Analgesics administered by intermittent bolus create broad swings in drug plasma levels. Constant rate intravenous infusions
(CRIs) are manually controlled titratable infusions. CRIs establish more consistent plasma levels and better overall control
of drug effects. Mu agonist opioids, lidocaine, and ketamine are the most frequently utilized analgesics providing an effective
multimodal benefit with low potential for adverse events. While delivery is possible through a standard gravity drip set,
most prefer the fine control provided by an IV fluid pump or syringe pump. Calculators are available to eliminate the worry
associated with drug delivery calculations. Analgesic CRIs not only contribute to compassionate pain relief, they can also
help contribute to a balanced anesthetic program, reducing maintenance agent requirements, frequently improving patient blood
pressures and ventilation. This session will focus on the details of how you can easily integrate analgesic CRIs into daily
practice in the primary care setting.
Overview of presentation
Constant Rate Infusions have become a frequently utilized analgesic delivery method in the primary care setting. Delivering
drugs via this route allows for a more consistent drug delivery that avoids the peaks and valleys associated with intermittent
administration. CRIs also allow for much greater control of the medications' effects, both positive and negative.
Common Manual Infusion Drugs and Dose Rates
Target controlled infusions (TCIs) are more sophisticated infusion methods requiring computerized delivery systems. TCIs require
a detailed understanding of the given drug's species specific pharmacokinetics. You might think of a TCI as a precision vaporizer
with a specific drug delivery scale while a CRI is more like a non-precision vaporizer providing more general low, medium,
high range guidance. Most importantly, unlike TCIs, CRIs are a practical method for any practice.
Drugs commonly delivered through CRIs include morphine, hydromorphone, fentanyl, ketamine, midazolam, lidocaine, and dexmedetomidine.
The opioids provide titratable analgesia that benefits the patient at the peripheral and central levels. Nausea, clinically
relevant bradycardia and respiratory depression are not expected at normal analgesic dose rates. Ketamine enhances analgesia
via different mechanisms; NMDA antagonism reducing central sensitization, enhancing opioid analgesia, reducing the risk of
opioid induced hyperalgesia and opioid tolerance as well as possible direct analgesia via the D2 dopamine receptors. Midazolam
provides sedation and relaxation, central analgesia, as well as a MAC sparing effect that can be of major benefit for patients
experiencing problematic isoflurane or sevoflurane induced hypotension. Lidocaine is capable of enhancing analgesia while
providing anti-inflammatory, reperfusion, cerebral protectant and GI motility benefits. Dexmedetomidine is attractive as an
anxiolytic as well as a tool for enhanced patient analgesia.
CRIs are most commonly delivered through the IV fluid bag route or directly using a syringe pump. The IV fluid bag route is
attractive because it allows for precise delivery rates using equipment already available at most practices. The simplest
method involves a single fluid bag providing both the drug delivery as well as the patient's fluid needs. The downside to
this method is the inability to adjust the fluid rate without changing the drug delivery rate. To maximize the flexibility
of this method you generally need to pick low to midrange drug dose rates so that adjustments in the patient's fluid need
don't take you outside of recommended drug dose rates.
You can expand your flexibility by running separate fluid lines through different IV fluid pumps. In the two-pump model, the
CRI drugs would be delivered at a very low rate (ex. 1 ml/kg/hr) while the patient's additional fluid needs are separately
managed through the second line. This allows for greater flexibility of drug and fluid delivery but requires multiple pumps
and multiport IV access.
The greatest level of flexibility is gained when each analgesic is delivered through its own IV fluid bag. This allows for
variable adjustments of each drug independent of the other but requires the greatest equipment resources and IV access ports.
While the calculations of the various CRI delivery options may seem daunting, this headache has been eliminated by easy to
use calculators directly available online. These calculators allow you to vary the IV fluid bag size, fluid delivery rate,
and drug dose rates to satisfy any conceivable combination.
Although the author does not generally recommend using gravity administration for analgesic CRIs this may be a consideration
in some settings. Delivering gravity administered CRIs through a buretrol maximizes control over drug delivery while also
setting a limit to the maximum amount of drug delivered.