Infusion Pump Mechanics & the Significance of Inaccurate Flow Rates
I am working on another post relating to FDA regulation of pain pumps, but expect it may be a week or two before I am able to complete it. In the meantime, I thought I would share an article I recently came across----Disposable Infusion Pumps by Skryabina and Dunn. It contains useful explanations of how some of the most common types of these pumps actually function.
These devices are used for many purposes beyond that of a "pain pump"--delivering local anesthetics post-operatively at or near a surgical site. These include the delivery of other medications, including chemotherapy, antimicrobials, antibiotics, as well as the delivery of anesthetics or analgesics by other routes, eg., continuous epidural, peripheral nerve block, and i.v.. The authors provide concise descriptions of the mechanisms of several types of non-electric pumps including elastomeric, positive-pressure (spring-powered and gas-pressured powered), negative-pressure (vacuum), and patient-controlled analgesia (PCA) pumps.
The flow rates of medications through disposable pumps are significantly inaccurate--typically within +/-15% or even +/-20%. (Compared to +/-3% with electronic syringe pumps and +/-5% with electronic volumetric pumps). I believe most pain pump manufacturers include the +/-15% figures in their written materials--see, eg. the flow rate table included in the product Insert for the On-Q Pump with Fixed Flow Rate.
Nonetheless, I assume most surgeons who use a pain pump labeled with a 2 ml/hr flow rate are likely to believe that the device delivers only 48 ml of local anesthetic in the first 24 hours. In fact, the same flow rate table in the Insert for the On-Q Pump (along with all other documents for the On-Q I've seen) shows that a 100 ml pump will actually deliver 65 ml in the first 24 hours. For the first several hours, the flow rate is actually 2.5 ml or higher.
If the anesthetic is 0.50% Marcaine, 65 ml means 325 mg. The maximum 24 hour dose for Marcaine is 400 mg and, as I've stressed before, this is based on the risk of systemtic toxicity (neurological or cardiac) and not local (tissue) toxicity. Surgeons commonly use additional injections of local anesthetics around surgical sites during procedures. Thus, it would take only an additional 15 ml of 0.50% Marcaine, along with a 100 ml 2 ml/hr. pump, to reach a 400 mg dose in the first 24 hours after surgery.
I believe many pain pump manufacturers have blithely assumed that any 24 hour dose of Marcaine as long as it remains less than 400 mg, in most any part of the body, regardless of the concentration, and regardless of the route of administration (continuous infusion vs. others), is inherently safe. For all who have sustained injuries from pain pumps caused by local anesthetic toxicity, this has been a tragically flawed assumption.
