The aortic valve & aortic valve disease

The mammalian heart comprises four chambers, the upper left and right atria and the corresponding ventricles. Blood flow from the atria into the ventricles and out of the heart via the pulmonary artery and aorta is regulated via pressure differentials in blood flow and the presence of valves which prevent the backflow of blood. The semilunar valves are the aortic and pulmonary valves located between the left ventricle and base of the aorta, and the right ventricle and pulmonary artery respectively. These valves permit blood flow into the arteries from the ventricles and prevent their backflow into the heart. The valves themselves comprise a structure known as a cusp, these form a ‘three-doored gateway’. Contraction of the ventricle causes a rise in pressure, subsequently opening the cusps of the aortic valve and allowing blood to flow into the aorta.  When the pressure drops in the ventricle, the valve closes which causes the sound of the second heartbeat.

The mitral and tricuspid valves ‘gate’ the atrium and ventricle chambers of the left and right respectively. Collectively, both are referred to as the atrioventricular valves.

Aortic valve disease (AVD) is characterized by an impairment in the proper function of the aortic valve, causing symptoms such as fatigue, shortness of breath, weakness, chest pain or dizziness. AVD can be either congenital (hereditary) or acquired with age. In Aortic insufficiency or regurgitation, the aortic valve does not seal completely when it closes and can allow blood to flow back into the left ventricle. Aortic stenosis is a dysfunction caused by tightening of the valve creating a restricted opening which limits blood flow from the heart. In some instances, AVD can develop as a combination of both insufficiency and stenosis. Left untreated, AVD leads to sever infection, heart atrophy and failure.¹ The aortic valve can develop in some people as a bicuspid valve (congenital dysfunction). In these cases, the patient can live normally without experiences any symptoms until they reach adulthood

As the closing of the aortic valve contributes to the rhythmic output of the heartbeat, abnormal function causes a heart murmur, which can be detected using a stethoscope during a physical examination. Non-invasive ultrasound imaging or an echocardiogram is also routinely used to diagnose AVD as well as to determine the extent of disease severity.¹

The Ross Procedure

AVD is not treated with any type of medication, and correcting aortic valve dysfunction involves surgical intervention. Surgical aortic valve replacement (SAVR) is performed to either replace or reconstruct the damaged valve, and this is done either with a prosthetic or tissue replacement (human or porcine).

The Ross procedure is the latter. So called for its introduction by surgeon Donald Ross in the 1960s, the Ross procedure involves the removal and replacement of the aortic valve with the pulmonary valve from the same heart and then inserting either a prosthetic valve or donor pulmonary valve (pulmonary homograft) in its stead.

It is seemingly counterproductive to opt to treat a single valve dysfunction by purposely creating a ‘two valve disease’. However, the main rationale for the Ross procedure was anchored in the hypothesis that using a living substitute for the aortic valve, as opposed to a mechanical one will produce results that best mirrored normal physiological behavior. While at the time, this was a point of discourse for most practicing surgeons, in recent years, long-term post operative studies has shown that the theory (at least compared to other procedures) holds true.^2,3,4,5

The Ross procedure is particularly advantageous for treating young children and adults with AVD and for patients whom the use of a prosthesis is unsuitable. The use of a tissue replacement in young children allows for the flexibility and adaptation demanded of the organ as the child grows into adulthood. As a procedure, surgical outcomes are generally good with approximately 76% of all patients having undergone the Ross procedure surviving up to 25 years.² Other well known advantages of the Ross procedure are that compared to other options, it often results in better stabilization of blood pressure and heart function (haemodynamics), there is a lowered infection risk resulting in endocarditis and, due to the reduced risk of patients developing blood clots from the procedure, it negates the need for anticoagulation medications.

