How to determine the starting dose and drug concentration when initiating intrathecal therapy?
The factors to consider when initiating intrathecal therapy in a patient include patient-related factors (e.g., reactions to systemic medications, medical comorbidities) and device-related factors. Conservative dosing regimens should be used for patients with multiple and/or severe comorbidities. All drug delivery systems have a finite volume and delivery rate; some systems have rates that can be adjusted. Drug reservoir volume, drug delivery rate, and drug dose are the device-related factors to consider when formulating intrathecal drugs to be administered to the patient. For example, with a 20-mL pump reservoir volume, a delivery rate of 0.5 mL/day will produce a reservoir refill interval of approximately 40 days compared to a delivery rate of 1 mL that will require a refill of approximately 20 days. Higher delivery rates will negatively impact the life span of intrathecal systems that rely on battery-powered modes of delivery. Ideally, the drug should be delivered at a rate of between 0.25 mL/day and 0.5 mL/day to maximize efficacy, system life-span, and patient convenience. When a desired drug dose is determined (typically measured as dose/day), a delivery rate is chosen, and these two will dictate the drug concentration to be formulated to fill the pump reservoir.