Coronary artery bypass graft surgery (CABG) remains the standard of treatment for selected patients with coronary artery disease (1). Significant long-term angiographic and clinical benefits of CABG have been attributed to the left internal mammary artery (LIMA)-to-left anterior descending artery (LAD) graft, with historical studies demonstrating survival benefits over long-term follow-up beyond 15-years (2,3). Histopathological studies have identified particular characteristics of the LIMA endothelium, such as fewer fenestrations, lower intercellular junction permeability, greater anti-thrombotic molecules and higher nitric oxide production, all of which make LIMA more resistant to atherosclerosis compared to saphenous vein conduits (4). The superiority of the LIMA-to-LAD graft is one of the main indications for surgical revascularization in the current era of drug-eluting stents (5).
Over the past two decades, minimally invasive surgery has evolved to influence many surgical specialties, including both cardiac and thoracic surgery (6-8). Complex cardiac operations such as mitral valve surgery and CABG have been performed with safety and efficacy through smaller incisions compared to the conventional sternotomy approach (7). Since the 1990s, robotic systems have been developed to facilitate minimally invasive cardiac surgery. Historical operating systems such as the Automated Endoscopic System for Optical Positioning (AESOP) and the Zeus Robotic Surgical System have evolved to the current da Vinci Surgical System (Intuitive Surgical, Inc., Sunnyvale, CA, USA), which was approved for use by the Food and Drug Administration (FDA) in 2000.
Despite encouraging reports from small retrospective institutional studies, there remains a paucity of robust clinical data on robotic-assisted CABG procedures. Heterogeneous classification of ‘robotic CABG’ techniques and non-standardized reporting of endpoints have further hindered any meaningful analysis of the existing literature. The current systematic review aimed to assess the clinical outcomes of robotic-assisted CABG procedures according to three defined surgical techniques: totally endoscopic coronary artery bypass without cardiopulmonary bypass (TECAB off-pump); TECAB with cardiopulmonary bypass (TECAB on-pump); and robotic-assisted LIMA harvesting followed by off-pump manual anastomosis of LIMA-to-LAD through minimally invasive direct coronary artery bypass (robotic MIDCAB). The primary endpoint was perioperative mortality, and secondary endpoints included perioperative morbidities, anastomotic complications and long-term survival.
Search strategy and study selection
Electronic searches were performed using Medline, EMBASE and Central Register of Controlled Trials from their dates of inception to January 2016. The search terms (“CABG” or “coronary artery bypass”) and (“robotic OR robot”) were combined as both keywords and MeSH terms. This was supplemented by manually searching the reference lists of key reviews and all potentially relevant studies. Two reviewers (S.V. and P.I.) independently screened the title and abstract of records identified in the search. Full-text publications were subsequently reviewed separately if either reviewer considered the manuscript as potentially eligible for inclusion. Disagreements regarding study selection were resolved by discussion and consensus.
Selected studies included those reporting peri-operative mortality after robotically assisted CABG procedures. These studies were categorized according to surgical techniques as (I) TECAB off-pump; (II) TECAB on-pump; (III) robotic MIDCAB. Studies that reported a mixture of surgical techniques were included for statistical analysis only when separate datasets were reported for patients in each group. All publications were limited to those involving human subjects and written in English, and studies with fewer than ten patients were excluded. When duplicated studies with accumulating numbers of patients or increased lengths of follow-up were identified, only the most complete reports were included for assessment.
Following intubation with a dual-lumen endotracheal tube, the patient was positioned supine with the left side elevated to 30 degrees and the left arm remaining at the side. With selective ventilation of the right lung and carbon dioxide insufflation of the left pleural space, three ports were inserted into the left thorax to optimize visualization of the surgical field and to maximize the range of motion for the robotic arms. Exact positioning of the ports depended on the surgeon’s preference, target vessels, and patient body habitus. The robotic-assisted LIMA harvest was then performed via the two working ports under endoscopic vision through the third port. Upon completion of LIMA harvest and systemic heparinization, occluding bulldog clamps were be applied to the proximal LIMA, prior to grafting through a totally endoscopic approach, including either TECAB on-pump or TECAB off-pump, or via a left anterior mini-thoracotomy by robotic MIDCAB.
Cardiopulmonary bypass was typically established via femoral venous and femoral or axillary arterial cannulation. Aortic occlusion could be achieved by an endovascular occluding balloon, placed and inflated in the ascending aorta under transesophageal ultrasound guidance. Cardioplegia could be delivered directly into the aortic root through a distal channel in the endoballoon. Alternatively, an endoscopic trans-aortic clamp could be applied via a port site in the chest wall and antegrade cardioplegia delivered via an endoscopically-placed vent needle in the proximal ascending aorta.
