How to calculate stroke volume, ejection fraction and heart rate for this question?

Question

Q: Assume that one ventricle of a child’s heart has an EDV of 90mL and ESV of 60mL and has a total cardiac output of 2.55L/min. What is the child’s stroke volume, ejection fraction and heart rate?

This is what I have so far, but I'm stuck as to how to get the heart rate?
STROKE VOLUME: stroke volume = cardiac output / heart rate sv = 2.55 L/min / ????
EJECTION FRACTION: E=(sv/edv)*100 E= ?? /90 mL *100
HEART RATE:????

Answer 1

I know that it has been awhile since this question has been asked. Perhaps you have already received input that has allowed you to successfully understand the concepts and answer the questions. Either way, I am more than happy to discuss this scenario and the related concepts...as these concepts are often challenging - until described in a manner that is accessible.

So...here we go.

We are given the End - Diastolic Volume (EDV) as being 90 ml, and the End - Systolic Volume (ESV) as being 60 ml. Recall that during diastole there are a number of important events occurring, despite the tendency to 'label' it as the resting phase. The two most important processes that occur during diastole are:

1. Filling of the ventricles in preparation for systole
2. Perfusion of the myocardium via the coronary arteries

Given the above, the EDV is the volume of blood in the ventricles after maximal filling during diastole (and immediately before systole). In our scenario, we are told that this equals 90 ml. Systole (myocardial contraction) is about to occur, and technically, there is now 90 ml of blood available in the ventricle for ejection. If our heart as a pump were 100% efficient, then all 90 ml. of blood would be ejected during systole. This however is not the case. At best, about 60 - 70% of the blood available for ejection at the end of diastole is ejected during systole. Not only does this statement make a point about the efficiency of our heart as a pump (60 - 70% efficient), this is also the definition of the Ejection Fraction (EF). For the EF is nothing more than the specific efficiency of our patient's heart as a pump. In other words, the EF = how much blood is ejected during systole, aka the STROKE VOLUME (SV) / the volume potentially available for ejection at the end of diastole (EDV). Just as mechanical engineers use this formula to measure the efficiency of mechanical pumps (eg. imagine a basement filled with 100 gallons of water. In order to remove the water from the basement, there is a sump pump. The water that needs to be removed (or available for ejection) is analogous to the EDV. The pump is turned on, and after a given amount of time, ends up pumping 75 gallons of water out of the basement - leaving 25 gallons behind. Well, for our purposes, let's call the 75 gallons ejected by the pump, the STROKE VOLUME (SV). Therefore, the efficiency of the sump pump, analogous to the EF, is 75 gallons / 100 gallons...or 75%. - starting to make sense...)

So....back to our patient. There was 90 ml. available in the ventricle at the end of diastole (EDV = 90 ml.). As we rarely measure the ESV directly, this may be confusing to some - but it is simply an indirect way of telling us what the STROKE VOLUME is. Recall that the SV is the amount of blood expelled from the ventricle during systole. If we started with 90 ml, and after systole...had 60 ml of blood still left in the ventricle, then 30 ml. of blood was pumped out. This is the STROKE VOLUME. The term, stroke, comes from our mechanical engineering friends, for imagine the pump turning a stroke with each revolution of the engine.

So....now we have:

1. The STROKE VOLUME = 30 ml
2. The EJECTION FRACTION = SV or 30ml / EDV or 90 ml = 33 % *note: while pediatric measurements and normal values can vary by age, weight, heart rate and other factors - adult normals for EF are generally considered to be between 60 - 70 % (varies by institution). Thus a heart having en efficiency of 33%, is seriously deficient as a pump. It is important to remember, however, that comparisons between values of EF are most useful and meaningful when used to evaluate the cardiac function of a specific individual. For example, say John Smith had an EF of 66%, discovered on a routine echocardiogram at age 57. At the age of 62, John had a significant myocardial infarction (damaging his heart muscle). A repeat echocardiogram, performed after the MI, shows an EF of 41%. 66% before, 41% after - one can see how comparing these EF values can give us an approximate quantitative idea regarding how much John's heart was damaged by the MI.

OK.....Finally! Perhaps the most tricky part of this question is calculating the Heart Rate (HR). However....I'm hopeful that after the following discussion, the word 'tricky' won't even be relevant! As noted, we are given the patient's Cardiac Output. By definition, the CARDIAC OUTPUT (CO) = SV (the volume of blood (ml) ejected during systole (by convention, one systolic contraction or beat.) x HEART RATE ( measured in beats per minute (BPM)). In other words, what is the total volume of blood pumped out by the heart to the body in one minute. For our patient, we know the STROKE VOLUME (30 ml.) and the CARDIAC OUTPUT (2.55 liters / minute). Therefore, the HEART RATE (HR) = CARDIAC OUTPUT (CO) / STROKE VOLUME (SV). (Algebraic rearrangement of the cardiac output equation, solving for heart rate.). Entering the values we know:

CO = 2.55 liters/minute DIVIDED BY SV = 30 ml (or 0.03 liters) = 85 BPM

In Summary, our STROKE VOLUME = 30 ml. Our EJECTION FRACTION = 33% And our HEART RATE = 85 BPM.