Normally the value should be less than 12mm, and it is a measurement of the left ventricular diastolic (filling phase) pressure. The significance, if high, it suggests that your heart is not emptying itself of enough blood during systole (pumping cycle) or that your heart can't relax well. If a medical problem the symptoms would be shortness of breath with little exertion and fatigue. I don't believe a value of 12mm would be a problem for low cardiac output. The low CO is what causes the symptoms.