Improved waitlist and comparable post-transplant outcomes in simultaneous heart-kidney transplantation under the 2018 heart allocation system.

OBJECTIVE: This study aimed to investigate the clinical trends and the impact of the 2018 heart allocation policy change on both waitlist and post-transplant outcomes in simultaneous heart-kidney transplantation in the United States. METHODS: The United Network for Organ Sharing registry was queried to compare adult patients before and after the allocation policy change. This study included 2 separate analyses evaluating the waitlist and post-transplant outcomes. Multivariable analyses were performed to determine the 2018 allocation system's risk-adjusted hazards for 1-year waitlist and post-transplant mortality. RESULTS: The initial analysis investigating the waitlist outcomes included 1779 patients listed for simultaneous heart-kidney transplantation. Of these, 1075 patients (60.4%) were listed after the 2018 allocation policy change. After the policy change, the waitlist outcomes significantly improved with a shorter waitlist time, lower likelihood of de-listing, and higher likelihood of transplantation. In the subsequent analysis investigating the post-transplant outcomes, 1130 simultaneous heart-kidney transplant recipients were included, where 738 patients (65.3%) underwent simultaneous heart-kidney transplantation after the policy change. The 90-day, 6-month, and 1-year post-transplant survival and complication rates were comparable before and after the policy change. Multivariable analyses demonstrated that the 2018 allocation system positively impacted risk-adjusted 1-year waitlist mortality (sub-hazard ratio, 0.66, 95% CI, 0.51-0.85, P < .001), but it did not significantly impact risk-adjusted 1-year post-transplant mortality (hazard ratio, 1.03; 95% CI, 0.72-1.47, P = .876). CONCLUSIONS: This study demonstrates increased rates of simultaneous heart-kidney transplantation with a shorter waitlist time after the 2018 allocation policy change. Furthermore, there were improved waitlist outcomes and comparable early post-transplant survival after simultaneous heart-kidney transplantation under the 2018 allocation system.

Donation after circulatory death improves probability of heart transplantation in waitlisted candidates and results in post-transplant outcomes similar to those achieved with brain-dead donors.

OBJECTIVE: To quantitate the impact of heart donation after circulatory death (DCD) donor utilization on both waitlist and post-transplant outcomes in the United States. METHODS: The United Network for Organ Sharing database was queried to identify all adult waitlisted and transplanted candidates between October 18, 2018, and December 31, 2022. Waitlisted candidates were stratified according to whether they had been approved for donation after brain death (DBD) offers only or also approved for DCD offers. The cumulative incidence of transplantation was compared between the 2 cohorts. In a post-transplant analysis, 1-year post-transplant survival was compared between unmatched and propensity-score-matched cohorts of DBD and DCD recipients. RESULTS: A total of 14,803 candidates were waitlisted, including 12,287 approved for DBD donors only and 2516 approved for DCD donors. Overall, DCD approval was associated with an increased sub-hazard ratio (HR) for transplantation and a lower sub-HR for delisting owing to death/deterioration after risk adjustment. In a subgroup analysis, candidates with blood type B and status 4 designation received the greatest benefit from DCD approval. A total of 12,238 recipients underwent transplantation, 11,636 with DBD hearts and 602 with DCD hearts. Median waitlist times were significantly shorter for status 3 and status 4 recipients receiving DCD hearts. One-year post-transplant survival was comparable between unmatched and propensity score-matched cohorts of DBD and DCD recipients. CONCLUSIONS: The use of DCD hearts confers a higher probability of transplantation and a lower incidence of death/deterioration while on the waitlist, particularly among certain subpopulations such as status 4 candidates. Importantly, the use of DCD donors results in similar post-transplant survival as DBD donors.

Utilization of cardiac graft with single coronary artery for orthotopic heart transplantation.

BACKGROUND: Anomalous coronary arteries arise in a small subset of the population, with each configuration conveying a varying degree of long-term risk. The utilization of cardiac grafts with these anomalies have not been well described. CASE PRESENTATION: An anomalous single coronary artery with the left main coronary artery arising from the right coronary ostium was discovered in a 40-year old male evaluated for cardiac donation. After evaluation, this heart was successfully procured and utilized for orthotopic heart transplantation. CONCLUSION: In this report, we demonstrate that in select cases, a cardiac graft with single coronary artery anatomy can be successfully procured and transplanted with excellent outcomes.

