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What is continuous non-invasive arterial pressure (CNAP)?

Posted by Gary Martinez | Dec 12, 2018 10:24:01 AM

This is a clinical education blog focused on non invasive hemodynamic monitoring.  It explains how continuous noninvasive arterial pressure (CNAP) works and the clinical evidence that supports it.

What is CNAP?

Continuous noninvasive arterial pressure (CNAP) is the method of measuring arterial blood pressure in real-time without any interruptions (continuously) and without cannulating the human body (non invasive).1

Traditionally, non invasive blood pressure is measured intermittently by an upper arm cuff and “beat-to-beat” monitoring has required the placement of an arterial catheter. CNAP provides the advantage of not only measuring standard blood pressure like an upper arm cuff but, provides continuous blood pressure measure in real time.1,2

Additionally, CNAP pulse waves can be used to accurately determine hemodynamic indices such as stroke volume (SV) and cardiac output (CO).3

  • This can be of clinical value since the majority of surgical cases do not employ invasive blood pressure monitoring.4
  • This is particularly so with regard to preventing unnecessary hypotensive during surgery and providing hemodynamic optimization via goal directed therapy.5,6

Therefore, a high demand for an accurate and easy to apply medical device exists.
One such non-invasive continuous beat-to-beat blood pressure device is provided by LiDCO. 

How does it work?

CNAP technology is directly connected to the LiDCO monitor in order to display arterial blood pressure wave form data.

The LiDCO CNAP Module uses established CNAP technology to obtain the arterial blood pressure waveform continuously and non-invasively for patients that are not otherwise indicated for invasive arterial blood pressure monitoring. The LiDCO rapid then uses this data to derive nominal stroke volume and heart rate using the PulseCO algorithm.

CNAP is a non-invasive method for measuring the continuous blood pressure waveform in adult and pediatric patients from the age of 4 years. A patient’s blood pressure waveform is recorded by the CNAP Module by means of a double finger cuff with an integrated InfraRed (IR) light sensor and air chambers.

The measured IR signal – similar to a pulse oximeter – helps to measure the blood volume in the finger, which is kept constant by means of CNAP: beat to beat a counter pressure in the finger cuff is built up, which fluctuates between the systolic and diastolic blood pressure.

CNAP diagram-1

A NBP cuff (oscillometric blood pressure measurement) measures absolute blood pressure values which are used to calibrate the relative blood pressure in the finger, thus ensuring absolute accuracy. The NBP cuff can be placed on the patient’s upper arm either on the same or on the other arm as the CNAP double finger cuff. NBP measurement is essential to ensure absolute accuracy of the recorded blood pressure values.

The CNAP monitor upgrades the gold standard upper arm measurement to a continuous measurement with the help of a finger sensor. This double finger sensor produces a continuous blood pressure signal which is calibrated to an initial oscillometric value by means of a special transfer function.

cnap steps 1 to 4 v2

The CNAP monitor upgrades the gold standard upper arm measurement to a continuous measurement with the help of a finger sensor. This double finger sensor produces a continuous blood pressure signal which is calibrated to an initial oscillometric value by means of a special transfer function.

Calibration can be programmed manually or automatically. Intelli Mode tracks blood pressure changes, and if special blood pressure patterns are detected, a new calibration is performed.

What is the vascular unloading technique?

The CNAP technique used is a volume clamp method known as “vascular unloading”. It is based on the work of Czech physiologist Jan Penaz (1973).7

CNAP vascular unloading penaz

The Czech physiologist Jan Peňáz introduced the vascular unloading technique on the finger in 1973 by means of an electro-pneumatic control loop. The control loop is shown in the block diagram: a cuff is placed over the finger, as it is the most suitable and an easily accessible region. Inside the cuff, the blood volume in the finger arteries is measured using an infrared light source (L) and a light detecting photocell (PC).

The plethysmographic signal (PG) – the light signal compared to constant C1 – is an electronic measure for finger blood volume. PG is fed into a control unit having proportional-integral-differential characteristics (PID). The PID-signal is added to a constant set point (C2), amplified and fed to an electro-pneumatic transducer (EPT). EPT produces a pressure in the cuff, which, again, alters finger blood volume.

Block diagram of Peňáz’ system with single control loop: F – finger, L – lamp, PC – photo cell, S – segments of transparent pressure cuff, C1 – average of PC-signal, DA – difference amplifier, V(PG) – plethysmographic signal, PID – correcting network, C2 – set point SP, SW – switch between open and closed loop, PA – power amplifier, EPT – electro-pneumatic transducer, M(CP) Pressure measured with Manometer. (Constructed with respect to Peňáz’ original drawing).

