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.