Prospective Evaluation of Coronavirus Disease 2019 (COVID-19) Vaccine Responses Across a Broad Spectrum of Immunocompromising Conditions: the COVID-19 Vaccination in the Immunocompromised Study (COVICS).

BACKGROUND: We studied humoral responses after coronavirus disease 2019 (COVID-19) vaccination across varying causes of immunodeficiency. METHODS: Prospective study of fully vaccinated immunocompromised adults (solid organ transplant [SOT], hematologic malignancy, solid cancers, autoimmune conditions, human immunodeficiency virus [HIV]) versus nonimmunocompromised healthcare workers (HCWs). The primary outcome was the proportion with a reactive test (seropositive) for immunoglobulin G to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor-binding domain. Secondary outcomes were comparisons of antibody levels and their correlation with pseudovirus neutralization titers. Stepwise logistic regression was used to identify factors associated with seropositivity. RESULTS: A total of 1271 participants enrolled: 1099 immunocompromised and 172 HCW. Compared with HCW (92.4% seropositive), seropositivity was lower among participants with SOT (30.7%), hematological malignancies (50.0%), autoimmune conditions (79.1%), solid tumors (78.7%), and HIV (79.8%) (P < .01). Factors associated with poor seropositivity included age, greater immunosuppression, time since vaccination, anti-CD20 monoclonal antibodies, and vaccination with BNT162b2 (Pfizer) or adenovirus vector vaccines versus messenger RNA (mRNA)-1273 (Moderna). mRNA-1273 was associated with higher antibody levels than BNT162b2 or adenovirus vector vaccines after adjusting for time since vaccination, age, and underlying condition. Antibody levels were strongly correlated with pseudovirus neutralization titers (Spearman r = 0.89, P < .0001), but in seropositive participants with intermediate antibody levels, neutralization titers were significantly lower in immunocompromised individuals versus HCW. CONCLUSIONS: Antibody responses to COVID-19 vaccines were lowest among SOT and anti-CD20 monoclonal recipients, and recipients of vaccines other than mRNA-1273. Among those with intermediate antibody levels, pseudovirus neutralization titers were lower in immunocompromised patients than HCWs. Additional SARS-CoV-2 preventive approaches are needed for immunocompromised persons, which may need to be tailored to the cause of immunodeficiency.

The predictive value of preimplant pulmonary function testing in LVAD patients.

BACKGROUND: The predictive value of preoperative pulmonary function testing (PFT) in left ventricular assist device (LVAD) patients remains unknown. This study evaluates the relationship between abnormal PFTs and postimplant outcomes in LVAD patients. METHODS: LVAD implants from January 2004 to December 2018 at a single institution were included. Patients were stratified based on the presence of abnormal preoperative PFTs, and the primary outcome was respiratory adverse events (AE). Secondary outcomes included 1-year overall postimplant survival, and complications including bleeding, renal failure, thromboembolism, and device malfunction. RESULTS: The total of 333 patients underwent LVAD implant, 46.5% (n = 155) with normal PFTs and 53.5% (n = 178) with abnormal PFTs. Patients with abnormal PFTs were noted to have higher rates of respiratory AEs (25.9% vs. 15.1%, p = .049). In multivariable analysis, the impact of PFTs was most significant when forced expiratory volume in 1 s/forced expiratory volume (FEV1/FVC) ratio was less than 0.5 (hazard ratio [HR] 16.32, 95% confidence interval [CI], 1.70-156.78). The rates of other AEs including bleeding, renal failure, right heart failure, and device malfunction were similar. One-year overall postimplant survival was comparable between the groups (56.8% vs. 68.8%, p = .3183), though patients in the lowest strata of FEV1 (<60% predicted) and FEV1/FVC (<0.5) had elevated risk-adjusted hazards for mortality (HR 2.63, 95% CI, 1.51-4.60 and HR 18.92, 95% CI, 2.10-170.40, respectively). CONCLUSIONS: The presence of abnormal preoperative PFTs is not prohibitory for LVAD implantation although it can be used for risk stratification for respiratory AEs and mortality, particularly in patients with severely reduced metrics. The importance of careful patient selection should be underscored in this higher risk patient subset.

Twenty-year survival following orthotopic heart transplantation in the United States.

