Measuring left ventricular size and wall thickness is a standard part of the routine echo examination. There are normative values for LV wall thickness, and the trainee sonographer is taught basic pattern-recognition in the early phases of training to identify patients with left ventricular hypertrophy. It is often assumed that Thick walls = hypertrophy. However, the influence of left ventricular volume to wall thickness is completely overlooked in that equation. Today’s post looks at the relationship (or interaction) between wall thickness, LV volume, and LV hypertrophy.
The 2015 ASE guidelines for chamber quantification describe a normal LV wall thickness (IVS or PW) as 0.6-1.0 cm (male) and 0.6-0.9 cm (female). This is measured at the blood-tissue interface using either 2D or m-mode, typically from the PLAX view. Increased wall thickness may be suggestive of LV hypertrophy, however, as an isolated number we have no indication about LV modeling, or left ventricular mass. During a recent heart dissection tutorial, one of my students made the observation that the cow’s heart they had cut open had very thick LV walls. This was a true statement, but the blood had been emptied from the heart and the remaining cavity size was much less than the usual diastolic chamber size. It is a bit like a helium balloon…when the balloon is empty, the balloon is very small and the wall of the balloon is quite thick (too thick to see through). When we inflate the balloon, the cavity increases in size, but the wall thickness is stretched to be quite thin and translucent. The wall is remodeled to accommodate the increase in chamber size. This is exactly what is happening in the heart.
But who cares? I never look at empty hearts…
True statement, but we do look at a lot of underfilled hearts, particularly in a postoperative setting. In this example, we see a septal and posterior wall thickness measuring 11mm.
Original PLAX view. LV mass – 214g.
The following echo was then performed 6 months later in the setting of marked hypotension post-septic knee washout. Note the drastic increase in wall thickness, now measuring 14 mm. The wall muscle has not hypertrophied in this time. It has not had an increase in the number of fibers in the myocardium. The ventricle has remodeled due to the drastically reduced blood volume in the LV. The walls are like that of a deflated balloon. The right ventricle has become severely hypokinetic and the left ventricle is underfilled as a result (was 51mm – now 40 mm).
PLAX view 6 months later in ICU. Note the increase in wall thickness, but no significant change in LV mass – 209g.
A4C view – note the severely hypokinetic right ventricle and underfilled left ventricle.
Looking solely at the wall thickness we may draw the wrong conclusion…but let’s look at the LV mass in this patient. The left ventricular mass looks at the total heart volume (epicardial volume) and subtracts the volume of blood that is in the cavity (endocardial volume) to be left with a myocardial volume. The volume is multiplied by the specific gravity of tissue (a standard factor of 1.05) to estimate a left ventricular mass.
There are several techniques for measuring mass by echo, but for simplicity of this case, I will use the linear method (cubed equation). I don’t want to get bogged down in pros/cons of the various techniques… I just want to focus on the big picture and this is the easiest way to make the point. Read the 2015 ASE guidelines for chamber quantification (table 5) for more information on the different techniques… and remember, if you are going to report a mass, it should ALWAYS be indexed to patient size.
Using the cubed formula in this case, we can see that the mass is within 5g of each other between the 2 studies, despite a marked increase in absolute numbers of wall thickness. Yes, the wall thickness has increased, but this is not due to hypertrophy or an increase in mass. It is just that the LV is underfilled. More than likely (based on the mass calculation) if we inflated the LV to 51mm, the walls would stretch out to be closer to 11mm, rather than 14mm.
The take home point is to consider the serial data carefully. An increase in wall thickness must always be taken in the context of changes to LV cavity size.
Not the point of this post, but still an important point… I always suggest the use of contrast enhancement (Definity) if you are going to report increased wall thickness. There are too many examples of inadequate border delineation with standard 2D imaging which can be easily resolved with contrast.