Real Time Intraoperative Monitoring of Blood Loss with a Novel Tablet Application

INTRODUCTION

Currently there is a lack of a standardized approach of objectively measuring intraoperative blood loss that is accurate and can be completed in real time. Blood is gathered intraoperatively with the use of surgical sponges and vacuum suction, however, confounding variables like saline, bodily fluids or tissues, and intravascular volume status (hemodilution, hemoconcentration, and third body spacing) are rarely accounted for in calculating intraoperative blood loss. We are left with three commonly cited strategies for quantifying surgical blood loss: (1) Subjective assessment by physicians, (2) gravimetric analysis, and (3) photometric analysis. Subjective measurement by physicians is the most commonly used method due to convenience and efficiency. It relies on the surgeon’s or anesthesiologist’s calculation of hemorrhage and other fluid loss, blood captured via surgical sponges, vacuum suction canister, blood on the floor and other surfaces, and amount of fluid is used for irrigation intraoperatively. It becomes clear that taking these numerous variables into consideration can be extremely challenging and likely the reason why subjective assessment is inaccurate and unreliable [1-7].It has also shown that surgical experience of the individual has had little impact on improving accuracy in estimating blood loss [8].

Gravimetric analysis is a relatively simple concept that assumes 1mL of blood equals 1gram, and the dry weights of surgical sponges are subtracted from the wet weights of blood soaked sponges. This method is also not immune to confounding variables like saline irrigation, suctioned devices, other body fluids, and solid body tissues that can get captured by surgical sponges and weighed at the end, resulting in yet another unreliable method [5, 9, 10]. Photometric analysis and laboratory testing have been shown to be reliable techniques for measuring blood loss in the research setting and are currently discussed as the “gold standard” for accurately quantifying blood loss [11, 12]. The process involves rinsing and assaying blood soaked products to determine hemoglobin concentration through spectrophotometry and intraoperative laboratory work-up of a patient’s blood samples. These processes are laborious, time consuming and costly ways to monitor blood loss and as a result are rarely performed. Therefore, a dependable, real-time technique for direct measurement of estimated surgical blood loss would provide multiple benefits: (1) Improved delivery of intraoperative and postoperative care, (2) a quality assurance tool for measuring blood loss for surgeons as part of performance processes are established by surgical departments for re-certification, (3) point of care testing to evaluate when interventions (i.e. transfusion, pharmacologic intervention) need to be initiated, and (4) proper documentation of accurate measure of intraoperative blood loss to satisfy quality measures for hospital reporting and accreditation.

A novel tablet computer application that includes Feature Extraction Technology (FET), and utilizes computer vision algorithms to assess hemoglobin concentration contained in surgical sponges through photographic analysis, may satisfy the measures stated above.

Our study hypothesis is that this novel tablet application using FET would be as accurate in measuring hemoglobin loss, as more laborious subjective validated hemoglobin measurement tools with respect to surgical sponges.

MATERIALS AND METHODS

This study was approved by our Institutional Review Board. Data was gathered in a consecutive series of 50 orthopaedic joint arthroplasty cases. Enrollment was initiated in June 2013 and continued through December 2013 at our clinical site. Preoperative hemoglobin and hematocrit levels as measured by standardized laboratory procedure for each patient were recorded.

Gauss Surgical Inc. (Los Altos, CA) has developed a novel tablet computer application technology, Feature Extraction Technology (FET), that utilizes computer vision algorithms to assess hemoglobin concentration contained in used surgical sponges through photographic analysis. FET software first pre-processed the captured image to both isolates on the sponge surface and the blood-containing regions of interest within the sponge. Given that not all operating rooms contained the same ambient lighting conditions or backgrounds, the program then normalized all images so as to numerically minimize the effects of ambient light. From the pre-processed image, the software extracts a set of geometric and pixel-level features and utilizes a proprietary mathematical model that maps clusters of these features to known hemoglobin (Hb) mass values. These algorithms specifically account for variations in the saline volume absorbed on a sponge, which acts as a diluent and can modify the appearance of a sponge’s color.

During the procedures the surgical sponges were removed from the surgical field and photographed “live” (intra-operatively) using a portable tablet, equipped with the Triton system (Gauss Surgical Inc., Los Altos, CA) featuring FET technology, by members of the surgical team resulting in a measurement of hemoglobin mass contained within the sponges (Fig. 1). To operate the Triton System, the Triton application is installed onto the iPad 2 (Apple Inc., Cupertino, CA), a touch-screen operated mobile device with a front-facing camera which is mounted onto IV pole in the operating room. The Triton application captures scans of surgical sponges via the iPad’s built-in camera. The capture of each image can be actuated by several means, including a Bluetooth-connected foot-pedal or other methods. Scans of each sponge collected during the surgical case are available for review and editing as needed in real-time. These images are stored on both an onboard and a remote server for processing after the surgical case is completed. FET employs computer vision and machine-learning algorithms are used to automatically filter out the effects of non-sanguineous components absorbed in each sponge or mixed into each canister and computes Hb mass from the image in real time. The Hb mass on each sponge was measured and summed for each case.

