How QLVA Works

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Topics on this page:

Why Do We Perform Quantitative Left-Ventricular Angiography?;
  Left-Ventricular Measurements; Measuring Left-Ventricular Volume;
  Left-Ventricular Wall Motion

 


Why Do We Perform Quantitative Left-Ventricular Angiography?

LV angiography is relatively easy to do and tells us quite a bit about the condition of the heart. By just viewing the angiogram, we can qualitatively assess the size of the left ventricle and its pumping efficiency, the condition of the mitral and aortic valves and something about the adequacy of the coronary artery blood supply. If we quantitatively measure left-ventricular volumes, we can calculate cardiac output and ejection fraction - important indicators of the heart's health and even the probability of survival of the patient. Although coronary angiography can tell us whether blood is flowing to a region of the LV, only the analysis of the ventriculogram can tell us whether this part of the heart muscle is contracting effectively.

By performing repeated QLVA, we also can assess the adequacy and effectiveness of interventions such as CABG and PTCA. This is important because, although coronary angiography can tell us about an improvement in blood supply to the heart muscle, it cannot tell us whether this improvement was accompanied by improvement in cardiac function.

Left-Ventricular Measurements

We begin by tracing the outline of the LV shadow at end-diastole (when it is the biggest) and end-systole (when it is the smallest). 

We then can use these outlines to calculate end-diastolic volume (EDV) and end-systolic volume (ESV).  (In truth, we can't calculate the volumes - there is not enough information in one or two X-ray projections. We actually estimate the volumes using mathematical models and assumptions about the symmetries of the LV cavity.)

These numbers are then plugged into a bunch of formulas to calculate other, more interesting, parameters, such as:

Stroke Volume (SV) = EDV - ESV
Ejection Fraction (EF) = SV / EDV
Cardiac Output (CO) = SV x Heart Rate

Measuring Left-Ventricular Volume

 There are two fundamentally different methods of estimating LV volume: Simpson's Rule and the Dodge-Sandler Area-Length Method. However, both methods also have much in common. Both depend on the accurate determination of the LV border. This is usually done manually by an operator because automated LV edge-detection does not work very well. Both methods also require the determination of the long axis of the LV. The long axis is typically defined as a line between the mid-point of the aortic valve and the LV apex . Because the apex can be defined as the point farthest from the mid-point of the AO valve, the long axis can be determined automatically.

 

Simpson's Rule Method

Simpson's Rule is a fundamental mathematical principle. It is based on the idea that the volume of an object can be determined by "cutting" the object into thin "slices", measuring the volume of each slice and summing the volumes of all slices. Simpson's Rule is applied to the LV by slicing the LV into "discs" along the long axis, as shown in figure A. If a single-plane angiogram is used, each disc is assumed to be circular, since only one diameter is known. If biplane angiography is used, each disc is assumed to be an ellipse, with a major axis determined from one plane and the minor axis determined from the other plane. The area of each disc is calculated and multiplied by the disc's thickness to determine its volume. Simpson's Rule works pretty well for determining LV volume, but it turns out that there is a simpler and better way.

 

Dodge-Sandler Area-Length Method

Many years ago, Drs. Harold Dodge and Harold Sandler observed that the left ventricle looks elliptical when viewed in the 30-degree RAO projection. To make things easy, they made the long axis of the ellipse coincide with the long axis of the LV. If  an ellipse is spun around its long axis, it forms an ellipsoid of revolution. The calculation of the volume of an ellipsoid of revolution only requires knowledge of the length of the axis of revolution and the area of the ellipse. The way Dodge and Sandler defined things, the length of the axis of revolution was just the length of the long axis of the LV. Easy! Determining the area of the ellipse was a bit stickier. This is where the real intellectual breakthrough happened. They asked "Why not measure the area of the LV shadow and use that area for the area of the ellipse?" The area of the LV can be measured mechanically by tracing its border with a planimeter.

