Blood pressure is among the most important vital signs monitored in surgical and non-surgical settings. Because high blood pressure, known as hypertension, is associated with adverse conditions such as heart disease and stroke, blood pressure is typically monitored as part of an annual physical exam and before many medical procedures. This type of blood pressure monitoring is non-invasive and usually involves the use of an oscillometric cuff: an inflatable cuff is placed around the upper arm, inflated to a point where blood flow is momentarily stopped, after which the cuff pressure is deflated and blood flow resumes.¹ This allows for systolic and diastolic blood pressure readings. Hypertension and hypotension (low blood pressure) are also both not ideal in the surgical setting, and blood pressure is monitored along with other vital signs throughout the perioperative period. Arterial blood pressure monitoring is an uncommon method that is used in a few specific situations due to its unique characteristics.

Monitoring blood pressure during surgery is critical, because anesthesia and surgery can cause rapid changes in the functioning of bodily systems, which may also result in and can be detected as dramatic changes to blood pressure.² Non-invasive blood pressure monitoring can be used in perioperative care; the American Society of Anesthesiologists recommends that blood pressure should be monitored at five minute intervals in anesthetized patients.³ However, there are some drawbacks to the non-invasive technique of blood pressure monitoring, such as the need for a large enough cuff size and the complications that can arise from prolonged inflation of the cuff and the resulting pause in arterial blood flow.⁴

Invasive blood pressure monitoring uses a catheter inserted into an artery with the pressure readout being displayed on an electronic monitor.⁵ The invasive approach allows for continuous “beat-to-beat” monitoring of blood pressure, which is useful in patients undergoing operations such as vascular surgery that can cause sudden changes in blood pressure.⁵ However, the invasive technique suffers from a number of potential complications. The presence of a catheter in the artery can lead to occlusion and the formation of clots, likely due to changes in artery wall integrity.⁶ Furthermore, poor insertion technique and inadequate disinfection at the insertion site can lead to infection, though this risk can be mitigated by selecting catheter materials more resistant to bacterial strains of concern.⁷

Continuous, non-invasive, perioperative blood pressure monitoring is thus of interest. The Volume Clamp method, developed by Czech physiologist Jan Penaz in 1973, is an example of such an approach. The volume of finger arteries is first assessed using a technique called photoplethysmography, after which blood volume is held constant by a fast-moving retractable and inflatable cuff.³ Blood pressure can be derived, by use of mathematical modeling, from the pressure needed to maintain constant blood flow. This method, while efficient and accurate, can cause “blue finger syndrome” when cuff pressure exceeds the finger’s blood pressure.

Other continuous non-invasive blood pressure monitoring technologies have been developed in the decades since Penaz’s debuted his method, many of which improve upon the method. The CNAP system uses two fingers and periodically switches between them to avoid vascular congestion. It also uses occasional upper arm oscillometric cuff readings to calibrate the system and ensure accurate results.⁸ Some researchers predict that in the coming decades, “smart technologies,” such as real-time computational analysis of patient data, will be coupled to perioperative blood pressure monitoring systems.

References

1. Lewis, P. S. Oscillometric measurement of blood pressure: a simplified explanation. A technical note on behalf of the British and Irish Hypertension Society. J. Hum. Hypertens. 33, 349–351 (2019), DOI: 10.1038/s41371-019-0196-9

2. Bartels, K., Esper, S. A. & Thiele, R. H. Blood Pressure Monitoring for the Anesthesiologist: A Practical Review. Anesth. Analg. 122, 1866–1879 (2016), DOI: 10.1213/ANE.0000000000001340

3. Stenglova, A. & Benes, J. Continuous Non-Invasive Arterial Pressure Assessment during Surgery to Improve Outcome. Front. Med. 4, 202 (2017), DOI: 10.3389/fmed.2017.00202

4. Wax, D. B., Lin, H.-M. & Leibowitz, A. B. Invasive and concomitant noninvasive intraoperative blood pressure monitoring: observed differences in measurements and associated therapeutic interventions. Anesthesiology 115, 973–978 (2011), DOI: 10.1097/ALN.0b013e3182330286

5. MEMSCAP | Invasive blood pressure. http://www.memscap.com/applications-and-market-segments/medical-and-biomedical/invasive-blood-pressure.

6. Scheer, B., Perel, A. & Pfeiffer, U. J. Clinical review: complications and risk factors of peripheral arterial catheters used for haemodynamic monitoring in anaesthesia and intensive care medicine. Crit. Care Lond. Engl. 6, 199–204 (2002), DOI: 10.1186/cc1489

7. Lopez-Lopez, G., Pascual, A. & Perea, E. J. Effect of plastic catheter material on bacterial adherence and viability. J. Med. Microbiol. 34, 349–353 (1991), DOI: 10.1099/00222615-34-6-349

8. Lakhal, K. et al. The CNAPTM Finger Cuff for Noninvasive Beat-To-Beat Monitoring of Arterial Blood Pressure: An Evaluation in Intensive Care Unit Patients and a Comparison with 2 Intermittent Devices. Anesth. Analg. 123, 1126–1135 (2016), DOI: 10.1213/ANE.0000000000001324