Measures of surgical sponge estimated blood loss (EBL) from the Triton system were compared with the estimates of blood loss via the gravimetric method and photometric analysis which were processed as the following. After the images were scanned by the application, operative sponges were transported to a medical waste room for further assessment. Sponges were rinsed in heparinized solution and dried, with the effluent collecting in a separate container. The container was weighed (recorded in grams) and the hemoglobin concentration (recorded in g/ml) was measured with HemoCue Plasma/Low Hb photometric system (HemoCue, Brea, CA). Using conversion mathematics, we converted the mass of effluent into milliliters, assuming an average fluid density of 1.0 g/ml. The recovered mass of hemoglobin contained within the effluent was then calculated by multiplying the concentration of hemoglobin by the total volume of effluent collected. The recovered Hb mass was scaled by a factor of 85% to reflect the residual Hb on sponges, based on previously reported recovery rates of 82% to 93% in studies that employed similar methods to recover blood from cotton products [13-16]. This process was completed for each patient.

Since photometric analysis (also referred to as the assay method) is considered the current most accurate method to determine Hb concentration [11, 12] it was used as the reference method to which both the Triton method and Gravimetric method were compared to. To compute estimates of blood loss for the Triton system and the assay method, values of Triton Hb mass and Assay Hb Mass were divided by the recorded laboratory-derived subject Hb, resulting in values of Triton EBL and Assay EBL. One case was excluded from the analysis of gravimetric method because sponge weights were not recorded. Triton EBL and Gravimetric EBL were then compared to Assay EBL for both correlation and concordance (Bland-Altman).

Software application calculations were then compared to the photometric analysis to assess accuracy and reproducibility. No patient contact was necessary for data acquisition and analysis.

RESULTS

DISCUSSION

Our results showed a significant positive correlation between Triton tablet system and the referenced photometric analysis with respect to intra-operative hemoglobin and blood loss at 0.92 and 0.91, correlations respectively. Although there was a bias of 6.4 g of hemoglobin between the Triton system and photometric analysis, as evidenced through a Bland-Altman plot, this value approximates to one-tenth of a standard unit of whole blood (450 ml) with a hemoglobin concentration of 12.7 g/dl, which was the average Hb value in our study. This discrepancy is not clinically significant because 45 ml of blood represents 0.9% of the circulating volume of the standard 70kg male with five liters of intravascular volume.

When the tablet software and gravimetric method were compared to the photometric analysis for direct measurement of intraoperative blood loss, the tablet proved superior with 0.91 versus 0.66 correlations, respectively. As discussed above, confounding variables like serous bodily fluids, saline irrigation and solid tissue that can alter the “wet weight” of surgical sponges are the likely cause for the decreased correlation of the gravimetric method. Although the gravimetric method has proven to be less labored intensive than photometric analysis, our results show that it was significantly less accurate when compared to the tablet software, with biases of 283 ml versus 53 ml, respectively. This six-fold increase in bias makes the gravimetric method unreliable in the presence of a more accurate technique in the measurement of intraoperative blood loss.

Despite these methods for measuring hemoglobin and total blood loss, it is still commonplace to rely on visual estimation due to convenience and ease. This method has proven to be inaccurate and often underestimates the amount of blood loss in multiple studies, yet remains the standard in many operating rooms [6, 7, 9, 19]. It has been shown that simulations and didactic training can mildly improve estimation methods, however these benefits are short-lived and experience of the physician does not improve estimation values overall [3, 20].

One limitation of this study was the lack of a true “gold standard” in measuring blood loss, since the rinsing of sponges to assay Hb content (photometric analysis) does not guarantee 100% recovery of Hb. However, we attempted to control for the inherent bias of this method by (1) standardizing the rinsing protocol (cycle times, diluent volumes, centrifuge dimensions), and (2) adjusting the assayed values by a factor of 15% based on prior studies [13-17]. Such limitations notwithstanding, this novel tablet technology provides surgeons, anesthesiologists, and other operative personnel with a real-time measurement of blood loss so that intraoperative actions can be taken based on objective real time measurements rather than subjective analysis that can be confounded by numerous variables like intravenous fluid flow rate, amount and rate of flow of blood loss, intraoperative pharmaceuticals, and other factors that can directly impact the circulatory drive of the patient.

We, therefore, conclude that our results support the accuracy of this novel tablet software as a real-time measurement to accurately quantify intraoperative blood loss that has been gathered on surgical sponges. Further studies are currently underway using the Triton system for accurate assessment of hemoglobin content within suction canisters which will allow calculation of to total intraoperative blood loss.

References