A Laboratory Balance Becomes a Surgical Monitor


Scientists are rational thinkers, but they can also be very creative when trying to find answers to real-life questions.

A recent example of this was a study initiated by bioengineer Aaron Chang, CEO of Renalert, which incorporated an Ohaus™ balance into a new device to more closely monitor patients during heart surgery.

Acute Kidney Injury

Up to 30 percent of patients who undergo cardiac bypass surgery experience kidney damage or Acute Kidney Injury (AKI). The procedure is long and can affect blood circulation. Patients who develop AKI may have longer hospital stays, develop other illnesses and may even die.

“AKI is associated with more deaths than heart failure, prostate cancer, breast cancer, and diabetes combined every year,” said Chang.

Earlier studies showed that patients who developed AKI after heart surgery had lower than expected rates of urine production and flow rates. Monitoring urine flow rates during surgery helps to check the patients’ kidney condition and lets the medical team optimize the amount of fluid being given to the patient.

Monitoring Urine Output

Before surgery, a catheter is inserted into the patient’s urethra. The catheter is then attached to other tubing that directs the patient’s urine to a collection bag.

At regular intervals during surgery, the operating room (OR) nurses or anesthesiologists manually record the amount of urine that a patient produces. The volume markings on the bags are not exact, so the amount is an approximation. The actual urine volume may vary from the visual estimate by as much as 26 percent and also does not indicate the rate of urine flow.

A New Design

Some devices already exist for measuring urine flow rate, but they do not necessarily connect to the typical urinary catheter systems.

As biomedical engineering students at Johns Hopkins University, Chang's team created a new device that uses a digital balance to constantly measure urine flow. They created a platform with a 3D printer, attached it to an aluminum stand, and placed it on an OHAUS™ STX Touch-Screen Digital Balance.

Testing the Process

In an initial feasibility study, 30 patients gave their permission to use the device during their surgeries. Their urine bags were put on top of the new weighing device, which was placed under the operating room table.

The portable balance measured and transmitted the weights to a computer where software calculated the urinary flow rates. The OR staff continued to also record their urine estimates in the patients’ medical records.


The new device was used successfully with all patients. Since the device and urine bag measured a little over a square foot, it did not interfere with the OR workflow.

The median percent error between the automated and manual urine measurements was just under 20 percent. Data from the new device was more accurate than visual readings and having the real-time urine flow numbers can improve patient management.

The success of this study allowed Chang and several clinicians to launch the startup Renalert, where they plan to further their work by shifting the treatment of AKI towards active prevention.

Discussion Questions

  • What is cardiac bypass surgery?
  • How does the body produce urine?


  • Acute
  • Balance
  • Catheter
  • Prototype