The distinction between Flow and Pressure is important, especially when selecting from the different technologies available for Cardiac Output monitoring. Flow-driven technologies derive data through either direct or indirect means:
- Direct flow measurement using Doppler ultrasound
- Indirect methods back-calculate flow data from mathematical algorithms using arterial blood pressure (ABP)
Direct flow measurement, by way of Doppler ultrasound in humans, has the precision and responsiveness to guide Intraoperative Fluid Management (IOFM) using the clinically proven >10% change Stroke Volume Optimization (SVO) algorithm. Direct flow measurement requires no additional calibration, just occasional focusing. Note however that in some scenarios the signal is not always continuously available, making the technology less ideal for long term continuous monitoring.
EDM+Vet utilizes both flow and pressure measurements, ensuring continuous access to calibrated waveforms.
Limitations of Pressure-Derived Monitoring
Systems that use pressure as a surrogate for flow have their limitations, particularly in periods of hemodynamic instability. Methods using ABP lines have been shown to be prone to drift due to changes in vascular tone, arterial compliance and the consequent arterial blood pressure variation. These changes have been reported to be clinically significant and prevent such devices from successful use of the >10% SVO algorithm. As a result, they are commonly described as offering Pulse Pressure Variation and/or Stroke Volume Variation as their modality for guiding intervention.
Measuring variation demands elimination of other variables. This means requiring the patient to be:
- in sinus rhythm
- fully mechanically ventilated
- tidal volume ≥7-8 mL/kg – higher tidal volumes elicit higher variations
- heart rate:respiratory rate ratio ≥4
- vulnerable to increasing PEEP (results in higher variations)
- generally stable: changes in lung or chest compliance, or patient position and right ventricular dysfunction or abdominal insufflation may affect readings
Pressure-based technologies can only be used to make a continuous record of cardiac output when the hemodynamics are stable.
Advantages of Flow-Derived Monitoring
Direct flow Doppler, unique to EDM, is the preferred source of data to guide intervention with fluid and drugs. Importantly EDM can do this effectively even in the hemodynamically challenging environment of the Operating Room, where anesthesia and surgery result in rapid and frequent changes in compliance. So while pressure-based technologies are useful in stable postoperative patients, they are limited in their ability to guide interventional treatment.