Clinical studies.
So much for the theory, does it actually work? We looked at both normal healthy patients and patients with LVF to see. Normal patients have an SMII of 1.6 – 2.2W/m2. Patients with cardiac failure have SMII values ranging between 0.4 and 1.0W/m2. This should come as no surprise. When we looked at the Frank-Starling curve above, the difference in stroke volume between the normal curve and the heart failure curve was about three-fold. Severe heart failure patients have an SMII of about one third of normal, at around 0.6W/m2.
Age v Inotropy.
In healthy subjects, younger patients have higher SMII values. Kids between 3 and 15 years of age have an average SMII of 1.92 W/m2. In subjects between 16 and 35 the average SMII was 1.87 W/m2 whilst for subjects over 50 years the figure was 1.68 W/m2. So guess what? Your heart gets weaker as you get older!
It seems that the approach of assessing cardiac power by “looking backwards” from the point of view of the circulation is valid. What’s more, the circulation doesn’t know or care why the heart is not delivering enough PE and KE to it. It matters nothing whether the problem is myocardial infarction, valvular disease, arrhythmias or anything else.
The circulation is a bit like a young child sitting at the dinner table. He doesn’t care if the farmer didn’t plant the crop, the supermarket was shut, the car broke down or mom forgot to go shopping. All he knows is that he’s hungry and will make his displeasure known in an all too obvious way! Whatever the problem, if the heart can’t deliver enough PE (blood pressure) and KE (blood flow) to the circulation then the heart has failed, period. SMII shows us the magnitude of the problem.
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Basic Science
