2.1. Conventionally risk prediction scores
In 2008, Matthews et al. used multivariate logistic regression in patients whom underwent mostly pulsatile LVAD implantation [
56]. They defined right ventricular failure (rate 35%) as; the need for post-operative IV inotrope support for >14 days, inhaled nitric oxide for ≥48 hours, right-sided circulatory support or hospital discharge on an inotrope. The multivariate technique used in this study allowed the simultaneous evaluation of many risk factors that may suggest different results when evaluated separately. An elevated ALT, vasopressor requirement, high bilirubin and creatinine levels were predictors of RHF in multivariate analyses. Additionally, the Michigan right ventricular failure risk scoring (vasopressor requirement 4 points; AST ≥80 IU/l 2 points; bilirubin ≥2.0 mg/dl → 2.5 points, creatinine ≥2.3 mg/dl 3 points; renal replacement therapy 3 points) was formulated to better stratify RV failure risk in LVAD candidates. The Michigan right ventricular failure risk score (RVFRS) is the first model for pre-operative risk stratification of RV failure in LVAD candidates. RVFRS was ≤3.0 in 88% of patients who did not develop right ventricular failure (specificity = 88%). Right ventricular failure was developed in 80% of patients with RVFRS ≥5.5 (positive predictive value = 80%).
In the same year Fitzpatrick et al., reviewed patients whom underwent mainly pulsatile LVAD implantation [
57]. According to the findings on echocardiography performed during preoperative examination, right ventricular dysfunction (rate 37%) is divided into three classes: mild, moderate and severe. This study identified that preoperative low cardiac index (CI) (≤ 2.2l/min/m2) and right ventricle stroke work index (RVSWI) (≤ 0.25mmHg/l/m2), severe pre-VAD RV dysfunction, high creatinine level (≥1.9mg/dl), previous cardiac surgery, and hypotension (≤96mmHg) independently associated with the need for biventricular support. For each of these variables, if a patient met the “high risk” criteria, he or she was given a score of 1 for that variable. By calculating these risk factors together, Penn RVAD risk score was created: 18. (CI)+ 18. (RVSWI) + 17. (creatinine)+16. (previous cardiac surgery) +16. (RV dysfunction) +13. (systolic blood pressure). After analysis of this risk scoring, it was revealed that successful LVAD support was predicted in patients with a score below 50, while the probability of BiVAD placement was high in patients with a score of 50 and above (83% and 80% sensitivity and specificity, respectively).
In one of the previous study, which used multivariate logistic regression analysis, have pointed 3 preoperative factors that seemed were significantly associated with RVF after LVAD implantation: (1) need of intra-aortic balloon counterpulsation before the operation, (2) increased in pulmonary vascular resistance, and (3) the device implantation as a destination therapy [
58]. The authors defined RVF (rate 44%) as the need for inhaled NO for >48 hours, the need for IV inotropes for >14 days, and/or RVAD implantation. The risk score (Utah RV Risk Score) was calculated as the sum of the points assigned for the existence some perioperative variables. Intra-aortic balloon counterpulsation (IABP), 4 points; pulmonary vascular resistance (PVR), quartile 1 (≤1.7 Wood units), 1 point; quartile 2 (1.8 –2.7 Wood units), 2 points; quartile 3 (2.8 – 4.2 Wood units), 3 points; and quartile 4 (≥4.3 Wood units), 4 points; inotrope dependency, 2.5 points; DT patients were given 3.5 points; beta-blocker, 2 points. angiotensin-converting enzyme inhibitor and/or angiotensin II receptor blocker, -2.5 points; and obesity, 2 points. The developed RVF risk score effectively predicted the risk of RV failure. They also showed that the presence of perioperative RVF was strongly associated with postoperative mortality in patients undergoing LVAD implantation. Additionally, their results revealed a significant reduction in survival at days 30, 180, and 365 after LVAD implantation using the risk score model. The subgroup with a risk score of 12.5 had significantly lower 1-year survival compared to the subgroups with a lower risk score.
