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:
      • Peripheral 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 calculated area by aortic compliance
      Compliance estimated by proprietary algorithm each time the device is calibrated.
  • SVR is continuously 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.