How To Use Constant Rate Infusions

What are the benefits of CRI, and why should you use them? Find out here.


What are the benefits of a CRI?

The problem with intermittent boluses of pain medications (e.g. buprenorphine every eight hours) is that it creates peaks and valleys above and below the preferred medication plasma level. In comparison, constant rate infusions, or CRIs, provide more consistent medication plasma levels.

In addition, CRIs allow you to maintain surgical patients at a lower anesthetic gas rate. This gas-sparing effect should help maintain blood pressure.

How do you set up a CRI?

I really prefer the expression "manually controlled infusion” (MCI) to CRI. The delivery of such infusion rates is rarely "constant.” We generally vary the delivery rate manually, based on the patient’s needs, the intended drug effect and observed drug influences.

It is very important to understand that long-term infusions, even with short duration drugs like fentanyl, lead to depot effects. This can increase plasma levels of the drug if you do not make gradual reductions in the delivery rate.

In addition, although one can deliver MCIs via gravity drip sets (i.e. counting drops), actual drug delivery is likely to be inconsistent and therefore riskier.

Does the type of IV fluid matter?

Any replacement or maintenance crystalloid would suffice. In other words, you could use LRS, Plasmalyte, 0.9 percent saline, etc.

So what’s the best way to deliver a CRI … or an MCI?

There are two basic options, each with its own pros and cons. Most practices have the necessary equipment to perform MCIs with the most common method: the IV fluid bag delivery. For that, you need a fluid pump, an IV bag and an IV line. The benefit is that only one pump is needed to manage this system.

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The simplest setup delivers the MCI via the patient’s supportive IV fluids. It provides less flexibility, since at times, the patient’s fluid requirements will vary.

This, in turn, affects the drug delivery rate. So here you may be forced to select lower drug delivery rates to allow the freedom of higher fluid rates, if needed, in order to avoid excessive drug delivery.

The disadvantage is that you are locked into a fixed drug delivery as all the drugs and "maintenance” fluids are in the same bag.

Is there a way to separate them?

Indeed. The next step in sophistication is to separate the supportive fluids from drug delivery fluids. In this scenario, you select one rate for the drugs and one rate for the supportive fluid. This means that you need two pumps.

I would suggest using the smallest available IV bag to add drugs. Some colleagues separate each drug into independent delivery systems. This provides complete control over each drug but requires four or more pumps per patient.

What’s the other way to deliver an MCI?

My preferred delivery method is using syringe pumps. Because the fluid amounts are much smaller, there is no risk of fluid overload.

Colleagues may assume that a syringe pump is expensive. The reality is that you can purchase a syringe pump for a few hundred dollars these days, used or even new. I prefer the high-pressure lines as they generally are low volume.

Again, drugs can be combined in one syringe (with a single delivery IV line) or separated into separate pumps (with several syringe pumps and IV lines).

The drugs we use in an MCI are so inexpensive that I typically combine the drugs in one syringe, with one IV line. If I want to change ratios or drop a drug from the infusion, I simply dispose of the previous product and start a new syringe.

Do you just "piggyback” the various IV lines?

I am not a fan of piggybacking IV lines because there is a delay between the change of a drug delivery rate at the pump and what happens at the patient level when drugs enter at a point higher up the delivery chain.

To facilitate the delivery of multiple elements, I prefer to use T-ports with multiple connection options (see photo).

What are the disadvantages of an MCI?

Besides the cost of the equipment and the potential for fluid overload, I can’t say that I would call anything a real disadvantage. The risk of any of the chosen drugs is low, as long as the drug choices and rates are carefully considered. This is similar to using any drug in any other application.

Colleagues and technicians new to MCIs may find that some patients are slower to extubate during recovery. This is usually a matter of becoming familiar with when to reduce the MCI rate. There is an art or "feel” to the process.

Should we worry about light sensitivity of morphine and lidocaine? Should we cover the IV bag and line?

I don’t cover bags and lines when I use a CRI for a few hours. However, I would wrap the product, e.g. with foil, if I were using it over many hours to a few days.

If a colleague uses an MCI and wants to transfer to the ER overnight, it is legal to transfer the IV bag so the client doesn’t have to pay for a "fresh” one?

It may sound like a generous idea, but you cannot legally "give” any practice any scheduled drug, and certainly not schedule II drugs. It is for your use in your practice only. Regardless, I would certainly advise against it. You do not want those drugs at risk for diversion.

In fact, many ERs decline to use IV product of any kind (with drugs inside or not) sent along with the patient. I would say that ERs and referral practices should be suspicious of product that they do not prepare themselves.

Can you use a pain MCI outside of the OR?

You certainly can, for a number of medical indications, including post-op pain management, trauma patients and severe pancreatitis. Any painful patient would benefit from an MCI.

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