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.