Following conduit harvest and preparation, a fourth port was inserted in the left subcostal or subxiphoid plane for the insertion of tissue stabilizing devices. The da Vinci telemanipulation system has a range of Endowrist instruments, (Intuitive Surgical Services, Sunnyvale, CA, USA) including a tissue stabilizer that could be docked to and manipulated by the robotic system. Other endoscopic tissue stabilizers include the Octopus Stabilizer, the CTS system (CardioThoracic Systems Inc., Cupertino, CA, USA), or the Elite Endoscopic Stabilizer (Genzyme Surgical Products, Fall River, MA, USA). Positioning devices such as the Starfish NS (Medtronic Inc., Minneapolis, MN, USA) could be used to manipulate and position the heart for multi-vessel anastomosis.
Following the harvest of the LIMA as described above, the robotic system was undocked and CO2 insufflation ceased to allow the heart to return to its natural position. An endoscope was used to identify the intended site of anastomosis and a spinal needle could be inserted through the chest wall to identify the precise location for the mini-thoracotomy incision. Alternatively, the previous port sites could be extended directly. A soft tissue retractor was used to provide exposure through the interspace, and the anastomosis was performed using endoscopic tissue stabilizers and standard off-pump anastomotic techniques. To maintain a consistency of surgical techniques, only off-pump robotic MIDCAB studies were included for detailed statistical analysis in the present systematic review.
Baseline characteristics and operative details were presented as raw values (%), mean ± standard deviation or median unless otherwise indicated. Pooled values for clinical outcomes were calculated using DerSimonian-Laird random-effects model and reported with 95% confidence intervals (CI) (9). For all studies, overall survival referred to freedom from death of any cause, and was calculated from the time of surgery. All statistical analyses were performed using Comprehensive Meta-analysis v2.2 (Biostat Inc., Englewood, NJ, USA). All P values were two-sided, and values <0.05 were considered statistically significant.
Using the predefined systematic search criteria, a total of 978 unique records were identified through the database and bibliographic searches, with one additional study identified through other sources. After exclusion of duplicated studies and 546 irrelevant articles based on abstracts and titles, 126 studies remained for full-text evaluation. Of these, 44 studies met the inclusion criteria, including nine studies in the TECAB off-pump group (10-18); two studies in the TECAB on-pump group (19,20); and 16 studies in the robotic MIDCAB group (10,21-33). One study provided separate data for both the TECAB off-pump and robotic MIDCAB groups, and was included for analysis in both groups (10). Eighteen additional studies reported on a mixture of different surgical techniques (18,34-53). A summary of the study selection process is presented in the PRISMA chart in Figure 1, and a summary of study characteristics is presented in Table 1.
All of the studies were observational studies, including clinical data on a total of 8,034 patients who underwent CABG. Twenty-one studies were excluded from the final analysis due to a mixed dataset for different surgical techniques (42-53) or use of cardiopulmonary bypass (36-41), or use of an outdated robotic system other than the da Vinci system (18,34,35). Of the patients who were included for statistical analysis in the selected studies on off-pump TECAB, on-pump TECAB and robotic MIDCAB, the mean age ranged from 58 to 67, and 51–88% of patients were male. The presence of hypertension, previous myocardial infarction (MI) and diabetes mellitus were reported in 50–92%, 7–56% and 13–51% of patients, respectively. Preoperative estimated left ventricular ejection fraction (LVEF) ranged from 48–63% across studies. A summary of baseline patient characteristics is presented in Table 2. The reported follow-up duration ranged from the peri-operative period to eight years, and 20 studies reported routine angiographic follow-up 1–96 months postoperatively by either angiography or computed tomography coronary angiography (CTCA).
The mean number of grafts per CABG procedure ranged from 1–2.8 in all three surgical techniques. However, there was a trend of more grafts being performed in robotic MIDCAB procedures compared to TECAB procedures. The LIMA-to-LAD graft was performed in all patients who underwent on-pump or off-pump TECAB, apart from one study. Circumflex and right coronary artery (RCA) grafts were performed in selected patients in two off-pump TECAB studies and six off-pump MIDCAB studies. Robotic-assisted LIMA harvest duration ranged from 26 to 60 minutes. The anastomosis time for the TECAB off-pump and TECAB on-pump groups ranged from 9.6–12.6 and 28–60 minutes, respectively. The mean overall operative time for the off-pump TECAB and off-pump MIDCAB groups ranged from 161–326, and 166–444 minutes, respectively. Pooled conversion rates from the intended incision were 7.0% (95% CI, 2.8–16.9%), 8.9% (95% CI, 1.3–42.7%) and 4.4% (95% CI, 2.9–6.6%) in the off-pump TECAB, on-pump TECAB, and robotic MIDCAB groups, respectively. A summary of these intraoperative outcomes is presented in Table 3.
