What Factors Affect Heparin Dosage?

When administering heparin, the correct dosage is extremely important because too little will not have enough of an anticoagulant effect for the patient’s medical condition, while too much can cause hemorrhaging, possibly resulting in death. Correct heparin dosage depends on several factors, including the patient’s body weight, general condition, and active partial thromboplastin time (aPTT), which is the amount of time in seconds it takes for blood to clot. Other variables of body weight can be used to calculate weight-related heparin dosage, including adjustments made for obese patients. Finally, during administration, changes in condition, the presence of significant bleeding, and changes in hemodynamic stability can indicate a need to adjust heparin dosage.

Since heparin was first discovered and became widely used in the early 20th century, differing opinions about how to determine dosage have been expressed. Physicians have debated the usage of weight-based versus non-weight-based dosing, with current prevailing opinion in favor of weight-based dosage. Non-weight-based heparin dosage indicates an initial bolus of 5,000 units with infusions of 1,000 units per hour.

Current weight-based guidelines indicate an initial bolus administration of 80 units per kilogram of actual body weight (ABW) and maintenance infusion of 18 units per kilogram ABW per hour when aPTT is less than 35 seconds. Adjustments are made based on periodic assessments of the aPPT, usually about every six hours. Assessment of aPPT reveals how well heparin is working within the body. Platelet counts and complete blood counts (CBC) can also help determine whether a heparin dosage adjustment is needed.

Another weight-based heparin dosage protocol is sometimes used based on calculations involving actual body weight and lean body weight in kilograms. The patient’s height in inches is calculated. Sixty is subtracted from the total. The answer is multiplied by 2.3 and then 50 is added for males, and 45 is added for females. The answer equals the patient’s lean body weight (LBW) in kilograms.

If the ABW is greater than 1.4 times the LBW, the patient is considered obese. A different formula is used to account for the increased blood volume of an obese patient. In this case, the dosing weight is calculated by taking the difference between ABW and LBW, multiplying the result by 0.4, and adding the result to LBW. Some doctors do not use this formula, stating that the increased blood volume in an obese patient has a minimal effect on heparin dosage.