Non Invasive BP Measurement

1. Non Invasive BP Measurement Medical Physics

2. History of BP Measurement • 1733 Reverend Stephen Hales first to measure BP • Measured the height of a column of blood after cannulating the carotid artery in a horse with a brass pipe. Dr Rishi Mehra

3. History of BP Measurement • The brass pipe was attached to a 12 inch glass tube. • Tube was connected to the pipe via trachea of a goose! • First to observe respiratory variation and pulsatile nature • 100 years later, Poiseuille used a mercury manometer, connected to the artery in a dog to measure BP Dr Rishi Mehra

4. History of BP Measurement • Both of these methods were invasive • First attempts at non-invasive BP measurement began in the late 1800’s Dr Rishi Mehra

5. History of BP Measurement • In late 1890s, Italian physician – Scipione Riva Rocci invented sphygmomanometer • Sphygmos = Pulse (Greek) • Manometer = Pressure meter Dr Rishi Mehra

6. History of BP Measurement • Consisted of mercury column manometer • Silk covering a rubber sleeve, which is filled with air • Squeeze bag to inflate rubber sleeve Dr Rishi Mehra

7. History of BP Measurement • 10 years later, a Russian surgeon – Nikolai Korotkoff discovered that sounds were present as cuff was deflated • Works on principle that normal blood flow is laminar, and hence silent • As cuff pressure drops, some blood begins to flow past compressed area of cuff • Blood flow is turbulent, non-laminar and audible Dr Rishi Mehra

8. History of BP Measurement • This method of measuring BP is called the Auscultatory method • Roy and Adami (1890) developed oscillometric method • With cuff being deflated, onset of oscillations in mercury height were noted • Called onset of oscillations = Systolic BP Dr Rishi Mehra

9. History of BP Measurement • Maximum oscillations = Diastolic (Nb Incorrect) • Hence all indirect BP measurement involves use of Riva-Rocci cuff, either via auscultatory measurement or via oscillometric methods. Dr Rishi Mehra

10. Automated Auscultatory Method • First automated systems used a microphone to replace need for manual auscultation • Analysis of sound produced was used to estimate SBP and DBP • Problems with this approach - External noise can lead to artefact - Difficulty obtaining BP where sounds may be muted eg hypotension Dr Rishi Mehra

11. Automated Auscultatory Method • Method used largely in 1960’s • Generally found to be unreliable, and often gave wide variations in BP • No longer used in isolation, but is incorporated in several systems in conjunction with oscillometric systems. Dr Rishi Mehra

12. Automated Oscillometric Method • Oscillometric method involves measuring oscillations • Oscillations are a result of application of cuff in order to compress artery • Principle of measurement involves tranducer • Measuring oscillations over time with decreasing cuff pressure enables estimation of BP Dr Rishi Mehra

13. Automated Oscillometric Method Dr Rishi Mehra

14. Automated Oscillometric Method • Cuff pressure at point of maximum oscillations is taken to be MAP • Various methods of estimating systolic and diastolic BP from MAP • Auscultatory method = measure SBP and DBP and estimate MAP • Oscillometric method = measure MAP and estimate SBP and DBP Dr Rishi Mehra

15. Sources of error • Cuff size - Oscillations are direct result of BP cuff application - Cuff too small = overestimation of BP - Cuff too large = underestimation of BP • Cuff position – Bladder needs to be placed over brachial artery Dr Rishi Mehra

16. Sources of error • Movement Dr Rishi Mehra

17. Sources of error • Beat to beat variation- Oscillometric method requires reasonably stable BP during cuff deflation- Required for detecting maximum aplitude of oscillations- If BP varies due to arrhythmia, difficult to estimate BP accurately Dr Rishi Mehra

18. Sources of error • Beat to beat variation cont… Dr Rishi Mehra