Now our two Harolds had a method, but how would they know if it was accurate? Measuring LV volume in-vivo was not practical. So they collected some cadaver hearts and injected the LV chambers with a mixture of silicone rubber molding compound and barium powder. The result was a set of anatomically and dimensionally accurate radiopaque casts of human left ventricles. It's pretty easy to measure the true volume of a cast - you just plunge it into a beaker of water and see how much water is displaced. They then X-rayed the casts and did the area-length calculation. The results were very good! Thus, the Area-Length method was born and soon there was a planimeter in every cath lab. These days, planimetry is performed with a computer, but the principle is exactly the same.

But wait... there's more!

It turns out that both volume methods can be improved a bit more through the use of statistics. This is done by performing a linear regression which compares true and measured volumes of the set of models. The regression coefficients can then be used to correct any new volume that we calculate. Several sets of regression coefficients exist today, each with its own set of adherents. Actually, they are all quite good. 

 


(Kids! Don't try this at home!)

 

Two Personal Asides

  1. I know about much of this because many years ago Drs. Dodge and Sandler were nice enough to help me develop what probably was the first computerized version of their method. Dr. Sandler actually loaned me their original set of LV casts to use in my validation! The casts sat in a cardboard box in my office at Stanford for many years. Unfortunately, I didn't appreciate their historical importance until it was too late - at some point they disappeared.
  2. The LV volume system that I developed was based on the use of a video disc recorder and light-pen. It was sufficiently novel that it was actually featured in TV Guide! My 15 minutes of fame came (and went) pretty early in my career. The guy with the sideburns pushing the catheter is Dr. Ed Alderman, my friend and co-conspirator for most of my career. Some of you old-timers may remember X-ray systems like this one - the gantry was stationary and we rotated the patient!

Left-Ventricular Wall Motion

Volume measurements give us a good idea about how well the entire left ventricle is functioning. However, the left ventricle is supplied by two major coronary arteries, each with many branches. Each of the branches supplies blood to a distinct region of left-ventricular muscle. It is very useful to look at the performance of the LV in each of these regions separately, so that we can estimate adequacy of the blood supply from each of the branches. We do this by measuring how "well" each point on the LV contour contracts during systole.

This idea raises two interesting questions:

  1. Given a point on the end-diastolic contour, how do we identify the location of the same point on the end-systolic contour?
  2. How do we know what is the normal amount of contraction for any given point?

Looking at some random point on the end-diastolic LV shadow, it is impossible to know where the point will be on the end-systolic shadow. However, we can predict where the point will be through the use of a model. Many contraction models have been proposed through the years. 

 

The Orthogonal Models

The earliest and simplest types of models are illustrated in figure A. In these models, each point on the LV wall moves on a path orthogonal (perpendicular) to the long axis of the LV. However, although these models are appealing in their simplicity, it is easy to see that they do not work at all well near the apex of the LV.

 

The Radial Models

The next models to be developed were the so-called radial models, illustrated in figure B. In the radial model, all points move toward a single point somewhere in the center of the LV. Although the exact location of the point is debatable, radial models were a vast improvement over the orthogonal models. The most well-known radial model is probably the one developed at Stanford by Ed Alderman and myself. It was based on a body of really amazing work done with implanted myocardial markers at Stanford by Craig Miller, Neil Ingels and George Daughters.

 

The Centerline Model

Some might argue that all points on the LV probably do not move toward a single point - and they would be right! To eliminate the need for such a "magic point", Florence Sheehan, Ed Bolson and colleagues at the University of Washington developed the centerline model. In the centerline model, an artificial line is constructed midway between the end-diastolic and end-systolic contours. The model assumes that all points on the LV wall move perpendicular to this centerline, as illustrated in figure C. Both the Stanford method and the Centerline method are quite accurate.

The Truth About LV Wall Motion

In the early days, there was controversy over the question of whether the LV also "rocked and rotated" as it contracted and whether there should be compensation for this rocking by realigning the ED and ES contours. The myocardial marker studies have put this controversy to rest, as well as a lot of other erroneous conceptions about LV wall motion. In fact, we now know that LV inward contraction is only one factor. Of more importance in normal hearts is thickening and twisting of the muscle, making the cavity smaller. But contraction is what we still call it, and as long as we all know what we mean, that's ok.

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