Discussion

Isolated aortic valve replacements comprised 10% of cardiothoracic operations in 2016, with 6% performed alongside coronary artery bypass grafting (CABG) and 1% alongside mitral valve replacement (MVR).[7] AVR surgery serves to provide symptomatic relief and improve prognosis in patients with severe valve obstruction.[8] Hence, most common indications for elective surgeries is aortic valve stenosis (AS) and aortic regurgitation (AR). [9] As a result of an ageing population, there has been an increasing incidence of AS and AR now with up to 275,000 to 370,000 per year who require AVR in the modern world. [10] Most AVR are elective surgeries and are performed for AS, in which the procedure currently provide a five-year survival rate of 78.4% and fifteen-year survival rate of 39.7%.[11] On the other hand, the most common indications for emergency AVR surgery are critical aortic stenosis and left ventricular failure, which typically presents as cardiogenic shock and multiple organ failure.[12] Advances in surgical techniques and understanding of the diseases produced notable improvements in treatment outcomes, lowering mortality rates in hospital from 6.4% overall in 2000, to 3.1% in 2015.[13,14]
The oldest manuscript was by Professor Alain Cribier who, in 1986, first trialled the use of percutaneous transluminal balloon catheter aortic valvuloplasty elderly patients with severe aortic valve stenosis in 1986.[15] This was introduced as an alternative intervention for those who are unfit for the traumatic surgical approach. While it was performed on only three patients and is too early to ascertain its efficacy, it laid the foundation for researchers to eventually develop the now widely used transcatheter aortic valve implantation (TAVI). This is reflected in the ever-increasing amount of literature investigating TAVI, with 29 of the top 100 specifically examining it alone. In comparison, only 8 manuscripts focused solely on surgical techniques. This increasing attention in AVR is reflected in the rising number of top manuscripts per year, from less than 5 per year before 2000 to peak in the 2010s, averaging more than 10 per year.
The most cited paper, by Leon et al ., investigated TAVI on high risk, severe aortic stenosis candidates who are not suitable for surgical replacement. This trial by PARTNER includes a specific cohort of patients taking place in a multi-centre, randomized clinical trial. Introduced in 1989 as a less invasive method of treatment for high-risk patients, transfemoral TAVI is found to significantly reduced 1-year mortality (30.7% vs 49.7%), cardiovascular-related mortality (19.6% vs 41.9%), repeat hospitalisation (22.3% vs 44.1%) and significant symptomatic relief [16]. However, the study also identified an increased number of severe stroke (7.8% vs 3.9%) and vascular events (32.4% vs 7.3%) in the 12 months following TAVI. Leon et al.concluded that such vascular complications may be attributed to large femoral access sheath insertions and so novel lower profile valves and support frames are being developed. This paper proposes TAVI as the best treatment for high risk severe aortic stenosis patients with complications unsuitable for standard surgery and identified the areas of improvement to help perfect TAVI.
The second most cited study was a similar study by Smith et al. , a randomised control trial comparing TAVI and standard approach but on high-risk patients who are suitable for surgery. High risk severe aortic stenosis patients showed similar 1-year mortality between standard surgical replacement and TAVI. TAVI cohort had shorter ICU stays (3 vs 5 days) and as well as hospital stay (8 days vs 12 days). Major bleeding was also less common in TAVI, with 14.7% vs 25.7%. However, neurological events such as stroke and/or transient ischemic attacks are nearly doubled in TAVI (8.3% vs 4.3%). This study also associated TAVI with more procedural complications compared to normal surgery, with increased vascular complications (18% vs 4.8%). The authors concluded that the outcome of TAVI for male patients was similar to the surgical approach but offers survival mortality benefits in women or patients with a coronary bypass graft.
The third most cited study was by Birkmeyer et al. , which investigated the relationship between hospital volume, and the number of procedures performed and their effects on postoperative mortality. Analysis of 2.5 million procedures, including 6 types of cardiovascular surgeries, revealed hospitals with larger volumes had lower mortality rates. With higher hospital volumes, observed mortality rates of AVR decreased: hospitals with very low, low, medium, high and very high volume shown a decreasing mortality rate from 9.9% to 7.6%. Similar trends can be seen in mitral valve replacements and carotid endarterectomies. This study only included patients on Medicare within the USA and the majority were above 65 years old. The authors concluded that the apparent mortality reduction in specialised procedures may be resultant of more specialised and experienced healthcare professions who are equipped with greater resources.
Apart from the number of citations and manuscripts published under each journal, the impact factor of the journals themselves also helps us understand the quality and significance of the papers. The impact factor is based on the citations of published articles in each journal, specifically the average amount of citation by a journal’s publications. As such, journals with higher impact factor tends to publish work of higher quality and importance which is cited more often. Hence, journals with an impact factor above 45 (JAMA, Lancet and NEJM ) accounted with over a third of the total citation counts with 25 manuscripts. Furthermore, only 7% and 16% of articles from the list were from journals with impact factors less than 5 and 10, respectively. This demonstrates that research in aortic valve replacement is largely dominated by publications in journals with high to very high impact factors. However, it would be useful for future research to be able to assess the clinical significance of these articles to determine the correlation between citation count and clinical applicability.