BACKGROUND: This study evaluated 20-year survival after adult orthotopic heart transplantation (OHT). METHODS: The United Network of Organ Sharing Registry database was queried to study adult OHT recipients between 1987 and 1998 with over 20-year posttransplant follow-up. The primary and secondary outcomes were 20-year survival and cause of death after OHT, respectively. Multivariable logistic regression was used to identify significant independent predictors of long-term survival, and long-term survival was compared among cohorts stratified by number of predictors using Kaplan Meier survival analysis. RESULTS: 20,658 patients undergoing OHT were included, with a median follow-up of 9.0 (IQR, 3.2-15.4) years. Kaplan-Meier estimates of 10-, 15-, and 20-year survival were 50.2%, 30.1%, and 17.2%, respectively. Median survival was 10.1 (IQR, 3.9-16.9) years. Increasing recipient age (>65 years), increasing donor age (>40 years), increasing recipient body mass index (>30), black race, ischemic cardiomyopathy, and longer cold ischemic time (>4 h) were adversely associated with a 20-year survival. Of these 6 negative predictors, presence of 0 risk factors had the greatest 10-year (59.7%) and 20-year survival (26.2%), with decreasing survival with additional negative predictors. The most common cause of death in 20-year survivors was renal, liver, and/or multisystem organ failure whereas graft failure more greatly impacted earlier mortality. CONCLUSIONS: This study identifies six negative preoperative predictors of 20-year survival with 20-year survival rates exceeding 25% in the absence of these factors. These data highlight the potential for very long-term survival after OHT in patients with end-stage heart failure and may be useful for patient selection and prognostication.

Nationwide variability in the use of induction immunosuppression for adult heart transplantation.

BACKGROUND: Institutional factors have been shown to impact outcomes following orthotopic heart transplantation (OHT). This study evaluated center variability in the utilization of induction therapy for OHT and its implications on clinical outcomes. METHODS: Adult OHT patients between 2010 and 2018 were identified from the United Network for Organ Sharing registry. Transplant centers were stratified based on their rates of induction therapy utilization. Mixed-effects logistic regression models were created with drug-treated rejection within 1 year as primary endpoint and individual centers as a random parameter. Risk-adjusted Cox regression was used to evaluate patient-level mortality outcomes. RESULTS: In 17,524 OHTs performed at 100 centers, induction therapy was utilized in 48.6% (n = 8411) with substantial variability between centers (interquartile range, 21.4%-79.1%). There were 36, 30, and 34 centers in the low (<29%), intermediate (29%-66%), and high (>67%) induction utilization terciles groups, respectively. Induction therapy did not account for the observed variability in the treated rejection rate at 1 year among centers after adjusting for donor and recipient factors (p = .20). No differences were observed in postoperative outcomes among induction utilization centers groups (all, p > .05). Furthermore, there was a weak correlation between the percentage of induction therapy utilization at the center-level and recipients found to have moderate (r = .03) or high (r = .04) baseline risks for acute rejection at 1 year. CONCLUSIONS: This analysis demonstrates that there is substantial variability in the use of induction therapy among OHT centers. In addition, there was a minimal correlation with baseline recipient risk or 1-year rejection rates, suggesting a need for better-standardized practices for induction therapy use in OHT.

Sex-Based Heart Transplant Outcomes After Bridging With Centrifugal Left Ventricular Assist Devices.

BACKGROUND: Prior studies demonstrated that female sex is associated with an increased mortality after orthotopic heart transplantation (OHT). The impact of sex on OHT outcomes after bridging with newer-generation durable left ventricular assist devices (LVADs) remains unclear. METHODS: The United Network for Organ Sharing database was queried to study OHT recipients bridged with a newer-generation LVAD (ie, HeartMate III or HeartWare) between 2010 and 2018. The primary outcome was mortality at 30 and 90-days and 1-year. Secondary outcomes included rates of posttransplant complications. Propensity score matching and Cox multivariable analysis were used to assess comorbidity-adjusted sex differences in outcomes. RESULTS: A total of 3010 patients (76.7% male) bridged with newer-generation LVADs underwent OHT. After adjusting for relevant covariates, both age and heart failure etiology, but not sex, were independent predictors of mortality. In the matched cohorts, sex did not affect posttransplant outcomes, including renal failure, cerebrovascular events, allograft rejection, functional status, or mortality (all P > .05). Survival at 1-year after OHT was 90.5% in males and 92.8% in females (P = .058). CONCLUSIONS: Among 3010 OHT recipients, matched females bridged with newer-generation HeartWare or HeartMate III LVADs have comparable posttransplant outcomes compared with males. Furthermore, survival at 1-year follow-up was not affected by sex; instead, it was driven by well-established risk factors including increased age, worse preoperative renal function, and heart failure etiology. These data suggest that considerable progress has been made in mitigating sex differences in heart failure outcomes in the modern era.

