Set 4: cardiothoracic ratio-

the hard way

In order to correct for overall body size, instead of using an absolute measurement for heart size, we use a ratio:  the cardiothoracic ratio, as shown here.  First you measure the widest inside diameter of the thoracic cavity (shown in blue) and then you measure the widest diameter of the heart.  Normally, the heart should be less than or equal to HALF of the thoracic diameter.  In this case, the cardiothoracic ratio was 0.4, which is normal.  You will notice that this is VERY difficult to determine by visual inspection.