How it Works
The ODM+ monitor measures central blood flow directly by way of a Doppler ultrasound probe. In addition the device takes the standard Arterial Blood Pressure (ABP) measurement and calculates a range of Pressure-Based Parameters. It is unique in that it can take the Cardiac Output as measured by Doppler flow to calibrate a Pulse Pressure Waveform Analysis algorithm. The PPWA algorithm provides secondary pressure based measures of Stroke Volume and Cardiac Output as well as new flow and pressure combined parameters.
By combining both flow and pressure-based measurements, ODM+ allows the user to quickly and simply calibrate and recalibrate from the Doppler flow-based measurements of Cardiac Output. This minimises the inherent weaknesses of PPWA systems.
Access the Training Workbook for a deeper understanding of how to use this technology.
- Flow and Pressure Monitoring Modes
The CardioQ-ODM+ user interface provides two new screens for Flow Monitoring Mode and Pressure Monitoring Mode.
Users selecting Pressure Monitoring Mode initially calibrate the parameters by pressing the calibrate button. In a matter of seconds the proven Liljestrand and Zander Pulse Pressure Waveform Analysis algorithm is calibrated from the current flow based Cardiac Output.
- Pulse Pressure Waveform Analysis (PPWA)
PPWA is a group of methods which use Arterial Blood Pressure (ABP) as a means of estimating Cardiac Output. The techniques have been called Pulse Contour Analysis, Pulse Pressure Analysis and Pulse Power Analysis, however all are variations in their approach to use ABP to estimate flow. Any measure from the artery includes the changes in pressure associated with changes in arterial function (compliance, impedance etc). Pressure based estimates of Cardiac Output are inherently limited by confounding blood pressure changes which may accompany actual output flow staying the same or decreasing.
Physiologic or therapeutic changes in vessel diameter are assumed to reflect changes in flow. Pulse Pressure (PP) methods measure the combined performance of the heart and the vessels thus limiting the application of PP methods for measurement of flow. This has to be compensated for by regular calibration of the waveform (as CardioQ-ODM+) to another flow measurement method (such as Doppler) and then monitoring the PP waveform. Recalibration is recommended after changes in patient position, therapy or condition, including known or suspected arterial compliance changes.
These methods use characteristics of the pulse pressure waveform to calculate aspects of heart function. The basic Pulse Contour method provides the following information; a. relates to myocardial contractility (similar to Mean Acceleration from Doppler), b. the area under the curve can be calibrated against another source of cardiac output to estimate cardiac output, c. systolic time (similar to Flow Time from Doppler), d. diastolic time.
Uncalibrated pulse pressure devices utilise an algorithm that is based on the principle that pulse pressure is proportional to Stroke Volume (SV). The algorithm uses statistical analysis of the arterial pressure wave to generate an estimate of stroke volume. These systems use a multivariate polynomial equation that continuously quantifies arterial compliance and vascular resistance. By analysing the shape of the arterial pressure waveform, the effect of vascular tone is assessed allowing calculation of SV and Cardiac Output is then derived.
All PPWA systems have limitations in that measurement of pressure in the artery to calculate the flow in the heart is physiologically irrational, simply pressure and flow do not necessarily rise and fall together and often are in opposition. For example, as impedance increases pressure may rise and flow may fall.
The dynamic variables SVV and PPV displayed with arterial pressure monitoring is also limited to patients with an invasive arterial line, who are fully mechanically ventilated, with closed chest, a tidal volume of >7-8 ml/Kg, are in sinus rhythm. Changing the patient’s position or altering PEEP or tidal volumes may alter the ‘cut off’ or ‘grey zone’ threshold.