Temporary left ventricular assist devices as a bridge to heart transplantation.

BACKGROUND: To create equitable access to donor organs for the highest mortality patients, the cardiac transplant allocation system now prioritizes patients with surgically implanted temporary left ventricular assist devices (T-LVADs). The outcomes following a direct bridge from a T-LVAD to orthotopic heart transplant (OHT) are not well delineated. AIM: This study investigates the T-LVAD waitlist outcomes and compares the posttransplant outcomes in patients bridged to OHT with surgically implanted T-LVADs to patients bridged with durable continuous-flow left ventricular assist devices (CF-LVADs). METHODS: Adults recorded in the United Network for Organ Sharing registry bridged to OHT with a durable CF-LVAD and T-LVADs, with or without temporary right ventricular assist devices (T-RVADs), between 2010 and 2018 were included. Propensity matching and multivariable Cox regression were utilized to compare outcomes. RESULTS: Of 504 patients waitlisted with T-LVADs, the majority were transplanted (50%), bridged to CF-LVAD (17%), or recovered (9%). A total of 9047 recipients were bridged to OHT during the study period with 8875 CF-LVADs and 172 T-LVADs. Early survival in propensity-matched T-LVAD ± T-RVAD patients was similar to CF-LVAD ± T-RVAD patients but reduced at a 1-year follow-up. This difference in survival at 1-year follow-up was attributable to significantly reduced survival in patients with combined T-LVAD + T-RVAD support when compared with CF-LVAD, isolated T-LVAD and combined CF-LVAD + T-RVAD support (80% vs 90% vs 90% vs 91%; P = .005). CONCLUSIONS: This study demonstrates that most patients waitlisted with a T-LVAD are successfully bridged to durable therapy or recover, and those bridged to OHT have acceptable posttransplant outcomes, particularly when T-RVADs are not required.

Concomitant abdominal exploration with durable left ventricular assist device implantation.

A 45-year-old male presented in decompensated heart failure from nonischemic cardiomyopathy and was implanted with a right transaxillary intra-aortic balloon pump. He was listed for heart transplantation but after 9 days, he developed abdominal pain with evidence of embolization on computed tomography scan despite a well-positioned balloon pump. He underwent concomitant abdominal exploration with small bowel resection and durable left ventricular assist device implantation. He recovered well and was discharged home. This case highlights the challenges of decision-making in the current era where intra-aortic balloon pumps are being utilized more frequently as a bridge to heart transplantation.

In-hospital outcomes of a minimally invasive off-pump left thoracotomy approach using a centrifugal continuous-flow left ventricular assist device.

BACKGROUND: Minimally invasive left thoracotomy (MILT) and off-pump implantation strategies have been anecdotally reported for implantation of the HeartWare ventricular assist device (HVAD). We analyzed our experience with off-pump MILT implantation techniques and compared early in-hospital outcomes with conventional on-pump sternotomy (CS) implantation strategy. METHODS: Between January 2013 and February 2014, 51 patients underwent HVAD implantation and were included in this study. Thirty-three patients had CS, whereas 18 patients underwent off-pump MILT. To compare outcomes of these techniques, a multivariate analysis using propensity score modeling was performed after adjusting for age, INTERMACS, Kormos and Leitz-Miller (LM) scores. RESULTS: Mean age at implant was 57 (range 18 to 69) years, and overall in-hospital mortality was 8%. Univariate analysis revealed a statistically significant reduction in days on inotropes (p = 0.04), and a trend toward reduced intra-operative blood product administration (p = 0.08) in the MILT group. There was no difference in intensive-care-unit length of stay (p = 0.5), total length of stay (p = 0.76), post-operative blood product administration (p = 0.34) and total time on mechanical ventilation (p = 0.32). After adjusting for age, INTERMACS profile and Kormos and LM scores, no statistically significant differences were observed between the MILT and CS groups. CONCLUSIONS: An off-pump MILT implantation strategy can be utilized as a safe surgical approach for patients undergoing HVAD implantation. Further large collaborative studies are needed to identify advantages of the MILT approach.

