Pressure Waveform Analysis
Describe the invasive and non-invasive measurement of
blood pressure andcardiac 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
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 can be classified based on whether they are:
Initial estimation is refined using a dilution technique.
- Dilutions may be by:
- 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
- Dilutions may be by:
Not corrected for a measured 'true' cardiac output.
- Inaccurate for short term changes in arterial properties
- Not validated in:
- Hepatic surgery
Due to changes in arterial tone.
- Cardiac surgery
Rely on a (usually femoral) arterial catheter.
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
- 3-element Windkessel
- 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
- Conservation of mass
- Compliance inferred from biometric data
Proprietary, non-validated auto-calibrating algorithm.
- Uses radial applanation tonometry
- Jozwiak M, Monnet X, Teboul J-L. Pressure Waveform Analysis. Anesth Analg. 2017.
- Francis, SE. Continuous Estimation of Cardiac Output and Arterial Resistance from Arterial Blood Pressure using a Third-Order Windkessel Model. MIT. 2007.