The pooled peri-operative mortality following off-pump TECAB was 1.7% (95% CI, 0.9–3.2%). The rates of peri-operative MI, stroke and acute kidney injury (AKI) were 1.1% (95% CI, 0.5–2.6%), 1.1% (95% CI, 0.4–2.7%) and 3.4% (95% CI, 1.2–9.7%), respectively. Re-operation for bleeding was required in 3.2% (95% CI, 1.6–6.3%) and peri-operative atrial fibrillation was reported in 11.7% (95% CI, 8.0–16.8%). Pooled post-operative ventilation time was 8.7 hours (95% CI, 1.5–15.9), and the pooled length of hospital and intensive care unit (ICU) stay was 5.6 days (95% CI, 3.5–7.7) and 32.0 hours (95% CI, 14.6–49.4), respectively.
The pooled peri-operative mortality following robotic MIDCAB was 1.0% (95% CI, 0.6–1.6%). The rates of peri-operative MI, stroke and AKI were 1.2% (95% CI, 0.7–2.1%), 0.7% (95% CI, 0.3–1.5%) and 1.8% (95% CI, 1.1–2.9%), respectively. Re-operation for bleeding was required in 2.7% (95% CI, 1.9–3.8%), peri-operative atrial fibrillation was reported in 12.5% (95% CI, 9.2–16.8%) and wound infection occurred in 1.9% (95% CI, 1.2–3.1%). Pooled post-operative ventilation time was 8.7 hours (95% CI, 5.6–11.8), and the pooled length of hospital and ICU stay was 4.7 days (95% CI, 3.7–5.7) and 26.7 hours (95% CI, 19.9–33.4), respectively. A summary of these perioperative outcomes is presented in Tables 4,5 and 6. There was insufficient data to statistically summarize clinical outcomes following on-pump TECAB from the limited number of studies.
Anastomotic complications and long-term outcomes
Anastomotic complications were reported using heterogeneous endpoints, such as graft occlusion, graft failure and graft stenosis. Postoperative routine imaging of coronary grafts by angiography or CTCA was performed in 12 out of 25 studies. In the remaining studies, graft complications were detected by investigation of symptomatic patients. The reported rates of graft occlusion ranged from 0–6.7%, and the rates of graft or anastomotic stenosis ranged from 0.7–13%. Due to the variable timing of follow-up imaging and definitions of anastomotic complications, formal statistical analysis of this endpoint could not be performed. A summary of the reported data on graft complications is presented in Table 7. Mortality beyond 30 days was reported at variable time frames ranging from three months to eight years, ranging from 0–5.4%. Due to the lack of standardized time intervals, detailed statistical analysis could not be performed. Reported outcomes for long-term mortality are summarized in Table 4.
The ultimate goal of robotic CABG is to perform safe and effective coronary anastomoses through a minimally invasive approach to enhance recovery and minimize trauma. To achieve this goal, a number of techniques and robotic systems have been developed over the past 15 years, culminating in a totally endoscopic approach using the da Vinci operating system, without the need for cardiopulmonary bypass, also known as the off-pump TECAB technique. Although LIMA was commonly grafted onto the LAD as a single TECAB graft, robotic harvesting of bilateral mammary arteries was feasible and all territories of the heart could be grafted robotically (14). From a historical perspective, the AESOP system was initially developed as a voice-activated robot used to hold an endoscope, and approved by FDA for minimally invasive surgery in 1994. This was followed by the Zeus Robotic Surgical System (Computer Motion Inc., Goleta, CA, USA), which consisted of two robotic arms mounted to the patient bed, controlled via a satellite control unit. The Zeus system was discontinued from clinical use in 2003, following the introduction of the da Vinci system, which consisted of three principle components: (I) a surgical console; (II) a computer controlled system; and (III) robotic manipulations. The surgeon was positioned at the console to grasp specially designed instrument handles. The surgeon’s motions were then relayed to a computer processor, which digitized the surgeon’s hand motions. The digitized information from the computer control system was then relayed in real time to robotic manipulators, which were attached to the operating room table. These manipulators held the endoscopic instrument tips, which were inserted through small ports, performing all aspects of the CABG procedure without sternotomy or thoracotomy (31).