  • Mills, E. (2013). CNAP Technology: Continuous Non-Invasive Blood Pressure [Power Point slides]. (LiDCO)
  • Fortin, J., Marte, W., Grüllenberger, R., Hacker, A., Habenbacher, W., Heller, A., Wagner, C., et al. (2006). Continuous non-invasive blood pressure monitoring using concentrically interlocking control loops. Computers in biology and medicine, 36(9), 941–57.

However, along with all noninvasive technologies, vascular unloading is subject to vascular tone changes related to vasodilation, vasoconstriction, and use of vasoactive drugs. LiDCOrapid utilizes a proprietary algorithm known as VERIFY to auto correct for changes in arterial tone.

verifi cnap

What clinical validation exists for CNAP non invasive hemodynamic monitoring?

evidence table

The accuracy and clinical value of CNAP was tested in several international clinical trials. Studies have compared CNAP beat-to-beat BP with both non-invasive upper-arm BP (NBP) but also invasive arterial line measurement (IBP).

Jeleazcov et al. (2010) have first reported data on the comparison between CNAP and IBP during general anaesthesia. Eighty-eight patients undergoing elective abdominal surgery, cardio-, or neurosurgery were included in the study. They reported a bias and standard deviation in mean arterial pressure of -1,6/11,0 mmHg and conclude that CNAP provides real-time estimates of arterial pressure comparable with those generated by an invasive intra-arterial catheter system during general anaesthesia. Jeleazcov, C. et. al. Precision and accuracy of a new device (CNAP®) for continuous noninvasive arterial blood pressure monitoring: assessment during general anaesthesia. BJA.105(3):264-272 (2010).

Ilies et al. (2012) have also compared CNAP to arterial line in ninety patients both during induction and maintenance of general anaesthesia. The bias was -4,3 mmHg with a standard deviation of 10,4 mmHg. They conclude that CNAP may be a potentially valuable monitor for patients in whom there is not an absolute need for invasive pressure monitoring, but beat-to-beat AP measurement would be of value. Ilies, C., Investigation of the agreement of a continuous non-invasive arterial pressure device in comparison with invasive radial artery measurement. BJA. 108(2):202-10. doi: 10.1093/bja/aer394 (2012).

Also, in Intensive Care Medicine, different research groups tested the accuracy of CNAP compared to IBP.  E.g. Jagadeesh and his colleagues (2012) have evaluated the system in cardiac surgical patients. Even in those critically ill patients the precision of CNAP is very high with a bias of -0,04mmHg and a standard deviation of 2,05mmHg compared to IBP monitoring. Jagadeesh, AM., A comparison of a continuuous noninvasive arterial pressure (CNAPTM) monitor with an invasive arterial blood pressure monitor in the cardiac surgical ICU.  Ann Card Anaesth. Jul-Sep;15(3):180-4. doi: 10.4103/09719784.97973 (2012).

  • Monnet, X. et al., Prediction of fluid responsiveness by a continuous non-invasive assessment of arterial pressure in critically ill patients: comparison with four other dynamic indices. British Journal of Anaesthesia (2012).
  • Neuner M et al. Clinical validation of a continuous non-invasive haemodynamic monitor (CNAPTM 500 ) during general anaesthesia. British Journal of Anaesthesia. 2012;(Table 1):1-5.
  • Biais, M., Vidil, L., Roullet, S., Masson, F., Quinart, A., Revel, P., & Sztark, F. Continuous non-invasive arterial pressure measurement: evaluation of CNAP device during vascular surgery. Annales françaises d’anesthèsie et de rèanimation, 29(7-8), 530–5. doi:10.1016/j.annfar.2010.05.002 (2010).

Further studies have been contacted that show the superiority of CNAP compared to gold standard intermittent NBP measurement.  Siebig at el. (2009) point out that continuous non-invasive BP monitoring with CNAP should be the preferred choice during interventional endoscopy. They could show that a substantial number of hypotensive episodes are detected with delay or are even mist when monitoring BP with an upper arm cuff.

