Pressure Waveform Analysis

Describe the invasive and non-invasive measurement of blood pressure and cardiac output including calibration, sources of errors and limitations

Analysis of arterial pulse contour is:

  • Real-time and continuous
  • Used to estimate cardiac output
    Less accurate but also less invasive (e.g. thermodilution) or technically demanding (e.g. echocardiography) than other methods.
    • Therefore also calculate (and often display) stroke volume variation and pulse pressure variation

Principles

All models recognise that the amplitude of the systolic upstroke is:

  • Directly proportional to stroke volume
  • Inversely proportional to arterial compliance

Other principles used by some (but not all) devices include:

  • Three-element Windkessel model
    Characterises the arterial tree as having three major features:
    • Aortic Impedance
    • Arterial Compliance
      Predicted using patient characteristics.
    • Systemic Vascular Resistance
  • Conservation of Mass

Devices

Devices can be classified based on whether they are:

  • Calibrated/Uncalibrated
    • Calibrated
      Initial estimation is refined using a dilution technique.
      • Dilutions may be by:
        • Thermodilution
          • Cold saline injected into SVC
            Using an IJV or SCV CVC.
          • Temperature changed measured at the femoral artery
        • Lithium dilution
          • Small amounts of lithium chloride injected into a central vein
          • Change in lithium concentration measured in radial artery
          • CO by calculated Stewart-Hamilton equation
      • Periodically recalibrated to correct for drift
    • Uncalibrated
      Not corrected for a measured 'true' cardiac output.
      • Inaccurate for short term changes in arterial properties
      • Not validated in:
        • Shock
        • ARDS
        • Hepatic surgery
          Due to changes in arterial tone.
        • Cardiac surgery
  • Invasive/Non-invasive
    • Invasive
      Rely on a (usually femoral) arterial catheter.
    • Non-invasive
      Rely on the volume clamp method:
      • Inflatable cuff wrapped around finger
      • Plethysmograph estimates blood volume in the digital arteries
      • Cuff inflates and deflates throughout the cardiac cycle, keeping the volume of the arteries constant
        Arterial pressure is proportional to cuff pressure.
      • Inaccurate in:
        • Periopehral oedema
        • Vasoconstricted states

Common Devices in Use

  • PiCCO/VolumeView/FloTrac
    • Calibrated
    • Invasive
    • 3-element Windkessel
    • Mechanism:
      • Calculates area under systolic part of the arterial curve
      • Divides calcualted area by aortic compliance
        Compliance estimated by proprietary algorithm each time the device is calibrated.
    • SVR is continuosly estimated from calculated CO and measured BP
  • LiDCO
    • Calibrated
    • Invasive
    • Conservation of mass
    • Compliance inferred from biometric data
  • Clearsight/CNAP
    • Uncalibrated
    • Non-invasive
  • T-Line
    • Calibrated
      Proprietary, non-validated auto-calibrating algorithm.
    • Non-invasive
    • Uses radial applanation tonometry

References

  1. Jozwiak M, Monnet X, Teboul J-L. Pressure Waveform Analysis. Anesth Analg. 2017.
  2. Francis, SE. Continuous Estimation of Cardiac Output and Arterial Resistance from Arterial Blood Pressure using a Third-Order Windkessel Model. MIT. 2007.
Last updated 2019-02-16

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