Kormos et al. evaluated the incidence, risk factors, and effect on outcomes of right ventricular failure in patients implanted with continuous-flow LVAD (HeartMate II) [
59]. RVF (rate 20%) is defined as RVAD that lasts >14 days after implantation, requires IV inotropes, and/or requires initiation of inotropic support more than 14 days after implantation. Multivariate analysis showed that central venous pressure/pulmonary capillary wedge pressure ratio (CVP/PCWP) greater than 0.63, need for preoperative ventilator support, and BUN level greater than 39 mg/dL were independent predictors of right ventricular failure after HeartMate II implantation. Authors also concluded that, rates of RVF and RVAD need in patients with the HeartMate II are low relative to previous results with pulsatile LVADs and support the use of new generation continuous flow device in end-stage heart failure.
In another study, Atluri et al. defined severe right ventricular dysfunction based on echocardiographic parameters, taking into account right ventricular contractility, tricuspid regurgitation, and tricuspid annular motion [
60]. In multivariate logistic regression analysis, CVP>15 mmHg, severe RV dysfunction, preoperative intubation, severe Tricuspid regurgitation and Tachycardia were determined to be major criteria predicting the need for biventricular support. Based on this analysis, they established the CRITT score as a predictor of RVF. In the “CRITT” score, each of the five variables is given a score of 0 or 1. Ninety-three percent of patients with a score of 1 or lower underwent successful isolated LVAD therapy. However, 80% of patients with a score of 4 or above required biventricular assistance. They concluded that patients with a score of 2 or 3 were in the gray zone and could undergo isolated LVAD under appropriate pharmacological or temporary RVAD support. The ability to quickly calculate the CRITT score at the bedside without the need for complex calculations is an advantage that increases its applicability.
Many of the first proposed RV risk scoring systems did not use detailed imaging parameters to aid risk stratification. Raina et al. combined echocardiographic variables such as right ventricular fractional area change (RV FAC), left atrial (LA) volume index, and estimated right atrial (RA) pressure into an echocardiographic scoring system to estimate RVF [
10]. Three points were assigned for LA volume index <38 mL/m
2, 2 points for RV FAC <31%, and 2 points for estimated RA pressure >8 mm Hg. Thus, a score scale that can range from 0 to 7 points emerged. Prolonged RVF after LVAD implantation (rate 29%) was defined as the need for inotropes for more than 14 days after LVAD implantation or the need for temporary RVAD placement. Initial BiVAD criteria (rate 23%) were stated as severe RV dysfunction on transthoracic echocardiography, severe pulmonary hypertension with PVR >5 WU or RAP >15 mmHg, and persistence of ventricular arrhythmias causing hemodynamic instability. When RVF and non-RVF groups were compared, the echocardiographic score was significantly higher in the RVF group. They concluded that, combining echocardiographic variables into a simple easily interpreted echocardiographic scoring system significantly improved prediction of RVF versus any one echocardiographic variable alone, and, importantly, the echocardiographic score remained predictive after inclusion of clinical variables in multivariate models, despite the small sample size in their study.
After a while, a study from Germany proposed the ARVADE score, which consists of echocardiographic parameters ([
61]. In this study, RVF (rate 57%) was defined as the need for placement of a temporary RVAD or the use of inotropic agents for at least 14 days. Multivariable analysis identified INTERMACS level 1, the Em/SLAT ratio ≥18.5 (Em: pulsed Doppler transmitral E wave; SLAT: tissue Doppler lateral systolic velocity), and the basal RVEDD ≥50 mm (right ventricular end-diastolic diameter) as independent predictors. An ARVADE score was calculated as the sum of points attributed according to the values of three variables: 3.0 points for Em/SLAT ≥ 18.5, 2.0 points for basal RVEDD≥50mm and 1.5 points for INTERMACS level 1. An ARVADE score >3.0 was predictive of post-implantation RVF, with a sensitivity of 89% and a specificity of 74%. Authors concluded that, the ARVADE score, calculated as the sum of scores for one clinical and three echocardiographic measures reflecting LV global systolic and diastolic dysfunction and RV congestion, may estimate suitability for LVAD implantation.