Perioperative blood product use: a comparison between HeartWare and HeartMate II devices.

BACKGROUND: The HeartWare (HW) (Framingham, MA) and the HeartMate II (HM II) (Thoratec Inc, Pleasanton, CA) continuous-flow left ventricular assist devices (CF-LVADs) are commonly used to bridge patients to transplantation. We hypothesized that there are differences in perioperative blood product (BP) use and chest tube (CT) output between CF-LVAD types. METHODS: We retrospectively evaluated BP use in 71 patients who were implanted with a CF-LVAD (HM II = 38; HW = 33) by median sternotomy for bridge to transplantation (BTT) indications from 2009 to 2013. Detailed BP use data were collected during the intraoperative and postoperative periods and included packed red blood cells, platelets, fresh frozen plasma, and cryoprecipitate. RESULTS: Preoperative characteristics (age, left ventricular ejection fraction, previous sternotomy, ischemic cause), and risk stratification scores (Interagency Registry for Mechanically Assisted Circulatory Support [INTERMACS]) profile, Leitz-Miller score, Kormos score) were comparable between groups (all p > 0.05). Total average intraoperative and postoperative BP use was different between device types: HW = 8.3 ± 13 versus HM II = 12.6 ± 14.0 units (p = 0.002) and HW = 6.1 ± 12.0 units compared with HM II = 13.5 ± 24.1 units (p = 0.022), respectively. Average postoperative CT output for HW (3,231 ± 3,648 mL) and HM II (3,463 ± 3,050) (p < 0.008) were different between device types. Multivariate analysis revealed that a higher preoperative Leitz-Miller score, implantation of an HM II CF-LVAD, previous sternotomy, and a longer duration of cardiopulmonary bypass (CPB) time were independently associated with increased need for BP use, whereas only use of the HM II device and a longer bypass time predicted a greater CT output. CONCLUSIONS: Compared with HM II, implantation of the HW CF-LVAD was associated with reduced intraoperative and postoperative BP use and decreased CT output. Increased awareness of device-related differences in bleeding and BP use may improve CF-LVAD patient outcomes.

Does renal dysfunction and method of bridging support influence heart transplant graft survival?

BACKGROUND: Renal insufficiency is common in status 1B patients supported with inotropes or a continuous flow left ventricular device (CF-LVAD) as a bridge to heart transplantation. We evaluated the association of renal function and inotrope versus CF-LVAD support on posttransplant graft survival in status 1B patients. METHODS: The Scientific Registry for Transplant Recipients database was analyzed for posttransplant survival in status 1B patients bridged with inotropes or CF-LVAD who underwent transplantation between 2003 and 2012. Pretransplant renal function was measured by estimating glomerular filtration rate (GFR) and was stratified as less than 45 mL · min(-1) · 1.73 m(-2), 45 to 59, and 60 or greater. Univariate Kaplan-Meier and multivariate Cox regression models were used to evaluate the main effects of GFR strata and inotropes versus CF-LVAD, and the interaction effect of GFR strata by CF-LVAD, on graft survival. RESULTS: This study included 4,158 status 1B patients (74% male, aged 53 ± 12 years). Of those, 659 patients had a CF-LVAD (HeartMate-II [Thoratec, Pleasanton, CA], n = 638; HVAD [HeartWare, Framingham, MA], n = 21), and 3,530 were receiving inotropes (31 CF-LVAD patients were also receiving inotropes). Kaplan-Meier analyses demonstrated reduced graft survival (p = 0.022) in patients with pretransplant GFR less than 45 versus GFR 45 to 59 (p = 0.062) and versus GFR 60 or greater (p = 0.007), and no effect of inotrope versus CF-LVAD support on graft survival (p = 0.402). Multivariate analysis demonstrated that, after adjusting for the main effects of GFR stratum, CF-LVAD, and inotropes, status 1B patients bridged with a CF-LVAD and GFR in the lowest stratum had reduced graft survival (interaction effect p = 0.040). CONCLUSIONS: Pretransplant renal insufficiency was associated with reduced posttransplant graft survival in status 1B patients. This risk is increased for patients bridged with a CF-LVAD (versus inotropes) who have GFR in the lowest stratum.

