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Flow vs. Pressure

Many different technologies have been used for Cardiac Output monitoring. However, the two most utilised are

  • Direct flow measurement using Doppler ultrasound
  • Arterial blood pressure-derived flow using mathematical algorithms

Direct flow measurement, by way of Doppler ultrasound, has the precision and responsiveness to guide Intraoperative Fluid Management (IOFM) using the clinically proven >10% change Stroke Volume Optimisation (SVO) algorithm. Direct flow requires no additional calibration. Direct flow measurement by Doppler requires occasional focusing and in some scenarios, the signal is not always continuously available, making the technology less ideal for long term continuous monitoring. However, the Deltex Medical ODM+, which utilises both flow and pressure measurements, ensures continuous access to calibrated waveforms.

Pressure-based systems, using pressure as a surrogate for flow, have their limitations, particularly in periods of haemodynamic instability. Pressure-based 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, such devices actually offer Pulse Pressure Variation and/or Stroke Volume Variation as their modality for guiding intervention.

These parameters have significant limitations in use as they require 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;
  • increasing PEEP results in higher variations;
  • changes in lung or chest compliance, or patient position and right ventricular dysfunction or abdominal insufflation may affect readings.

Despite these limitations, in periods where the patient has stable haemodynamics, pressure-based technologies can be used to make a continuous record of cardiac output.

Direct flow Doppler is therefore the preferred source of data to guide intervention with fluid and drugs. This is especially true in the haemodynamically challenging environment of the Operating Theatre, where anaesthesia and surgery result in rapid and frequent changes in compliance.

Pressure-based technologies are useful in stable postoperative patients but are limited in their ability to guide interventional treatment.  A combination of both technologies, as featured on Deltex Medical’s ODM+, can truly be said to deliver the best of both worlds.