One of the foundational lessons in echocardiography is, How to measure the cardiac output ?
Echo formula for estimating cardiac output is
(HR X LVOT area X LVOT-VTI*)
* VTI (Velocity time Integral -a Link ) is complex sounding, but a simple parameter. It is expressed in CMs .In simple terms, it tells how many cm, the blood will move per second ? with a single ventricular contraction.
Image courtesy : http://www.pocus101.com
Sample calculation
If the normal LVOT diameter is about 2cm. Area will be πr2 . Since the radius is 1cm, area will be same as value of π ie 3.14
Normal LVOT VTI is about 20 cm & HR is 70
Stroke volume = 3.14 X 20 =63 ml . Cardiac out put = 63 X 70 = 4410 ml
It can be converted to cardiac Index with body surface area.
Stroke volume measurement is not a big deal .
All currently available echo machine has automated soft ware .It beams the cardiac index live. How reliable it is ? It is as bad or as good as our trust levels. We can Imagine, how the machine vision traces the borders of LVOT. But, carefully done manually measurement is always reliable . Even then, many of us are reluctant to use echo for stroke volume . Here is an Important paper that compared cardiac output calculated by Echo with that of the the gold standard of thermo-dilution technique. Zhang Y, Wang Y, Shi J, Hua Z, Xu J. Cardiac output measurements via echocardiography versus thermodilution: A systematic review and meta-analysis. PLoS One. 2019 Oct 3;14(10)
Such a simple calculation, why is it not popular among cardiologists. ? (Many ER physicians still use it effectively )
I guess being a humble and simple, works against it. It is surprising, the awareness about EF% is all to pervasive (which is a crude and load dependent parameter) while the info about stroke volume per beat finds difficult to enter bed side cardiology. Generally, we tend to trust complex things and ready to spend ( waste) lots of time with equations, such as in PISA, EROs , DVIs , PAPIs, , stroke work etc . Mind you the probability of error increase directly with number of factors you take in a calculation
The potential bed side usage of stroke volume data
1.Cardiac Index is one of defining criteria for cardiogenic shock. I rarely see my fellows documenting LV stroke volume in any major STEMI, that can detect an impending cardiogenic shock.
2. It will help assess the efficacy of inotropes .Find, objectively how much the Dobutamine really worked in LVF ? and correlate it with clinical assessment of S3, rales and BP. (Mind you BP is very poor surrogate marker. for stroke volume. )
3. More Importantly, Similar to LV stroke volume, we can measure RV stroke volume by RVOT area x VTI x HR. This can be of critical value in the management of RV infarct, where two ventricular stroke volumes often mis-match often. Once RV stroke volume equals the LV , we can presume recovery.
4.The enigma of high output cardiac failure can be studied with this simple formula.
5.In any mechanical assisted circulation LV, stroke volume is going to give important prognostic info.
6.Finally, assessing stroke volume will aid in fluid therapy in ER with any cause of systemic hypotension.
.Final message
Its time, we concentrate & fine tune the echo derived stroke volume as a definitive circulatory volume data in day to day practice.
A request to all ER physicians and cardiology fellows, try measuring the cardiac output whenever you see patient in shock and hypotension .Make it a habit to measure the stroke volume. You need just two minutes. It is cheap, you can repeat as many times as you want in follow up. It can totally change the way you understand hemodynamics of cardiac failure and shock.
Reference
Further reading
There is one more methodology, called EIT electrical Impedance tomography, that can provide live online stroke volume , comes inbuilt in ECMO and other MCS systems
Dr.S.Venkatesan MD 