By Mark Ard
Welcome to a new type of mnemonics I like to call “napkin drawings.” This term describes diagrams that literally look like they were drawn on a napkin, devoid of proper axis labeling. They serve to highlight a few key concepts without getting bogged down in the details. Mine are drawn on Penultimate, but you should be able to copy them to your study resources (once you understand them!).
Let’s start with one of those simple on the surface concepts that the USMLE asks 101 different ways – Frank Starling and PV Curves.
First let’s look at pressure-volume loops (left part of the diagrams). I’m not going into isovolumetric vs. rapid ejection vs. early/late filling. Let’s just appreciate they go in a loop (counter-clockwise…hopefully). When you change things, you get a new loop.
Next we have the dreaded Frank-Starling curves (right part of the diagrams). They are a superposition of two separate yet interrelated concepts. The bottom-left to top-right going line is basically a reflection of contractility (inotropy). The Y-axis is Stroke Volume (SV). The X-axis is Left Ventricular End Diastolic Pressure (LVEDP). In the words of Starling, “The heart will pump what it receives.” But you ask, how do I know what it received? Where are we on the line?
Overlay the top-left to bottom-right going line. Let’s call this the preload line. The Y-axis is Venous Return (VR) and the X axis is Right Atrial Pressure (RAP). Notice that if VR is zero, meaning the heart ain’t pumping, then the X-intercept is the mean systemic pressure exerted by all the blood vessels (in the absence of a functioning heart). That’s a test question.
Now combine the two graphs (by blatantly disregarding units and utterly confusing people) and you have a relationship between contractility and preload! Where they cross is what’s actually going on.
Diagram 1: You increased preload (Normal Saline bolus). You are now on a different preload line. Contractility didn’t change; you just kinda slide along the contractility line. SV went up (yeah) but your LVEDV went up too. Watch out for the danger zone where congestive symptoms occur. Notice the imaginary line called Arterial Elastance. It is parallel from the old loop to the new. This is how you know it’s a preload change.
Diagram 2: Now let’s look at a decrease in contractility. We slide down the preload line and are at an increased LVEDV (hello congestion) and stroke volume decreases (no marathons). The overall volume didn’t change, but the start/stop slid to the left in the pressure-volume loop. Another imaginary line, the ESPVR, moves when contractility changes. To understand the effects of Digoxin, literally go backwards from dotted to solid.
Diagram 3: Finally we have a combo deal. Afterload increased, say from hypertension. Your contractility and preload curves implode like a poorly build house of cards, so the only big change is a straight drop in Stroke Volume. In the loop, you see the isovolumetric contraction (sorry, I went there) phase is elongated because it has to overcome high pressure. Also, notice the Arterial Elastance line moves differently than a preload change.
Give these a try for increase/decrease in preload, afterload, and contractility. Then list all the things that affect those three parameters.
Got a “napkin drawing” you’d like to share with the First Aid Team? Email it to us at [email protected]
Good luck studying.