|Although CO is the most valuable haemodynamic parameter, assessment of ventricular filling is also believed to be important in the management of perioperative and critically ill patients. Unlike the use of PAC thermodilution for CO determination, there is unfortunately no bedside gold standard for determining optimal ventricular filling. In the absence of a good measure, the pulmonary capillary wedge pressure (PCWP) using a PAC is commonly used. However, the PCWP is subject to a number of technical and disease related problems. Oesophageal doppler waveform analysis has been increasingly evaluated as a method for determining optimum cardiac preload.
The key preload parameter of interest is the flow time; the time required from the start of the waveform upstroke to return to baseline. Flow time represents the duration of left ventricular systole and makes up one third of the cardiac cycle or cycle time. Since the flow time (FT) is heart rate dependent it is typically corrected (FTc) to a rate of 60 bpm and compensates for the change in duration of systole.
It is analogous to the corrected QT interval on the ECG.
FTc = FT ÷ vcycle Time
|Heart Rate (bpm)
||Cycle Time (s)
||Flow Time (s)|
A low FTc value (<330 msec) is seen with a vasoconstricted circulation e.g. hypovolaemia, excess arterial constriction (e.g. vasoconstrictors), major circulatory obstruction (e.g. pulmonary embolus). A raised FTc (>360 msec) is seen with a vasodilated circulation e.g. sepsis, hyperpyrexia