Discussion
Port-access MV surgery is nowadays a valid approach to treat all kinds of MV disease according to contemporary techniques, resulting in a clinical outcome equal to the conventional sternotomy approach. This technique has been implemented in our department since more than 10 years now for the surgical management of most MV pathologies, with or without the need for additional treatment of associated lesions as atrial fibrillation and tricuspid valve dysfunction. Even after including the obligatory learning phase, the clinical outcome as well as the quality of MV procedure itself appeared to be non-inferior to that of the standard MV surgery. Hence, the effect of the learning period during the adoption of a new approach is not negligible. In a large volume center, Holzhey et al. demonstrated that approximately 75 to 125 operations are required to achieve optimal results with MIVT, characterized by a net decrease of bleeding complications, reoperation for early valve failure and a nearly obsolete conversion rate12.
Moreover, some complications are typically related to the surgical approach. The risk of stroke is often estimated to be increased due to the retrograde systemic perfusion and the potential risk for aortic dissection, inadequate aortic de-airing and the deficient tactile manipulation of the ascending aorta, regardless of the mode of aortic clamping13-14. The incidence of major neurological injury in our study was similar to others, revealing only one case of retrograde aortic dissection. Here, we underscore the importance of examining the vascular access – specifically the arterial side – rigorously to limit such devastating complication as much as possible.
Considering the lower traumatic impact of a mini-thoracotomy on the thorax compared to a sternotomy, one would expect a favorable effect on respiratory dynamics and so, shorter ventilation time. In this propensity-matched comparison, the ventilation times were identical as well as the ICU stay duration. In an equivalent study design comparing 350 port-access patients and 365 sternotomy patients, Suri et al. found a slight decrease of ventilatory support duration despite of significantly longer procedural times15. However, it is commonly known that the criteria for extubation may vary in-between centers, and that even in one single center, the decision for extubation may vary among ICU physicians, advancing thereby the subjective aspect of this parameter.
A recurrent finding of many studies on this topic is the notification of longer aortic cross-clamp and cardiopulmonary bypass times5-11. The adverse relationship between these factors and secondary organ dysfunction is well-known in the field of cardiac surgery16-17. Through analysis of specific organ function biomarkers, routinely sampled at the postoperative stage, we found no significantly different effect between both approaches. Biomarkers of myocardial and liver origin increased similarly in both groups, but one has to consider that, through lack of specificity, it is difficult to know what the real impact of each procedure on respectively myocardial and liver function is. Interestingly, the creatinine-kinase level was significantly increased after port-access surgery. Bearing in mind that this count also represents global muscle damage, one can relate this to a condition of relative limb ischaemia initiated by the cannulation of the femoral artery and vein. As the duration of cardiopulmonary bypass is rarely exceeding the critical duration threshold for irreversible ischemia and rhabdomyolysis, it generally remains clinically insidious. However, this issue needs to be accounted for optimization of the peripheral cannulation strategy, by using specifically designed arterial cannulas allowing distal leg perfusion or by avoiding combined ipsilateral femoral artery and vein cannulation.
Kidney biomarkers are both specific and sensitive to renal function. Our study demonstrated a rise of creatinine and ureum level, together with a decrease of glomerular filtration, during the early postoperative period, but independent of the used surgical approach. Nevertheless, the incidence of effective kidney failure, requiring renal replacement therapy, was low. Data on this organ-specific outcome are controversial, some showing a clear advantage for the minimally invasive technique, while others were not able to found any effect15,18. One part of this confounding is related to the use of different study designs including selection bias and other statistical methods to analyze renal outcome.
Regarding the systemic inflammatory reaction, CRP counts remained largely inferior during the first 48 hours after port-access MV surgery than after conventional sternotomy. This suggests that the MIVT technique is associated with a significant trauma reduction to the body, with less activation of inflammatory mediators. Watt et al. reported similar results in a systematic review on CRP post-surgery, and identified that CRP counts were higher after bigger, more invasive surgical procedures like thoracic surgery compared to smaller surgeries such as cholecystectomy19. The results of Paparella et al. confirmed that CRP and interleukine-6 counts were lower after MIVT compared to sternotomy MV surgery20.
The lack of detrimental effect of longer duration of cardiopulmonary bypass and cardiac arrest on secondary organ function, as observed after port-access MV surgery, points to eventual intra-operative measures, directly related to the management of cardiopulmonary bypass. The conduct of cardiopulmonary bypass during MIVT surgery is commonly facing flow restrictions, due to the use of smaller caliber cannulas via peripheral vascular access, which on top of it, need to be properly positioned, depending on adequate guidance via intra-operative transesophageal echocardiography. Using kidney function as most robust organ function endpoint, our group recently demonstrated that an equivalent organ-specific as well as clinical outcome between MIVT and conventional MV surgery is achievable through targeting an minimal oxygen delivery during the duration of cardiopulmonary bypass. However, in order to cope with the frequent inferences of intra-operative flow restrictions over a longer procedural time during MIVT, other means need to be applied as the tolerance of lower body temperature, the pursuit of a higher intra-operative hematocrit level and the use of blood preserving measures21. Moreover, operative times may probably be less decisive for patient outcomes, as long as they are not excessively long.