SARS-CoV-2 has caused 117 799 584 cases of COVID-19 across the world,1  and to date it has caused 529 301 deaths in the United States.2  The care of critically ill patients with COVID-19 infection has rapidly evolved, and health care workers have adapted to the complex care these patients require.3  One of the various challenges has been the delivery of medications by infusion pumps placed outside of patients’ rooms, and we read with interest the manuscript by Blake et al.4  By placing infusion pumps outside of patients’ rooms, the distance between the pumps and the patient has significantly increased. Unstable critically ill patients with COVID-19 infection are receiving medications (eg, sedatives, vasopressors, fluid boluses) administered by infusion pumps through very long intravenous (IV) tubing. The time it takes for the medications to travel the additional distance imposed by the long IV tubing may vary. Adverse clinical effects may develop, especially during titration of doses to their desired effect. Anecdotally, while using infusion pumps outside of patients’ rooms, we have noted that patients may remain critically hypotensive despite increases in the dose of the selected vasopressors at the pump level. At the same time, some patients become dangerously hypertensive after several attempts of up-titration of vasopressors. Similar situations have been noted with continuous infusions to control heart rate and sedation. Here we explore the different variables that may play a role in the time it takes for medications to be delivered from their infusion pump to the patient.

The Hagen-Poiseuille equation describes the variables involved in the flow of liquids through IV tubing.

$Q=ΔPπr48ηL$

Where Q = flow of liquid through tubing, Δ = difference in pressure, π = pi, r4 = radius of tubing to the fourth power, η= viscocity of liquid, and L = length of tubing.

Infusion pumps typically move fluid forward by different mechanisms. The flow rates and patterns of flow produced by the mechanism of pump operation can be of clinical importance. For example, when highly concentrated solutions of short-acting vasoactive drugs are given by continuous infusion at low flow rates, there may be unexpected clinically significant hemodynamic fluctuations.5,6

The radius of the IV tubing can be rather standard. However, accidental compression of the tubing can cause a significant decrease in flow.

The flow rate at which medications travel is inversely related to the length of the IV tubing. With an increase in the length of the tubing, a proportional decrease in the flow rate might occur and therefore an increase in the time that it takes for medications to travel from the infusion pump to the patient.

## Conclusion

Several factors are implicated in the delivery of drugs from the infusion pump to patients. The length of IV tubing may be an important factor involved in the delivery of medications. There may be clinically significant adverse effects because of very low flow rates of concentrated vasoactive drugs. This is based on anecdotal observations at the bedside. Animal studies and gravimetric computer models may be necessary to confirm these findings.

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## Footnotes

Letters to the editor raising points of current interest or commenting on articles previously published in the journal are welcome. Letters should be 400 words or less with no more than 5 references and are subject to editing for length and clarity. Address letters to aacnacc@aacn.org.

FINANCIAL DISCLOSURES

None reported.