Valves exist in the blood vessels of the human body to ensure one way flow of blood.
There are a great many valves in the body. The heart has four – the Aortic, Pulmonary, Mitral and Tricuspid valves. The aortic valve is the valve that sits between the heart and the main artery of the body – the aorta.
As the heart squeezes blood is ejected from the lower chambers of the heart, called the ventricles, into the arteries that then distribute blood around the rest of the body. The right ventricle supplies blood to the lungs via the pulmonary artery and the left ventricle supplies blood to the rest of the body via the aorta.
Degeneration of the aortic valve occurs naturally through life as we age. Most people do not notice the aging process affecting the aortic valve. However, in around 2% of the population the aging process takes more of a toll and the valve can stiffen or loosen.
Restriction of blood flow through a tight valve is called “stenosis”.
Back flow of blood from a loose valve is called “regurgitation”.
As the valve becomes tighter the flow of blood across the valve quickens similar to when a finger is placed over a hosepipe. Instead of smooth flow of blood the blood whooshes across the valve at a faster pace and higher pressure causing a turbulent spray or jet. This puts more pressure on the aorta to cope with a high pressure jet of blood.
Where the aortic valve is formed abnormally from birth it is thought that the stress on the valve becomes greater leading it to wear quicker. A common example of this is the bicuspid aortic valve (BAV) which affects around 1% of the population. In patients with BAV, the aortic valve comprises of two parts rather than the usual three. A similar presentation can occur where a three part valve fuses together to make two parts. This is called a pseudo BAV. The BAV is an example of a congenital heart defect and is found more commonly in males and can run in families, although the genetic basis for this is poorly understood.
Classification of severity
The severity of aortic stenosis can be classed as mild, moderate or severe.
The assessment of valve function and therefore severity is usually by an ultrasound scan of the heart to look at the area of the valve, the velocity of the jet of blood ejected through the valve and the pressure difference (or pressure gradient) between the left ventricle and the aorta.
The aortic valve area is usually greater than 2cm2 in size and is classed as severe when it gets to the around the size of a golf tee or smaller.
The normal pressure gradient across the aortic valve is usually very small at 0 – 2 mmHg. As the heart struggles to squeeze blood across the tight valve, the pressure difference starts to develop.
It’s important to understand the severity as it helps to plan when treatment is necessary. In some circumstances an MRI of the valve might be useful if there is doubt over the accuracy of the ultrasound findings.
As aortic stenosis becomes more severe one would expect to develop symptoms associated with this. It is unusual to develop symptoms before the stenosis is severe. Some may never develop symptoms at all and the aortic stenosis may only be found by chance during review for another condition.
The classical symptoms associated with severe aortic stenosis include –
Shortness of breath
As the severity of the stenosis increases one would expect to see increased strain on the heart as it pumps. Over time, this extra workload can cause thickening of the muscle on the wall of the left side of the heart. This is called left ventricular hypertrophy.
Frustratingly, this thickening reduces the space available in the left ventricle to fill with blood which should then be squeezed around this body. In time, left ventricular hypertrophy leads to a reduced ability of the left side of the heart to empty its load into the aorta. This condition is called left ventricular failure. Left ventricular failure can cause fluid to accumulate in the lungs which may provoke a cough with frothy white sputum, very significant shortness of breath on exertion, shortness of breath when laying flat and severe episodes of shortness of breath when sleeping causing the sufferer to wake in panic. A patient presenting with these symptoms needs to be treated urgently.
It is not fully understood why patients with aortic stenosis faint. The two predominant theories include –
Failure of the left ventricle to eject enough blood into the aorta to meet the requirements of the brain when required.
AND / OR
A reflex mechanism caused by the jet of high pressure blood hitting the wall of the aorta. This in turn causes the body to think that the blood pressure is too high resulting in dilation of blood vessels and drop in blood pressure.
A stenotic aortic valve that is causing symptoms requires replacement or repair which can be done either by minimally invasive surgery or via open heart surgery. Replacement is also indicated when the valve is classed as severe irrespective of the presence of symptoms. When replacement is indicated it should usually be done soon to avoid the risk of sudden deterioration in valve function which is associated with the risk of sudden death.
In patients where the risks associated with an operation are high the option of minimally invasive valve repair is preferred.
Where the surgical risk is less so the valve can usually be replaced.
The replacement valve can be –
Autologous (meaning from one’s own self) – such as in the Ross procedure where the Pulmonary valve is swapped into the aortic position
Porcine – replaced with a tissue valve from a pig
Prosthetic – man made
The pros and cons of each are not discussed here but should be discussed in detail with your surgeon.
Other co-existing medical problems may also interact with the processes that determine the speed of degeneration of the valve and as such should also be addressed as a routine part or caring for a patient with a stenotic aortic valve.