In 2018, Loforte et al. introduced a simple and easily memorized risk stratification tool (ALMA score) to determine whether isolated LVAD (continuous flow device) implantation could be tolerated in 258 patients from two centers [
62]. Patients in the BiVAD cohort included those who had sudden RVF after isolated LVAD implantation and required early application of a temporary or long-term RV assist device (RVAD). Severe RVF defined as receiving short- or long-term right-sided circulatory support despite maximal dosage of continuous inotropic support and NO ventilation. A five-point risk score was developed based on the clinical variables identified in the multivariate logistic regression analysis. Then, 1 or 0 point was assigned for each of the variables in the institutionally defined “ALMA” score: Destination therapy (DT) intention, pulmonary artery pulsatility index (PAPi) <2, right ventricular stroke work index (RVSWi) <300 mm Hg/ml/m2, RV/ LV ratio >0.75, and Model for End-Stage Liver Disease Excluding International Normalized Ratio (MELD-XI) score >17. The predicted rate of RVF was increased from 9% for a score of less than 2, to 57% for a score of 2–3, and to 100% for a score of 4–5. In the resulting ROC curves, a score of 3 points provided sensitivity and specificity higher than 80% for the entire cohort. Based on this model, the Authors recommended isolated LVAD implantation for patients with a score of 0 or 1 and BiVAD for patients with a score of 4 or 5. They placed patients with a score of 2 in the gray area and stated that LVAD implantation could be performed for these patients accompanied by appropriate pharmacological and/or temporary RVAD support or tricuspid valve repair.
Historically, older RVF risk scores were developed in the era of pulsatile flow LVADs. The lack of validation studies has made it difficult for these models to accurately predict RVF in the current continuous-flow LVAD population. To more accurately predict RVF, models that use retrospective, predominantly single-center, primarily continuous-flow LVAD data have been developed. However, a common shortcoming of both old and new risk scoring models is that they are subject to limited external validation and had modest predictive value.
In 2018 Soliman et al. developed and validated a simple score to predict early RHF after continuous-flow LVAD implantation in a large population from the EUROMACS Registry [
63]. Definition of RVF (rate 21.7%) was receiving short- or long-term RVAD support, continuous inotropic support for ≥14 days, and NO ventilation for ≥48 hours. They examined 82 potential preoperative predictors and CPB time as a operative variable for the association with RHF. The EUROMACS-RHF risk score is composed of severe RV dysfunction, 2 points; ratio of RA/PCWP ≥0.54, 2 points; advanced INTERMACS class 1-3, 2 points; need for ≥3 intravenous inotropes, 2.5 points; and hemoglobin ≤10 g/dL, 1 point. Composite 5-point score predicted early RHF after LVAD implantation; Moreover, as the score increased, the risk of both RHF and mortality increased. ROC curve analysis of the EUROMACS-RHF risk score was compared with previous risk scores and with individual known markers of RHF. They claimed that, The EUROMACS-RHF risk score outperformed previously published scores and known individual echocardiographic and hemodynamic markers of RHF. Finally, they validated the risk model in the validation cohort. The c index was 0.70 in the derivation versus 0.67 in the validation cohort. The Hosmer-Lemeshow goodness-of- fit P value was 0.61 in the validation cohort, which reflects an appropriate fit for the data in this cohort.
Early studies examining risk factors associated with RVF and developing various risk scores were generally based on the weighted sum of 4-7 risk factors contributing modest sensitivity or specificity. In addition, accurate prediction of patients at risk of RVF after LVAD implantation depends on the multidimensional and variable interaction of many perioperative variables that cannot be adequately captured by traditional multivariate modeling techniques. As a result, generalized recommendations for patient selection obtained from relatively small single-center patient groups have limited usefulness in practice.
Prediction models that we have summarized so far were conventional statistical analysis methods. As AI began proving itself more within healthcare, heart failure subgroups specific research increased as well, where considerable LVAD and heart transplant subjects related AI literature began populating journals more and more. Although studies mentioned above evaluated risk factors regarding post LVAD RHF, due to the fact that this is a multifactorial problem makes it especially hard to effectively investigate this issue properly through conventional means. It is for this reason that AI and ML enables a more comprehensive avenue of research on this topic.
