Diastology

Diastology

In this post I’m going to talk you through a little cardiac physiology.For those of you who have interest in this sort of thing, read on.The remaining 90% of you can simply click the “back” button in your browser and move on to checking your Facebook profile or browsing the latest news about celebrities acting badly.

Every once in a while I feel the urge to write about something that’s actually educational, in contrast to previous posts about drug names, anxiety, and poetry dedicated to toxic flowers.With this in mind I’d like to educate you on the concept of congestive heart failure (CHF) and ejection fraction.

CHF, as most of you already know, is the condition that arises when the heart cannot adequately cycle the needed amount of blood volume to your limbs and organs.The symptoms of CHF arise from an inability to push blood out to the body (fatigue, lightheadedness) or to process the blood that returns from the body (swelling in legs, fluid in lungs, labored breathing).It’s a common condition that can arise from multiple causes.

The term ejection fraction (EF) should not be confused with rejection fraction, a phrase that refers to percentage of girls who turned me down for dates in high school (my RF was somewhere around 93%).No, EF is actually a heart-related thing.It refers to the amount of blood that is squeezed out of the left ventricle (LV) of the heart (the main pumping chamber) with each beat.Intuitively, you would think that a normal EF would be 100% but it turns out that the heart doesn’t ring every drop of blood out of its cavity with every contraction.A normal EF ranges from about 55 to 70 percent (in other words, about two thirds of the volume of blood that has filled the LV between contractions actually gets forced out with each beat; a third stays behind).

A weak heart (or, more precisely, a weak LV) is a common cause of CHF.Instead of having an EF of 60% a patient with CHF symptoms may be found to have an EF of only 20% and is therefore left with a heart that is about a third as strong as it should be.If only 20% of the blood in the ventricle is pushed out to the body with each beat it doesn’t take much imagination to understand how that type of patient may suffer from the symptoms I described above.

In order to demonstrate this concept I’ve created a series of visually stunning and anatomically accurate diagrams representing the highly complex mathematical principles associated with EF (in other words, I made some circles):

You’ll note two words you may not be familiar with: systole (SIS-tuh-lee) and diastole (dye-ASS-tuh-lee).Systole is Greek for “drawing together” and diastole translates to “drawing apart,” and respectively these words refer to ventricular contraction and relaxation.It’s not really important that you know what these words mean in Greek; it just gives me a chance to show off my mastery of ancient languages (and you wonder why I couldn’t get dates in high school).

Now let’s throw in some math.A typical heart fills with about 120 ml of blood in between beats (during diastole) and squeezes out all but about 40 ml during systole.The difference between those two values is called the stroke volume and is the amount of blood that is expelled with each beat (80 ml per beat and 60 beats per minute equals 4.8 liters of blood pumped each minute-a relatively normal resting value).We calculate the EF by dividing the stoke volume by the diastolic volume (80 ml ÷ 120 ml) and in this case come up with an EF of 67%.

Here are diagrams that represent a sick and weak heart.The stroke volume (160 ml minus 120 ml) is only 40 ml, half of what it should be.You’ll note that the size of the LV is somewhat different than in the normal example above: the heart is larger and the volume of blood is greater.This is a normal (albeit unhealthy) compensatory mechanism that allows the heart to continue to produce an acceptable stroke volume despite a lower EF.

To calculate EF we simply divide the stroke volume by the diastolic volume, and in this example we come up with a quotient of 0.25, or 25%.A patient with an EF of 25% could easily develop CHF symptoms and would be a candidate for aggressive medical management.

Now comes the really interesting part.Half of all patients presenting to hospitals in the U.S. with the diagnosis of congestive heart failure actually have a normal EF.How is this possible, you ask?Doesn’t a normal EF imply normal heart function?

The whole concept of EF is a fabricated construct that allows us cardiologists to quantify the strength of the LV contraction but it really has very little physiological bearing.The body doesn’t really care what the EF is-it’s only interested in how much blood gets pumped out with each beat.The stroke volume is the value that determines whether the brain receives enough blood to keep a patient lucid and comfortable.If the LV can squeeze out 80ml with each beat then the kidneys, liver, lungs and legs all stay happy; less than that and the patient develops CHF.

Let’s now introduce the concept of diastolic CHF, but in order to do this properly I need to briefly remind you all about blowing up balloons when you were kids.Do you remember how hard it was to expand a balloon that came straight out of the package?You blow with all your might but the stiffness of the latex resists expansion; only after you pull and stretch on the deflated balloon are you able to push any air into it.Later, after you’ve inflated and deflated the balloon several times, it becomes easy to blow it up to its maximal size.

When you’re young, your heart muscle is pliable and easy to stretch, much like a worn-in latex balloon.It accepts a large volume of blood with nearly no resistance.Furthermore, the young and healthy muscle cells actually participate in the process of relaxation between heart beats by actively stretching to accommodate the incoming blood volume.

As you age, your ventricular walls become gradually stiffer and don’t relax and expand quite as quickly or easily.Persons with poorly controlled high blood pressure compound the problem by developing thickened heart muscle that decreases the effective ventricular cavity during diastole.The heart takes on the characteristics of the stiff new balloon fresh out of the package.

Take a look at the next diagram.This is the heart of an elderly patient who has spent his life ignoring his doctor’s advice to take his blood pressure pills.The heart muscle is thickened and poorly compliant.The overall size of the LV is larger than normal, but due to the thickness of the wall the interior volume is diminished (60 ml versus the normal 120 ml).Ventricular contraction isn’t a problem, and the EF in this patient is robust at 67%.If you were to judge this patient’s heart function on EF alone you’d conclude that it’s quite healthy.

But remember, the really important number to the body is the stroke volume, and in this situation it is a measly 40 ml per heart beat-exactly the same as in our patient with a weak heart, and about half of what it should be.This number worsens anytime the patient develops a rapid heart rate (as with atrial fibrillation) or excessively high blood pressure.When that happens, the stroke volume drops even further, fluid builds up in the lungs and legs, and the patient comes to the emergency room with the exact same symptoms as the patient with an EF of 25%.

There are various ways we can assess the diastolic function of a person’s heart but the most common is with cardiac ultrasound (echocardiography).The science of ventricular relaxation has become quite a hot topic over the last decade and is something of a cottage industry for many academic cardiologists.I attended a conference a few years ago hosted by several self-designated “diastoligists,” experts in the arena of echo-based diagnosis and assessment of diastolic function.That’s right: there are cardiologists who specialize to such a degree that they limit their realm of study to the diastolic portion of the cardiac cycle.Not contraction and relaxation-just relaxation.That’s something like an auto mechanic who specializes in problems of the left front wheel.Of European cars.That are blue.And only during right-hand turns.

With our current epidemic of obesity (which leads to hypertension and diabetes) we’re likely to see a continuing rise in cases of CHF due to diastolic LV dysfunction.The mainstay of treatment is control of blood pressure, following a sensible low-salt diet, and good compliance with prescribed medication-all factors that require active participation on the part of the patient.

So, if you’re a patient with high blood pressure and likes to ignore your doctor’s advice, I’ll give you this warning: Take your meds, watch your salt, do your exercises, and eat your greens.And above all, do not make me call in the diastologist!