One of the burning questions that I never got a straight answer to through my MBBS and my residency was - why do murmurs happen?
I tried to make sense of them in this article here. But it doesn’t really answer why murmurs happen. Instead it explains a way to make sense of murmurs.
All my seniors, professors included, told me that it was because of turbulent flow of blood. And for all practical purposes, thats all it was. Turbulent blood flow through damaged valves or non compliant heart chambers lead to led to murmurs.
But I was not convinced and I wanted to know how murmurs actually happened? Why do you hear this whooshing sound when you auscultate? Was turbulence really the answer?
Turns out in 1959, another curious fellow set out to answer this question. I’ve linked the original article below. But I’ve summarised the findings for you. So here we go.
Prerequisites for a murmur
In order for you to be able to hear a murmur when you place your stethoscope on your patient’s chest, a few things need to happen. Firstly, something needs to happen to the flow of blood to ensure that it creates enough energy to vibrate the vessel walls. The vessel walls will then conduct those vibrations through the surrounding soft tissue to the precordium or chest wall. And finally these vibrations are picked up by your stethoscope.
Our goal is to explain what causes the blood to create enough enough energy to vibrate the walls of the blood vessels.
Theories Explaining Murmurs
There are three prevalent theories that have been proposed to explain murmurs.
Theory of turbulence
Theory of cavitation
Theory of Aeolian frequencies
Let’s look at each one.
Theory of Turbulence
What is turbulence?
Normal flow of fluid is laminar. This means that fluid flows in calm and parallel lines. As the velocity of the fluid increases and as the fluid is subjected to external pressures, this parallel configuration is altered. Fluid tends to form eddies and vortices which interact with each other. This is called turbulent flow.
This is important because, turbulent flow sucks the kinetic energy out of a fluid and we need to pump more energy into the system to get turbulent fluid to flow. It’s inefficient.
How do you know that a fluid is turbulent?
The Reynold’s number tells us if a fluid is flowing in a lamellar or turbulent fashion. It’s given by this formula-
R = (diameter)(velocity)/viscosity
R>2000 is suggestive of turbulent flow.
R<2000 is suggestive of lamellar flow.
Does blood get turbulent?
Constantly, yes.
In a stenotic valve, the velocity of the blood would increase. Therefore the Reynold’s number would be higher, and the flow would be turbulent.
In patients with anemia, the viscosity of the blood would fall and again, the Reynold’s number will increase. And again, you would have turbulence.
In patients with high out put states, the velocity of the blood would increase, and hence, blood would be turbulent.
So if we were to just take it for granted that turbulence causes murmurs, then in theory, it could explain all the major causes of murmurs.
Does turbulence explain murmurs?
Here is the problem though. Turbulence just does not create enough energy to be translated to noise simply because turbulence itself consumes too much energy.
All the pent up potential energy that laminar flow has is lost to the chaotic kinetic energy in turbulence. Very little is left over to cause vibrations in vessel walls which can be transmitted.
But lets say, just for the sake of argument, that turbulence could cause sound. How much turbulence would be needed for blood to cause a murmur?
Well in the Reynold’s number equation, the diameter blood vessel and viscosity of blood are fairly constant. The only thing that changes is the velocity. And it is estimated that blood would have to have a velocity of about 2000 cm/s in order to create enough turbulence to cause murmurs.
Unfortunately, our blood only travels at about 150 cm/s. That’s the fastest that blood travels at the peak of systole (aka peak systolic velocity or PSV)
So turbulence can’t really explain everything about murmurs.
Theory of Cavitation
What is cavitation?
Cavitation is really cool. It’s something that you should read more about.
But here is the gist. When a fluid flows through an area of high pressure and then comes into an area of low pressure, it passes a threshold called the vapour pressure.
When this happens, bubbles are formed. And as the pressures equalise, the bubbles burst. And the theory is that the bursting of thousands of these bubbles in unison leads to murmurs.
Cool right?
What is the problem with cavitation?
Simply put, in order for cavitation to explain murmurs, the pressure gradient has to be constant. In our blood, the pressure is constantly fluctuating.
So thats another theory thats out.
Theory of Aeolian Tones and Fluctuating Wakes
What are Aeolian Tones?
There is a well documented phenomenon that objects places in the path of a fluid lead to the formation of a wake. Now if the object is placed in the centre of the flow, the waves generated have a predictable frequency.
And this can lead to the production of sound.
Ancient civilisations used this idea to create Aeolian harps which are just sculptures with strings placed in the path of wind in such a way that sounds are generated. There is no plucking of the strings required. The air that travels creates the sounds.
How does it work?
Well then an object is blocking a fluid, gas or liquid, it creates vortices. The vortices move away from centre in a process called vortex shedding. And they are shed one after the other in an alternating pattern.
Now the line of vortices that is created is called a Von Karman vortex street or trail. The frequency of the Aoelian tone is the same frequency at which the vortex pairs are shed.
So the sound is produced by vortices?
It is the not the vortices themselves that produce the sound. They are actually wattless. The vortices leave little gaps int he fluid. It is the surrounding fluid that rushes to fill the space left by moving vortices that leads to the formation of sounds.
If the flow is turbulent, will Aoelian tones still be formed?
In vortex shedding, we normally assume laminar flow. But, as the flow velocity increases, this regular pattern of vortex shedding is lost because of turbulence.
But this does not occur till the Reynolds number exceeds 200,000.
This means that even if the flow is relatively turbulent, vortices can form in a periodic fashion. This, in turn, can generate sound which is transmitted by the vibrating vessel walls.
Are Aoelian tone loud enough?
It turns out that are close distances of about 5 cm (typical distance while auscultating a murmur), a velocity of 150 cm/s can also lead to audible murmurs. This means that theoretically, it is a possible explanation of murmurs.
Aoelian tones and the pitch of murmurs.
In Gallavardin phenomenon, there is a low pitched harsh murmur heard at the carotid and over the aortic area, and a high pitched musical murmur heard at the apex.
Have you ever been given a satisfying explanation for this? I am about to.
As the blood moves from the point of shedding, the frequency of the periodic waves generated reduces which leads to a lower pitch.
The frequency is higher closer to the origin of the sound.
So the carotid, which is downstream has a low pitched and harsh murmur, while the apex, which is upstream has the high pitched musical murmur.
So thats about it. That’s the best explanation that we have for why murmurs occur. I found it supremely fascinating to research this article and I hope you found it interesting too!