Potential to Kinetic Energy Ratio
PE:KE Ratio - PKR.
Potential energy gives us blood pressure. Kinetic energy gives us blood flow. As with so much in haemodynamics, the balance is important. Pressure is meaningless unless there is flow, but flow alone can’t maintain the vital organs. We need both in adequate amounts.
In the septicaemic example above, we started with high flow but low pressure. We then had good pressure but low flow after the noradrenaline. Finally we achieved adequate pressure and flow. The PE:KE ratio (PKR) is very informative here. The normal ratio is around 30:1, so far more of the ventricular power goes to generating blood pressure rather than flow. At the time of the first USCOM reading the PKR was just 3:1, plenty of flow but not much pressure. At the second USCOM reading the ratio had reversed to 64:1, enough pressure but inadequate flow. By the third reading the PKR was down to 42: 1. We’re getting close to the right balance. Finally, after the increase in dobutamine to 10mcg/kg/min the PKR was 33:1, close enough!
In arterial hypertension due to excessive vasoconstriction (as opposed to excessive cardiac output) the PKR is in the range of 60:1 to 150:1. Appropriate therapy with vasodilating medication such as ACE inhibitors or calcium channel blockers can reduce the PKR to near normal. Conversely, in hypertension due to excessive cardiac output, the PKR is around 10-15:1, and increases with appropriate therapy, such as a β blocker, back towards normal. Not only do we have a simple tool to identify the underlying cause of the hypertension, we also have a method of optimizing therapy other than just looking at the BP with no regard to blood flow. How cool is that?
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Balancing Inotropes
