|Once blood flow velocity in the descending thoracic aorta has been determined, the technology combines this with an estimated aortic cross sectional area to produce stroke volume and cardiac output measurements. Thus, linear variables are converted to volumetric variables. Stroke Distance calculation is a key step in this process. It is described as the distance a column of blood travels down the descending thoracic aorta with each systole (cm). It is also represented by the area under the velocity time waveform.
In a tube:
Volume = Length (distance) x cross sectional area
Compare with CardioQ measurements:
Stroke volume = SD x Aortic cross-sectional area
There are three primary assumptions used in the calculation of the Left Ventricular Stroke volume:
CardioQ nomogram: The CardioQ uses a fixed stored nomogram that provides an estimate of the patients aortic cross sectional area. The patient's biometric data consisting of age, height and weight are imported manually by the operator. The limits accepted by the nomogram are given below:
Table of nomogram (Well validated in the adult population)
Age 16-99 years
Weight 30-150 Kg (66-330 lb)
It is assumed that the imported biometric data is correct and that the patient's actual aortic dimensions are accurately represented by the stored nomogram. A 2mm miscalculation of a 25 mm aortic diameter will result in an automatic 16% error in cross sectional area (and
Constant angle of insonation: The angle of the CardioQ oesophageal probe to blood flow in the descending aorta is 45 degrees. Velocity measurements from the doppler equation use the cosine of this value. A wider angle will result in a greater reduction in measured velocity compared to true velocity.
Constant proportion of left ventricular output: The CardioQ assumes that descending aortic flow represents 70% of left ventricular output and that this remains constant throughout the spectrum of haemodynamic changes. During some procedures e.g. during aortic cross clamping, there may be redistribution of blood to the supra-aortic regions. This leads to underestimation of the complete cardiac output.