Background: Patients scheduled for thoracic cancer surgery are eligible for goal-directed fluid therapy, but cardiac output monitoring remains challenging in that specific setting.

Objective: We aimed to compare cardiac output as measured with chest bioreactance with that measured by calibrated pulse contour analysis; the hypothesis being that both methods would be interchangeable.

Design: A prospective monocentre observational study.

Setting: A tertiary university hospital.

Patients: Fifty adult patients undergoing thoracic cancer surgery over a one year period.

Main outcome measures: Simultaneous measurements of cardiac index (CI) with bioreactance (CI-NICOM) and arterial pulse contour analysis calibrated by transthoracic echocardiography (CI-PCA) were performed at eight pre-specified intra-operative time points and following fluid challenge and/or vasoactive agents. Relationships between absolute values and changes in CI were assessed by linear regression. Interchangeability was tested with Bland-Altman analysis and percentage error calculation. A four quadrant plot was used to evaluate trending ability.

Results: There was a significant difference between CI-PCA and CI-NICOM: 2.4 ± 0.8 (range: 0.9 to 5.8) l min^-1  m^-2 vs. 2.9 ± 0.9 (range: 0.9 to 7.2) l min^-1  m^-2 , respectively (P  < 0.001). A positive relationship was found between both techniques: y = 0.29x + 2.19; r ²  = 0.08 (P  < 0.001). Taking CI-PCA as the reference method, there was a systematic overestimation of CI-NICOM by 21% (0.5 l min^-1  m^-2 ) and limits of agreement were large: -2.49 to 1.47 l min^-1  m^-2 . The percentage error was 77% and concordance rates were 75 and 70% with and without an exclusion zone of 0.5 l min^-1  m^-2 .

Conclusion: Chest bioreactance is feasible and well tolerated in patients undergoing thoracic surgery for cancer. When compared with calibrated PCA over a wide range of CI values, the technique is moderately correlated, not interchangeable, and provides moderate trending ability.

Trial registration: NCT04251637.