This is the first proof-of-concept study to show the feasibility of continuous, non-invasive delivery of oxygen monitoring in cardiac surgical patients with our novel algorithm that simultaneously integrates CO readings from the ECS with SpHb and SpO2 readings from the MSPC to produce continuous, real-time DO2 readings non-invasively. Moreover, each patient’s preoperative baseline DO2 value served as their reference point, providing ease for the real-time interpretation of oxygen delivery and allowing for monitoring of the DO2 trend.

Utility of DO2 monitoring

The ultimate goal of hemodynamic monitoring and management is to provide adequate DO2 to meet the metabolic oxygen demands of the tissues and organs, to avoid anaerobic metabolism and accumulation of lactic acidosis. However, there are limitations to the use of DO2 monitoring in current clinical practice and functional hemodynamic monitoring is largely used as a surrogate for DO2 instead.

Addressing current limitations

Our novel algorithm can have real-time DO2 monitoring and seeks to address the current limitations faced. For non-surgical and non-cardiac surgical patients, DO2 is not routinely measured as CO is commonly obtained via minimally invasive (e.g. pulse pressure analysis method) or invasive means (e.g. pulmonary artery catheter bolus thermodilution method), and Hb sampling is done only intermittently via point-of-care or laboratory tests. The utility of DO2 is limited as it is cumbersome due to the need for invasive lines and the lack of continuous DO2 readings and trends.

Successful integration of CO, Hb and SpO2 readings to obtain DO2 readings at 1- and 10-minute intervals using the Quantum Perfusion System and Dideco software system respectively, during cardiac surgery has been previously reported [5, 14]. However, this can only be done when the patient is on CPB when CO is dependent on the pump flow setting. Maintaining DO2 above 272 mL/min/m2 during CPB has been shown to improve postoperative outcomes [15] but DO2 trending cannot be extended to the post-CPB period due to technical limitations. Instead, post-CPB monitoring relied largely on functional hemodynamic monitoring which cannot track end-organ DO2. Our solution of a non-invasive, continuous DO2 monitoring system is the first to demonstrate that it can be used to track DO2 postoperatively and to demonstrate its use in predicting poor outcomes after cardiac surgery.

Our setup is easy to use, non-invasive, and continuous, allowing real-time trending of DO2. This ensures that the physician is alerted early to a possible downturn in patients’ parameters, and this DO2 trending can be expanded to a large array of at-risk patients across different disciplines and clinical care settings.

Robustness of novel algorithm

While recognizing concerns about expanding the use of monitors beyond their original purpose, the robustness of our novel algorithm is built upon the established reliability of the proprietary algorithms embedded in the ECS and MSPC systems.

The non-invasive CO monitor by ECS has been evaluated and validated to be equivalent to its invasive counterpart during and after cardiac surgery – the pulmonary artery catheter bolus thermodilution method [16]. Simultaneously, given the unparalleled ability to have continuous and non-invasive SpHb monitoring unlike any other available hemoglobin monitors, the SpHb monitor by MSPC has been highly recommended as a trend monitor and allows for more effective blood utilization in cardiovascular surgery [17]. Additionally, we corroborated the accuracy of hemoglobin measurements obtained by MSPC through a comparison with laboratory test results.

However, it is important to note that calibration of the ECS and MSPC systems pre- and postoperatively should be carried out by the manufacturer’s recommendations to prevent system drift and inaccurate readings.

DO2 and AKI

We have presented both statistical and graphical comparisons of DO2 trends between cardiac surgical patients with and without postoperative AKI. As anticipated, patients with postoperative AKI exhibited poorer DO2 values overall. These results lead to the important question regarding the critical DO2 threshold in the post-CPB period which would reduce the incidence of AKI. Having demonstrated the feasibility of DO2 trending, our next focus would be to demonstrate and validate its robustness as a monitor for detecting AKI-associated DO2 trends and/or critical threshold. This endeavor aims to enhance the optimization of DO2 to mitigate and reduce postoperative adverse outcomes.

