Question 18

Compare and contrast non-invasive oscillometric and invasive arterial blood pressure monitoring.

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College Answer

There were some good answers, though invasive BP measurement was better answered than oscillometry. Many candidates provided extensive detail in one area i.e. the workings of a Wheatstone bridge, to the detriment of a balanced answer. Few seemed to have a structure consisting of "equipment, method, sources of error, advantages, disadvantages" or similar and missed providing important information as a result. Several described auscultatory non-invasive blood pressure measurement, rather than oscillometry, which although related in principle is a different process

Discussion

As it appears that the CICM examiners wanted their answer in the format of "equipment, method, sources of error, advantages, disadvantages", that is the structure I chose for this table:

Blood Pressure Measurement:
Invasive vs Non-Invasive Methods
Domain	Invasive (arterial catheter)	Oscillometric cuff measurement
Equipment	Arterial catheter Incompressible tubing Pressure transducer Monitoring Counterpressure fluid	Inflatable cuff Cuff manometer Release valve
Physical principles and method	Pressure wave transmitted via fluid column Pressure changes are converted to resistance changes in a Wheatstone bridge transducer The resulting change in current is displayed as a graph By calibrating the sensor against a known range of pressures, this can be converted to a graph of pressure over time	Counterpressure is applied to a perfused limb Pulse from the limb arterial supply is detected (eg. by auscultation) Increasing counterpressure is applied to the limb This counterpressure decreases the amplitude of the detected pulse until the pulsations are no longer detected The counterpressure at which pulse is eliminated is recorded as the systolic pressure; Maximum counterpressure at which there is no pulse amplitude change is recorded as the diastolic pressure
Practical advantages	Thought to be the "gold standard" Allows continuous monitoring Gives access to the bloodstream for sampling Waveform is a source of information	No invasive procedures required Cheap and reusable Requires minimal training Requires no monitoring equipment or electronics Minimal moving parts, robust setup, durable Does not require regular recalibration
Practical disadvantages	Requires arterial puncture Non-reuseable kit Monitoring equipment is required for display Requires regular re-zeroing and re-levelling Training is required for staff Transducers can drift Relatively expensive parts	Less reliable measurements at pressure extremes Continuous monitoring is not possible Can become painful if set to repeat too frequently Can give rise to pressure areas Maximum accuracy requires manual operation (i.e. automatic modes are unreliable in unstable patients)
Sources of error	Catheter malposition Transducer mislevelling Inaccurate zero reading Failure of counterpressure Transducer/monitor malfunction or miscalibration	Arrhythmia Faint pulse Peripheral vascular disease Pneumatic leak from balloon or valve Manometer malfunction

References

Ward, Matthew, and Jeremy A. Langton. "Blood pressure measurement." Continuing education in anaesthesia, critical care & pain 7.4 (2007): 122-126.

Meidert, Agnes S., and Bernd Saugel. "Techniques for non-invasive monitoring of arterial blood pressure." Frontiers in medicine 4 (2018): 231.

Sorvoja, Hannu, and Risto Myllyla. "Noninvasive blood pressure measurement methods." Molecular and quantum acoustics 27 (2006): 239-264.

Geddes, Leslie Alexander. Handbook of blood pressure measurement. Springer Science & Business Media, 2013.

Geddes, Leslie A. "Counterpressure: the concept that made the indirect measurement of blood pressure possible." IEEE Engineering in Medicine and Biology Magazine 17.6 (1998): 85-87.

Lewis, Philip S. "Oscillometric measurement of blood pressure: a simplified explanation. A technical note on behalf of the British and Irish Hypertension Society." Journal of human hypertension 33.5 (2019): 349-351.

Juroszek, Barbara. "The sources of errors in the oscillometric blood pressure measurements." Metrology and Measurement Systems 15.4 (2008): 539-549.

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