**5. Transfer function**

The electrical bioimpedance-based method for central aortic pressure waveform reconstruction allows long-term monitoring of the CAP and obtaining of haemodynamic parameters like the augmentation index (AI) [41] (see **Figure 15**).

#### **Figure 15.**

*Finding of augmentation index (AI) from the CAP waveform with diastolic and systolic pressures DP and SP accordingly, the CAP waveform is a sum of the forward (green line) and reflected (light red) pressure waves.*

#### **Figure 16.**

*Demonstration of the CAP cycle reconstruction with transfer function (TF) from the EBI cardiac cycle waveform.*

#### **Figure 17.**

*The impedance variation ΔZ(t), obtained by integration of dZ(t)/dt wave (Figure 7) using trapezoidal method with dt = 0.0025 s and scaling the amplitude by factor of 20.*

To make this possible, the measured EBI waveform is transformed into the waveform of CAP, and for that, the transfer function (TF) approach is used (see **Figure 16** for illustration). Over 100 measurements of the EBI and invasive CAP waveforms were provided in East-Tallinn Central Hospital (Estonia) to collect data for this research work. In the beginning, the EBI measurements were carried out using a wireless multichannel impedance cardiograph CircMon BT101 [22] with additional channel for simultaneous acquiring of invasively measured CAP data using the PVB's XTRANS pressure sensor.

One possible algorithm for estimating a generic TF between the EBI and CAP waveforms (**Figures 17** and **18**) is a period-wise estimation of individual transfer functions for each patient and ensemble averaging to get a generic TF [42]. Another approach is to use adaptive algorithm to find a generic TF directly by matching all available patients' signals [43]. Both approaches give similar generic TF between the EBI and CAP of cardiac cycle waveforms. Despite the efforts made, the problems related to removing the artefacts remain. This causes corruption of the reconstructed CAP waveform due to the fact that all uncleaned artefacts are transformed into the reconstructed CAP. Regardless of that, the EBI signal-based noninvasive

**25**

*Noninvasive Acquisition of the Aortic Blood Pressure Waveform*

estimation of the central aortic blood pressure waveform is still highly promising

*The CAP waveform reconstructed digitally from the radial artery impedance waveform ΔZ(t) given in* 

Blood pressure variations inside the aorta during cardiac cycles (known also as central aortic blood pressure curve) is an important source for diagnosing patient's cardiovascular system. Classical approach, catheterization, is technically demanding and costly medical procedure. Therefore, different noninvasive methods have been studied and taken into use. The present chapter discusses the possibilities to bring in wearable techniques by sensing blood pressure variations with electrical bioimpedance changes on the radial artery. Two versions of wearable devices were designed and different electrode systems studied by simulations and experiments. Provisional human experiments were carried out at the hospital in limited extent but evidently with positive results. Further work will concentrate in developing the

The research was supported by Estonian ICT Center of Research Excellence EXCITE, Estonian Research Council (grant IUT1911), European project H2020 Flag-ERA JTC 2016 CONVERGENCE and Estonian IT Academy scholarships. The work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 668995. This material reflects only the authors' view, and the EC Research Executive Agency is not responsible for

generalized transfer function algorithms and electrode system.

The authors declare no conflicts of interests.

any use that may be made of the information it contains.

*DOI: http://dx.doi.org/10.5772/intechopen.86065*

alternative to the applanation method.

*Figure 17 by applying the transfer function TF (Figure 16).*

**6. Summary**

**Figure 18.**

**Conflict of interests**

**Acknowledgements**

*Noninvasive Acquisition of the Aortic Blood Pressure Waveform DOI: http://dx.doi.org/10.5772/intechopen.86065*

**Figure 18.**

*Wearable Devices - The Big Wave of Innovation*

**Figure 15.**

**Figure 16.**

*waveform.*

**Figure 17.**

To make this possible, the measured EBI waveform is transformed into the waveform of CAP, and for that, the transfer function (TF) approach is used (see **Figure 16** for illustration). Over 100 measurements of the EBI and invasive CAP waveforms were provided in East-Tallinn Central Hospital (Estonia) to collect data for this research work. In the beginning, the EBI measurements were carried out using a wireless multichannel impedance cardiograph CircMon BT101 [22] with additional channel for simultaneous acquiring of invasively measured CAP data using the PVB's XTRANS

*The impedance variation ΔZ(t), obtained by integration of dZ(t)/dt wave (Figure 7) using trapezoidal* 

*method with dt = 0.0025 s and scaling the amplitude by factor of 20.*

*Finding of augmentation index (AI) from the CAP waveform with diastolic and systolic pressures DP and SP accordingly, the CAP waveform is a sum of the forward (green line) and reflected (light red) pressure waves.*

*Demonstration of the CAP cycle reconstruction with transfer function (TF) from the EBI cardiac cycle* 

One possible algorithm for estimating a generic TF between the EBI and CAP waveforms (**Figures 17** and **18**) is a period-wise estimation of individual transfer functions for each patient and ensemble averaging to get a generic TF [42]. Another approach is to use adaptive algorithm to find a generic TF directly by matching all available patients' signals [43]. Both approaches give similar generic TF between the EBI and CAP of cardiac cycle waveforms. Despite the efforts made, the problems related to removing the artefacts remain. This causes corruption of the reconstructed CAP waveform due to the fact that all uncleaned artefacts are transformed into the reconstructed CAP. Regardless of that, the EBI signal-based noninvasive

**24**

pressure sensor.

*The CAP waveform reconstructed digitally from the radial artery impedance waveform ΔZ(t) given in Figure 17 by applying the transfer function TF (Figure 16).*

estimation of the central aortic blood pressure waveform is still highly promising alternative to the applanation method.