Left thoracotomy HeartWare implantation with outflow graft anastomosis to the descending aorta: a simplified bridge for patients with multiple previous sternotomies.

Advances in mechanical circulatory support have been critical in bridging patients awaiting heart transplantation. In addition, improvement in device durability has enabled left ventricular assist device therapy to be applied as destination therapy in those not felt to be transplant candidate. Because of the increasing complexity of patients, there continues to be a need for alternative strategies for device implantation to bridge high-risk patients awaiting heart transplantation, wherein the risks of numerous previous sternotomies may be prohibitive. We present a unique technique for placement of the HeartWare ventricular assist device via left anterior thoracotomy to the descending aorta in a patient awaiting heart transplantation with a history of multiple previous sternotomies.

Mechanical circulatory support and heart transplantation: donor and recipient factors influencing graft survival.

BACKGROUND: Mechanical circulatory support is an accepted strategy to bridge patients to heart transplantation (HTx). Among mechanical circulatory support patients who go on to HTx, factors associated with improved graft survival have not been fully elucidated. METHODS: Using the Scientific Registry for Transplant Recipients, we identified adults who were treated with a left ventricular assist device (LVAD) or total artificial heart (TAH) before HTx. Kaplan-Meier and multivariate Cox regression models were used to identify patient, donor, and device characteristics associated with graft survival. RESULTS: Between January 1997 and February 2012, 2,785 adults underwent HTx. Before HTx, 2,674 patients were treated with a LVAD (HeartMate XVE, 724; HeartMate II, 1,882; HeartWare, 68), and 111 were treated with a TAH. Follow-up averaged 25 ± 24 months. Gender mismatch occurred in 23%. Graft survival did not differ between LVAD groups (all p > 0.168), but TAH was associated with reduced graft survival compared with LVADs (p < 0.001). After controlling for device type (LVAD vs TAH), lower recipient pulmonary vascular resistance, shorter ischemic time, younger donor age, donor-to-recipient gender match, and higher donor-to-recipient body mass index ratio were independent predictors of longer graft survival (all p < 0.05). CONCLUSIONS: TAH was associated with reduced graft survival after transplant, and survival did not differ between the LVAD device groups. Additional variables that were independently associated with graft survival were donor age, recipient peripheral vascular resistance, ischemic time, gender match, and donor-to-recipient body mass index ratio. Recognition of these factors may inform decisions regarding device support and donor suitability.

Benefits of ambulatory axillary intra-aortic balloon pump for circulatory support as bridge to heart transplant.

OBJECTIVE: Axillary intra-aortic balloon pump therapy has been described as a bridge to transplant. Advantages over femoral intra-aortic balloon pump therapy include reduced incidence of infection and enhanced patient mobility. We identified the patients who would benefit most from this therapy while awaiting heart transplantation. METHODS: We conducted a single-center, retrospective observational study to evaluate outcomes from axillary intra-aortic balloon pump therapy. These included hemodynamic parameters, duration of support, and success in bridging to transplant. We selected patients on the basis of history of sternotomy, elevated panel-reactive antibody, and small body habitus. Patients were made to ambulate aggressively beginning on postoperative day 1. RESULTS: Between September 2007 and September 2010, 18 patients underwent axillary intra-aortic balloon pump therapy. All patients had the devices placed through the left axillary artery with a Hemashield side graft (Boston Scientific, Natick, Mass). Before axillary placement, patients underwent femoral placement to demonstrate hemodynamic benefit. Duration of support ranged from 5 to 63 days (median = 19 days). There was marked improvement in ambulatory potential and hemodynamic parameters, with minimal blood transfusion requirements. There were no device-related infections. Some 72% of the patients (13/18) were successfully bridged to transplantation. CONCLUSIONS: Axillary intra-aortic balloon pump therapy provides excellent support for selected patients as a bridge to transplant. The majority of the patients were successfully bridged to transplant and discharged. Although this therapy has been described in previous studies, this is the largest series to incorporate a regimen of aggressive ambulation with daily measurements of distances walked.