LVET variability?

[fredrik]

I've got limited knowledge on human physiology.

 

Given that there must be calibrations for individuals etc, does anyone know what the typical variability of the LVET(left ventricular ejection time) is as a function of the relevant variables. I guess assuming normal heart function.

 

I presume LVET = f(HR,...?) with some calibration paramters

 

HR = heart rate.

 

I'm working on a bioimpediance application to do a non-invasive estimate cardiac output, and the standard prescription is to put in a typical fixed LVET. But I am doubtful how sensible that is. Should I at minimum include a linear correction vs bpm? There will be simultanous ECG measurement too.

 

Any "typical" relations?

 

/Fredrik

 

In the unlikely event that someone else is interested I found this old paper.

 

"Heart rate--left ventricular ejection time relations. Variations during postural change and cardiovascular challenges", Br Heart J. 1976 December; 38(12): 1332–1338.

-- http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=483176

 

It suggest a linear correction against bpm, and that LVET drops with increasing bpm, where the constant furthermore depends on the type of activity.

 

I think the solution is to replace the constant with a dynamical beat-to-beat evaluation of the parameter, computer from some of the available signals. Possibly heart sound.

 

/Fredrik

[fredrik]

Just to finish this thread. The better algorithms identify the so called X and B points within each cardiac cycle in the dZ/dt curve, which works for normal hearts, time difference beeing LVET.

 

It seems, apart from the formula itself, the variability of blood resistivity with for example haematocrit values and other variables, is another issue that is usually ignored in the standard formula for stroke volume(SV), but which seems hard to correct for by non-invasive methods.

 

[math]SV = \rho_{blood} * (L/Zo)^2 *LVET* |dZ/dt|_{max}[/math]

 

L = distance between sensing electrodes.

 

So Zo, dZ/dt and LVET are measured and computed from the electrode voltage. The other parameters are assumed constant.

 

/Fredrik