Question

1. Background: Critically ill patients in a cardiovascular intensive care unit must be intensely monitored for any number of conditions. One of these conditions is hypoperfusion, which is an overall decrease in blood flow throughout the body. Low stroke volume is one indicator of hypoperfusion. The pressure-volume (PV) graph below shows three possible loops for the heart. The control loop shows a healthy heart while the other two loops show reduced or increased preload (assuming that afterload and contractility remain fixed). 200 Control Preload LV Pressure (mmHg) | Preload RK '15 200 0 . 0 100 LV Volume (ml) (a) What is the stroke volume for the control patient? (b) A healthy heart rate (HR) is measured as 80 beats per minute. What is the control patient's cardiac output CO? mL Control patient's CO: min (c) Using the control PV loop, estimate the work done by the heart (in J) for a healthy individual during a single heart beat. Select the answer that is closest to your estimate. Hint: What are the units of work? What are the units of the PV loop? Assume the PV loop as roughly a rectangle, with the height of the rectangle - 75 mmHg, and the width of the rectangle 75 ml. (d) If a patient is suffering from hypoperfusion, which of the above graphs best represents their PV Loop?

Answer 1

a) stroke volume for a control patient is = ESV- EDV= 70ml

b) Cardiac output = HR $\times$ stroke volume = 80 $\times$ 70 = 5600 ml/min = 5.6 L/min

c) work done at constant pressure = W = P$\bigtriangleup$V = 0.6N/cm² $\times$ 100cm³ = 60 N cm = 0.6 N m = 0.6J

d) The graph that depicts hypoperfusion is the reduced preload( green portion). This is because during hyperfusion blood volume and pressure decreases resulting in ischemia and hence the preload which is eqqual to the ventricular filling also decreases because decreased preload is characteristic of low blood pressure and blood volume.