  • Siebig, S., Rockmann, F., Sabel, K., Zuber-Jerger, I., Dierkes, C., Brünnler, T., & Wrede, C. E. (2009). Continuous non-invasive arterial pressure technique improves patient monitoring during interventional endoscopy. International journal of medical sciences, 6(1), 37–42. Retrieved from http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2631161&tool=pmcentrez&rendertype=abstract

Ilies and colleagues (2012) have run a similar protocol in women undergoing spinal anaesthesia for caesarean delivery. They have applied both NBP and CNAP measurement in 90 patients before placement of the catheter for spinal anaesthesia. CNAP was superior to NBP in detecting episodes. Furthermore, fetal acidosis defined by an umbilical vein pH under 7,25 did not occur when the lowest systolic AP measured by CNAP was above 100 mmHg. The authors conclude that CNAP detected more hypotensive episodes after spinal anaesthesia and significantly lower arterial pressure values compared with an upper arm cuff. Arterial pressure monitoring based on CNAP may improve haemodynamic management in this patient population with potential benefit for the fetus.

Even for the automated guidance of vasopressor therapy during caesarean section, CNAP was shown to be the preferred method. The authors have developed a novel vasopressor delivery system that automatically administers phenylephrine or ephedrine based on CNAP continuous non-invasive BP monitoring.

  • Siebig, S., Rockmann, F., Sabel, K., Zuber-Jerger, I., Dierkes, C., Brünnler, T., & Wrede, C. E. (2009). Continuous non-invasive arterial pressure technique improves patient monitoring during interventional endoscopy. International journal of medical sciences, 6(1), 37–42. Retrieved from http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2631161&tool=pmcentrez&rendertype=abstract
  • Sia, a T. H., Tan, H. S., & Sng, B. L. (2012). Closed-loop double-vasopressor automated system to treat hypotension during spinal anaesthesia for caesarean section: a preliminary study. Anaesthesia, 1–8. doi:10.1111/anae.12000

In which cases would CNAP not be suitable?

In some cases, CNAP measurement is not suitable:

  • Do not use CNAP and NBP in patients with vascular prostheses in the arm.
  • Weak signal shown via the Perfusion Index indicator (Low PI <1).
  • Reduced peripheral blood flow (e.g. peripheral shock, hypothermia extreme centralization, extreme hypothermia).
  • Arterial vascular diseases (arteriosclerosis, Raynaud’s syndrome, endarteritis obliternans, collagenosis, extremely advanced vascular diseases (PAOD).

Start course on non invasive monitoring

References

1) Continuous noninvasive arterial pressure. Retrieved from, https://en.wikipedia.org/wiki/Continuous_noninvasive_arterial_pressure.
2) https://www.frontiersin.org/articles/10.3389/fmed.2017.00231/full
3) Wesseling, K. H., Jansen, J. R., Settels, J. J., & Schreuder, J. J. (1993). Computation of aortic flow from pressure in humans using a nonlinear, three-element model Computation of aortic flow from pressure in humans using a nonlinear, three-element. Journal of Applied Physiology (1993), 74(5), 2566–2573.
4) von Skerst B: Market survey, N=198 physicians in Germany and Austria, Dec.2007 - Mar 2008, InnoTech Consult GmbH, Germany
5) Ilies, C., et al. Detection of hypotension during Caesarean section with continuous non-invasive arterial pressure device or intermittent oscillometric arterial pressure measurement. British Journal of Anaesthesia (2012), 3–9.
6) Lopes, R., et al. Goal-directed fluid management based on pulse pressure variation monitoring during high-risk surgery: a pilot randomized controlled trial. Critical Care (2007), 11(5), R100.
Penaz J, Voigt A, Teichmann W. [Contribution to the continuous indirect blood pressure measurement]. Z Gesamte Inn Med (1976) 31:1030–3. 
7) Mills, E. (2013). CNAP Technology: Continuous Non-Invasive Blood Pressure [Power Point slides]. (LiDCO)
8) LiDCO Hemodynamic Monitoring. Non-Invasive Setup Guide [Handout].
9) LiDCO Hemodynamic Monitoring. How to Set Up Noninvasive [Video]. Retrieved from, www.lidco.us
10) LiDCO Hemodynamic Monitoring. (2017, v2). LiDCO Unity User’s Manual.

Topics: hemodynamics, ICU, intensive care units, stroke volume variation, svv, arterial blood pressure changes, non invasive, continuous non-invasive, noninvasive hemodynamic monitoring, Continuous noninvasive arterial pressure, arterial blood pressure, non-invasive blood pressure, beat-to-beat” monitoring, CNAP pulse waves, Cardiac Output, CNAP technology, PulseCO Algorithm, counter pressure, NBP cuff, oscillometric, Intelli Mode, vascular unloading, Perfusion Index

Written by Gary Martinez

Gary Martinez is a registered nurse in the US with many years of critical care clinical experience with a background in education. Gary is a clinical specialist working for LiDCO in Houston, Texas and enjoys golf and learning new things.

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