Assessing LV function is the most common indication for doing echocardiography .Sinus tachycardia is the most common cardiac arrhythmia in humans. So, it is no surprise we encounter the above situation very often in echo labs.
The principle behind echo assessment of LV function is to measure the left ventricular wall thickening and the resultant reduction in LV cavity size.
The LV wall thickens in systole and returns to baseline thickness in diastole.When the heart rate increases the rate of thickening has to be faster, as do the rate of subsequent thinning in diastole. The endocardial segment of systole and thin segment of diastole tend to overlap at the base . borders are too shaggy and many times the thick So identifying true reference point for endocardium become a difficult task.
It is ironical , inspite of M mode echo being termed outdated and obsolete by most Echo schools , it remains the most utilized modality to measure LV function . It is highly unlikely , M-Mode derived LV EF will be replaced in the near future .This is because “simplicity will always prevail” over quality and accuracy .
It is all the more important , the already poor index of M-Mode – LV EF % becomes further error prone at high heart rates.
It need to be emphasised , the impact of tachycardia in confounding the true EF is greatest in patients with preexisting LV dysfunction .
In a normal heart the errors are less and it can be safely stated , tachycardias rarely result in clinically important LV function errors
Does 2D derived EF by modified Simpson overcome the problem of tachycardia related errors ?
To a certain extent , “yes ” . Here again the endocardial excursion is so fast one might have difficulty in marking the border.Automated border detection algorithms are never corrected for heart rate related errors.
Other issues in LV function assessment during tachycardia
In the presence of CAD , the coronary arteries often have varying degrees of obstruction. Hence ,the myocardial segments also exist in varying degrees of ischemia.
In patients with significant CAD ,tachycardia due to any cause (Compensatory /Non compensatory -Fever, anxiety etc) can be considered a stress to myocardium . (By all means , can we consider it an equivalent of dobutamine stress echo! ?)
We know , dobutamine stress echo , has a variable effect on the contractility of LV. It can either depress , argument, or have neutral effect .Different lesions have different response depending upon the baseline viability of myocardium . For example a 70% lesion subtending a infarcted – viable segment may improve , while a 90% lesion supplying a normal LV segment may either worsen or hypercontractile .
What is poor man’s viabilty test ?
Grossly differing LV EF % in two different echocardiograms at two different heart rates may be an indirect clue for the presence of viable myocardium.(Poor man’s PET or Thallium !)
The final message
The above concepts remind us the complexity of measuring the true EF in the presence of CAD. Purists, may even question the existence of a ” true normal EF” in a given patient
So, in the presence of marked tachycardia what is the ideal advice ?
- Aviod measuring EF % during tachycardia , atleast in patients with CAD.
- Use simpson method whenever possible
- Atleast attempt it few times, you will overcome the laziness !