DOPPLER SPECIFIC PARAMETERS
Anatomy of a Waveform
Flow Time Corrected (FTc)
Flow Time (FT) is the duration of time of the flow from the left ventricle during systole. Flow Time corrected (FTc) is Flow Time duration of blood flow in the aorta normalised to 60 beats/min using Bazett’s equation.
Typically FTc is one third of the cardiac cycle. When standardised to 60 beats/min one cycle is one second. FTc is then 0.33 seconds or 333 milliseconds. Thus typical values for normally hydrated resting healthy individuals is 330 – 360 milliseconds.
FTc is inversely related to afterload/resistance. A low FTc is most commonly related to a low preload and can also be seen when afterload is high. If the FTc is low, consideration generally should be given to a fluid challenge in the first instance, as it will likely improve perfusion. If fluid does not improve the Stroke Volume (check caveats on the Decision Tree), other causes of an increased afterload/resistance should be considered.
Note: An FTc less than 330ms is never normal but the upper limit depends on the clinical scenario.
A high FTc (greater than 360ms) is usually seen in low afterload/resistance states such as the vasoactive effects of drugs and sepsis.
Peak Velocity (PV)
Peak Velocity is a Doppler only parameter and is available on both the CardioQ-ODM and CardioQ-ODM+ as the maximal velocity of the blood.
PV is an indicator of contractility and typical values change with age. An approximate guide would be 140 – age.
The following are more detailed approximate values:
20 yrs – 90 -120cm/s
30 yrs – 85 -115cm/s
40 yrs – 80 -110cm/s
50 yrs – 70 -100cm/s
60 yrs – 60 – 90cm/s
70 yrs – 50 – 80cm/s
80 yrs – 40 – 70cm/s
90 yrs – 30 – 60cm/s
Thus a PV markedly below the typical expected value may be an indicator of increased afterload or decreased cardiac function. A higher than normal PV may be indicative of decreased afterload and may be seen when inotropic drugs are used or as compensation when cardiac output is falling.
Stroke Distance (SD)
Stroke Distance is a Doppler flow based parameter only. SD is simply the distance the blood ejected by the left ventricle travels down the aorta every beat. It is measured in centimeters per second. If the aorta approximates to a cylindrical pipe then the SD can be represented as a series of cylinders of blood moving down the aorta as each pulse of the left ventricle propels them more distally into the arterial system.
The CardioQ-ODM is distinct from other ODM devices in possessing the ability to calculate Stroke Volume (SV) and so Cardiac Output (CO) from its own patient nomogram. The patient nomogram was created as a result of research by Prof. Mervyn Singer. The patient nomogram is a calibration of Stroke Distance against the total Cardiac Output as measured by a Pulmonary Artery Catheter (PAC) for patients of both genders and various races, ages, weights and heights. SD is the basic parameter for IOFM and SV is automatically calculated by the monitor. The patient’s age, weight and height are input during the monitor set up. This information accesses the nomogram which effectively provides the dynamic aortic root diameter.
SV = SD x Aortic Root Diameter
As a result ODM has been found to be equivalent in terms of accuracy to a PAC. However it is the precision of the oesophageal Doppler monitor in tracking change that is key to recognising how and why it guides Stroke Volume Optimisation (SVO) so effectively, and why this guidance has resulted in an unparalleled evidence base.
Minute Distance (MD)
MD is simply the distance blood moves in one minute down the aorta.
MD = SD x HR
MD is inversely affected by afterload.
Mean Acceleration (MA)
MA is the average speed on the upstroke of the waveform and is measured in cm/s.
MA can also be used as an indicator of contractility, e.g. a lower MA may be seen in left ventricular dysfunction and the waveform may be ‘rounder’ in shape.