Heparin therapy needs to be adjusted to maintain the patient in an anticoagulated state sufficient to prevent thrombosis, yet not result in excessive hemorrhage. Based upon changes in monitoring tests, heparin administration can be adjusted to keep the patient within the desired therapeutic range.
The aPTT is widely available for monitoring, with the decision points given as multiples of the mean for the reference range. However, different commercial aPTT reagents vary in their responsiveness to heparin, so that this process can be misleading. Therefore, it is recommended that the heparin response curve be studied, where a given test system's aPTT results can be plotted versus heparin levels.
Antifactor Xa levels can also be monitored, but this testing is often not readily available.
Initial heparin dosing:
(1) Give a bolus of 80 IU per kilogram.
(2) Start an infusion of 18 IU per kilogram per hour.
(3) Order an aPTT 6 hours after heparin bolus given.
Adjusting Infusion based on aPTT
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aPTT
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Adjustment
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< 1.2 times mean of reference range
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repeat 80 IU/kg bolus, then increase infusion rate by 4 IU/kg per hour
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1.2-1.5 times mean of reference range
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give 40 IU/kg bolus, then increase infusion rate by 2 IU/kg per hour
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1.5-2.3 times mean of reference range
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no change (therapeutic range)
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2.3-3.0 times mean of reference range
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decrease infusion rate by 2 IU/kg per hour
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> 3 times mean of reference range
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hold infusion 1 hour, then decrease infusion rate by 3 IU/kg per hour
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where:
• The target for aPTT monitoring is reported as a multiples of mean for reference range.
• The therapeutic range is often given as 1.5-2.3 times the mean of the normal range.
• Check to see if actual body weight or ideal body weight should be used for calculations.
Ongoing monitoring:
(1) Order an aPTT 6 hours after any dosage change.
(2) Once 2 consecutive aPTTs are within the therapeutic range, monitor the aPTT every 24 hours and adjust infusion accordingly.
Example:
(1) reference range for aPTT: 20-36 seconds, with mean 28 seconds
(2) multiples of mean for aPTT: 1.5 times is 42 seconds, 2 times is 56 seconds, 2.3 times is 64 seconds.
(3) The recommended therapeutic range: 42-64 seconds