Current limitations to the robotic CABG technique include its cost, heterogeneous clinical outcomes, limited training opportunities and evolving instrumentation for the endoscopic technique. From a technical perspective, one of the greatest challenges to robotic-assisted CABG is the anastomosis of the LIMA-to-LAD graft. To perform this critical step, the distal end of the LIMA is skeletonized prior to anastomosis, and the target coronary artery is identified and positioned. If TECAB is performed off-pump, silastic bands or saddle loops can be placed either side of the anastomotic site for vessel occlusion prior to arteriotomy. Some authors advocated the use of ischemic preconditioning (14) or intra-coronary shunt placement (18,26). The target vessel can be opened using an endoscopic knife and the arteriotomy performed with endoscopic Potts scissors. The distal anastomosis can be performed using robotic endoscopic sutures, U-clips (Medtronic, Minneapolis, MN, USA), or anastomotic devices such as the C-Port Flex A (Cardica, Redwood City, CA, USA). Distal anastomotic patency can be checked intraoperatively via angiography or ultrasonography (11,14), or postoperatively using CTCA (17), Doppler ultrasonography (18) or formal angiography. Overall, it has been acknowledged that robust clinical outcomes, as well as improved instrumentation at affordable costs will be critical to the future development of the TECAB technique.
The present systematic review found that the majority of studies on robotic-assisted CABG procedures involved patients who underwent off-pump TECAB or robotic-assisted MIDCAB techniques. This was not an unexpected finding, as the use of cardiopulmonary bypass was acknowledged as a ‘stepping stone’ to off-pump TECAB for surgeons who were overcoming their technical learning curves (20). Although cardiopulmonary bypass and cardioplegia allowed the surgeon to operate on an arrested heart, the addition of peripheral cannulation and aortic clamping prolonged the operative duration, and added complications such as groin wound infections and vascular injuries (20). The patients included in the selected studies in the present systematic review were relatively young, with a mean age in the 60s, and well-preserved preoperative left ventricular function, with a mean ejection fraction of >55% in the majority of studies. In addition, the numbers of anastomoses were relatively few, especially for the TECAB groups, which averaged less than two grafts in all of the studies, and almost exclusively for the LIMA-to-LAD graft. Keeping these considerations in mind, results of the present systematic review demonstrated relatively safe perioperative outcomes for both the off-pump TECAB and robotic MIDCAB groups, with a pooled perioperative mortality rate of 1.7% and 1.0%, respectively. Dhawan and colleagues reported a relatively high perioperative mortality rate of 3.8% from a cohort of 106 patients, with four deaths within the perioperative period related to cardiogenic shock, fibrillatory arrest or bleeding (13). Other perioperative outcomes, such as MI, stroke, AKI, re-operation for bleeding and atrial fibrillation were comparable to contemporary data for conventional CABG procedures (54,55). Other key findings of the study included anastomotic complication rates that ranged from 0.7–13%. However, routine follow-up varied in relation to angiographic assessment technique, timing, and definition of endpoints. Long-term survival was also difficult to assess due to limited and non-systematic follow-up of patient cohorts, but the available data reported favorable outcomes, including one report of 4% mortality at 8-year follow-up.
Some of the unexpected results can be rationalized with detailed examination of the surgical technique performed. For example, Zaouter et al. reported a relatively long anastomosis time of 60 minutes (19). However, this duration was measured from the time of LIMA occlusion, and included the period of time spent on target vessel identification and preparation prior to anastomosis. Similarly, although Srivastava, Poston and Subramanian reported mean total operative times of 311, 348 and 444 minutes, respectively, they also averaged 1.9–2.2 grafts, accounting for the increased operative time spent on conduit harvesting and performing the additional anastomoses (29,30,32). In addition, studies of on-pump TECAB, such as the multi-institutional study by Argenziano and colleagues, reported longer operative times due to the need for peripheral cannulation and use of a cardiopulmonary circuit (20). Finally, perioperative outcomes such as ventilation time, ICU stay, and total hospitalization time were dependent on clinical pathways specific to each institution, and prolonged times may have been multi-factorial (10).
A number of important limitations to the present systematic review should be acknowledged, and reported outcomes should be interpreted with caution. Firstly, only non-randomized, observational studies were identified, and robust comparisons with conventional CABG through the sternotomy approach were not possible using directly comparative data. Secondly, it should be noted that the patient cohort included in the present systematic review were a highly selected subgroup, with relatively favorable baseline patient characteristics, and should not be generalized to all patients with coronary artery disease, especially those who present with acute coronary syndrome. Thirdly, surgical techniques evolved over time, partly due to the learning curve of individual surgeons and institutions, and also partly due to the evolution of instrumentation available in clinical practice. Variations in surgical techniques and experience may have accounted for some of the differences between the selected observational studies.
Overall, the present systematic review examined the current evidence for robotic-assisted CABG procedures, which were mostly focused on the off-pump TECAB and off-pump MIDCAB techniques. Statistical analysis demonstrated relatively safe outcomes using perioperative mortality as the primary endpoint. However, there remains a paucity of robust clinical data related to anastomotic complications, and future studies should follow standardized, routine angiographic follow-up at predefined time intervals to demonstrate technical efficacy. Results of the present systematic review should be considered as a useful benchmark for future studies on robotic CABG, until further data is reported from randomized studies.
Conflicts of Interest: The authors have no conflicts of interest to declare.
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