2.2. AI based studies/risk scores
The use of Bayesian statistical modeling was proposed by Loghmanpour et al. to overcome the limited predictive capacity of risk scores obtained from existing multivariate analyses [
64]. This recommendation of the Authors is based on the hypothesis that it is essential to consider the relationships and conditional probabilities between independent variables to achieve satisfactory statistical accuracy. In this context, Bayesian Network (BN) algorithms can account for nonlinear interactions between variables by identifying groups of risk factors and their conditional interdependencies. The Bayesian models reported in this study are particularly suitable for combining large sets of risk factors because they are based on conditional probabilities of the likelihood of RVF for a given combination of interrelated variables. The authors suggested that these algorithms better reflect prioritization of dynamic clinical information when using data provided by the INTERMACS registry. To the authors' knowledge, this is the first report of a prognostic RVF model following continuous-flow LVAD using the INTERMACS database and adopting ML methods for statistical analysis. They extracted 34 preoperative variables from INTERMACS data base of 10909 patients from 2006 to 2014 in order to predict RVF after LVAD implantation. The definition for RVF was based on the INTERMACS definition prior to 2014. Overall 2024 patients were diagnosed with RVF (18.5%), 293 with acute (<48 hours after implant) RVF (2.7%), 1036 with early (48 hours to 14 days) RVF (9.5%), and n=695 with late onset (>14 days) RVF (6.4%). Systolic PAP, pre-albumin, LDH and RVEF parameters were found to have the most predictive value among all the preoperative variables. The authors acknowledge that a retrospective study with incomplete data is not ideal for a more detailed analysis where RVF severity could have been also considered. Patients already considered too risky for LVAD implantation for RVF possibility and thus never received LVAD were unavoidably omitted from data set, perhaps skewing results.They analyzed accuracy, area under the ROC curve (AUC), sensitivity and specificity, respectively. According to their findings, The AUC of the Bayesian model was 0.90 for acute RV failure, 0.84 for early RV failure, and 0.88 for late RV failure after LVAD implantation, significantly outperformed all previously published risk scores.
In a 2018 study, Samura et al. utilized a supervised ML model in order to predict right ventricular assist device (RVAD) requirement for patients that will undergo LVAD implantations [
65]. They used 42 preoperative clinical and hemodynamic parameters of 115 patients that proceeded to be implanted with a continuous flow LVAD between years 2013 and 2017. As a result of their study, 5 parameters were highlighted as having most predictive value, left ventricular end-diastolic dimension, left ventricular end-systolic dimension, left ventricular ejection fraction, etiology of dilated phase of hypertrophic cardiomyopathy, and less-distensible right ventricle. 8 different ML algorithms were tested in order to obtain the best results and they declared that a derived Naïve Bayes model achieved a great accuracy of 95 % and area under curve (AUC) of 0.85. Researchers concluded that this method was useful and feasible in order to preoperatively predict which patients would likely need RVAD implantation.
Bellavia et al. used ML approach to find out the association between regional right ventricular and right atrial strain for prediction of right ventricular failure in both early and late postoperatively period [
66]. Michigan risk score along with CVP and apical longitudinal systolic strain of the right ventricular free wall were found to be the most important predictors of acute RHF. For the chronic RHF, the most prominent predictors were right ventricular free wall systolic strain of the middle segment, right atrial strain and tricuspid annular plane systolic excursion.
Shad et al. used combining greyscale video data and optical flow streams from the video data with a three dimensional 152-layer deep learning ML algorithm. 1909 scans from 723 patients were evaluated in order to predict RHF development of LVAD patients [
67]. The researchers used two clinical risk score systems; CRITT and PENN scores to identified potential risky patients for RHF after LVAD implantation. Subsequently they compared their deep learning and ML systems performance of against to risk scores. The study included 941 LVAD patient; separated in two; group one (n:182) presented RHF, group two (n:541) without RHF. While the researchers checked the area under curve (AUC) they found that AUC of CRITT and PENN scores are 0,616 and 0,605 respectively. The AUC of their AI systems is 0,729 which means the newly developed deep learning system is presented more accuracy to predict of RHF. Although they worked with small and limited data set the results were challenging and on account of that they believed that AI will find wider working place in cardiovascular disease especially for doing prediction studies. They further argue that, when RVAD implantation is planned beforehand and perhaps concurrently with LVAD implantation as opposed to emergent RVAD implantation after patient condition deteriorates. Therefore, being able to predict eventual RHF development for patients before LVAD implantation will improve patient survivability.
In 2021, Kilic et al. utilized extreme gradient boosting which is an ensemble ML algorithm in order to investigate preoperative data association with postoperative adverse events which translates to 90 days and 1-year survival rates [
68]. This study involved 16120 patients from 170 centers where the data set was acquired from INTERMACS. It includes patient demographics, comorbidities, laboratory parameters, clinic visit measurements, interval events during hospitalization prior to LVAD insertion, and concomitant operative procedures. Post-LVAD data collected in INTERMACS includes adverse events and survival. Examples to these adverse events include thrombosis, RHF, infection and bleeding. Reportedly, end result of this study found that there was an improvement of 48.8% (p<0.001) in 90-day mortality prediction and 36.9% (p<0.001) improvement in 1-year mortality prediction with ML compared to usual logistic regression data analysis. ML models derived using the XGBoost algorithm were well calibrated and had improved discrimination over logistic regression. Based on these findings, they concluded that ML may have an important role in risk prediction in LVAD treatment, both independently and in addition to traditional modeling approaches such as logistic regression. Further study that focuses on specific adverse events prediction, such as RHF, may be conducted in order to better understand the underlying mechanisms of these clinical outcomes and which would translate to creating a better patient treatment plan accordingly.
