Corrected Flow Time FTc.
Corrected flow time (what the flow time would be if the heart rate were 60/minute, rather like QTc in the ECG) can also give us some guidance as to fluid loading, as a low figure is found in hypovolaemia. The reason for this is that the heart contracts at a force mainly determined by its inotropy and preload. For any given inotropy level, the time the heart takes to eject a stroke volume depends on how large that stroke volume is, which in turn depends on how full the ventricle was at the end of diastole. A low LVEDV leads to a low SV and a short flow time, whilst a high LVEDV leads to a high SV and a long flow time. Simple! Well, most of the time in healthy subjects, FTc and LVEDV are pretty close.
If inotropy is fairly normal then this relationship between preload and FTc holds, but it’s actually not quite so simple once we start looking at sick patients with lower SMII values.
The main problem with this concept is that the flow time will, as mentioned above, depend on inotropy. As inotropy increases the flow time will become shorter, the ventricle will eject more rapidly. Inotropy changes with preload, increasing as preload increases, so the relationship between FTc and LVEDV is far from linear. It is often claimed in transoesophageal Doppler studies that FTc is a reliable indicator of preload, but clearly this cannot be true if inotropy is not constant. The other big problem is that flow time will also depend on afterload. The higher the afterload the higher the flow time will become as the heart has to work harder against the back pressure from the aorta. This can lead to a change in inotropy which can be either positive or negative depending on the exact pressures and ventricular size, and the resting inotropy. The relationship is clearly not simple. A high FTc may indicate a high preload but is also found in cardiac failure (well OK, we could say that that represents a high preload too) but either way, it suggests that more fluid is not wise.
Left Bundle Branch Block.
With LBBB the ventricle depolarizes abnormally and invariably leads to a prolongation of systolic flow time. Under these circumstances can we still use FTc as an indicator of preload? The short answer is no, but it may still give us some guidance in therapy by looking at the response to preload changes. This is far from easy however, and it is probably better not to use FTc if the ECG shows LBBB.
FTc and SVV can therefore be regarded as useful guides to preload but must be interpreted in the light of the full clinical and haemodynamic picture. If only we could measure or calculate the true LVEDV then the problem would become a lot more straightforward. SMII can help us here.
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Measurement of Preload
