (segs) on a lab report
