Two different techniques for aortic occlusion can be used during minimally invasive cardiac surgery – external (transthoracic) aortic clamping or endoaortic balloon occlusion. Data on the use of these aortic occlusion techniques comes mainly from retrospective observational studies, and few direct comparisons have been reported [6]. We performed a PS-matched analysis of patients undergoing MIMVS at our centre to describe the outcomes of endoclamping in patients where exoclamping could not be used. Key findings included high primary MV reconstruction rates and good long-term MV competency in both groups; no significant between-group differences in complication rates or survival, which may need to be interpreted with caution because of the low absolute number with events; and shorter X-clamp and total procedure times with endoclamping.
At our institution, transthoracic clamping is more commonly performed, provided the aorta can be safely dissected and clamped; endoclamping is used in cases where the ascending aorta cannot be safely mobilised. A similar approach is used at other centres [2]. An endoclamp may also be preferred for re-operative procedures, where scarring around the aortic root can make it difficult to apply an external clamp [7,8,9]. There is a learning curve for the technique associated with using an endoaortic balloon, with outcomes generally improving as the number of completed procedures increases [10]. However, with good patient selection, a highly competent team, use of an appropriate insertion technique and imaging, and careful monitoring, the risk of adverse outcomes can be minimised [8, 11].
We compared outcomes after endoclamping and exoclamping using PS-matched groups to take account of baseline characteristics. We found some differences between the PS-matched groups in terms of the specific procedures that were performed: anterior mitral valve leaflet reconstruction, left atrial appendage closure and patent foramen ovale closure were all more common in the exoclamp group, whereas posterior mitral valve leaflet reconstruction was more common in the endoclamp group. The reasons for this difference are not clear, but it is possible that surgeons did not consider the endoclamp procedure to be appropriate in some patients undergoing more complex procedures. Other studies have not reported differences in the type of operation performed between endoclamp and exoclamp groups [12, 13].
Our analysis found no significant difference in the overall procedure time or in cardiopulmonary bypass time between the two groups, but the median X-clamp time was shorter (by 11 min) in the endoclamp group. Some previous studies have found no significant difference in cardiopulmonary bypass and X-clamp times between exoclamping and endoclamping groups [12,13,14,15], whereas others have reported longer operating, cardiopulmonary bypass and X-clamp times with endoclamping [7, 16, 17]. Meta-analyses have generally found no significant difference in X-clamp or cardiopulmonary bypass times between endoclamping and exoclamping [6, 18, 19]. However, a subgroup analysis found some differences depending on the cannulation used for endoclamping: X-clamp and cardiopulmonary bypass times were shorter with exoclamping compared with endoclamping with femoral cannulation; however, X-clamp time was shorter with endoclamping with aortic cannulation compared with exoclamping, and cardiopulmonary bypass time did not differ between exoclamping and endoclamping with aortic cannulation [19].
In our study, ICU and hospital stays were slightly longer in the endoclamp group than the exoclamp group (by 2 and 1 days, respectively), but the differences were not statistically significant. This is consistent with most previous studies which have found no significant difference in the length of ICU stay [13,14,15] or hospital stay [6, 7, 14, 15, 17], although one study reported a significantly longer hospital stay (by 2 days) after endoclamping [16] and another a significantly longer hospital stay (by 1 day) after exoclamping [13].
Successful primary mitral valve reconstruction rates were high and long-term mitral valve competency was good in both groups in our study. However, there was trend towards a better outcome with respect to valve competency in the endoclamp group, which achieved statistical significance during long-term follow-up. This is a somewhat surprising finding. Other studies have found no significant difference in the severity of mitral insufficiency between endoclamp and exoclamp groups at the time of discharge from hospital [7, 13, 15], but comparative data for this parameter after long-term follow-up are scarce.
We found no significant difference in the rates of peri-procedural complications or adverse outcomes at 30 days post-procedure between the two groups. However, 30-day event rates were low, which precludes meaningful p-values for the comparison at this timepoint. The rate of all-cause mortality at 30 days was numerically slightly higher in the exoclamp group (4/63; 6.3%) than in the endoclamp group (2/63; 3.2%). Previous studies and meta-analyses have not found a significant difference in 30-day all-cause mortality rates between endoclamping and exoclamping groups [6,7,8, 12, 13, 18,19,20].
Strokes occurred in numerically more patients in the endoclamp group (6/63; 9.5%) than in the exoclamp group (1/63; 1.6%) in our study, but this was not statistically significant. While further investigation is needed to better understand why the occurrence of stroke was higher in patients in the endoclamp group in this analysis, previously published reports have suggested a possible increased risk of neurological complications with the use of endoaortic occlusion compared with external clamping [12, 21]. Conversely, other studies have not reported this finding [8]. Recent meta-analyses found no significant difference in the risk of cerebrovascular accidents between the two approaches [18, 19]. Potential reasons for an increased stroke risk with endoclamping include the risk of emboli mobilisation by the guidewire, occlusion of arteries by balloon catheters that have migrated, and re-positioning of balloons without partial deflation [18]. One meta-analysis found that the relative risk of neurological events with endoclamping versus exoclamping was lower in more recent publications than in older reports, and the authors suggested this may be due to the availability of improved devices and cannulas, greater surgeon experience and improved echocardiography guidance [18]. Despite this, the long-term (7-year) survival rates for patients were similar in both groups at our centre.
Endoaortic balloon occlusion is a versatile tool with applicability for a range of cardiac surgeries [22]. In general, certain measures are vital to ensure the safe performance of endoclamping. Viability should be confirmed using preoperative imaging [22]. Transoesophageal echocardiographic monitoring is vital to control guidewire advancement and ensure correct positioning of the endoclamp [4, 10, 22]. Changing the balloon position after the initial endoclamping has been undertaken is possible under echocardiographic surveillance and with partial deflation of the balloon to avoid damaging the aortic wall [22]. It has been shown that MIMVS using endoclamping is a safe procedure even during the initial learning curve [11]. Our study confirms the feasibility and safety of endoclamping during MIMVS.
Some authors have suggested that endoclamping might be associated with higher costs than exoclamping at their centres, due to device costs, the technical complexity and a longer operative time [14, 15], whereas others have found that the endoclamping approach may be more economic [1]. However, no studies have reported specific cost data comparing the different approaches.
Limitations
The main limitation of this observational study is its retrospective nature, making it subject to potential selection bias. Propensity-score matching was used to mitigate this but did not eliminate it completely. For example, surgeon expertise, and the date at which surgery was performed during the 6-year study period, were not taken into account. The aim of the study was to report on the outcomes of patients receiving endoclamping during minimally invasive mitral surgery, but the use of propensity-score matching matched these patients to a control of patients receiving exoclamping to put the results into perspective. As a result, the data has become a comparison of endo- versus exoclamping rather than focusing on the outcomes of endoclamping alone. The relatively small number of patients in the endoclamp cohort meant that the study had a small overall sample size after propensity matching, and therefore lacked power to fully address some issues, such as whether there was a difference in the incidence of stroke. The final limitation of this study is that it was a single-centre study, which limits the generalisability of the results.
Additional studies, including randomized controlled trials and prospective observational studies with a larger sample size and adequate power are needed to address some issues further, such as the incidence of stroke with the endoclamp procedure. Such studies may also help to further define the subgroup of patients for whom endoaortic clamping is appropriate, and clarify the technical aspects of the procedure that are key for optimising patient outcomes.