The PrevAKI randomized controlled trial has previously shown that using functional hemodynamic monitoring with early GDT optimization can significantly reduce postoperative moderate to severe AKI [7]. Our proposed solution of DO2 trending not only looks at the functional hemodynamic parameters but also addresses the adequacy of oxygenation and blood management which will enhance GDT optimization.

Advancing patient-centric care: personalized critical DO2 threshold

Critical DO2 values of < 280 ml/min/m2 during CPB have consistently been shown to be associated with AKI after cardiac surgery [4]. Currently, there is a paucity of studies on the optimal postoperative critical DO2 threshold, which reduces the risk of postoperative complications. Published critical DO2 values so far are based on the study cohort baseline characteristics, the outcome measures, and the statistical modeling method used [5, 14].

To address this gap, our innovative setup empowers clinicians with unprecedented access to continuous, real-time DO2 monitoring. This technological leap facilitates the identification of a patient’s unique baseline DO2 and the analysis of DO2 trends in relation to postoperative outcomes. This capability unlocks the potential for personalized critical DO2 thresholds, marking a paradigm shift towards a patient-centric model of care. The real-time availability of personalized DO2 readings serves as an unparalleled reference point, enabling clinicians to monitor trends and detect potential complications early. The personalized baseline not only enhances precision in identifying subtle changes in oxygen delivery tailored to each individual but also allows for interventions based on the patient’s distinct physiological response. As we continue to pursue technological refinements, the prospect of trending DO2 off-site and implementing customized alarms for each patient becomes increasingly significant. This not only ensures timely interventions but also empowers healthcare providers to proactively address deviations from a patient’s personalized baseline using early GDT strategies, ultimately improving postoperative adverse outcomes.

Limitations

There are a few limitations to our study. Our study was conducted within a single centre and involved a relatively small number of cardiac surgical patients. To enhance the external validity of our observations and extend the generalizability of our conclusions beyond the confines of cardiac surgical care, future studies should strive for broader inclusivity by encompassing non-cardiac surgical patients. Expanding the scope to include a more diverse patient population will contribute to a more comprehensive understanding of the implications of continuous DO2 monitoring across various clinical contexts.

Additionally, continuous, real-time DO2 monitoring introduces specific challenges. Firstly, 2 separate proprietary monitors (ECS and MSPC) are currently required to integrate and obtain DO2 readings, though each monitor is not bulky. Secondly, the ECS finger cuffs and MSPC adhesive sensors are costly, however, the cost may potentially be reduced if these consumables are purchased in bulk when DO2 monitoring is routinely done. Thirdly, the maximum duration of continuous DO2 readings is limited to 8 h at one stretch according to manufacturers’ recommendations. Lastly, the use of these peripheral monitors is less useful in very low perfusion states, e.g. during CPB, but continuous DO2 readings can be obtained from the perfusion pump machine.

Future works and applications

Our novel setup and algorithm show great promise in revolutionizing patient monitoring systems to include non-invasive DO2 trend monitoring as the gold standard for hemodynamic optimization. Further refinement to our algorithm to allow the inclusion of user inputs of a patient’s body surface area will provide real-time indexed DO2 values to allow for better referencing, standardization, and comparability between patients due to variations in body surface area. Also, incorporating an intelligent alert system will provide timely audio and visual cues to draw the attention of the clinician when DO2 values are below a critical threshold. In addition, future studies should validate non-invasive ECS-derived CO measurements against established methods like bolus thermodilution or echocardiography, extending the utility of our novel algorithm to include the use of individual DO2 values.

The vast amount of continuous DO2 data and its components will allow us to build a robust predictive model of postoperative outcomes utilizing machine learning and artificial intelligence. This will allow for a tailored response, such as optimizing Hb and/or hemodynamic parameters, for individual patients at specific timepoints, significantly improving postoperative outcomes.

Apart from the intensive care setting, this non-invasive DO2 trending can be easily performed in all clinical care areas for at-risk patients as an early trend monitor to guide decisions on when to escalate care and aid in appropriate utilization of scarce resources – all of which value adds the clinical care being delivered.