Using the statistical computing tool called “R”, Kilic et al. evaluated data from ENDURANCE trials in 2020, which included 564 patients [
69]. This study aimed to analyze the risk of major adverse events after LVAD implantation and how they transitioned to each other. These events were device malfunction, bleeding, infection, neurological / renal / respiratory dysfunction and RHF. They identified that most common adverse events were bleeding and infection. Interestingly, they found that RHF is one of the top three adverse events, bleeding and infection being the other two, that leads to further adverse events most often. The highest transition probabilities were found to be infection to infection (0.34), bleeding to bleeding (0.31), RHF to bleeding (0.31), RHF to infection (0.28), and bleeding to infection (0.26). Additionally, they found that RHF has the lowest median time to first adverse event with 3.5 days. Highlighting the importance of RHF for overall mortality rates post LVAD implantation, Patients with RHF are shown to have 50% mortality rates. RHF was also identified to be significantly linked with bleeding and infections, which it then follows that RHF prediction is vital to successful long term LVAD patient survivability.
2550 patient data from International Registry for Mechanically Assisted Circulatory Support (IMACS) database were utilized by Nayak et al. in 2022 in order to analyze 41 pre-implant variables of patients with acute post LVAD RHF [
70]. An unsupervised ML model was used in their work, identifying 4 RHF phenotypes, where severe shock phenotype had worst clinical outcomes. Ischemic cardiomyopathy (ICM) with low grade shock and non ICM without shock were two other phenotypes identified. Best clinical outcomes were observed with ICM without shock phenotype. The notion of classifying patients into phenotypes may prove useful for future researches as applying separate ML based prediction or analysis models to significantly differing pathophysiologies for RHF could improve predictive capabilities of pre-implant evaluations overall.
The study that designed by Bahl et al. was one of the newest studies which focused on ML and RHF [
71]. They preferred an “explainable” ML method called Boosted Decision Trees for analyze the preimplant patient factors in nonlinear interactions with RHF after LVAD implantation. The study includes the patients in INTERMACS registry who were implanted with their first durable LVAD between 2008 to 2017. A total of 186 potential risk factors were analyzed from 19,595 patients as unbiased and comprehensively as possible. This study was aimed at better quantifying and understanding how different clinical variables impact each other and the complex mechanism that leads to RHF after LVAD implantation. The study showed that in 19.1% of patients severe RHF was developed within the first 30 days. Thirty top predictors of RHF were identified. INTERMACS profile, Model for End-stage Liver Disease score, the number of inotropic infusions, hemoglobin, and race were the first five top factors. Additionally, many of these top factors showed nonlinear relationships with key risk inflection points such as INTERMACS profile 2-3, right atrial pressure of 15 mmHg, pulmonary artery pressure index of 3, and prealbumin of 23mg/dl. They claimed that, ML offers a number of algorithms that are far more flexible and are well equipped for high dimensional, nonlinear, and interacting relationships. They also believed that this study could open new era for researchers to formulate patient optimization strategies before LVAD implantation.
Using a Convolutional Neural Network (U-Net), Just et al. evaluated preoperative CT scan data of 137 patients in order to assess their body composition and then predict postoperative major complications after LVAD implantation [
72]. Body composition evaluation included visceral and subcutaneous adipose tissue areas, psoas and total abdominal muscle areas and sarcopenia. The body composition parameters were correlated with postoperative major complication rates, such as postoperative infections, in hospital mortality and overall quality of life. They found that Adipose tissue distribution / concentration was a good predictor of postoperative infections, in-hospital mortality, impaired 6-minute walking distance and quality of life within 6 months postoperatively. While the study focuses on all cause related outcome prediction, RHF is one of the poor outcome classes that is present in the data set. Therefore, a focus study on the usefulness of AI in RHF prediction using a similar data set might be warranted. Method and performance summary of reviewed publications presented in
Table 1.