The Ross procedure indubitably, is an exhaustive process and technically demanding. As such, in very young infants (less than 1 year) with AVD, it is not the first option for treatment and is often employed as a secondary procedure when the first procedure has poor clinical outcome. The rationale behind this being to first buy some time to allow the infant to reach an age when the predicted positive clinical outcomes of a Ross procedure are much higher, avoiding infant death.³ 

Although there are many reports that confirm the longevity of aortic valve replacement using the Ross procedure,^2,3,4 this seemingly is dependent on the patient’s age and the severity of AVD. In a cohort of patients who had congenital AVD with an average age of 34 years, a 20 year survival was estimated to be as high as 93%.⁴ However, as alluded earlier in neonates and infants the Ross procedure for correcting aortic valve dysfunction carries a high risk of infant death (16 – 22%) and the preference to use the Ross procedure as a secondary procedure after attempting an initial valve repair procedure.³ Mortality risk is also higher in older patients who may not have a congenital defect and may suffer from pre-existing conditions.

This individual variability also affects the ability to conclusively come to consensus on what factors significantly affect the longevity of the autograft and allograft used for a Ross procedure. Therefore accurately determining the deterioration of a transplanted valve and the ideal time for reoperation in a patient can be particularly challenging, leaving both the physician and patient facing a great deal of uncertainty.^4,6

In general, the ideal candidate considered for a Ross procedure would be under 60 years of age, have been highly active, and did not suffer from any pre-existing conditions such as an autoimmune disease or coronary disease which may affect a positive long term clinical outcome for the patient.

Alternatives to the Ross Procedure

While the Ross procedure is the favored option for patients within a specific age group, alternatives must be considered for those who for any reason do not meet the criteria. The most common option is the use of a prosthesis. Comparatively, mechanical prostheses have better durability than the use of living tissue constructs, the patient does not suffer a secondary injury and as the surgery requires the replacement of a single valve, the technique is less cumbersome for both physician and patient. However, there are significant limitations as patients are prone to bleeding, are at higher risk of developing infection (prosthetic valve endocarditis) and require lifelong anticoagulation medication⁵.

Aortic valve neocuspidalization, also referred to as the Ozaki procedure, has gained interest in the past decade as a viable alternative to the Ross procedure. First reported in 2011, the Ozaki procedure involves the use of part of the lining of the heart wall or pericardium to replace the cusps in the aortic valve. As the procedure does not make use of any donor or fabricated materials Ozaki and colleagues categorize the procedure as a valve repair and not replacement.⁷ Long term outcomes are yet to be discerned for the Ozaki procedure, however a study published in 2021 by Polito et al reporting on the Ozaki procedure having generally safe and effective outcome in pediatric patients.

Transcatheter aortic valve replacement (TAVR) is a technique used to treat aortic stenosis that does not require open heart surgery, and is a viable option for patients where open-heart surgery may not be suitable. Patients with preexisting cardiovascular disease or who already suffer from other valve dysfunction, for example. The replacement valve is introduced through a small incision and guided into place (delivering to the aortic valve) using a catheter via a blood vessel. Catheters can be introduced by the femoral blood vessel (Trans femoral), the base of the heart (Transapical), or the aorta (Trans aortic).

Summary

The Ross procedure is a surgical technique introduced in the 1960s by Donald Ross and is used to replace faulty aortic heart valves. It is an open heart procedure in which the aortic valve is removed and replaced with the pulmonary valve from the patient’s heart, and the pulmonary valve is subsequently replaced with a donor. Although the Ross procedure invariably introduces a secondary injury to the heart, the procedure is still a favorable option in patients who qualify for the procedure, as the method produces the most stable postoperative haemodynamics, low risk of endocarditis infection and negates the need for the patient to take anticoagulants. The Ross procedure, however, is a taxing procedure and the patients that qualify are restricted to an specific age range (18-40) who do not suffer any comorbidities. As such, alternative methods which improve on this, like the Ozaki procedure, are still an attractive option for most surgeons. Although promising, whether the long term clinical outcomes of newly developed techniques can match or surpass that reported by the Ross procedure are yet to be ascertained.