Monitoring Matters, Non-Invasive and Minimally Invasive Cardiac Index Devices Don’t Always Align: suggests study

The Quest for Safer, Less Invasive Monitoring

Cardiac surgery requires precise haemodynamic monitoring to guide life-saving therapies. Traditionally, invasive catheters have been the gold standard for measuring cardiac output, but their risks have spurred the development of non-invasive alternatives. Yet, are these new technologies up to the challenge in the high-stakes environment of cardiac surgery?

A recent pilot study from Hyderabad, India, published in the Indian Journal of Anaesthesia, set out to compare two such methods: the Noninvasive Cardiac System (NICaS™), which uses bioimpedance, and FloTrac™ pulse contour analysis.

Putting the Technologies Head-to-Head

Researchers retrospectively analysed 16 adult patients undergoing elective cardiac surgery with cardiopulmonary bypass. At six key surgery phases, cardiac index (CI) readings from both the NICaS™ and FloTrac™ were taken at both radial and femoral arterial sites. The goal? To see how closely these devices agreed and whether trends in patient status were tracked similarly.

Crunching the Numbers: Gaps in Agreement and Precision

The findings were revealing:

FloTrac™ consistently reported higher cardiac index values than NICaS™ at nearly all time points, especially after patients came off bypass.

Bland–Altman analysis revealed wide limits of agreement and high percentage error (>30%), far exceeding the threshold for clinical interchangeability.

Four-quadrant and polar plot analyses, which examine how well devices track trends, showed poor concordance (69% for radial, 63% for femoral; well below the 90% benchmark).

NICaS™ tended to underestimate cardiac index compared to FloTrac™, with greater divergence during the most dynamic post-bypass period.

Why the Discrepancy?

Both devices have their quirks. FloTrac™ can be thrown off by changes in vascular tone—common during and after bypass—while NICaS™ bioimpedance is highly sensitive to fluid shifts, hypothermia, and tissue changes that are hallmarks of heart surgery. These factors mean that while non-invasive bioimpedance works well in stable patients, its accuracy drops during the haemodynamic rollercoaster of cardiac surgery.

The Takeaway: Caution Advised in the OR

This study suggests that NICaS™ bioimpedance and FloTrac™ pulse contour analysis are not interchangeable for monitoring cardiac index in cardiac surgery. Each has limitations, and neither should be relied upon as the sole decision-making tool during complex procedures. Larger, multicentre studies using gold-standard measurements are needed to validate these findings.

Key Takeaways

NICaS™ bioimpedance underestimates cardiac index compared to FloTrac™, especially after bypass.

Wide limits of agreement and high error rates mean the methods are not clinically interchangeable.

Both devices are affected by physiological changes common in cardiac surgery.

Trending ability (tracking status changes) is poor between the two technologies.

Clinicians should use caution and consider multiple data sources for haemodynamic management.

Citation: Patro A, Ayya SS, Renuka DR, Ramachandran G. Comparison of cardiac index values between bioimpedance and pulse contour analysis in patients undergoing cardiac surgery under cardiopulmonary bypass. Indian J Anaesth 2026;70:443-51. https://journals.lww.com/ijaweb