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Selected Clinical Calculations
Chapter 10
- Heparin is a heterogeneous group of
muco-polysaccharides that have
anticoagulant properties (slows clotting
time).
- Heparin salt, as heparin sodium, are
standardized to contain 140 USP
heparin Units in each milligram.
- Heparin salts administered as sterile
aqueous solutions by intravenous
infusion, intermittent intravenous
injection, or deep subcutaneous
injection for the prophylaxis and
treatment of venous thrombosis.
Heparin-Dosing calculations
- Dosage of Heparin sodium is adjusted according to the level of
blood coagulation or partial thromboplastin time (PTT)
- Doses from 5000 Units up to 120 , 000 Units (patients with
massive pulmonary emboli).
- In pediatric use, initial dose 50 mg/kg by IV drip, followed by
maintenance doses of 100 mg/kg every 4 hours or 20 , 000
units/m
2
/ 24 hours.
- A variety of low-molecular-weight heparins (fragments of
heparins) are also used as antithrombotic agents e.g. enoxaprin
sodium (Lovenox) and dalteparin sodium (FRAGMIN).
Heparin-Dosing calculations
Example: An intravenous infusion contained 20 , 000 units of heparin sodium in
1000 ml of D 5 W. The rate of infusion was set at 1600 units per hour for a 160 - lb patient. Calculate
a) the concentration of heparin sodium in the infusion, in units/ml
b) the length of time the infusion would run, in hours
c) the dose of heparin sodium administered to the patient, on units/kg/minute basis.
a) 20 , 000 units 1000 ml
X units 1 ml X = units/ml
b) 1600 units 1 hour
20 , 000 units X hour X = hours
c) 160 / 2. 2 = 72. 7 kg AND 12. 5 hour = 750 minutes
20 , 000 units 750 minutes
X units 1 minute X = units/minute
- 67 units 72. 7 kg
X units 1 kg X = units/kg/minute
Heparin-Dosing calculations
20 1000
20000 1
- 5 1600
20000 1
- 67 750
20000 1
37
7
67 1
Example Calculations of Ideal Body Weight
- Calculate the ideal body weight for a male patient weighing 164
lb, and measuring 5 ft. 8 in. in height.
IBW = 110 lb + ( 8 x 5 lb.)
= 110 lb. + 40 lb. = 150 lb., answer.
- Calculate the ideal body weight for a female patient weighing 60
kg and measuring 160 cm in height.
160 cm = 63 in. = 5 ft. 3 in.
IBW = 45. 5 kg + ( 3 x 2. 3 kg)
= 45. 5 kg + 6. 9 kg = 52. 4 kg, answer.
Drugs are eliminated from the body through two major mechanisms
Hepatic (liver) metabolism and Renal (kidney) excretion.
- Polar drugs are eliminated predominantly by renal excretion.
- The kidneys receives about 20 % of the cardiac output and filter
approximately 125 ml per minute of plasma. If the function is lost
drug clearance will decrease.
- The filtration rate of the kidney can be estimated by a number of
methods. One of the most useful, is the estimation of the
creatinine clearance rate (CrCl) through the use of empiric
formulas.
- Creatinine is break-down product of muscle metabolism,
produced in a constant rate depends on the muscle mass and
eliminated by renal filtration (kidney).
- The normal adult value of serum creatinine is 0. 7 - 1. 5 mg/dl
Dosage Calculations Based on Creatinine Clearance
- Creatinine clearance rate (CrCl, ml/min):
Is the volume of blood plasma that is cleared of creatinine by
kidney filtration per minute.
1 - Jellife equation:
For males:
For females: CrCl = 0. 9 X CrCl determined using formula for males
2 - Cockcroft-Gault equation
For males:
For females: CrCl = 0. 85 X CrCl determined using formula for males
Dosage Calculations Based on Creatinine Clearance
serumcreatinineinmg dl
patient sageinyears CrCl /
serumcreatinineinmg dl
patientsageinyears Bodyweightinkg CrCl 72 /
3 - Sanaka equation (for patients over 60 years of age) :
For males:
For females:
4 - Schwartz equation (for pediatric and adolescent patients from
neonates to 17 years of age) :
K is proportionality constant ranging from 0. 33 (for neonates) to 0. 7 (for
adolescent males)
Dosage Calculations Based on Creatinine Clearance
serumcreatinine mg dl
patient sweight kg Plasma g dl CrCl 100 /
' 19 albumin / 32
serumcreatinine mg dl
patient sweight kg Plasma g dl CrCl 100 /
' 13 albumin / 29
serumcreatinine mg dl
k patient sheight cm
CrCl
- Clinical laboratory tests (chemistries) analyze samples for chemicals as,
glucose, cholesterol, total lipids, creatinine, bilirubin, sodium, potassium,
carbon dioxide and other substances, including drugs following their
administration.
- The usual amount of a chemical substance is a common range, e.g. glucose in
serum is 65 - 99 mg/dL and for creatinine is 0. 5 - 1. 4 mg/dL
- Units may change from mg/dL to SI units of millimoles per liter (mmol/L) of
blood plasma
If a patient is determined to have a serum cholesterol level of 200 mg/dL, what
is the equivalent value expressed in terms of mmol/L.
M.W. of cholesterol = 387
1 mmol = 387 mg and 200 mg/dL= 2000 mg/L
387 mg 1 mmol
2000 mg X mmol X = 5. 17 mmol/L
Calculations Involving Clinical Laboratory Tests
Practice Problems
Page 163 : 1 and 3.
Page 164 : 5 , 7 , 9 , 11 , 13 , 15 , 17.
Page 165 : 19.
